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Let's take a look at the simplest alkenes, so that would just be two carbons that are double bonded to each other. And then I put the hydrogens around each carbon like that. Let's compare that to a two carbon alkanes, so a two- carbon alkane but then have six hydrogens around it like that. So when we did our alkane nomenclature we called a two carbon alkane-- we called this molecule on the right ethane. So this over here is ethane. So you can see the -ane ending for an alkane. And the eth-, of course, since there are two carbons. So if I look at this two carbon alkene over here on the left, I know that the root is going to be eth- so I can write eth- here. The ending, it's an alkene, it's not an alkane, so instead of an -ane ending it's going to get an -ene ending. So this molecule over here is ethene. So an -ene ending. The other name for this molecule is ethylene, so ethene or ethylene, either one is fine. Let's take a look at the molecular formulas for these molecules. So on the right, ethane has two carbons and six hydrogens, like that. So we have the general formula, if you have n carbons you have 2n plus 2 hydrogens, like that. On the left, ethene has a molecular formula C2 H4 and, therefore, its general formula would be if you have n carbons then you have H2n hydrogens, like that. So we can see that ethene doesn't have as many hydrogens as ethane does. Ethane has as many hydrogens as is possible for these carbon atoms, so ethane is said to be saturated with hydrogen. So let's go ahead and write that. This one over here is-- this molecule is saturated with hydrogens. It can't get any more hydrogens. Ethene over here only has four hydrogens, so it could actually add two more. So that's some chemistry that we haven't gotten to yet. But it right now is not completely saturated with hydrogen, so it is said to be unsaturated. So we have unsaturated versus saturated since we have four hydrogens over here and six hydrogens over here. So in future video, we'll see how saturated and unsaturated relate to saturated and unsaturated fat that everyone's heard of before. So let's look at these molecules some more. Let's look at the hybridization states of the carbon atoms. So let's look at the hybridization state of this carbon atom. Well, they are only single bonds around that carbon, therefore that must be an SP3 hybridized carbon, like that. And the exact same situation for this carbon. So each SP3 hybridized carbon is going to exhibit to tetrahedral geometry. What about the carbons over here on the left? Well, this carbon has a double bond, so therefore it be SP2 hybridized. This carbon over here is also SP2 hybridized. SP2 hybridized carbons are trigonal planar in terms of their geometry. So the molecule on the left is flat, the molecule the right is tetrahedral. All single bonds in the molecule on the right. So if I look at all of these bonds, they're all single bonds. They're all sigma bonds, so the bond between my two carbon atoms here is a sigma bond. And sigma bonds allow free rotation. So on the right there is free rotation between the two carbon atoms, and since there's free rotation between those two carbon atoms you can get several different conformations for the ethane molecule. So we saw an earlier video about conformations for ethane. So sigma bonds allow free rotation. On the right, in this double bond, one of these bonds is sigma. So if I look at this double bond here, one of those-- one of those bonds is a sigma bond, and the other bond is a pi bond. So the pi bond helps to stabilize that bond and it prevents any free rotation. So there is no free rotation over here on the left for my double bond. So that's a very important difference. Double bonds have no free rotation. Single bonds, for single bonds on the right, do have free rotation. All right, let's look at some more alkenes. Over here on the left, you can see that we have all hydrogens bonded it to my double bond like that. Let's take off one of those hydrogens and let's put an R group on there. So let's go ahead and put an R group. So I still have my double bond, I still have hydrogens attached to my carbon. I'm going to take off one of those hydrogens and I'm going to put an R group. So I have some sort of-- the rest of the molecule over here-- so it's substituted one time. So I'm going to call this a mono-substituted alkene. Let's take off another hydrogen and let's put on another R group. So I have H, H, R, and then I'll make this R prime, so it's a different R group. So this is a di-substituted alkene. So a mono-substituted alkene, a di-substituted alkene and you can see where I'm going with this. I'll keep this hydrogen, keep this R group, make this R prime, I'll make this R double prime. So this is obviously a tri-substituted alkene. And then, of course, one more. I have one more spot where I can substitute an R group in. So this R, this is R prime, this is R double prime, and then finally this is R triple prime. So I have four different substituents, right? So it's tetra-substituted. Now, the question is, out of these four alkenes, which one is the most stable? So it turns out that the tetra-substituted alkene is the most stable. So the more substituent, more alkyl groups you have, the more stable that makes your molecule. So tetra-substituted is the most stable, followed by tri-substituted alkene, followed by a di-substituted alkene, and finally, a mono-substituted alkene is the least stable out the ones that we have talked about here. So these alkyl groups allow for de-localization of electron density throughout the molecule, and that helps to stabilize the molecule. So the actual explanation to this is fairly complicated. It involved some quantum mechanics. It's very similar to the effect called hyperconjugation, which we will discuss in a later video, the video on carbocations. So this will sort of be explained in that later video. Let's look at two examples and let's try to identify the degree of substitution and identify the stability of some these molecules. So let's take a look at some alkenes here. So let's take this alkene, and then let's take this alkene, and let's compare these two in terms of their stability. So first, if I look at the molecule on the left, I need to think, OK, what degree of substitution does this have? Is this mono-substituted, di-substituted, tri-substituted or tetra-substituted? And the way to do is to focus in on your double bond and think to yourself about what else is in this molecule. Well, I know that there are-- there's a hydrogen here and there's a hydrogen here on that side. So there are two alkyl groups coming off of my double bond. So I think it's obvious that this becomes di-substituted, so this is a di-substituted alkene. Once you've drawn your hydrogens it's much easier to see. On the right over here, if I look at my double bonds, well, I know that this must be a hydrogen right here. So how many different alkyl groups do I have? I have three different alkyl groups coming off of my double bond. So on the right, this one is tri-substituted. So I have a di-substituted alkene on the left, a tri-substituted alkene on the right. The more substituted an alkene is, the more stable it is. So this molecule over here on the right is the more stable of the two.