4.7 Introduction

Alkynes are hydrocarbons that contain carbon-carbon triple bonds. Many of the reactions of alkynes are similar to the corresponding reactions of alkenes because both involve π-bonds between two carbon atoms. Like the π-bond of an alkene, the π-bonds of an alkyne are also electron rich, and readily undergo addition reactions.

4.8 Nomenclature of Alkynes

• Find the longest continuous chain of carbon atoms that includes the triple bond and change the - ane ending of the parent alkane to - yne .
• The chain is numbered to give the carbon atoms of the triple bond the lower possible numbers.
• The hydrogen attached to the carbon of the triple bond is called the acetylenic hydrogen atom.
• The anion obtained when the acetylenic hydrogen is removed is known as an alkynide ion or an acetylide ion.

Alkynes are relatively nonpolar and quite soluble in most organic solvents. Acetylene, propyne, and the butynes are gases at room temperature. Alkynes have one σ-bond and two π-bonds. Hybridization of the s orbital with one p- orbital gives two linear sp hybrid orbitals that are used to form the σ-bond with each carbon atoms and with the hydrogen s orbitals. Two π-bonds result from overlap of the two remaining unhybridized p- orbitals on each carbon atom (Figure 1).

Figure 1

The triple bond is relatively short because of the high s character of the sp hybrid orbitals which also account for the slightly shorter C-H bonds in acetylene compared to ethylene and ethane (Figure 2).

Figure 2

Terminal alkynes are much more acidic than other hydrocarbons. Abstraction of an acetylenic proton gives a carbanion that has the lone pair of electrons in the sp hybrid orbital. Hydroxide ion and alkoxide ions are not strong enough bases to deprotonate alkynes but very strong bases such as sodium amide, deprotonate terminal acetylenes to form acetylide ions (Scheme 1).

Scheme 2