Chapter 3, Problem 38P

Interpretation Introduction

Interpretation: Poor selectivity in the formation of the brominated products when equimolar mixture of ${\text{Br}}_{\text{2}}$ and ${\text{Cl}}_{\text{2}}$ compared to the case when ${\text{Br}}_{\text{2}}$ alone is taken should be explained.

Concept introduction: The monobromination performed with ultraviolet light proceeds via radical chain mechanism.

In general, tertiary $\text{C – H}$ bonds react in halogenations more readily than secondary those, in turn, are faster to react than primary. This can be attributed to stability of radical formed upon $\text{C – H}$ bond cleavage via hyperconjugation.

The phenomenon of hyperconjugation refers to donation of $\text{σ}$ electrons into the partly vacant $p$ orbital lobes of radical carbon. More will be number of $\text{σ}$ orbital capable of donation of electrons more is the stabilization.