A Free Radical Substitution Reaction
This page gives you the facts and a simple, uncluttered mechanism for the free radical substitution reaction between methylbenzene (previously known as toluene) and chlorine.
Methylbenzene has a methyl group attached to a benzene ring. The hexagon with the circle inside is the standard symbol for this ring. There is a carbon atom at each corner of the hexagon, and a hydrogen atom on each carbon apart from the one with the methyl group attached.
The reaction we are going to explore happens between methylbenzene and chlorine in the presence of ultraviolet light - typically sunlight. This is a good example of a photochemical reaction - a reaction brought about by light.
The organic product is (chloromethyl)benzene. The brackets in the name emphasizes that the chlorine is part of the attached methyl group, and isn't on the ring.
One of the hydrogen atoms in the methyl group has been replaced by a chlorine atom, so this is a substitution reaction. However, the reaction doesn't stop there, and all three hydrogens in the methyl group can in turn be replaced by chlorine atoms. Multiple substitution is dealt with on a separate page, and you will find a link to that at the bottom of this page.
The mechanism involves a chain reaction. During a chain reaction, for every reactive species you start off with, a new one is generated at the end - and this keeps the process going. The over-all process is known as free radical substitution, or as a free radical chain reaction.
- Chain initiation: The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals.
- Chain propagation reactions: These are the reactions which keep the chain going.
- Chain termination reactions: These are reactions which remove free radicals from the system without replacing them by new ones. If any two free radicals collide, they will join together without producing any new radicals. The simplest example of this is a collision between two chlorine radicals.
Jim Clark (Chemguide.co.uk)