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).

stereochemistry of reactions 01

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

initiation stage

Stage 2. Propagation

propagation stage

[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.

The generated radical is flat and the lobes are attacked with equal probability by molecular bromine, producing racemic 2-bromobutane.
Halogenation of (S)-2-Chlorobutane to C3
The bromo halogenation reaction of (S)-2-Chlorobutane on carbon C 3 has the following form:
halogenation c3
The propagation stages that determine the stereochemistry of the product formed are:
spread c3
[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.

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