Alkynes

Alkynes are hydrocarbons that have at least one triple bond between carbon atoms. The alkynes are traditionally known as acetylenes, though the name acetylene is also used to refer specifically to the simplest member of the series, known officially as ethyne.

Physical properties

Unlike an alkane an alkyne is unstable and very reactive. This gives rise to the intense heat of the acetylene flame used in welding.

Alkynes also readily polymerise, like alkenes.

Saturation

Alkynes contain two units of unsaturation.

Synthesis

We start from a dihalogenized substance.

If we add NaOH to this, it will dissociate into Na⁺ and OH^-. Na⁺ will not react, but OH^- will pull off an electron from the substance.

• 1st possibility

2nd order elimination reaction. the 2 Cl will go to respect the tetravalent structure of the carbon.

• 2nd possibility

As before, 2nd order elimination reaction. the 2 Cl will go to respect the tetravalent structure of the carbon.

Examples

The simplest alkyne is ethyne (acetylene):

Reactions

Terminal alkynes

1. terminal alkyne + strong base → anion of terminal alkyne

1. • anion of terminal alkyne + methyl or primary alkyl halide → new carbon-carbon bond

1. • anion of terminal alkyne + ketone → tetrahedral carbonyl addition compound (alkoxide)

1. • • alkoxide + aqueous acid → hydroxyalkyne

1. terminal alkyne + dialkylborane + hydrogen peroxide → aldehyde

Synthesis

1. alkene + bromine → dibromoalkane

1. • dibromoalkane + strong base → alkyne

Addition

1. alkyne + 1bromine → (E)-dihaloalkene.

1. • (E)-dihaloalkene + 1bromine → tetrahaloalkene

1. alkyne + 2hydrogen halide → geminal dihaloalkane

Reduction

1. alkyne + hydrogen gas + transition metal catalyst → alkane

1. alkyne + hydrogen gas + Lindlar catalyst → cis alkene

1. alkyne + hydrogen gas + alkali metal → trans alkene

Other

1. alkyne + borane → trialkenylborane

1. • trialkenyl borane + hydrogen peroxide → enol

1. • • enol ↔ ketone

1. alkyne + water + mercury salt + acid catalyst → enol

1. • enol ↔ ketone