Q13 P. - Organic Chemistry (Mcmurry)

Question

Predict whether each of the following substitution reactions is likely to be ${\mathbf{S}}_{\mathbf{N}}\mathbf{1}$or${\mathbf{S}}_{\mathbf{N}}\mathbf{2}$:

(a)

(b)

Step-by-Step Solution

Answer

(a) This reaction occurs by an ${\mathrm{S}}_{\mathrm{N}}1$ mechanism.

(b) This reaction occurs by an ${\mathrm{S}}_{\mathrm{N}}2$ mechanism.

Nucleophilic unimolecular substitution reaction S N 1

The ${\mathbf{S}}_{\mathbf{N}}\mathbf{1}$ reaction is a nucleophilic substitution reaction where carbon-halogen bond is broken which results in positively charged carbon and then nucleophile attacks the carbocation and forms a new bond. Only one reacting species is involved in the rate determining step of the reaction. The term ${\mathrm{S}}_{\mathrm{N}}1$ stands for substitution nucleophilic unimolecular. The hydrolysis of tert-butyl bromide with aqueous NaOH solution is an example of ${\mathrm{S}}_{\mathrm{N}}1$ reaction.

Nucleophilic bimolecular substitution reaction S N 2

The ${\mathrm{S}}_{\mathrm{N}}2$ reaction is a nucleophilic substitution reaction where bond is broken and another is formed simultaneously. Two reacting species are involved in the rate determining step of the reaction. The term ${\mathrm{S}}_{\mathrm{N}}2$ stands for substitution nucleophilic bimolecular.

                                                                    ${\mathrm{S}}_{\mathrm{N}}2$reaction

Determine the substitution of the following reaction

(a)

This reaction probably occurs by an ${\mathrm{S}}_{\mathrm{N}}1$ mechanism. HCl converts the poor –OH leaving group into an excellent $-{\mathrm{OH}}_2$ leaving group, and the polar solvent stabilizes the carbocation intermediate.

(b)

This reaction takes place with a negatively charged nucleophile ina polar, aprotic solvent. It is very likely that the reaction occurs by an ${\mathrm{S}}_{\mathrm{N}}2$ mechanism.