Chapter 17, Problem 17.59P

Interpretation Introduction

Interpretation:

From the given molecules, the nucleophiles that will add reversibly and irreversibly to a polar $\pi$ bond are to be determined.

Concept introduction:

A polar $\pi$ bond, that is, alpha, beta -unsaturated is susceptible to nucleophilic attack at both the electron-poor atom of the polar $\pi$ bond and the beta carbon. Attack at the carbonyl carbon yields the direct addition (or 1, 2—addition) product whereas attack at the beta carbon yields the conjugate addition (or 1, 4-addition). Direct addition to an alpha, beta -unsaturated polar $\pi$ bond dominates if the nucleophile adds irreversibly to the polar $\pi$ bond. Otherwise, the conjugate addition is favored.

Nucleophilic addition tends to be irreversible if the negative charge that develops in the adduct is significantly better stabilized than it is in the nucleophile. This occurs in the nucleophile that has the negative charge located on a carbon or hydrogen atom, and the negative charge is not stabilized by resonance or inductive effects. Therefore, nucleophilic addition to a carbonyl carbon tends to be irreversible when it involves a very strong ${\text{R}}^{\text{-}}$ or ${\text{H}}^{\text{-}}$ nucleophile. Otherwise, the nucleophilic addition tends to be reversible.