Chemistry » Organic Chemistry » Hydrocarbons

# Alkynes

## Alkynes

Hydrocarbon molecules with one or more triple bonds are called alkynes; they make up another series of unsaturated hydrocarbons. Two carbon atoms joined by a triple bond are bound together by one σ bond and two π bonds. The sp-hybridized carbons involved in the triple bond have bond angles of 180°, giving these types of bonds a linear, rod-like shape.

The simplest member of the alkyne series is ethyne, C2H2, commonly called acetylene. The Lewis structure for ethyne, a linear molecule, is:

The IUPAC nomenclature for alkynes is similar to that for alkenes except that the suffix -yne is used to indicate a triple bond in the chain. For example, $${\text{CH}}_{3}{\text{CH}}_{2}\text{C}\equiv \text{CH}$$$${\text{CH}}_{3}{\text{CH}}_{2}\text{C}\equiv \text{CH}$$ is called 1-butyne.

## Example

### Structure of Alkynes

Describe the geometry and hybridization of the carbon atoms in the following molecule:

### Solution

Carbon atoms 1 and 4 have four single bonds and are thus tetrahedral with sp3 hybridization. Carbon atoms 2 and 3 are involved in the triple bond, so they have linear geometries and would be classified as sp hybrids.

Chemically, the alkynes are similar to the alkenes. Since the $$\text{C}\equiv \text{C}$$$$\text{C}\equiv \text{C}$$ functional group has two π bonds, alkynes typically react even more readily, and react with twice as much reagent in addition reactions. The reaction of acetylene with bromine is a typical example:

Acetylene and the other alkynes also burn readily. An acetylene torch takes advantage of the high heat of combustion for acetylene.

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