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The Earth is now starting to get closer to being hospitable to people like us or animals like us. In the last video, we saw during the Proterozoic Eon, oxygen began to accumulate in the atmosphere. This actually caused this first snowball Earth and this mass extinction of all the anaerobic species. But it made conditions suitable for eukaryotic cells. And maybe even more important, these eukaryotic cells were able to form multicellular organisms. And we see where that starts right here on this chart, on this time clark. Multicellular life starts right over here.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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But it made conditions suitable for eukaryotic cells. And maybe even more important, these eukaryotic cells were able to form multicellular organisms. And we see where that starts right here on this chart, on this time clark. Multicellular life starts right over here. And I want to be clear. All of these things are a bit moving targets. As we discover more things in the geological record and we get more tools at our disposal, these numbers get tweaked.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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Multicellular life starts right over here. And I want to be clear. All of these things are a bit moving targets. As we discover more things in the geological record and we get more tools at our disposal, these numbers get tweaked. But they do give you a good sense, based on our current understanding, of when these things start to appear. And coinciding with multicellular life, and this is interesting in its own right because it has its own meta-level effect on evolution, you actually start also having sexual reproduction. And what's interesting about this, why this has such a big impact on evolution, and we talk about it a lot in the biology playlist, is before evolution, variation in DNA had to be completely dependent really on mutations and just random movement around within DNA or maybe some viruses.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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As we discover more things in the geological record and we get more tools at our disposal, these numbers get tweaked. But they do give you a good sense, based on our current understanding, of when these things start to appear. And coinciding with multicellular life, and this is interesting in its own right because it has its own meta-level effect on evolution, you actually start also having sexual reproduction. And what's interesting about this, why this has such a big impact on evolution, and we talk about it a lot in the biology playlist, is before evolution, variation in DNA had to be completely dependent really on mutations and just random movement around within DNA or maybe some viruses. Now with sexual reproduction, you had kind of a systematic mixing of DNA so that you got more variation in the gene pool, which allowed more selection for, or I guess you had more variants to select for. And so you kind of had an acceleration in the actual pace of evolution. So that's what we're talking.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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And what's interesting about this, why this has such a big impact on evolution, and we talk about it a lot in the biology playlist, is before evolution, variation in DNA had to be completely dependent really on mutations and just random movement around within DNA or maybe some viruses. Now with sexual reproduction, you had kind of a systematic mixing of DNA so that you got more variation in the gene pool, which allowed more selection for, or I guess you had more variants to select for. And so you kind of had an acceleration in the actual pace of evolution. So that's what we're talking. I've looked at a bunch of sources from, they say, 1.2 billion, 1.5 billion, a little bit over a billion if you score a little bit several hundred million years ago. You start having these multicellular life forms and sexual reproduction. The other thing that we talked about in the proterozoic eon is the accumulation of oxygen allowed the ozone layer to build up.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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So that's what we're talking. I've looked at a bunch of sources from, they say, 1.2 billion, 1.5 billion, a little bit over a billion if you score a little bit several hundred million years ago. You start having these multicellular life forms and sexual reproduction. The other thing that we talked about in the proterozoic eon is the accumulation of oxygen allowed the ozone layer to build up. Ozone is just three oxygen atoms. It is O3. And by the end of the proterozoic eon, so we're talking, I don't know, maybe 550 million years ago, give or take, tens of or maybe even 100 million years.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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The other thing that we talked about in the proterozoic eon is the accumulation of oxygen allowed the ozone layer to build up. Ozone is just three oxygen atoms. It is O3. And by the end of the proterozoic eon, so we're talking, I don't know, maybe 550 million years ago, give or take, tens of or maybe even 100 million years. These are all moving targets. The ozone layer was dense enough to protect the land from UV rays. We talked about that in the last video.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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And by the end of the proterozoic eon, so we're talking, I don't know, maybe 550 million years ago, give or take, tens of or maybe even 100 million years. These are all moving targets. The ozone layer was dense enough to protect the land from UV rays. We talked about that in the last video. The Earth is being bombarded with UV rays. And the ozone layer is the only thing that really keeps us from being seriously irradiated by the sun and allows land animals to actually live. And so coinciding with that time period, around 550 million years ago, you start to have life colonizing, especially significant life colonizing land.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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We talked about that in the last video. The Earth is being bombarded with UV rays. And the ozone layer is the only thing that really keeps us from being seriously irradiated by the sun and allows land animals to actually live. And so coinciding with that time period, around 550 million years ago, you start to have life colonizing, especially significant life colonizing land. So life colonizes land. And this was kind of an interesting, when I first learned it, it was kind of an aha moment. You always assume that trees and grasses are kind of part of the background.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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And so coinciding with that time period, around 550 million years ago, you start to have life colonizing, especially significant life colonizing land. So life colonizes land. And this was kind of an interesting, when I first learned it, it was kind of an aha moment. You always assume that trees and grasses are kind of part of the background. They come part and parcel with land. But it turns out that animals colonized land before plants did. Plants didn't come into the picture until about 450 million years ago, give or take a few tens of millions of years.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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You always assume that trees and grasses are kind of part of the background. They come part and parcel with land. But it turns out that animals colonized land before plants did. Plants didn't come into the picture until about 450 million years ago, give or take a few tens of millions of years. And so we're now entering the end of the Proterozoic Eon. Life has started to colonize land. We now have an ozone layer.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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Plants didn't come into the picture until about 450 million years ago, give or take a few tens of millions of years. And so we're now entering the end of the Proterozoic Eon. Life has started to colonize land. We now have an ozone layer. And what happens, and actually there's another snowball glaciation or snowball Earth near the end of the Proterozoic Eon, I should say. And there's a bunch of theories about why it came about and then why it disappeared. Maybe there were volcanoes, greenhouse gases, who knows.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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We now have an ozone layer. And what happens, and actually there's another snowball glaciation or snowball Earth near the end of the Proterozoic Eon, I should say. And there's a bunch of theories about why it came about and then why it disappeared. Maybe there were volcanoes, greenhouse gases, who knows. But as we enter the end of that, we start seeing life begin to flourish. And it starts to really flourish as we enter the Phanerozoic Eon. And it's not even labeled here.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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Maybe there were volcanoes, greenhouse gases, who knows. But as we enter the end of that, we start seeing life begin to flourish. And it starts to really flourish as we enter the Phanerozoic Eon. And it's not even labeled here. The Phanerozoic Eon is this chunk of time right over here. And let me write it out. This right over here is the Phanerozoic Eon.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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And it's not even labeled here. The Phanerozoic Eon is this chunk of time right over here. And let me write it out. This right over here is the Phanerozoic Eon. And so this chart, these divisions right here are eons. And then they jump into, instead of doing eons here, they then break into eras. Eras are subsets of eons.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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This right over here is the Phanerozoic Eon. And so this chart, these divisions right here are eons. And then they jump into, instead of doing eons here, they then break into eras. Eras are subsets of eons. They're hundreds of millions of years. So this is the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. And that's actually our current era.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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Eras are subsets of eons. They're hundreds of millions of years. So this is the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. And that's actually our current era. But perhaps the most interesting, well, I don't want to pick favorites here, but it's one of the most interesting times in the geologic era, is the first period in the Paleozoic Era, which is the first era in the Phanerozoic Eon. And that's the Cambrian Period. You might have heard of it before.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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And that's actually our current era. But perhaps the most interesting, well, I don't want to pick favorites here, but it's one of the most interesting times in the geologic era, is the first period in the Paleozoic Era, which is the first era in the Phanerozoic Eon. And that's the Cambrian Period. You might have heard of it before. The Cambrian Period. That's about this period of time right over here. And during this period of time, the Earth experiences what we call the Cambrian Explosion.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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You might have heard of it before. The Cambrian Period. That's about this period of time right over here. And during this period of time, the Earth experiences what we call the Cambrian Explosion. And that's because there's just this explosion in the number of species and genera that existed, the biodiversity, on the planet. It might just be that we had the ozone layer protecting us. Things were colonizing land.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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And during this period of time, the Earth experiences what we call the Cambrian Explosion. And that's because there's just this explosion in the number of species and genera that existed, the biodiversity, on the planet. It might just be that we had the ozone layer protecting us. Things were colonizing land. It was an oxygen-rich environment. We start seeing complex, multicellular organisms. It's about that time, if you fast forward maybe a few tens of millions of years, you start seeing the first fish, the first kind of pre-amphibians or proto-amphibians.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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Things were colonizing land. It was an oxygen-rich environment. We start seeing complex, multicellular organisms. It's about that time, if you fast forward maybe a few tens of millions of years, you start seeing the first fish, the first kind of pre-amphibians or proto-amphibians. You fast forward a little bit, as we get out of the Cambrian Period, we start seeing plants. So they actually draw it right over here on this land plants, or at this point right over here. And of course, these are moving targets, depending on what we discover in the fossil record.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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It's about that time, if you fast forward maybe a few tens of millions of years, you start seeing the first fish, the first kind of pre-amphibians or proto-amphibians. You fast forward a little bit, as we get out of the Cambrian Period, we start seeing plants. So they actually draw it right over here on this land plants, or at this point right over here. And of course, these are moving targets, depending on what we discover in the fossil record. And for me, the big aha moment here is so many of these things that you consider fundamental to what Earth is are relatively recent phenomena. Plants weren't on land until about 450 million years ago. Insects weren't on land, or did not even exist, until about 400 million years ago.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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And of course, these are moving targets, depending on what we discover in the fossil record. And for me, the big aha moment here is so many of these things that you consider fundamental to what Earth is are relatively recent phenomena. Plants weren't on land until about 450 million years ago. Insects weren't on land, or did not even exist, until about 400 million years ago. Reptiles didn't exist until about 300 million years ago. So we're about right over here now. Mammals didn't exist until about 200 million years ago.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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Insects weren't on land, or did not even exist, until about 400 million years ago. Reptiles didn't exist until about 300 million years ago. So we're about right over here now. Mammals didn't exist until about 200 million years ago. Birds didn't exist until about 150 million years ago. The whole dinosaur age, which we kind of consider in our distant past, that's essentially the Mesozoic Era right here. So this is the age of the dinosaurs right over here.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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Mammals didn't exist until about 200 million years ago. Birds didn't exist until about 150 million years ago. The whole dinosaur age, which we kind of consider in our distant past, that's essentially the Mesozoic Era right here. So this is the age of the dinosaurs right over here. When you look at your time clock, you can see it's a relatively recent time period. And it actually ends with, we currently believe, a huge rock, a six mile in diameter rock, colliding with what is now the Yucatan Peninsula in Mexico, or right off the coast of the Yucatan Peninsula. And it destroyed all of the large land life forms, especially the dinosaurs.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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So this is the age of the dinosaurs right over here. When you look at your time clock, you can see it's a relatively recent time period. And it actually ends with, we currently believe, a huge rock, a six mile in diameter rock, colliding with what is now the Yucatan Peninsula in Mexico, or right off the coast of the Yucatan Peninsula. And it destroyed all of the large land life forms, especially the dinosaurs. And to put all of this in perspective, and actually the thing that really was an aha moment for me, plants are 450 million years ago. Grass, I kind of use this fundamental thing in nature. But grass has only been around for about, I've seen multiple estimates, 40 to 70 million years.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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And it destroyed all of the large land life forms, especially the dinosaurs. And to put all of this in perspective, and actually the thing that really was an aha moment for me, plants are 450 million years ago. Grass, I kind of use this fundamental thing in nature. But grass has only been around for about, I've seen multiple estimates, 40 to 70 million years. Grass is a relatively new thing on the planet. Flowers have only been around for 130 million years. So there was a time where you had dinosaurs, but you did not have flowers, and you did not have grass.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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But grass has only been around for about, I've seen multiple estimates, 40 to 70 million years. Grass is a relatively new thing on the planet. Flowers have only been around for 130 million years. So there was a time where you had dinosaurs, but you did not have flowers, and you did not have grass. And so you fast forward all the way. And so when you look at this scale, it's kind of funny to look at. They say, OK, this is the time period where the dinosaurs showed up.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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So there was a time where you had dinosaurs, but you did not have flowers, and you did not have grass. And so you fast forward all the way. And so when you look at this scale, it's kind of funny to look at. They say, OK, this is the time period where the dinosaurs showed up. This whole brown line is where the mammals showed up. So the dinosaurs started to show up along with the mammals. And then, of course, the dinosaurs died out here.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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They say, OK, this is the time period where the dinosaurs showed up. This whole brown line is where the mammals showed up. So the dinosaurs started to show up along with the mammals. And then, of course, the dinosaurs died out here. Our ancestors, when the giant rock hit the Earth, must have been burrowed in holes or able to stash some food away or who knows what, and didn't get fully affected. I'm sure most of the large mammals were destroyed. But what's almost humbling or almost humorous or almost ridiculous when you look at this chart is they put a little dot.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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And then, of course, the dinosaurs died out here. Our ancestors, when the giant rock hit the Earth, must have been burrowed in holes or able to stash some food away or who knows what, and didn't get fully affected. I'm sure most of the large mammals were destroyed. But what's almost humbling or almost humorous or almost ridiculous when you look at this chart is they put a little dot. You can't even see it here. They say 2 million years ago, the first humans. And even this is being pretty generous when they say first humans.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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But what's almost humbling or almost humorous or almost ridiculous when you look at this chart is they put a little dot. You can't even see it here. They say 2 million years ago, the first humans. And even this is being pretty generous when they say first humans. These are really the first pre-humans, the first humans that are the same as us. If you took one of those babies and you brought them up in the suburbs and gave them haircuts and stuff, they would be the same thing as we are. Those didn't exist until 200,000 years ago, give or take.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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And even this is being pretty generous when they say first humans. These are really the first pre-humans, the first humans that are the same as us. If you took one of those babies and you brought them up in the suburbs and gave them haircuts and stuff, they would be the same thing as we are. Those didn't exist until 200,000 years ago, give or take. 200,000 to 400,000 years ago, I've seen estimates. So this is actually a very generous period of time to say first humans. It's actually 200,000 years ago.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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Those didn't exist until 200,000 years ago, give or take. 200,000 to 400,000 years ago, I've seen estimates. So this is actually a very generous period of time to say first humans. It's actually 200,000 years ago. And just to give you an idea of how new we are and how new our evolution is, it was only 5 million years ago. And I mentioned this in a previous video. It was only 5 million years ago.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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It's actually 200,000 years ago. And just to give you an idea of how new we are and how new our evolution is, it was only 5 million years ago. And I mentioned this in a previous video. It was only 5 million years ago. So this is just to get a sense. This is zero years. Homo sapiens sapien, only around for 200,000 years.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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It was only 5 million years ago. So this is just to get a sense. This is zero years. Homo sapiens sapien, only around for 200,000 years. The Neanderthals, they were cousin species. They weren't our ancestors. Many people think they were.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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Homo sapiens sapien, only around for 200,000 years. The Neanderthals, they were cousin species. They weren't our ancestors. Many people think they were. They were cousin species. We come from the same root. Although there are now theories that they might have remixed in with homo sapiens.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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Many people think they were. They were cousin species. We come from the same root. Although there are now theories that they might have remixed in with homo sapiens. So maybe some of us have some Neanderthal DNA. And it shouldn't be viewed as an insult. They had big brains.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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Although there are now theories that they might have remixed in with homo sapiens. So maybe some of us have some Neanderthal DNA. And it shouldn't be viewed as an insult. They had big brains. Well, they didn't necessarily have big brains. They had big heads. But that seems to imply a big brain.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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They had big brains. Well, they didn't necessarily have big brains. They had big heads. But that seems to imply a big brain. But who knows? We always tend to portray them as somehow inferior. But I don't want to get into the political correctness of how to portray Neanderthals.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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But that seems to imply a big brain. But who knows? We always tend to portray them as somehow inferior. But I don't want to get into the political correctness of how to portray Neanderthals. But anyway, this is a very small period of time. If you go 2 million years, then you get to the pre-human ancestors. And our family tree only diverged from the chimpanzees 5 million years ago.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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But I don't want to get into the political correctness of how to portray Neanderthals. But anyway, this is a very small period of time. If you go 2 million years, then you get to the pre-human ancestors. And our family tree only diverged from the chimpanzees 5 million years ago. If you draw that on this clock right here, it would be like 2 pixels, or maybe not even 2 pixels, is when we diverged from the chimpanzees. So hopefully that gives you a sense of things. At least for me, it really puts things in perspective.
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Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3
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So one definition for formal charge is the hypothetical charge that would result if all bonding electrons are shared equally. So let's go down to the dot structure on the left here, which is the dot structure for methanol, and let's assign a formal charge to carbon. We need to think about the bonding electrons, or the electrons in those bonds around carbon, and we know that each bond consists of two electrons. So the bond between oxygen and carbon consists of two electrons. Let me go ahead and draw in those two electrons. Same for the bond between carbon and hydrogen. Each bond consists of two electrons, so I can go around and put in all of my bonding electrons.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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So the bond between oxygen and carbon consists of two electrons. Let me go ahead and draw in those two electrons. Same for the bond between carbon and hydrogen. Each bond consists of two electrons, so I can go around and put in all of my bonding electrons. So if we want to assign a formal charge to carbon, we need to think about the number of valence electrons in the free atom, or the number of valence electrons that carbon is supposed to have. We already know that carbon is supposed to have four valence electrons. So I can put a four here, and from that four, we're going to subtract the number of valence electrons in the bonded atom, or the number of valence electrons that carbon has around it in our drawing.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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Each bond consists of two electrons, so I can go around and put in all of my bonding electrons. So if we want to assign a formal charge to carbon, we need to think about the number of valence electrons in the free atom, or the number of valence electrons that carbon is supposed to have. We already know that carbon is supposed to have four valence electrons. So I can put a four here, and from that four, we're going to subtract the number of valence electrons in the bonded atom, or the number of valence electrons that carbon has around it in our drawing. And since we're doing formal charge, we need to think about all those bonding electrons being shared equally. So we think about a covalent bond. So if we have two electrons in one bond, and those two electrons are shared equally, we could split them up.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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So I can put a four here, and from that four, we're going to subtract the number of valence electrons in the bonded atom, or the number of valence electrons that carbon has around it in our drawing. And since we're doing formal charge, we need to think about all those bonding electrons being shared equally. So we think about a covalent bond. So if we have two electrons in one bond, and those two electrons are shared equally, we could split them up. We could give one electron to oxygen, and one electron to carbon in that bond. We go over here to this carbon-hydrogen bond, and we could do the same thing. We have two electrons, we could split up those two electrons.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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So if we have two electrons in one bond, and those two electrons are shared equally, we could split them up. We could give one electron to oxygen, and one electron to carbon in that bond. We go over here to this carbon-hydrogen bond, and we could do the same thing. We have two electrons, we could split up those two electrons. We could give one to carbon, and one to hydrogen. And we go all the way around, and we do the same thing over here, split up those electrons, and the same thing here. So how many valence electrons do we see around carbon now?
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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We have two electrons, we could split up those two electrons. We could give one to carbon, and one to hydrogen. And we go all the way around, and we do the same thing over here, split up those electrons, and the same thing here. So how many valence electrons do we see around carbon now? So let me go ahead and highlight them. There's one, two, three, and four. So that's the number of valence electrons around carbon in our drawing.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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So how many valence electrons do we see around carbon now? So let me go ahead and highlight them. There's one, two, three, and four. So that's the number of valence electrons around carbon in our drawing. So four minus four is equal to zero. So zero is the formal charge of carbon. So let me go ahead and highlight that here.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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So that's the number of valence electrons around carbon in our drawing. So four minus four is equal to zero. So zero is the formal charge of carbon. So let me go ahead and highlight that here. So in this molecule, the formal charge for carbon is zero. Now let's move on to oxidation states. So you could also call these oxidation numbers.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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So let me go ahead and highlight that here. So in this molecule, the formal charge for carbon is zero. Now let's move on to oxidation states. So you could also call these oxidation numbers. So one definition for an oxidation state is the hypothetical charge that would result if all of those bonding electrons are assigned to the more electronegative atom in the bond. So let's go to the dot structure on the right of methanol, and let's assign an oxidation state to that carbon. We need to think about our bonding electrons again, so let's go ahead and put those in.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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So you could also call these oxidation numbers. So one definition for an oxidation state is the hypothetical charge that would result if all of those bonding electrons are assigned to the more electronegative atom in the bond. So let's go to the dot structure on the right of methanol, and let's assign an oxidation state to that carbon. We need to think about our bonding electrons again, so let's go ahead and put those in. So we know that each bond consists of two electrons, so I'm putting in the two electrons in each bond. And let's think about the oxidation state of that carbon. Well, first, we need to know the number of valence electrons in the free atom.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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We need to think about our bonding electrons again, so let's go ahead and put those in. So we know that each bond consists of two electrons, so I'm putting in the two electrons in each bond. And let's think about the oxidation state of that carbon. Well, first, we need to know the number of valence electrons in the free atom. So just like before, we know that carbon is supposed to have four valence electrons. So this would be a four. And from that, we subtract the number of valence electrons in the bonded atom, or the number of valence electrons that carbon actually has in the drawing.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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Well, first, we need to know the number of valence electrons in the free atom. So just like before, we know that carbon is supposed to have four valence electrons. So this would be a four. And from that, we subtract the number of valence electrons in the bonded atom, or the number of valence electrons that carbon actually has in the drawing. This time, we need to think about an ionic bond, so we're going to pretend like a covalent bond is an ionic bond because we're going to assign all of the bonding electrons to the more electronegative atom. So there's no more sharing here. The winner takes all, the more electronegative atom is going to get all of the electrons.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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And from that, we subtract the number of valence electrons in the bonded atom, or the number of valence electrons that carbon actually has in the drawing. This time, we need to think about an ionic bond, so we're going to pretend like a covalent bond is an ionic bond because we're going to assign all of the bonding electrons to the more electronegative atom. So there's no more sharing here. The winner takes all, the more electronegative atom is going to get all of the electrons. So let's think about the electronegativities of carbon versus oxygen. We know that oxygen is more electronegative than carbon, so oxygen takes both of those electrons in that bond, so oxygen gets both of those electrons. Next, let's think about the electronegativities of carbon and hydrogen.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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The winner takes all, the more electronegative atom is going to get all of the electrons. So let's think about the electronegativities of carbon versus oxygen. We know that oxygen is more electronegative than carbon, so oxygen takes both of those electrons in that bond, so oxygen gets both of those electrons. Next, let's think about the electronegativities of carbon and hydrogen. We know that carbon is a little bit more electronegative than hydrogen. So for these two electrons, carbon's going to take both of them since carbon is more electronegative than hydrogen. And the same thing for our other carbon-hydrogen bonds.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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Next, let's think about the electronegativities of carbon and hydrogen. We know that carbon is a little bit more electronegative than hydrogen. So for these two electrons, carbon's going to take both of them since carbon is more electronegative than hydrogen. And the same thing for our other carbon-hydrogen bonds. Carbon is more electronegative than hydrogen, so carbon takes those. Carbon is more electronegative than hydrogen, so carbon takes those. And so how many electrons do we have around carbon now?
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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And the same thing for our other carbon-hydrogen bonds. Carbon is more electronegative than hydrogen, so carbon takes those. Carbon is more electronegative than hydrogen, so carbon takes those. And so how many electrons do we have around carbon now? Let's count them up. That's one, two, three, four, five, and six. So now we have six electrons around carbon.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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And so how many electrons do we have around carbon now? Let's count them up. That's one, two, three, four, five, and six. So now we have six electrons around carbon. So four minus six gives us negative two. So here, in this example, carbon has an oxidation state of negative two. So there's no more sharing when you're doing oxidation states, right?
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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So now we have six electrons around carbon. So four minus six gives us negative two. So here, in this example, carbon has an oxidation state of negative two. So there's no more sharing when you're doing oxidation states, right? Think about the more electronegative atom and assign both electrons to the more electronegative atom. Both formal charge and oxidation states are just really extreme methods of electron bookkeeping. They're not perfect.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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So there's no more sharing when you're doing oxidation states, right? Think about the more electronegative atom and assign both electrons to the more electronegative atom. Both formal charge and oxidation states are just really extreme methods of electron bookkeeping. They're not perfect. They're certainly not perfect. We're assuming that the electrons are either shared equally, perfectly, or that one atom takes both electrons, and neither of those concepts is perfect in the real world. But it works when we're drawing our dot structures and we're thinking about chemical reactions.
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Comparing formal charges to oxidation states Organic chemistry Khan Academy.mp3
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So what happens is those two hydrogens from the hydrogen gas are added across your double bond, and they're added on the same side of where the double bond used to be. So it's a syn addition. Let's take a look at why this is a syn addition of hydrogens. So we have our metal catalyst over here. So let's go ahead and draw our flat metal catalyst. And these metals are chosen because they adsorb hydrogen really well, which means that if you bubble hydrogen gas through, the hydrogen is going to be adsorbed to the surface of that metal catalyst like that. And then your alkene comes along, and your alkene is also flat.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So we have our metal catalyst over here. So let's go ahead and draw our flat metal catalyst. And these metals are chosen because they adsorb hydrogen really well, which means that if you bubble hydrogen gas through, the hydrogen is going to be adsorbed to the surface of that metal catalyst like that. And then your alkene comes along, and your alkene is also flat. The portion of the molecule that contains the double bond. So these two carbons, this carbon, this carbon, are sp2 hybridized, which means that the stereochemistry around those two carbons, it's going to be flat. So this portion of the molecule is flat.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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And then your alkene comes along, and your alkene is also flat. The portion of the molecule that contains the double bond. So these two carbons, this carbon, this carbon, are sp2 hybridized, which means that the stereochemistry around those two carbons, it's going to be flat. So this portion of the molecule is flat. So you have one thing that's flat approaching something else that's flat. So the only way those hydrogens can add are to add them onto the same side. So if this carbon and this carbon, if you add this hydrogen to the carbon on the left and add this hydrogen to the carbon on the right, and then you go ahead and you draw the rest of the bonds, this would now be a wedge and then a dash, and then this would be a wedge and a dash.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So this portion of the molecule is flat. So you have one thing that's flat approaching something else that's flat. So the only way those hydrogens can add are to add them onto the same side. So if this carbon and this carbon, if you add this hydrogen to the carbon on the left and add this hydrogen to the carbon on the right, and then you go ahead and you draw the rest of the bonds, this would now be a wedge and then a dash, and then this would be a wedge and a dash. You can see those two hydrogens have added onto the same side. So these two hydrogens are these two hydrogens for our syn addition. Notice we're also changing from sp2 hybridization to sp3 hybridization over here on the right.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So if this carbon and this carbon, if you add this hydrogen to the carbon on the left and add this hydrogen to the carbon on the right, and then you go ahead and you draw the rest of the bonds, this would now be a wedge and then a dash, and then this would be a wedge and a dash. You can see those two hydrogens have added onto the same side. So these two hydrogens are these two hydrogens for our syn addition. Notice we're also changing from sp2 hybridization to sp3 hybridization over here on the right. So we have to think about stereochemistry for this reaction for your products as well. So let's take a look at an actual reaction here. And let's see if we can follow along.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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Notice we're also changing from sp2 hybridization to sp3 hybridization over here on the right. So we have to think about stereochemistry for this reaction for your products as well. So let's take a look at an actual reaction here. And let's see if we can follow along. So if this was my reaction, I want to hydrogenate this alkene. So I would add some hydrogen gas, and I could choose whichever metal catalyst I wanted to. I would add two hydrogens on the same side.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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And let's see if we can follow along. So if this was my reaction, I want to hydrogenate this alkene. So I would add some hydrogen gas, and I could choose whichever metal catalyst I wanted to. I would add two hydrogens on the same side. So I could add two hydrogens on the same side, just like I did up there. So we would get now everything changes from sp2 hybridization to sp3, so we have wedges and dashes to worry about. And usually you wouldn't see it drawn like this.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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I would add two hydrogens on the same side. So I could add two hydrogens on the same side, just like I did up there. So we would get now everything changes from sp2 hybridization to sp3, so we have wedges and dashes to worry about. And usually you wouldn't see it drawn like this. That's too much work, quite frankly. It would be much easier just to say, oh, well, all I have to do is take away the double bond, and there's my product. So for some of these reactions, they're very, very, very simple.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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And usually you wouldn't see it drawn like this. That's too much work, quite frankly. It would be much easier just to say, oh, well, all I have to do is take away the double bond, and there's my product. So for some of these reactions, they're very, very, very simple. Just take away the double bond, and you'll end up with your alkane-like product. Let's take a look at oxidation states for this reaction. So I'm going to redraw this reaction.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So for some of these reactions, they're very, very, very simple. Just take away the double bond, and you'll end up with your alkane-like product. Let's take a look at oxidation states for this reaction. So I'm going to redraw this reaction. And this time I'm going to draw in my atoms. And I'm also going to draw in my electrons here in a second. So I'm just drawing out all the atoms here, so I have all these methyl groups to worry about.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So I'm going to redraw this reaction. And this time I'm going to draw in my atoms. And I'm also going to draw in my electrons here in a second. So I'm just drawing out all the atoms here, so I have all these methyl groups to worry about. And then I have electrons in these bonds. Each one of these bonds consists of two electrons. I'm going to go ahead and put in all of my electrons here like that.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So I'm just drawing out all the atoms here, so I have all these methyl groups to worry about. And then I have electrons in these bonds. Each one of these bonds consists of two electrons. I'm going to go ahead and put in all of my electrons here like that. Now let's assign oxidation states to those two carbons that formed our double bond. So let's look at oxidation state for the top carbon. Remember, when you're doing oxidation states, you're worried about electronegativity.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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I'm going to go ahead and put in all of my electrons here like that. Now let's assign oxidation states to those two carbons that formed our double bond. So let's look at oxidation state for the top carbon. Remember, when you're doing oxidation states, you're worried about electronegativity. So oxidation states are all about electronegativity. So go back and watch the earlier video on oxidation states. So we have here comparing the electronegativities of carbon and carbon.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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Remember, when you're doing oxidation states, you're worried about electronegativity. So oxidation states are all about electronegativity. So go back and watch the earlier video on oxidation states. So we have here comparing the electronegativities of carbon and carbon. Well, obviously they're the exact same. So in the struggle for these electrons, it gets divided. Each one of these carbons is going to get one of these electrons.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So we have here comparing the electronegativities of carbon and carbon. Well, obviously they're the exact same. So in the struggle for these electrons, it gets divided. Each one of these carbons is going to get one of these electrons. So that's the case for all of these right here. So that carbon has four electrons around it. It's the exact same thing for this carbon.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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Each one of these carbons is going to get one of these electrons. So that's the case for all of these right here. So that carbon has four electrons around it. It's the exact same thing for this carbon. This carbon has four electrons around it. To assign an oxidation state, we take the number of valence electrons that atom usually has, which carbon normally has four, of course. And from that, we subtract the number of electrons we just drew around it for our dot structure.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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It's the exact same thing for this carbon. This carbon has four electrons around it. To assign an oxidation state, we take the number of valence electrons that atom usually has, which carbon normally has four, of course. And from that, we subtract the number of electrons we just drew around it for our dot structure. So that would be four. Each one of those carbons has four. So each of these carbons has an oxidation state of zero.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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And from that, we subtract the number of electrons we just drew around it for our dot structure. So that would be four. Each one of those carbons has four. So each of these carbons has an oxidation state of zero. Let's look at the product, and let's see if we can assign some oxidation states for the product. So our product over here on the right, we had a carbon, and we had some methyl groups bonded to that carbon. We added on a hydrogen.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So each of these carbons has an oxidation state of zero. Let's look at the product, and let's see if we can assign some oxidation states for the product. So our product over here on the right, we had a carbon, and we had some methyl groups bonded to that carbon. We added on a hydrogen. So each one of these carbons got a hydrogen added onto it. And let's go ahead and fill in our electrons in these bonds. So once again, each bond consists of two electrons like that.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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We added on a hydrogen. So each one of these carbons got a hydrogen added onto it. And let's go ahead and fill in our electrons in these bonds. So once again, each bond consists of two electrons like that. And now we have a single bond between our carbons. And let's assign some oxidation states. So once again, we know that the two carbons have the same electronegativity.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So once again, each bond consists of two electrons like that. And now we have a single bond between our carbons. And let's assign some oxidation states. So once again, we know that the two carbons have the same electronegativity. So the tug of war for these two electrons right here, it's a tie. So it's a tie. It's a tie.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So once again, we know that the two carbons have the same electronegativity. So the tug of war for these two electrons right here, it's a tie. So it's a tie. It's a tie. What about carbon versus hydrogen? Carbon is actually more electronegative than hydrogen. So in the war over the two electrons in the carbon-hydrogen bond, carbon wins because it's a little bit more electronegative.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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It's a tie. What about carbon versus hydrogen? Carbon is actually more electronegative than hydrogen. So in the war over the two electrons in the carbon-hydrogen bond, carbon wins because it's a little bit more electronegative. So we're going to assign this extra electron here to carbon. And then again, carbon versus carbon, so that carbon gets that electron as well. Same thing down here.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So in the war over the two electrons in the carbon-hydrogen bond, carbon wins because it's a little bit more electronegative. So we're going to assign this extra electron here to carbon. And then again, carbon versus carbon, so that carbon gets that electron as well. Same thing down here. So it's a tie. It's a tie. Carbon beats hydrogen.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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Same thing down here. So it's a tie. It's a tie. Carbon beats hydrogen. And over here, it's a tie. So in the dot structure on the right, the oxidation states, the normal number of valence electrons would be 4. From that, we subtract the number of electrons in our picture here, which would be 5 electrons.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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Carbon beats hydrogen. And over here, it's a tie. So in the dot structure on the right, the oxidation states, the normal number of valence electrons would be 4. From that, we subtract the number of electrons in our picture here, which would be 5 electrons. Each one of these carbons has 5 electrons around it. So it gained electron. So 4 minus 5 will give us a negative 1.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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From that, we subtract the number of electrons in our picture here, which would be 5 electrons. Each one of these carbons has 5 electrons around it. So it gained electron. So 4 minus 5 will give us a negative 1. So the oxidation states of these two carbons is negative 1. And we can look at our original oxidation states of being 0. Went from 0 to negative 1.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So 4 minus 5 will give us a negative 1. So the oxidation states of these two carbons is negative 1. And we can look at our original oxidation states of being 0. Went from 0 to negative 1. That's a decrease in the oxidation state. A decrease in the oxidation state means reduction. So this is a reduction reaction.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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Went from 0 to negative 1. That's a decrease in the oxidation state. A decrease in the oxidation state means reduction. So this is a reduction reaction. So the alkene is reduced by the addition of these two hydrogens. And you'll see other definitions for oxidation states. You'll see a gain in hydrogens is reduction.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So this is a reduction reaction. So the alkene is reduced by the addition of these two hydrogens. And you'll see other definitions for oxidation states. You'll see a gain in hydrogens is reduction. That's another definition that's often found in organic chemistry textbooks. And while that's true, to me it makes more sense to go ahead and assign your oxidation states and watch the oxidation states change as you add those hydrogens, as your molecule gains hydrogens. So this is a reduction.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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You'll see a gain in hydrogens is reduction. That's another definition that's often found in organic chemistry textbooks. And while that's true, to me it makes more sense to go ahead and assign your oxidation states and watch the oxidation states change as you add those hydrogens, as your molecule gains hydrogens. So this is a reduction. Let's look at the stereochemistry of the hydrogenation reaction. So let's do an example involving stereochemistry. So let's say your alkene, let's do that ring again.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So this is a reduction. Let's look at the stereochemistry of the hydrogenation reaction. So let's do an example involving stereochemistry. So let's say your alkene, let's do that ring again. Wasn't a very good one. So let's say your alkene looked something like this. And you're going to react that with hydrogen and with platinum.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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So let's say your alkene, let's do that ring again. Wasn't a very good one. So let's say your alkene looked something like this. And you're going to react that with hydrogen and with platinum. Well, your first thought might be, OK, this is simple. All I have to do is take away that double bond and I'm done. Well, sometimes that's true.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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And you're going to react that with hydrogen and with platinum. Well, your first thought might be, OK, this is simple. All I have to do is take away that double bond and I'm done. Well, sometimes that's true. But in this case, we actually formed two new chirality centers. So this top carbon here is a chirality center. And this bottom carbon here is also a chirality center.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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Well, sometimes that's true. But in this case, we actually formed two new chirality centers. So this top carbon here is a chirality center. And this bottom carbon here is also a chirality center. So sometimes it's not quite that simple. We need to think about the syn addition of those hydrogens. We need to think about the possible products that would result.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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And this bottom carbon here is also a chirality center. So sometimes it's not quite that simple. We need to think about the syn addition of those hydrogens. We need to think about the possible products that would result. So we're going to get two products here. Let's look at the one on the left. Well, one possibility is I could add those two hydrogens on the same side as a wedge.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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We need to think about the possible products that would result. So we're going to get two products here. Let's look at the one on the left. Well, one possibility is I could add those two hydrogens on the same side as a wedge. So I have one hydrogen as a wedge, the other hydrogen as a wedge. That's our syn addition. And that means that at this top carbon here, this ethyl group must be going away from me.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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Well, one possibility is I could add those two hydrogens on the same side as a wedge. So I have one hydrogen as a wedge, the other hydrogen as a wedge. That's our syn addition. And that means that at this top carbon here, this ethyl group must be going away from me. So there's my ethyl group going away from me. And down here at the bottom carbon, the methyl group must be going away from me. So that's one possible product.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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And that means that at this top carbon here, this ethyl group must be going away from me. So there's my ethyl group going away from me. And down here at the bottom carbon, the methyl group must be going away from me. So that's one possible product. The other possibility, instead of having my two hydrogens add as wedges, I can have my two hydrogens add as dashes. So there's a hydrogen, and then here's a dash, and there's a hydrogen. So at this top carbon here, now my ethyl group is coming out at me.
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Hydrogenation Alkenes and Alkynes Organic chemistry Khan Academy.mp3
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