The amino acid (S)-alanine has the physical characteristics listed under the structure.

a. What is the melting point of (R)-alanine?

b. How does the melting point of a racemic mixture of (R)- and (S)-alanine compare to the melting point of (S)-alanine?

c. What is the specific rotation of (R)-alanine, recorded under the same conditions as the reported rotation of (S)-alanine?

d. What is the optical rotation of a racemic mixture of (R)- and (S)-alanine?

e. Label each of the following as optically active or inactive: a solution of pure (S)-alanine; an equal mixture of (R)- and (S)-alanine; a solution that contains 75% (S)- and 25% (R)-alanine.

A molecule with a chiral center must have an enantiomer. The corresponding enantiomer has the opposite configuration.

The enantiomers have identical physical properties. For example, solubility, melting point, boiling point values are the same for enantiomers.

a. The R and S alanine amino acids are enantiomers. Therefore, the melting point (physical property) is the same for R alanine as S alanine. Hence, the melting point is $\text{297°C}$for the R alanine.

b. The racemic mixtures are different than the enantiomers. Therefore, the melting point is different than the S-alanine.

c. The specific rotation is also a physical property of the enantiomers. Therefore, R and S alanine have the same specific rotation value, but the sign is different due to the interaction of R and S in a different direction.

So, if the S-alanine has the specific rotation +8.5, the R alanine has the specific rotation value -8.5.

d. The optical rotation for the racemic mixture of R and S alanine is zero. This is because the racemic mixture contains an equal amount of R and S.

e. The optical activity for the given terms is as follows:

• Solution of pure (S)-alanine: optically active.
• An equal mixture of (R)- and (S)-alanine (racemic mixture): optically inactive.
• 75% (S)- and 25% (R)-alanine: optically active.