PROBLEM 7.67 - Organic Chemistry

Question

Question: Draw a stepwise, detailed mechanism for the following reaction. Use curved arrows to show the movement of electrons.

Step-by-Step Solution

Answer

Answer 

The given compound is a tertiary alkyl halide. It will readily undergo the S_N1  substitution. The reaction will take place in the following steps

Removal of the leaving group

Attack of the nucleophile

Abstraction of a proton

To find out if the reaction proceeds by the S N 1 or the S N 2 mechanism

The given alkyl halide is examined to know if it is a tertiary, secondary, or primary alkyl halide. Based on the nature of alkyl halide, the mechanism of the given reaction can be predicted. 

 

The primary alkyl halides are most reactive toward the S_N1  reactions. The tertiary alkyl halides are least reactive toward the S_N1  reactions. The tertiary alkyl halides are the most reactive toward the S_N2  reactions. The primary alkyl halides are the least reactive toward the S_N2 reactions.

The use of the solvent also helps to determine the mechanism of the   S_N1 and S_N2 reactions.

The mechanism of the given reaction

The given alkyl halide is a tertiary alkyl halide. It will undergo the S_N1  substitution reaction only. The reaction proceeds by the following mechanism:

The leaving group leaves the molecule resulting in the formation of the cyclic carbocation as shown in the following structure:

Removal of the leaving group

In the next step, there is an attack of the nucleophile. The nucleophile can attack from both above or below the carbocation as shown in the structure below:

Attack of the nucleophile

In the final step, there is an abstraction of H⁺ ion by  the Br^- ion from the molecule to finally produce the two isomers as shown in the structure below:

Abstraction of a proton

The S_N1 substitution will result in the formation of a racemic mixture. Therefore, a mixture of both the enantiomers will be obtained.