Chapter 9, Problem 9.9P

Interpretation Introduction

Interpretation:

It is to be determined whether the phenoxide ion ${\text{(C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{O}}^{\text{-}})$ is a strong or a weak base. It is to be determined whether it will tend to favor $\text{E1}$ or $\text{E2}$ mechanism.

Concept introduction:

The strength of a base can be determined on the basis of the pKa value of its conjugate acid. The weaker the conjugate acid, stronger the base. Higher the ${\text{pK}}_{\text{a}}$ value, weaker the conjugate acid and stronger the base. Generally a base is compared to the hydroxide ion, ${\text{HO}}^{\text{-}}$, to decide whether it is a strong or weak base.

An E1 reaction is a two-step mechanism. The first, rate-determining step involves only the substrate. Therefore, the strength of the base does not affect the rate of an E1 reaction. On the other hand, in an E2 reaction, both the substrate and the base are involved in the only (and therefore the rate-determining) step. Since the base initiates the reaction, its strength influences the rate of the reaction.

A weak base will therefore favor an E1 reaction while a strong base will favor an E2 reaction.