Acetylide ion can also add to carbonyl compounds. A primary alcohol can be synthesized by adding an acetylide to formaldehyde but other aldehydes give a secondary alcohol. Addition of an acetylide ion to ketones gives a tertiary alcohol (Scheme 3).

Scheme 3

Carbon-carbon triple bond can be generated by eliminating two molecules of HX from a dihalide under strong basic conditions. In the first step vinyl halide is formed by dehydrohalogenation of a geminal or vicinal dihalide. Second dehydrohalogenation occurs only under strong basic conditions since it involves dehydrohalogenation of a vinyl halide. Dihalide is usually heated to 200 °C with strong base such as fused KOH or alcoholic KOH. Sodium amide can also be used for the double dehydrohalogenation that can take place at a lower temperature (Scheme 4).

Scheme 4

4.9 Reactions of Alkynes

Many addition reactions of alkynes are similar to the corresponding reactions of alkenes since both involve π-bonds. Reagents add across the triple bonds of alkynes just as they add across the double bonds of alkenes and the reaction is usually exothermic.

4.9.1 Addition of Hydrogen

In the presence of a catalyst, hydrogen adds to alkynes to give alkanes. For example, when either of the 1-butyne isomers reacts with hydrogen in the presence of a platinum catalyst, n -butane is formed. Platinum, palladium, and nickel catalysts are commonly used for this transformation (Scheme 5).

Scheme 5

Lindlar's catalyst is a poisoned palladium catalyst, composed of powdered barium sulfate coated with palladium. The catalytic hydrogenation of alkynes is similar to the hydrogenation of alkenes, and both proceed with syn stereochemistry. In this process, the face of a π-bond contacts the solid catalyst, and the catalyst weakens the π-bond, allowing two hydrogen atoms to add on the same face of the alkyne ensures syn stereochemistry (Scheme 6).

Scheme 6

Nickel boride (Ni 2 B) is an alternative to Lindlar's catalyst that can be more easily made to give often better yields (Scheme 7).