Answer to Question #163865 in Mechanics | Relativity for Anand

Question

Complete the following reactions and write their mechanisms: 

(i) CH₃COCH₃ + CH₃MgBr in the presence of ether/ H₂O -----> ??

(ii) CH₃COCH₃ + Br₂ in the presence of (OH⁻)---> ??

Answer 1

1)

$$CH_3COCH_3+CH_3MgBr \to CH_3CH_3CH_3+COMgBr$$

2)

$$CH_3COCH_3+Br2 \to CH_3COCH_2+HBr$$

If a nucleophilic substitution occurs at an asymmetric carbon atom, then the configuration is reversed, since in the transition state three non-reactive groups and the central carbon atom are in the same plane, and the entering and leaving groups are located on the same straight line perpendicular to this plane. As a result, the structure turns inside out like an umbrella.

Monomolecular nucleophilic substitution occurs in two stages.

At the first stage, under the action of a solvent, a heterolytic cleavage of the bond in the substrate occurs, as a result of which a carbocation is formed. The process is slow and determines the speed of the reaction in general. In the second step, the carbocation quickly reacts with the nucleophile to give the substitution product.

The reactions have the following main features.

1. Kinetic

The reaction rate depends only on the concentration of the substrate since the nucleophile does not participate in the rate-limiting stage of the process.

2. Stereochemical sign

If a nucleophilic substitution occurs at an asymmetric carbon atom, then, as a rule, a racemic mixture is formed, since an attack by a nucleophile of a planar carbocation from both sides is equally probable.

Factors affecting the course of nucleophilic substitution

The ease of the reaction and its mechanism depend on many factors, among which the following can be distinguished:

the structure of the hydrocarbon radical of the substrate;

the nature of the leaving group;

nucleophile strength;

the nature of the solvent.