Chapter 11, Problem 7PP

Interpretation Introduction

Interpretation:

The factor behind the observation that tertiary alcohols react faster with HX than secondary alcohols and the factor behind the observation that methanol reacts with HX faster than primary alcohols, are to be explained.

Concept introduction:

Electrophiles are electron deficient species that have positive or partially positive charge. Lewis acids are electrophiles that accept electron pair.

Nucleophiles are electron rich species that have negative or partially negative charge. Lewis bases are nucleophiles that donate electron pair.

Substitution reaction: A reaction in which one of the hydrogen atoms of a hydrocarbon or a functional group is substituted by any other functional group is called substitution reaction.

Nucleophilic substitution reaction is a reaction in which an electron rich nucleophile attacks the positive or partial positive charge of an atom or a group of atoms to replace a leaving group.

${\text{S}}_{\text{N}}\text{2}$ reaction is the nucleophilic substitution reaction in which the two components are involved in the rate determining step.

The ${\text{S}}_{\text{N}}\text{2}$mechanism of substitution reaction occurs in secondary alkyl halides with an inversion of configuration.

The ${\text{S}}_{\text{N}}\text{2}$reaction takes place in one step. The nucleophile attacks the alkyl halide from the side opposite to the leaving group, resulting in an inversion of configuration.

An ${\text{S}}_{\text{N}}1$ reaction is a nucleophilic substitution reaction in which only one component is involved in the rate-determining step.

The nucleophilic substitutions in which a nucleophile replaces a leaving group are known as ${\text{S}}_{\text{N}}1$ reactions.

${\text{S}}_{\text{N}}1$ reactions are unimolecular as the rate of reaction is dependent on the concentration of a single reactant.

The stability of carbocation:

${\text{3}}^{\text{o}}\text{carbocation}\text{>}{\text{2}}^{\text{o}}\text{carbocation}{\text{>1}}^{\text{o}}\text{carbocation}\text{>methyl}\text{carbocation}$

Reaction of tertiary and secondary alcohols with HX proceeds via S_N1 pathway. In S_N1, the formation of carbocation is the rate determining step. Tertiary alcohols form more stable carbocation than secondary alcohols, and therefore, reacts faster with HX.

For primary alcohols and methanol, the reaction with HX proceeds via S_N2 pathway. The backside attack of the nucleophile and simultaneous departure of the leaving group is the rate determining step here. The primary alcohol being more crowded than methanol reacts slower with HX compared to methanol.