We are going to see how chemical reactions can introduce chirality in molecules, obtaining products in the form of racemic mixtures or mixtures of diastereoisomers.

Butane halogenates in the presence of bromine and light, at carbon 2, to form a mixture of enantiomers. The radical formed presents enantiotopic faces, which are halogenated with equal probability, giving rise to a racemic mixture (enantiomers in equal proportion).

The mechanism of this reaction consists of three stages: initiation, propagation and termination. Propagation is the step that determines the stereochemistry of the final product.

Butane halogenation

Stage 1. Initiation

Stage 2. Propagation

[1] H (Enantiotopic Hydrogens)

[2] Radical with enantiotopic faces

[3] Pair of enantiomers

The product is obtained as a racemic mixture, due to the formation of a planar radical that is halogenated on both faces. The enantiotopic hydrogens are chemically equivalent and are subtracted by bromine at the same rate.

Halogenation of (S)-2-Chlorobutane to C3

The bromo halogenation reaction of (S)-2-Chlorobutane on carbon C ₃ has the following form:

 

 

The propagation stages that determine the stereochemistry of the product formed are:

[1] H (diastereotopic hydrogens)

[2] Radical with diastereotopic faces

[3] Mixture of diastereoisomers

 

Halogenation of the C₃ position leads to diastereoisomers. The faces of the radical formed are not equivalent and are attacked at different rates by the bromine molecule. They are called diastereotopic faces and the hydrogens that bromine subtracts are called diastereotopic hydrogens.