Question

A ketone undergoes acid-catalyzed bromination, acid-catalyzed chlorination, racemization, and acid-catalyzed deuterium exchange at the $\alpha$-carbon. All of these reactions have similar rate constants. What does this tell you about the mechanisms of these reactions?

Short answer

In most cases, 50 to 70% of the product of a S_N1 reaction is the inverted product. If the reaction does lead to equal amounts of the two stereoisomers, the reaction is said to take place with complete racemization. When more of the inverted product is formed, the reaction is said to take place with partial racemization.

Step-by-step solution

Racemization

In most cases, 50 to 70% of the product of a S_N1 reaction is the inverted product. If the reaction does lead to equal amounts of the two stereoisomers, the reaction is said to take place with complete racemization. When more of the inverted product is formed, the reaction is said to take place with partial racemization.

Acid-catalyzed bromination

The reaction of propiophenone with Br₂molecule in the presence of acid is as follows:

The reaction of propiophenone with Cl_2Vmolecule in the presence of acid is as follows:

The compound 2-methyl-1-phenylbutan-1-one undergoes acid-catalyzed hydration to form its racemic mixture as follows:

The reaction of propiophenone with D₂O in the presence of acid gives the following product.

The mechanism of these reactions follows the first step the formation of enol and the second step is the electrophilic addition ofhalogen. In these two steps, the first stepis the slowest step. So, it is a rate-determining step. Thus, all the reactions proceed at the same rate because the slowest step in all reactions is the formation of enol.