Question

Assuming that strong acids add to alkynes in the same manner as they add to alkenes, propose a mechanism for each of the following reactions:

Short answer

The mechanisms of the reactions are given below.

a).

b).

c).

Step-by-step solution

Addition of   to alkyne

The mechanism of addition of halogen acids to alkynes is similar to addition to alkenes. Alkynes contain two pi bonds and hence two equivalents of acid can be added to one molecule of an alkyne. The general mechanism for addition to alkynes is given below. In the first step of the reaction a stable carbocation is generated. The nucleophile attacks the carbocation. The product formed after addition of one molecule of HX gives an alkene. Further addition takes place following the same mechanism to give germinal dihalide as the product.

X = Cl, Br

Addition   to Pentyne

Addition of hydrochloric acid takes place with carbocation formation. The mechanism for the reaction is given below

Addition HBr to But-2-yne

The addition of HBracid takes place with carbocation formation. The mechanism of the reaction is given below.

Addition HBr to phenylacetylene

Addition of hydrochloric acid takes place with an intermediate of carbocation.

Then lead to the formation of required product.