During the free radical chlorination of n-butane, substitution at the secondary carbon (C2) yields 2-chlorobutane, which contains a chiral centre. The reaction proceeds via the formation of a secondary free radical intermediate (sec-butyl free radical). This free radical is essentially planar ($sp^2$ hybridized). The subsequent attack by a chlorine molecule/radical can occur from either face of the planar intermediate with equal probability. As a result, an equimolar (50:50) mixture of both enantiomers (d- and l-forms) is formed, which is optically inactive and is known as a racemic mixture.