Chapter 9, Problem 9.39P

Interpretation Introduction

Interpretation:

The mechanisms for the competing ${\text{S}}_{\text{N}}\text{2}$, ${\text{S}}_{\text{N}}1$, $\text{E2}$, and $\text{E1}$ reactions when bromocyclohexane is treated with sodium cyanide are to be drawn.

Concept introduction:

The ${\text{S}}_{\text{N}}\text{2}$ reaction is a single step, bimolecular nucleophilic substitution reaction.

The nucleophile forms a bond with the electrophilic carbon in the substrate, with the bond between that carbon and the leaving group breaking at the same time. Since the leaving group hasn’t departed when the nucleophile-carbon bond is formed, the nucleophile must approach the substrate from a direction opposite to the leaving group. This results in an inversion of configuration if the reaction center is a chiral carbon.

The ${\text{S}}_{\text{N}}\text{1}$ reaction is a two-step, unimolecular nucleophilic substitution reaction. Only the substrate is involved in the rate-determining first step. The leaving group departs, along with the electron pair from its bond with the carbon. This results in the formation of a carbocation, which may be subject to a rearrangement. The nucleophile then forms a bond with the carbocation to form the substitution product.

The E2 is a single-step, bimolecular elimination reaction. A proton from a carbon adjacent to the one attached to the leaving group is extracted by a base. The $\text{C}-\text{H}$ bond pair moves toward the carbon attached to the leaving group to form a $\text{π}$ bond. At the same time, the leaving group breaks off along with the electron pair from its bond with the carbon atom to form the elimination product. Since this is a concerted action, the leaving group and the extracted proton must be anti coplanar. In an open chain compound, this is not an issue as a rotation about the $\text{C}-\text{C}$ bond achieves this conformation. In a cyclic compound, this restriction means both the proton and the leaving group must be in anti-positions.

An E1 reaction is a two-step, unimolecular reaction. The first step of E1 is the same as the first step of the ${\text{S}}_{\text{N}}\text{1}$ reaction. The leaving group departs with its bond pair, forming a carbocation in the first, rate-determining step. The carbocation may rearrange if it results in a more stable carbocation. In the second step, the base extracts a proton from an adjacent carbon, with the $\text{C}-\text{H}$ bond pair moving toward the carbocation a $\text{π}$ bond.