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Intro to Alkenes


This is MCAT Organic Chemistry One. We'll be covering alkenes and eliminations. Alkenes contain a carbon carbon double bond. Sometimes they are referred to as olefins. Because there's a carbon carbon double bond they're not fully hydrogenated and we would say they are unsaturated or they have a Unit of unsaturation in them.

To name an alkene we replace the ane ending on the parent chain with ene. So ethane becomes ethene, propane will become propene. Now these two have common names. Ethene is also called ethylene And propene is known as propylene. If we were gonna draw these, ethene would just be a carbon carbon double bond. Propylene is a single carbon, attached to this ethene.

Even though these have a y in them don't confuse them with the alkyne. Propylene and ethylene are not alkynes, they're alkenes. Now next in butane, the C4 compound it's the first time we're going to get isomerism. We've got 1-butene, cis-2-butene, trans-2-butene and isobutene. So from our C4 or four carbons, we can have, Trans-2-butene Cis-2-butene One butene they are no cis isomers because they are two hydrogen attached to this first carbon.

And in isobutene we just have a different configuration. Where the two methyl groups are attached to the same carbon. We have a few common alkene substituents that you will want to memorize. This first one here is a methylene group, over here we have the vinyl group, and the last one is an allyl group. The difference the vinyl group and the allyl group, is we've added a methylene group in between the point of attachment in the vinyl.

Now, let's discuss a few alkene properties. We'll start with terminal versus internal alkenes. A terminal alkene is where the alkene functionality is at the end, or the terminus of a molecule. And an internal alkene just has the alkene functionality somewhere in the middle. Now in general internal alkenes have higher polarity than terminal ones.

Because of this they have slightly higher boiling points. And trans versus cis isomerism. The trans molecule tends to have greater symmetry than cis. This gives a higher melting point and it tends to be more thermal dynamically favorable because the large groups are off side to the ring. So be more common as reaction product.

Now cis alkenes are more polar because you orient both of the bulky groups in the same side of the ring. This gives cis alkenes a higher boiling point, and because they're more polar, they have a higher solubility in water. Now let's compare the melting point and boiling point of a few alkene isomers. We take butane, which is a straight four carbon chain.

Then, we start to turning it into different isomers. One butene, cis-two-butene, trans-two-butene, and the iso-butene, we can see some definite difference in melting point and boiling point. The highest melting point occurs right here for the trans two butene. We said that symmetry causes the trans melting point to be greater than the cis melting point.

If we compare the boiling point, cis two butene has the highest of the boiling point at 3.7. That matches up with what we said earlier, cis is greater than trans. Now, our internal alkenes are the two butenes. And our terminal alkenes are 1-butene and our iso-butene. And you'll notice the iso-butene and the 1-butene Have the lowest boiling point.

So the internal alkene had a higher boiling point in the terminal one. Last thing to notice butane or alkane here that's splits down the middle for all of these. So there's no real trend for the alkane versus the alkene in this particular case. For member that as the molecule gets larger these sorts of forces diminish as Vanderall's forces take over primary driving force, because these functionalities like an alkene or an alcohol or an ether become smaller and less significant when the molecule becomes larger.

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