Chapter 7, Problem 56P

(a)

Interpretation Introduction

Interpretation: The explanation for poor nucleophilicity of the solvent should be suggested.

  

Concept introduction:Carbocation formation is relatively slower than acid-base reactions. Carbocations generated from alkyl halides have two fates; they can be either trapped by nucleophiles to give substitution product or may deprotonate to yield a small amount of alkene.

Bimolecular substitution or ${\text{S}}_{\text{N}}2$ proceeds via the single-step mechanism. Thus it is well known as the concerted mechanism. Nucleophile approaches carbon while the leaving group still departs from the rear side (opposite to leaving group). The transition state only illustrates the geometric orientation of the substrates and reagents as they pass through the maxima in the single-step mechanism.

A general ${\text{S}}_{\text{N}}2$ reaction mechanistic pathway is illustrated below:

  

Unimolecular substitution or ${\text{S}}_{\text{N}}1$ proceeds via a two-step mechanism. The first slow step that determines the rate is the removal of the leaving group from the substrate haloalkane and generates a carbocation. Since the rate is only governed by substrate alone and no other nucleophile or solvent it is termed as a unimolecular substitution. The final step is the attack of the nucleophile on carbocation generated and the formation of racemic products.

A general ${\text{S}}_{\text{N}}1$ reaction mechanistic pathway is illustrated below.

  

The most likely mechanisms for different kinds of alkyl halides reactions with various nucleophiles is given as follows:

  

(b)

Interpretation Introduction

Interpretation: The relative rates of the two steps should be found and compared to usual ${\text{S}}_{\text{N}}1$ reaction mechanistic steps.

  

Concept introduction: Carbocation formation is relatively slower than acid-base reactions. Carbocations generated from alkyl halides have two fates; they can be either trapped by nucleophiles to give substitution product or may deprotonate to yield a small amount of alkene.

Bimolecular substitution or ${\text{S}}_{\text{N}}2$ proceeds via the single-step mechanism. Thus it is well known as the concerted mechanism. Nucleophile approaches carbon while the leaving group still departs from the rear side (opposite to leaving group). The transition state only illustrates the geometric orientation of the substrates and reagents as they pass through the maxima in the single-step mechanism.

A general ${\text{S}}_{\text{N}}2$ reaction mechanistic pathway is illustrated below.

  

Unimolecular substitution or ${\text{S}}_{\text{N}}1$ proceeds via a two-step mechanism. The first slow step that determines the rate is the removal of the leaving group from the substrate haloalkane and generates a carbocation. Since the rate is only governed by substrate alone and no other nucleophile or solvent it is termed as a unimolecular substitution. The final step is the attack of the nucleophile on carbocation generated and the formation of racemic products.

A general ${\text{S}}_{\text{N}}1$ reaction mechanistic pathway is illustrated below.

  

The most likely mechanisms for different kinds of alkyl halides reactions with various nucleophiles is given as follows:

  

(c)

Interpretation Introduction

Interpretation: The manner carbocation stability and decreasing solvent nucleophilicity might affect the relative magnitudes of ${\text{rate}}_{\text{1}}$ and ${\text{rate}}_2$ should be explained.

  

Concept introduction: Carbocation formation is relatively slower than acid-base reactions. Carbocations generated from alkyl halides have two fates; they can be either trapped by nucleophiles to give substitution product or may deprotonate to yield a small amount of alkene.

Bimolecular substitution or ${\text{S}}_{\text{N}}2$ proceeds via the single-step mechanism. Thus it is well known as a concerted mechanism. Nucleophile approaches carbon while the leaving group still departs from the rear side (opposite to leaving group). The transition state only illustrates the geometric orientation of the substrates and reagents as they pass through the maxima in the single-step mechanism.

A general ${\text{S}}_{\text{N}}2$ reaction mechanistic pathway is illustrated below:

  

Unimolecular substitution or ${\text{S}}_{\text{N}}1$ proceeds via a two-step mechanism. The first slow step that determines the rate is the removal of the leaving group from the substrate haloalkane and generates a carbocation. Since the rate is only governed by substrate alone and no other nucleophile or solvent it is termed as a unimolecular substitution. The final step is the attack of the nucleophile on carbocation generated and the formation of racemic products.

A general ${\text{S}}_{\text{N}}1$ reaction mechanistic pathway is illustrated below:

  

The most likely mechanisms for different kinds of alkyl halides reactions with various nucleophiles is given as follows:

  

(d)

Interpretation Introduction

Interpretation: The complete mechanism for the indicated reaction should be written.

  

Concept introduction: Carbocation formation is relatively slower than acid-base reactions. Carbocations generated from alkyl halides have two fates; they can be either trapped by nucleophiles to give substitution product or may deprotonate to yield a small amount of alkene.

Unimolecular substitution or ${\text{S}}_{\text{N}}1$ proceeds via a two-step mechanism. The first slow step that determines the rate is the removal of the leaving group from the substrate haloalkane and generates a carbocation. Since the rate is only governed by substrate alone and no other nucleophile or solvent it is termed as anunimolecular substitution. The final step is the attack of the nucleophile on carbocation generated and the formation of racemic products.

A general ${\text{S}}_{\text{N}}1$ reaction mechanistic pathway is illustrated below:

  

The most likely mechanisms for different kinds of alkyl halides reactions with various nucleophiles is given as follows: