Question

Explain the following result. Although alkenes are generally more reactive than alkynes towards electrophiles, the reaction of with but-2-yne can be stopped after one equivalent of has been added.

Short answer

The addition of chlorine makes the pi bond less electron-rich and therefore less reactive with an electrophile.

Step-by-step solution

Reaction of alkynes with halogens

Alkynes react with electrophiles like halogen to form a trans product.

The trans product can react with one equivalent of the same electrophile to form a tetrahalide as alkenes have high reactivity with electrophiles.

Therefore, the final product of a reaction of an alkyne with electrophile is an alkane.

Reaction of but-2-yne with chlorine

Chlorine undergoes anti addition to alkynes. The reaction can be given as:

Generally, this alkene undergoes a further addition reaction to form an alkane because alkenes have high reactivity with electrophiles.

But when chlorine is used as an electrophile, the reaction can be stopped at the trans halide stage.

Reduction of reactivity of alkene

Chlorine is an electron-withdrawing group.

The trans chloride has two chloride groups attached to the carbon-containing double bond. This makes the pi bond less electron-rich.

Therefore, the reactivity of the alkene towards the electrophile decreases, and the reaction can be stopped at the trans chloride stage.