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.

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

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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:

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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.

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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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