Alkynes: Properties and Nomenclature

Study Notes

An alkyne is a hydrocarbon that contains a carbon-carbon triple bond.
Only relatively few alkynes are found in nature, with examples including capillin and ichthyothereol.
Alkynes have four fewer hydrogens than an alkane with the same number of carbons.
The general molecular formula for an acyclic alkyne is CnH2n-2, and for a cyclic alkyne is CnH2n-4.
Systematic names of alkynes are obtained by replacing the “ane” ending of the alkane name with “yne.”

The longest continuous chain containing the carbon-carbon triple bond is numbered in the direction that gives the functional group suffix as low a number as possible.
Alkynes with triple bonds located at the end of the chain are classified as terminal alkynes.
Alkynes with triple bonds located elsewhere along the chain are internal alkynes.
In common nomenclature, alkynes are named as substituted acetylenes.
The common name is obtained by stating the names of the alkyl groups (in alphabetical order) that have replaced the hydrogens of acetylene.

All hydrocarbons (alkanes, alkenes, and alkynes) have similar physical properties.
They are all insoluble in water but soluble in nonpolar solvents.
They are less dense than water and have boiling points that increase with increasing molecular weight.
Alkynes are more linear than alkenes, and a triple bond is more polarizable than a double bond.
These features cause an alkyne to have stronger van der Waals interactions and, therefore, a higher boiling point than an alkene with the same number of carbons.