Q29E - Organic Chemistry (Mcmurry)

Question

Terminal alkynes react with and water to yield bromo ketones. For example:

Propose a mechanism for the reaction. To what reaction of alkenes is

the process analogous?

Step-by-Step Solution

Answer

Mechanism of given reaction

1. Attack of nucleophile alkyne on the Bromine atom and formation of three membered ring

 2. Ring opening by a hydronium ion

 3. Tautomerization takes place in which alcohol is converted into ketone

Step 1: Formation of ketones

An alkyne acts as a nucleophile and attacks on the ${\mathbf{Br}}_{\mathbf{2}}$, resulting in the formation of a cyclic ring. The three-membered ring is highly strained and thus unstable. This is what causes the ring to open and then ketone to form.

Step 2: Mechanism of given reaction

1. Attack of nucleophile alkyne on the Bromine atom and formation of three membered ring

 2. Ring opening by a hydronium ion

 3. Tautomerization takes place in which alcohol is converted into ketone

Step 4: Analogous mechanism

Formation of a halohydrin is an analogous mechanism to given mechanism.