Nomenclature of Alkenes

Alkenes are hydrocarbons that have double bond(s). In the naming of a hydrocarbon, you use the suffix -ene. For instance, a straight chaine of six carbons would be called hexene.

When naming alkenes, you must designate where the double bond occurs. Numbering occurs on the lower numbered carbon.

nalke1.gif (1136 bytes)

This would be 2-hexene. NOT 3-hexene. 3-hexene would be:

nalke2.gif (1133 bytes)

This is pretty easy, until you start getting into substituents.

Naming substituents in a straight chain (also called an n- chain) is pretty basis. Name this compound:

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This would be called 2-methyl-3-hexene.

In organic, there is a whole "priority" in numbering. If you have a methyl- group and a -ene group, then the -ene group gets priority (meaning that the -ene must the lowest possible). For instance let's look at this molecule:

nalke4.gif (1134 bytes)

This would be called 4-methylcyclohexene. The -ene group has priority over the methyl- group, so the -ene is assumed to be one. Then, counting in a counterclockwise direction (clockwise would yield 5-methylcyclohexene, and lower numbers are preferred), the methyl group is located on the 4 carbon.

When you have more than one -ene, you add a prefix to the suffix. For instance, if you have TWO double bonds, the molecule would be -diene. Three double bonds would be called -triene. Easy shtuff. Let's try some examples:

nalke5.gif (1159 bytes)

Well, the -ene gets priority from the methyl-group,and we also know there are two double bonds, so we know to name this a methylcyclohexadiene. Now, in terms of numbering, we can either have 4-methyl-1,4-cyclohexadiene or 1-methyl-1,4-cyclohexadiene. Since we always want lower numbers, we choose the latter. Hence, this molecule is called 1-methyl-1,4-cyclohexadiene.

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2,5-heptadiene. Easy.

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Ignore the methyl for a second. We know that going both ways, it will be 2,5-heptadiene (try counting carbons from the left and then from the right). So, we'll name the left carbon "one" since the methyl group would then be 2-methyl as opposed to 6-methyl. Final answer? 2-methyl-2,5-heptadiene.

nalke8.gif (1338 bytes)

This is exactly like the last one, except we have one more double bond. I used this one to demonstrate the priority of the double bond over the methyl- substituent. This one has three double bonds, so it's a triene. It has 7 carbons, so it's a hepta-. However, this one is NOT named the same way going from the right or the left (unlike the last problem). Going from the right, this is a 1,2,5-heptatriene. Going from the left would be 2,5,6-heptatriene. So we MUST name from the right since it has lower numbering. Although the methyl- group will be a 6-methyl-, the -enes draw the priority. Sort of like the "pecking" order at a school. Teachers over students :)

Final answer? 6-methyl-1,2,5-heptatriene.


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