Chapter 25, Problem 25.45P

Interpretation Introduction

Interpretation:

In the halogenation of ${\text{H}}_{\text{2}}$, described in Problem 25.44, which halogen (F, $\text{Cl}$, Br, or $\text{ I}$) will react fastest is to be explained. The answer is to be defended using the appropriate bond energies from Tables 25-1 and 25-2.

Concept introduction:

Radical halogenation is a substitution reaction in which a hydrogen atom from an alkane is replaced by a halogen atom from a molecular halogen, ${\text{X}}_{\text{2}}$, producing a hydrogen halide as a by-product. Similar to alkanes, hydrogen gas can undergo radical halogenation. A free radical chain reaction involves free radicals in a cyclic sequence of steps, called propagation steps, which are responsible for converting overall reactants into overall products. Throughout the sequence of propagation steps, also called the propagation cycle, there is no net consumption of free radicals. The overall reaction is obtained by summing just the propagation steps. An initiation step is responsible for producing the free radical that enters the propagation cycle. A termination step removes free radicals from the propagation cycle and is responsible for slowing the overall reaction.