Chapter 10, Problem 10.42P

Interpretation Introduction

Interpretation:

It is to be explained why halogenation doesn’t readily takes place with ${\text{F}}_{\text{2}}$ at an alpha carbon of a ketone or aldehyde under basic conditions, and why it readily takes place if halogens are ${\text{Cl}}_{\text{2}}{\text{, Br}}_{\text{2}}$ or ${\text{I}}_{\text{2}}$.

Concept introduction:

When a ketone or aldehyde with an alpha hydrogen is treated with a strong base in the presence of a molecular halogen such as ${\text{Cl}}_{\text{2}}{\text{, Br}}_{\text{2}}$ or ${\text{I}}_{\text{2}}$, a halogen atom replaces alpha hydrogen. A strong base abstracts an alpha hydrogen atom from the ketone or aldehyde thus generating an electron-rich enolate ion which serves as a nucleophile. The halogen molecule is non-polar, and the three lone pairs of electrons surrounding each halogen atom is the electron rich site. However, ${\text{Cl}}_{\text{2}}{\text{, Br}}_{\text{2}}$ or ${\text{I}}_{\text{2}}$ are relatively highly polarizable due to their abundance of electrons (non-bonded and bonded). Consequently, when an electron-rich species like the enolate ion approaches, the electrons on the halogen molecule are repelled quite readily, generating a substantial induced dipole. This makes the halogen atom that is nearer the nucleophile relatively electron-poor.