Lecture Notes on Halogenation of Alkanes

HALOGENATION OF ALKANES

The enthalpy change for the reaction can be calculated from bond dissociation energies.
 
During chlorination, polyhalogenation can occur.

STRUCTURE & STABILITY OF FREE RADICALS

Homolytic cleavage produces free radicals

Heterolytic cleavage yields ions
A carbon free radical has characteristics similar to the carbocation.

The geometry is planar with the unpaired electron in an atomic p-orbital.

The species is electrophilic.
 
 
Alkyl groups stabilize free radicals for the same reasons that they stabilize carbocations.
Initiation Step:

Initiates the reaction by generating a free radical reactant. This requires an energy input for homolytic bond dissociation to occur.

A chlorine atom (free radical) must be generated to start the substitution process.

This energy is generally supplied by either heating the reaction or by exposing the reaction to ultraviolet light.
 
 
At the end of propagation step 2, a molecule of product has been formed along with a chlorine atom (free radical) that initiates another propagation step 1.
Each time this step occurs, a chlorine free radical is removed that could have continued the chain while a methyl free radical is consumed that would have caused another chlorine free radical to be generated.

HALOGENATION OF HIGHER ALKANES: REGIOSELECTIVITY

 
Since there are 10 hydrogens in this molecule, you would expect that each hydrogen has a 10% chance of being the collision site.
Experimentally, the secondary hydrogens are found to be replaced more frequently than the primary hydrogens.