Radicals - Introduction
Radical Chlorination of an Enantiomerically Pure Starting Product
The chlorination of 2-bromobutane is examplary of a radical reaction that results in racemization. If a pure enantiomer of 2-bromobutane is applied in chlorination, only the 2-bromo-2-chlorobutane racemate is obtained. Obviously, an achiral intermediate stage is formed during the reaction.
The abstraction of a hydrogen atom from S-2-bromobutane with the aid of a chlorine radical yields the planar, achiral, sp2 hybridized bromobutane radical. This radical has two enantiotopic faces. Subsequently, each of the bromobutane radical's enantiotopic faces may be attacked by a chlorine molecule with same probability. Therefore, the two enantiomers of the product are formed in equal amounts - that is, the reaction results in racemization. The product is optically inactive and the stereochemical information of the starting product has been lost.
The formation of the planar, sp2 hybridized bromobutane radical is illustrated by the three-dimensional, interactive animation (Chime) below. Here, one can clearly see that the distance between the three substituents of the radical increases according to the change of hybridization (sp3 to sp2) during radical formation. The increasing distance is the motive power that leads to a planar-like radical structure, as, this way, the steric repulsion between the substituents is minimized.