Question

When a ketone is treated with acid and a halogen, the a-monohalogenated product can be obtained in high yield. However, under basic conditions it is extremely difficult to isolate the mono halogenated product. Provide an explanation for this reactivity.

Short answer

When one hydrogen is replaced by halogen (such as chlorine), the other hydrogens become more acidic. The halogenated carbon can now easily donate the other hydrogens to the base. Then the enolate ion can further attack another halogen and form dihalosubstituted products. Therefore, the reaction cannot be stopped at monohalogenated product. Hence, the isolation of the monohalogenated product is so difficult.

Step-by-step solution

Mechanism of halogenation of ketone

Halogenation of a ketone in acid medium:

Halogenation of a ketone in a basic medium

Explanation   

When one hydrogen is replaced by halogen (such as chlorine), the other hydrogens become more acidic. The halogenated carbon can now easily donate the other hydrogens to the base. Then the enolate ion can further attack another halogen and form dihalosubstituted products. Therefore, the reaction cannot be stopped at monohalogenated product. Hence, the isolation of the monohalogenated product is so difficult.