Free radical halogenation of alkane
Abstract
Radical halogenation is a reaction that introduces a halogen atom (Cl or Br) onto a saturated carbon of an alkane via a free radical chain mechanism, using molecular halogen (Cl2 or Br2) activated by light (hν) or heat (Δ).
Keywords
Alkane, Radical reaction, Halogenation, Alkyl halide
Category
Alkane
Mechanism
The reaction proceeds via a radical chain mechanism consisting of three stages: initiation, propagation, and termination. Initiation: the radical chain is initiated by homolytic cleavage of the X-X bond of the molecular halogen, typically induced by ultraviolet light or heat. Propagation: a halogen radical abstracts a hydrogen atom from the alkane, generating a carbon-centered radical. This intermediate then abstracts a halogen atom from X2, yielding the alkyl halide and regenerating a halogen radical, which continues the chain reaction. Termination: the chain is terminated when two radicals combine, forming a stable product without generating new radicals. Examples include the recombination of two halogen radicals, two carbon radicals, or a carbon radical and a halogen radical.
It is important to note that the hydrogen abstraction is generally selective for the formation of the most stable carbon radical. As a result, radical stability follows the order tertiary > secondary > primary, analogous to the one of carbocations. In other words, halogenation occurs at the most substituted carbon atom.
