Question

(a)Optically active 2-bromobutane undergoes racemization on treatment with a solution of KBr. Give a mechanism for this racemization.

(b)In contrast, optically active butan-2-ol does not racemize on treatment with a solution of KOH. Explain why a reaction like that in part (a) does not occur.

(c)Optically active butan-2-ol racemizes in dilute acid. Propose a mechanism for this racemization.

Short answer

a)

b)Because an alkene rather than an alkane is produced as a byproduct of the reaction between optically active butane-2-ol and a solution of KOH, no racemization occurs.

c)

Step-by-step solution

Step 1:

Enantiomers are stereoisomers that have mirror images of one another but are not superimposable or identical to one another. Racemization is the process of creating an equal mixture of all potential enantiomers when a molecule is transformed. Racemic mixture is the name given to the resulting mixture.

Any substance's ability to rotate plane polarised light is known as the substance's optical activity.

a)

Through the SN1 process, 2-bromobutane racemization is taking place, producing a carbocation intermediate. Then, this intermediate carbocation is addressed from either the front or the back. As a result, the racemization of 2-bromobutane by the SN1 mechanism described above results in a racemic mixture.

The mechanism for the racemisation of 2-bromobutane with a solution of KBr is as follows,

b)

Because an alkene rather than an alkane is produced as a byproduct of the reaction between optically active butane-2-ol and a solution of KOH, no racemization occurs.

The mechanism for the racemisation of butane-2-ol with a solution of KOH is as follows,

c)

When optically active butane-2-ol is racemized in diluted acid, the protonation of the hydroxyl group—rather than the SN1 mechanism—leads to the formation of the carbocation, which causes water to attack the intermediate either from the front or the back. Butane-2-ol is produced in a racemic mixture as a result of this assault.

The mechanism for the racemisation of butane-2-ol with a dilute acid is as follows,