Question

Predict all the products of the reaction for butane and Cl2 in the presence of heat?

Answer 1

Chlorination of Butane in presence of heat/light

There are free radical mechanisms for substitution reactions of alkanes. These are of use for synthesis only in rather restricted cases, but the products we see are also controlled by competition in which the more stable intermediates are formed faster.

We see that a secondary hydrogen has been replaced by a chlorine more often than a primary hydrogen, even though there are six primary hydrogens and only four secondary hydrogens. This would make sense if the reaction mechanism went through a free radical, since we would expect a more stable secondary free radical to be formed faster (more often) than a less stable primary free radical. Such a mechanism would be:

Again the more abundant product is produced through the more stable (secondary) free radical rather than through the less stable (primary) free radical. Notice that this reaction is also a chain reaction, since the second step produces a chlorine atom which can react with another butane molecule to continue the chain. The reaction is initiated in this case by homolytic cleavage of the weak Cl-Cl bond by light to produce chlorine atoms.