Chapter 25, Problem 25.48P

Interpretation Introduction

Interpretation:

A mechanism for the chain reaction in Problem 25.47, including initiation, propagation, and two plausible termination steps is to be proposed.

Concept introduction:

Radical halogen substitution in alkanes is a chain reaction that consists of three steps, initiation, propagation, and termination. Initiation is the formation of a radical from the reagent, typically by UC irradiation. The irradiation results in the homolytic dissociation of the halogen (or halogenated alkane) molecule. The halogen radical then extracts a hydrogen atom (radical) from the substrate alkane forming an alkyl radical and hydrogen halide. If the alkane contains different types of carbons, the probability of alkyl radical formation increases in the order of the stability of radicals, ${\text{1}}^{\text{o}}{\text{ < 2}}^{\text{o}}{\text{ < 3}}^{\text{o}}$. In the second propagation step, the alkyl radical induces homolytic dissociation of another halogen molecule, forming alkyl halide and another halogen radical which then continues the reaction.

Termination steps, called radical coupling, consist of the combination of any two radicals.