The basic rules for naming alkenes are the same as those for the nomenclature of alkanes, except that we change the suffix to –ene.

Find the longest continuous chain of carbon atoms that contains the ##”C=C”## double bond. If you have two ties for longest continuous chain and both chains contain a ##”C=C”## double bond, then choose the chain with more substituents.

Give the lowest possible number to the ##”C=C”## double bond.

Add substituents and their positions to the name of the alkene as prefixes. Of course, remember to give them the lowest numbers possible and to write them in alphabetical order.

Identify stereoisomers. You can use the cis/trans notation when there is a hydrogen atom on each of the alkene carbons, but it is always safer to use the Cahn-Ingold-Prélog ##E//Z## notation. ##E## (entgegen) means the higher priority groups are on opposite sides of the double bond. ##Z## (zusammen) means the higher priority groups are on the same side of the double bond.

Functional Groups Have Higher Priority Than Double Bonds

The only groups with lower priority than ##”C=C”## are ##”NO”_2## groups, halogens, and alkynes.

Since ##”OH”## has higher priority than ##”C=C”##, alkenes containing ##”OH”## groups get the suffix -ol.

The parent chain must contain the alcohol group and go through as many ##”C=C”## bonds as possible (This may not happen to be the longest possible carbon chain).

Let’s apply these rules to the naming of geraniol, a primary component of rose oil that is commonly used in perfumes.

Its structure is

We start at the carbon bearing the ##”OH”## group and count through both double bonds to the end of the chain (8 carbons). This is then an octadienol (di for two double bonds).

The ##”OH”## is on ##”C1″## and the double bonds start at ##”C2″## and ##”C6″##.

We now have an octa-2,6-dien-1-ol (It is acceptable to call this a 2,6-octadien-1-ol, but it is now the preferred practice to put the locating numbers as close as possible to the groups they locate).

In addition, there are methyl groups at ##”C3″## and ##”C7″##, so the name becomes 3,7-dimethylocta-2,6-dien-1-ol.

Now we indicate stereochemistry.

We cannot use cis/trans for the ##”C2=C3″## double bond, so we must label it according to the Cahn-Ingold-Prélog ##E//Z## system.

The two higher-priority groups are the ##”CH”_2″OH”## on ##”C2″## and the ##”CH”_2## group on ##”C3″##.

Since they are on opposite sides of the double bond, the configuration is ##E##.

The full name for geraniol is (2##E##)-3,7-dimethylocta-2,6-dien-1-ol.

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