PROBLEM 7.63 - Organic Chemistry

Question

Question: Determine the mechanism of nucleophilic substitution of each reaction and draw the products, including stereochemistry.

a.

b.

c.

d.

e.

f.

Step-by-Step Solution

Answer

Answer 

a. The following reaction proceeds by the S_N1  mechanism as shown below:

The products obtained will be a racemic mixture.

b. The  ^-OCH₃ is a strong nucleophile, and DMSO is a polar aprotic solvent. Hence, the reaction will proceed by the S_N2  mechanism. The products obtained will have an inversion of configuration.

c. The reaction will proceed by the S_N1  mechanism only. The products will be a racemic mixture.

d. The reaction will take place at the stereogenic center by the S_N1  mechanism, which will result in the racemization of the product.

e. The presence of a strong nucleophile and the polar aprotic solvent favors the  S_N2 reactions..

f. The reaction will proceed by the S_N1  mechanism. The nucleophile can attack from both sides and will result in the formation of two diastereomers.

Mechanism and stereochemistry of S N 1 nucleophilic substitution

 S_N1 mechanism: 

• The reaction proceeds in a single step where the nucleophile attacks and the leaving group leaves at the same time by the formation of a transition state.
• The  S_N1 reactions are bimolecular in nature. 
• The  S_N1 reactions are stereospecific in nature. The nucleophilic attack always takes place at the stereocenter, and it always results in the inversion of configuration.

Mechanism and stereochemistry of S N 2 nucleophilic substitution

  S_N2 mechanism:

• The reaction takes place in two steps. The first step involves the formation of a carbocation intermediate. In the second step, the nucleophilic attack takes place on the planar carbocation.
• The nucleophilic attack can occur from both sides of the intermediate carbocation. It, therefore, results in the formation of a racemic mixture.

The mechanism and stereochemistry for a, b and c

a. The reactant is a primary halide. The following reaction proceeds by the S_N1  mechanism as shown below:

Formation of products in reaction a

The products obtained will be a racemic mixture.

b. The given reactant is a secondary alkyl halide. The secondary alkyl halide can undergo nucleophilic substitution by both S_N1  and  S_N2 mechanisms. The solvent used in this case is DMSO. The reaction will proceed in the following way:

Formation of products in reaction b

The ^-OCH₃ is a strong nucleophile, and DMSO is a polar aprotic solvent. Hence, the reaction will proceed by the S_N2 mechanism. 

Since the nucleophilic attack will take place at the stereogenic center, the products obtained will have an inversion of configuration.

c. The given halide is a tertiary alkyl halide. The reaction will proceed by the S_N1 mechanism only.

Formation of products in reaction c

The products will be a racemic mixture.

The mechanism and stereochemistry for d, e, and f

d.  The given halide is a secondary alkyl halide. The secondary halide can participate in both  S_N1 and S_N2  reactions. 

The nucleophile being used in this reaction is CH₃COOH  , which is a weak nucleophile that favors only S_N1 reaction.

Formation of products in reaction d

The reaction will take place at the stereogenic center, which will result in the racemization of the product.

 e. The alkyl halide used in the following reaction is a secondary alkyl halide which can participate in both  S_N1 and S_N2 reactions. The nucleophile used in this reaction is ^-OCH₂CH₃ which is a strong nucleophile. 

Formation of products in reaction e

DMF is a polar aprotic solvent that is being used in the above reaction. The presence of a strong nucleophile and the polar aprotic solvent favors the S_N2  reactions.

 

The above reaction is taking place at a stereogenic center, the inversion of the configuration will take place.

f. The given alkyl halide is a secondary alkyl halide that can participate in S_N1 and S_N2 reactions. The CH₃COOH  is a weak nucleophile that favors the S_N1  reactions.

Formation of products in reaction f

In the above reaction, the nucleophile can attack from both sides. Therefore, it will result in the formation of two diastereomers.