Chapter 17, Problem 17.73P

Interpretation Introduction

(a)

Interpretation:

The complete mechanism showing the formation of the Wittig reagent from $\text{5-bromo-1, 3-cyclopentadiene}$ using the reagents provided in the given reaction equation is to be drawn.

Concept introduction:

A Wittig reagent, also called a phosphonium ylide, is highly nucleophilic at the ${\text{C}}^{-}$ site. To generate a Wittig reagent, the alkyl halide is treated with triphenylphosphine, $\text{P}{\left({\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\right)}_{\text{3}}$, followed by a very strong base such as butyllithium.

Interpretation Introduction

(b)

Interpretation:

It is to be explained why the Wittig reagent generated from $\text{5-bromo-1, 3-cyclopentadiene}$ does not undergo nucleophilic addition with ketones or aldehydes.

Concept introduction:

A Wittig reagent, also called a phosphonium ylide, is highly nucleophilic at the ${\text{C}}^{-}$ site. To generate a Wittig reagent, the alkyl halide is treated with triphenylphosphine, $\text{P}{\left({\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\right)}_{\text{3}}$, followed by a very strong base such as butyllithium.

Cyclic planar compounds with cyclic conjugation, which obeys Huckel’s rule of $\text{(4n+2π)}$ electrons, are said to aromatic. The addition reaction destroys the aromaticity, which, in turn, lowers the stability; thus aromatic compounds do not undergo addition reactions to retain the aromaticity.