PROBLEM 7.34 - Organic Chemistry

Question

Question: For each reaction, use the identity of the alkyl halide and nucleophile to determine which substitution mechanism occurs. Then determine which solvent affords the faster reaction

a.

b.

c.

Step-by-Step Solution

Answer

Answer

1. The substrate and the nucleophile favor the S_N1  mechanism. CH₃OH is a polar protic solvent. It enhances the rate of the S_N1   mechanism.
2. The substrate and the nucleophile favor S_N2 the  mechanism. DMF is a polar aprotic solution, and it enhances the rate of the S_N2  mechanism.
3. The substrate favors the S_N1  mechanism, and the nucleophile favors the S_N2  mechanism. The solvent HMPA is a polar aprotic solution, so it enhances the rate of S_N2  the reaction.

S N 1 reactions

This is a two-step reaction.

 In these reactions, the leaving group leaves first and hence forming a carbocation. The carbocation is then attacked by the nucleophile. 

The rate of the reaction depends upon the leaving group and the nature of the nucleophile. 

The polar protic solvents enhance the rate of the S_N1 mechanism.

A weak nucleophile favors S_N1 mechanism.

S N 2 reactions

These reactions do not involve the formation of the carbocation. 

In this reaction, one bond is broken, and another bond is formed simultaneously. 

A strong nucleophile favors the S_N2 mechanism. The polar aprotic solutions enhance the rate of the S_N2 mechanism.

Determination of the reaction pathway

a. (CH₃CH₂)₂C(CH₃)Cl is a tertiary substrate, so it favors S_N1  the mechanism. CH₃OH is a weak nucleophile; hence, it favors S_N1  mechanism.

Since CH₃OH is a polar protic solvent, it enhances the rate of the         S_N1 mechanism.

b. CH₃CH₂Br is a primary substrate, so it favors the S_N2 mechanism.^-OH is a strong nucleophile; hence, it favors S_N2 mechanism.

Since DMF is a polar aprotic solution, it enhances the rate of the S_N2 mechanism.

c.  (CH₃CH₂)₂CCl is a secondary substrate, so it favors S_N1  the mechanism.

However, since CH₃O^- is a strong nucleophile, it favors the S_N1 mechanism. The solvent HMPA is a polar aprotic solution, so it enhances the rate of the S_N2 reaction.