Question

Write out a stepwise sequence that shows how a racemic mixture of leucine enantiomers can be resolved into optically active amino acids using $\left(R\right)\mathbf{-}\mathbf{\alpha }\mathbf{-}\mathbf{methylbenzylamine}$.

Short answer

Answer

Step-by-step solution

Enantiomers

Stereoisomers that are non-superimposable mirror images of each other are called enantiomers. Enantiomers have the same physical and chemical properties and are optically active.

Conversion of leucine to N-acetyl amino acid

A racemic mixture of amino acids such as (R)- and (S)-leucine, the racemate is first treated with acetic anhydride to form N-acetyl amino acids.

Formation of N-acetyl amino acid

Convert enantiomers to diastereomers

Both (R) and (S)-leucine enantiomers react with the R- isomer of the chiral amine form diastereomers. These salts have the same configuration around one stereogenic center but the opposite configuration about the other stereogenic center.

Formation of diastereomers

Separate diastereomers

The diastereomers are separated by physical techniques, such as crystallization or distillation.

Diastereomers

Regeneration of amino acid

The amides can be hydrolyzed with an aqueous base to regenerate the amino acids. The amino acids are now separated from each other. The optical activity of the amino acids can be measured and compared to their known rotations to determine the purity of each enantiomer.

Formation of S- and R-leucine