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About this Lesson
- Type: Video Tutorial
- Length: 12:26
- Media: Video/mp4
- Use: Watch Online & Download
- Access Period: Unrestricted
- Download: MP4 (iPod compatible)
- Size: 133 MB
- Posted: 07/01/2009
This lesson is part of the following series:
Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/biology. The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.
George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.
About this Author
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I have never understood why when people hear the word organic chemistry they want to run out of the room. Organic chemistry is not as bad as everybody makes it out to be. It is a fascinating subject and you need to understand what we mean by organic chemistry and what the implications of that meaning are.
Well, let's start out with what do we mean by organic chemistry? Organic chemistry is the chemistry of carbon, of carbon compounds that is what I want to talk about. Yes, it can get very complex because of the nature of carbon. You see, carbon can form an unbelievable variety of materials. But, that is the true secret to life, the true secret to life is the fact that you are mostly made out of carbon compounds. And, your organic compounds are what keep you going. I have three words I want you to remember when it comes to organic chemistry. Shape, shape, shape. That is right, shape is everything in biology. The shape of the molecule is what organic chemistry is all about.
Let's just take a look at carbon and compare it to some of the other atoms that are so important in living systems. We remember that bonding takes place from the electrons in the outer shells. And, I hope you remember that hydrogen has one in its outer shell, oxygen has two, and nitrogen has three. But, carbon is unique because in its outer shell there are four electrons, which makes carbon very, very prone to forming covalent bonds. Remember, these are called valence electrons and therefore, when you have a valence electron that is going to be shared and we are going to call that a covalent bond. There will be no overall transfer of electron because the electronegativity is not great enough to rip electrons. Generally speaking, carbon does not have that kind of electronegativity to rip electrons off of other molecules. So, carbon can form lots of different variations. In addition to all of that carbon can form chains and that is where things start getting very, very interesting.
Let's take a look at carbon, I happen to have one right here. This particular carbon, as you can see, carbon has four electrons in its outer shell and we are going to put these four little guys inhere to represent the four bonding electrons, the four valence electrons of carbon. Shape is everything and I want you to get a picture of this carbon and the way it is going to bond in your head, assuming that I can get all of these things the right length.
Now, that being said, remembering that carbon has this shape to it, we want to take a look at how carbon is going to bond. Now, in its simplest form carbon is just going to form something called CH, methane. If this were the end of the story we wouldn't be talking any more because this is just some kind of dead end molecule, it's not going to be bonding with anything because carbon is sharing electrons with four different entities. All four of which are hydrogen, and so this carbon, in sharing the four electrons with hydrogen, has filled its outer shell and that is the end of the story. But it's not, because you see what carbon can do is carbon can do some other things. For example, carbon can actually bond with other carbons and start to form these long chain-like substances.
For example, let's put two of these carbons together and when we can put two of these carbons together we can now bond other things to it. So, for example, we can form a two carbon chain with, look at that, remember, carbon has to have all four of its bonding sites filled. Well, in this case look what we have, C86, so carbon can form chains. Now, I don't have enough of these little beads to show you everything, but we can do some great things.
For example, here is something cool that we do in organic chemistry, we show these bonds with these shared pairs of electrons with lines. So, get used to that. I'm going to do that a lot where I'm going to show bonds, covalent bonds with lines, very typical. This molecule, the one I just built, is ethane. Well, there are other molecules that we can do with carbon too. For example, carbon obviously can add a third one, let's take a look at propane. One, two, three carbons in the chain, that is important. We can keep going, you want four, there's four - butane, but that's not all.
Carbons can do something else that is kind of cool too. One of the things, and boy, talk about stability, watch this. Sometimes carbon will form what are called double bonds. Now, let's see what we are going to do with this one. Let's take this carbon and now I'm going to change my little wooden prongs to springs, this particular carbon has there's two, and let me just throw the other bonds on there too, just so you know that I'm not cheating you here or anything. There we go, one, two, three, four, now, I put these springs in there because I have to illustrate something. There is no such thing as a spring bond or anything like that, but I have to bend them and I can't bend wood with my bare hands.
Now, what I've done here is, and remember carbon has four bonding sites, and now let's think about what is going on here. This carbon has one bonding site filled with this hydrogen and one bonding site filled with this hydrogen. Let's put two more hydrogens on here, one and another one, but look what happens. Now, this molecule looks a little different than the other ones we have had. This molecule will only have four hydrogens. Why? Because you have a double bond between the two carbons. Well, this extremely stable situation is going to cause some real big shape changes.
Now, let's take a look at this for example. Butene, now before you saw the molecule butane and we have the molecule butene, the difference, look there is a double bond right here in one butene and there is a single bond right here in butane. Trust me that is important, are you okay with double bonds? Double bonds are important and that's not all.
Carbon can form branches, now this is where it gets interesting. Let's go back to butane. One, two, three, four right in a line, right? But, it doesn't have to be in a line. For example, we can branch off and form another whole group of carbon chains right up in here. This happens to be something called isobutane, but, nevertheless, you see what is happening here carbon doesn't have to be one big straight line thing. It can be branches and from that branch could come other branches and on, and on, and on it goes.
And, lastly, carbon can form very complex molecules, some of which can be represented by rings. For example, here's carbon and notice it has formed an enclosed structure called a ring. This is called cyclohexane, because it has one, two, three, four, five, six carbons. So, cyclohexane, notice what we have here every carbon, if you count, one, two, three, four, one, two, three, four, I can go all the way around and count four bonds on every carbon. And, I better be able to do that because carbon will tend to have all of its bonds filled in covalent bonding. And, you can even combine some things, look, a ring with double bonds, benzene. How cool is that. You have the ring but notice you don't have as many hydrogens because some of the bonds are taken up by double bonds. Cool stuff.
One more thing I want to tell you about carbon, at least a generic thing about carbon, it forms structures called isomers. How important are isomers? Oh baby, isomers are important. What is an isomer? Well, there are three different kinds of isomer I want to tell you about. The first kind of isomer is called a structural isomer. A structural isomer, boy, we are going to come back to three important words here, shape, shape, shape. Think about it. Let's look at these two, what do you see that is different about them? Well, what you see is that you have the same, if you count carbons, one, two, three, four; one, two, three, four. Count hydrogens, one, two, three, four, five, six, seven, eight, nine, ten; one, two, three, four, five, six, seven, eight, nine, ten; they have the same formula. Yet, these are two completely different molecules that will behave differently. Why? Shape, they have a different shape. Structural isomers have a different arrangement of what we call covalent partners; their arrangements are different. This particular carbon instead of being on the end is in the middle and it makes all the difference in the world. That is a structural isomer.
Let's move to something called the geometric isomer and boy, these are important. In a geometric isomer we get this double bonding effect again. And, you see what happens when you double bond things is that you lose any kind of rotational ability around the axis of the bond. Let me say that again, you lose rotational ability around the bond axis. Whereas a single bond you can get some kind of rotation, the double bond is much more rigid. Now, because of this inflexibility you don't get the freedom of movement and there is no rotation of atoms. Here are two geometric isomers, now, there is no such thing as xs in chemistry, but these could be anything. In this particular geometric isomer notice you have two xs here, in other words, the xs have been redistribute; shape, shape, shape.
Let me give you an example of this. Most of you out there have eyes and do you realize that you have a chemical in your eye called rhodopsin? Now, there is a part of the rhodopsin molecule that is sensitive to light. Guess what happens? When light hits it that thing changes its shape, it actually isomerizes and it is no loner sensitive to light that is how we pick up light messages in our eyes. So, what happens is light beam hits it, the molecule isomerizes and then after a series of reaction changes back so we can pick up light again. Isomers are very important in biological processes.
The very, very last isomer I want to tell you about is enantiomers, sometimes called optical isomers, are kind of like your hands. If you look at your hand you say, oh, they are both the same aren't they? No, one is left and one is right. Try superimposing your hands on top of each other and you can't. They are mirror images of each other. Similarly we have optical isomers, and take a look at this. What has happened in this case is we have the CH, but look what has happened in this situation. The NH and the H have reversed position. See, on this side or on this molecule the NH is on the left and on this molecule it is on the right. Why does that matter in the shape? Well, imagine that this molecule had to sit in some kind of enzyme, shall we say, and the enzyme was expecting the NH to be on the left and you try to hit it with an isomer where the NH is on the right it is not going to work.
So, you see that this mirror image thing can determine how efficient a particular isomer is going to work. We find this a lot when we study pharmacology. If we design an isomer of a certain drug it doesn't work as well as the other one. This whole introduction to carbon becomes crucial. And, the most important thing is shape, don't forget that as we go through carbon chemistry shape is everything.
Inorganic and Organic Chemistry
Carbon Chemistry and Isomers Page [1 of 3]
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