20.18 : 自由基卤化：立体化学

The stereochemistry of radical halogenation depends on whether the reacting molecule is chiral or achiral.

For example, achiral n-butane, upon radical chlorination, gives 1- and 2-chlorobutane. The reaction introduces a new chiral center, as one of the products is a racemic mixture.

The formation of a racemic mixture is driven by the generation of an achiral trigonal planar radical intermediate, on which chlorine can attack from either face, yielding equal amounts of R and S enantiomers.

This makes carbon-2 hydrogens enantiotopic in nature.

Similarly, radical halogenation at an existing chiral center also forms a racemic mixture.

Here, the radical intermediate loses the reactant's configuration and becomes achiral, enabling the halogen attack from either side and producing equal amounts of enantiomers.

In contrast, a chiral compound with a chiral carbon not involved in radical halogenation gives diastereomeric products, introducing a second chiral center.

The existing chiral center makes the trigonal planar radical intermediate chiral. Consequently, chlorine attacks to a greater extent at one face than the other and produces chiral, diastereomeric products of unequal amounts.