Chapter 7.9, Problem 7.22E

(a)

Interpretation Introduction

Interpretation: Out of the substrates for the below reactions the one pair that will give a higher $\text{E2:E1}$ product ratio should be identified.

  

Concept introduction: Carbocation formation is relatively slower than acid-base reactions. Carbocations generated from alkyl halides have two fates; they can be either trapped by nucleophiles to give substitution product or may deprotonated to yield small amount of alkene. Stronger the base more is the probability of elimination over substitution over elimination. Further, if still higher concentration is employed reaction proceeds via bimolecular elimination. On the other hand, weak base waits until the carbocation is formed and the type of elimination with relatively weak base is two-step elimination or $\text{E1}$ .

Analogous to the case that strong nucleophiles are more favored to react via ${\text{S}}_{\text{N}}2$ pathway, greater concentration of strong bases are more favored for single-step elimination $\text{E}2$ pathway. Such strong bases include hydroxide ion or alkoxide ions. The reason of predominance of $\text{E}2$ elimination is that strong bases have high probabilities and high rates to abstract hydrogen from carbon adjacent to carbocation.

(b)

Interpretation Introduction

Interpretation: Out of the substrates for the reactions the one pair that will give a higher $\text{E2: E1}$ product ratio should be identified.

  

Concept introduction: Carbocation formation is relatively slower than acid-base reactions. Carbocations generated from alkyl halides have two fates; they can be either trapped by nucleophiles to give substitution product or may deprotonated to yield small amount of alkene. Stronger the base more is the probability of elimination over substitution over elimination. Further, if still higher concentration is employed reaction proceeds via bimolecular elimination. On the other hand, weak base waits until the carbocation is formed and the type of elimination with relatively weak base is two-step elimination or $\text{E1}$ .

Analogous to the case that strong nucleophiles are more favored to react via ${\text{S}}_{\text{N}}2$ pathway, greater concentration of strong bases are more favored for single-step elimination $\text{E}2$ pathway. Such strong bases include hydroxide ion or alkoxide ions. The reason of predominance of $\text{E}2$ elimination is that strong bases have high probabilities and high rates to abstract hydrogen from carbon adjacent to carbocation.