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Organic Chemistry Hydrocarbons Alkenes and Alkynes Page [1 of 2] We’Ve Already Talked About the Structure of Alkanes and the Activity of Alkanes

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Organic Chemistry Hydrocarbons Alkenes and Alkynes Page [1 of 2] We’Ve Already Talked About the Structure of Alkanes and the Activity of Alkanes Organic Chemistry Hydrocarbons Alkenes and Alkynes Page [1 of 2] We’ve already talked about the structure of alkanes and the activity of alkanes. Now we are going to talk about the structure and activity of alkenes and alkynes. Alkenes and alkynes are in related to the alkanes, in that they are hydrocarbons. They are composed of C and H in the molecules. But they differ in that they have pi bonds. An alkene contains one pi bond and one sigma bond. Alkynes contain two pi bonds and one sigma bond. This has very profound effects on the structure as well as the reactivity. First of all alkenes are plainer. Each carbon is hybridized, so that means that overall the molecule is consisting of two hybridized carbons and in order to form the pi bond, the p orbital of each of those two carbons is used to overlap to make the pi bond up. So what happens in the end is that in order for the p orbitals to overlap, the carbon and all the ligands on the carbon, all the substituants on the carbon, have to be in the same plane. And it looks something like this. So here would be one of the carbons of the alkene and here would be the other carbon of the alkene. Both of these carbons are hybridized and the p orbitals stick up and down like this and the overlap of these p orbitals will make the pi bond. Now here is another carbon attached and there is another carbon attached as alkenes. There’s a hydrogen there and the hydrogen there and you can see that all of these atoms the carbon, carbon, carbon, H, carbon and H are in the same plane. Now, what happens if we try to twist the molecule out of planarity? Well we can twist it like this, but what happens when we do that is that the two p orbitals that used to be forming the pi bond are now perpendicular to each other and as I have drawn down here. And when that happens, the overlap if formally zero, because this p orbital is in the nodal plane of the other p orbital. So when you do this you break the pi bond and as a consequence, there is an approximately 50 killikal per mole rotating around the pi bond. So when the sigma bonds are freely rotating, pi bonds are not. At room temperature, there are stuck like this. If you start with a pi bond like this, it can’t rotate at room temperature. Now, how do we name alkenes? Alkenes are named similarly to alkanes. We start with the Greek prefix for the number of carbons and append onto that the suffix ene. So ethene would be a two-carbon alkene. Propene is a two- carbon alkene. Butene is a four-carbon alkene. But once we get to butene, something else happens and that is that we have to identify the position of the double bond in the molecule. Here is 1-butene and the double bond is at the first carbon. Here is 2-butene and the double bond is at the second carbon. So what we do, are we indicate the position of the double bond by a number one, two or whatever. In this case we could never have 3-butene because 3- butene would be the same as 1-butene, and when you name these molecules, you always go with the lowest number. In other words, 1-butene is a lower number than 3-butene. There is no such thing as 3-butene; it is just called 1- butene. Now, alkenes, because they are planar, they can exist in two forms. These are called isomers. We are going to talk about isomers later on, but suffice it to say that at this point, isomers are molecules that have the same molecular formula, but different shapes. So here is an example of two isomers called cis and transbutene. So in the cis molecule, the two groups are on the same side, but in the transmolecule they are on opposite sides. So here would be transbutene, and if I were to switch these two, it would become cisbutene. And now the ethyl groups are on the same side. And again because of the various rotation is so high, they can’t rotate. The cis compound is stuck cis and the trans compound is stuck trans. Now the word cis means on the same side and the word trans means on opposite sides. That is why this guy is cis and that guy is trans. But there is also another nomenclature that is used and that is to call the cis the Z alkene and the trans would be the E alkene. So E-2-butene is the same things as trans-2-butene and Z-2-butene is the same as cis. So Z equals cis and E equals trans. Now all alkenes exist as cis trans isomers. There has to be a difference between the two groups on the end. So in this here molecule propene, we have H’s here and here, so it makes a difference whether the is on this side or that is on the other side, they are the exact same molecule. And all they are is the rotation of the molecule in space. The same is true with this compound here, this methylbutene compound. Two methyl, two butene, again there is no cis Organic Chemistry Hydrocarbons Alkenes and Alkynes Page [2 of 2] trans isomerism because the groups are the same, and this group is next to this guy or next to that guy, they are exactly the same molecule, just rotated in space. Alkenes are much more reactive than alkanes. Alkenes will undergo additional reactions with molecules like bromine. Bromine is a reactor molecule and it reacts with the double bond portion of an alkene to give you a Bromo alkane. By the way, I should mention, that sometimes alkenes are also called olefins, so the word alkene and olefin are synonymous. So the bromine reacts with the double bond of the alkene and gives you this bromo alkane. Now alkynes again are related to alkenes, the only difference is now that there are two pi bonds. So we have one sigma and two pi. And the carbon is SP hybridized. In the SP hybridized carbon we have two perpendicular p orbitals. So we have one p orbital that is indicated in blue that goes up and down, and one p orbital indicated in red, that goes left and right, and the sigma bonds of the carbon go back and forth, one goes toward the screen and one goes away from the screen. Now, in these two perpendicular p orbitals, when we have another carbon in back, we can form pi bonds between those p orbitals and I have a model of that right here. So the red would be the pi system that would form from the red p orbitals and the yellow is a pi system that forms from the yellow p orbitals. And you can see that they are perpendicular to each other and that is where we get the triple bond character. So that is basically what an alkyne looks like. And the carbon is SP hybridized and as a consequence, there is a 180-degree angle that is linear between C, C and H. And alkynes are similar to alkenes in their nomenclature. Again, you name them the same way, you should be getting familiar with at this point. A two-carbon alkyne is ethyne. Eth means two, the yne suffix means alkyne. Propyne is a three carbon and butyne, like before; we have to indicate the position of the alkyne. This is 1-butyne and that is 2- butyne. Pretty straightforward. Hexyne, three hexyne here, one here and the triple bond is at the third carbon. Two hexyne would have had the triple bond over here; one over here and one hexyne would have had the triple bond all the way at the end. That is how we name these things. By the way, you can’t have a one-carbon alkyne, because you have to have two carbons in order to have the pi bond, and so you can a one carbon alkane, methane, but a one carbon makes no sense because you have to have at least two carbons. Now the reaction of alkynes are also similar to those of alkenes. Basically, they will undergo additional reactions as well. And I have illustrated that with bromine. Once again, we have to start with our unsaturated triple bond containing molecule, the alkyne here, treat it with and the product is a bromo alkene, in this case we go from the alkyne to the alkene of the products. So, in summary we have seen that alkenes and alkynes are molecules that have carbon-carbon pi bonds. The pi bonds are composed of p orbitals. In the case of an alkene, two p orbitals that mix together with the two carbons. In the case of an alkynes, there is two p orbitals on each carbon for a total of two pi bonds. Alkenes are planar, the are hybridized, alkynes are linear and they are SP hybridized, and both of these classes of molecules react with bromine to give additional products. .
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