Chapter 25, Problem 25.55P

Interpretation Introduction

Interpretation:

A chain reaction mechanism is to be proposed for the elimination of HI from $\text{2-iodopropane}$ to form an alkene.

Concept introduction:

Haloalkanes can undergo the elimination of hydrogen halide (HX) to give an alkene when irradiated with UV light. The initiation step is similar to the one in the case of radical halogenation. The weakest bond in haloalkane, $\text{C}-\text{X}$, undergoes homolytic dissociation to form a pair of radicals, an alkyl radical and a halogen radical. In the first of the propagation steps, the halogen radical extracts a hydrogen atom (radical) from a carbon adjacent to the one bonded to the halogen. The resultant radical then undergoes homolytic dissociation of the $\text{C}-\text{X}$. The unpaired electrons on the adjacent carbons form a $\text{π}$ to produce the alkene. The eliminated halogen radical continues the chain reaction. The overall reaction is the elimination of a hydrogen halide molecule to form an alkene.

Chains can be terminated by the coupling of any two radicals present in the reaction mixture.