Alkene Nomenclature

Nomenclature of Alkenes

a. Alkenes are named by dropping the -ane ending of the parent and adding -ene. The parent structure is the longest chain containing both carbon atoms of the double bond.

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2-pentene J

3-heptene J

b. Give the double bond the lowest possible numbers regardless of substituent placement.

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4-methyl-2-pentene J

c. Common names that you should know are...

vinyl substituent H₂C=CH-

allyl substituent H₂C=CH-CH₂-

allene molecule H₂C=C=CH₂

isoprene

d. Endocyclic double bonds have both carbons in the ring and exocyclic double bonds have only one carbon as part of the ring.

Cyclopentene is an example of an endocyclic double bond.

Methylenecylopentane is an example of an exocyclic double bond.

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3-allylcyclohexene J

2-vinyl-1,3-cyclohexadiene J

e. Double bonds can exist as geometric isomers and these isomers are designated by using either the cis / trans designation or the modern E / Z designation.

cis...The two largest groups are on the same side of the double bond.

trans...The two largest groups are on opposite sides of the double bond.

E/Z nomenclature

E = entgegan ("trans") Z = zusamen ("cis")

· Priority of groups is based on the atomic mass of attached atoms (not the size of the group). An atom attached by a multiple bond is counted once for each bond.

fluorine atom > isopropyl group > n-hexyl group

deuterium atom > hydrogen atom

-CH₂-CH=CH₂ > -CH₂CH₂CH₃

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