Chapter 9, Problem 9.4YT

Interpretation Introduction

Interpretation:

The strongest base in Table 9-2 is to be identified based on the ${\text{pK}}_{\text{a}}$ value of its conjugate acid. How does the E2 reaction rate for this base compare with the others?

Concept introduction:

The ${\text{pK}}_{\text{a}}$ is the negative log of the ${\text{K}}_{\text{a}}$. It is related to the strength of the acid. The stronger an acid, the larger the ${\text{K}}_{\text{a}}$ value and, therefore, the smaller the ${\text{pK}}_{\text{a}}$ value. The weak acid, on the other hand, has a large ${\text{pK}}_{\text{a}}$. A strong base has a weak conjugate acid. In terms of ${\text{pK}}_{\text{a}}$, this means the conjugate acid of a strong base will have a large ${\text{pK}}_{\text{a}}$.

An E2 reaction is a bimolecular reaction, i.e., the base is also involved in the rate-determining step. The strength of the base, therefore, influences the rate of a reaction. The stronger the base, the more easily it will extract the proton to start the elimination step. Therefore, the stronger the base used, the faster the reaction.