Chemical Properties of Alkynes - Practice Questions & MCQ

Hydrohalogenation
Addition of one molecule of halogen gives vinyl halide which then adds another molecule of hydrogen halide to form gem-dihalide. This addition follows Markownikoff's rule. The reaction occurs as follows:

For example:

Halogenation
Alkenes combine with gaseous chlorine or bromine in the dark to form di or tetrahalides. Here the addition is Anti Markownikoff's. The reaction occurs as follows:

For example:

Hydration
Alkynes cannot be hydrated more easily than alkenes because of their low reactivity towards electrophilic addition reactions. The reaction occurs as follows:

Mechanism

For example:

Hydroboration-Oxidation reaction
Alkynes react with BH₃ (in THF) and finally converted into carbonyl compounds. This method is useful for preparing aldehyde from terminal alkyne, which is otherwise not possible by hydration. The reaction occurs as follows:

For example:
 

Reaction with carbonyls
This reaction is very useful for the preparation of alcohols. In this reaction, a salt like NaNH₂ is used which produces a carbanion as follows:

Now this carbanion reacts with the carbonyl group. Here, the carbanion binds with the carbonyl carbon and carbon-oxygen bond shifts to the oxygen giving it a negative charge. Now we use H₃O⁺ to bind with O^- and thus forms the OH or alcohol.

The complete mechanism is given below:

Addition of Hypochlorous acid
Alkynes when passed into hypochlorous acid solution forms dichloroacetaldehyde. The reaction occurs as follows:

For example:

Polymerisation
Alkynes polymerize to give the following compounds. The reactions occur as follows: