Alkene hydrocarbons (formerly called olefins) are more reactive than alkanes having the same number of carbons. This is because of the greater reactivity of the electrons in the carbon-carbon double bond. Alkenes also exhibit cis-trans isomerism, but that is beyond the scope of this article.
Alkenes are petrochemicals that exist in small amounts in crude oil. They are primarily obtained by cracking larger alkanes in refineries (Figure 1). Both small and large alkenes are excellent starting materials for the synthesis of many other organic compounds, including plastics, ethanol, and ethylene glycol (antifreeze).
Saturated and Unsaturated Hydrocarbons
Saturated hydrocarbons contain only single bonds between carbons. In other words, the carbons are "saturated" with as many hydrogen atoms as allowed by the bonding preferences of carbon (four bonds). The alkenes are "unsaturated," meaning they have one or more hydrogen atoms removed, with the electron pair that would exist between a carbon and a hydrogen collapsed into a carbon-carbon double bond.
Naming Linear Alkenes Containing One Double Bond
Naming linear alkenes has much in common with the naming of linear alkanes. As is the case with alkanes, the first step in naming an alkene is to count the carbons in the longest carbon chain. The name begins with a prefix based on the number of carbons. The common prefixes are:
- meth (although there are no 1-carbon alkenes)
- eth
- but
- prop
- pent
- hex
- hept
- oct
- non
- dec
The name of an alkene begins with the numerical identifying prefix, and ends in "-ene." There is only one two-carbon unsubstituted alkene, namely ethene (C=C). Unsubstituted means there are only hydrogens attached to the main chain carbons. There is only one unsubstituted three-carbon unsubstituted alkene, namely propene. This is because, even though one can draw propene two different ways (C=C–C or C–C=C), these structures are identical when rotated in space.
When the number of carbons exceeds three, a numbering system is implemented to identify the location of the double bond. The numbers are chosen to be as small as possible. For example, in the molecule C=C–C–C, the double bond is in the first bond position between the first two carbons in the chain, so this molecule is 1-butene. A butene with the double bond at the second position (C–C=C–C) is 2-butene. To give another example, C–C–C–C–C=C–C–C is 3-octene.
Naming Alkenes Containing More Than One Double Bond
Alkenes with two double bonds are alkadienes (dienes), ones with three carbon double bonds are alkatrienes (trienes), and so on. A straightforward extension to the numbering system is used to properly name an alkapolyene. So, C–C=C-C=C–C is 2, 4-hexadiene. Another example is 1,5-heptadiene (C=C–C–C–C=C–C), and 1,2,3-pentatriene (C=C=C=C–C).
Naming Branched Alkenes
Branched alkenes are named in a way similar to that of branched alkanes.
1. First, find the longest carbon chain. This is the main chain used to determine the name.
2. Assign numbers to the carbons where the side groups exist.
3. The double bond positions are named in the same way as for unbranched alkenes.
4. Give the side group carbon number and name first, then the location of the double bonds, followed by the main chain name.
Take as an example C–C–C=CC–C, where the CC indicates a methyl group attached to C-2 (not C-4 because number should be chosen to be as small as possible). It may help to draw it to show the branched methyl group. The longest carbon chain has five carbons (-pent). The methyl group is on C-2. The name of this molecule is 2-methly-2-pentene. One more example: C=C–CCNOv2–C=C–C is 3-methyl-3-nitro-1,4-hexadiene (where v indicates a subscript).
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