3.5 Alkenes and Alkynes

A hydrocarbon or alkyl group with one or more double bonds is called unsaturated. If it has one double bond, it is monounsaturated; if it has more than one, it is polyunsaturated. Unsaturated hydrocarbons are called alkenes; examples include ethylene CH₂=CH₂, emitted by plants and responsible for the blue haze in mountainous forests; propylene or propene CH₂=CHCH₃, the butenes CH₂=CHCH₂CH₃ and CH₃CH=CHCH₃, and so on.

Butadiene is an alkene with the structure CH₂=CHCH=CH₂, and it is made into polymers that have good bounce, for things like tires and toy balls. It resembles isoprene CH₂=C(CH₃)CH=CH₂, the precursor to natural rubber.

A hydrocarbon with a triple bond is called an alkyne. (The Y is pronounced as in "try".) We've already met acetylene CH≡CH, but its official name is ethyne. Alkynes are not very common.

There are also cycloalkenes such as cyclopropene, cyclobutene, cyclopentene, cyclohexene, and so on.

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