D8.5 Alkynes

An alkyne is a hydrocarbon with one or more carbon-carbon triple bonds. Two carbon atoms joined by a triple bond are bound together by one σ bond and two π bonds. The general molecular formula of an alkyne with one triple bond is CnH2n-2. The alkyne has four hydrogen atoms fewer than the corresponding alkane with same number of carbons, and hence it has two degrees of unsaturation.

The suffix -yne is used to indicate the presence of a triple bond. The simplest alkyne is ethyne, C2H2, commonly called acetylene. The Lewis structure for ethyne is:

The structural formula and name for ethyne, also known as acetylene, are shown. In red, two C atoms are shown with a triple bond illustrated by three horizontal line segments between them. Shown in black at each end of the structure, a single H atom is bonded.

Chemically, alkynes have reactivity similar to alkenes. Since the C≡C functional group has two π bonds, alkynes can react with twice as much reagent in an addition reaction. For example, acetylene can react with bromine in the following reaction:

Exercise: Formulas, Multiple Bonds, and Rings

Exercise: Bond Lengths in Hydrocarbons

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Chemistry 109 Fall 2021 by John Moore, Jia Zhou, and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.