1. This is angelic acid, or E-2-methylbut-3-enoic acid.

    Angelic acid.

  2. Like cyclopentane-1,2-diol, cyclohexane-1,3-diol has three isomers, two enantiomeric forms, and one meso form that is diastereomeric to the other two forms.

    Cyclohexane-1,3-diol.

  3. Which of the following statements are true?
    • Diastereomers are superimposable mirror images of each other. False: they are not mirror images. Superimposable mirror images are meso forms.
    • Diastereomers have different melting points. True: diastereomers differ in all their chemical and physical properties.
    • Diastereomers are different from one another at all chiral centres. False: enantiomers differ at every chiral centre. Diastereomers differ at only some (as a special case, epimers are multi-chiral compounds that only differ only at one chiral centre).
    • Enantiomers are have different binding constants to enzymes. True: enantiomers behave differently in chiral environments, such as that provided by proteins made only from L-amino acids.
    • Enantiomers are superimposable mirror images of each other. False: that is meso forms. Enantiomers are non-superimposable mirror images.
    • A 50:50 mixture of two diastereomers will have no effect on the plane of polarised light. False (in general): a racemic mixture of two enantiomers will have this property, a mixture of diastereomers almost certainly will not.
  4. This is the S isomer of erythrulose, S-1,3,4-trihydroxybutan-2-one.
  5. Specific rotation = (100 × 0.0699° ) ⁄ ( ( 15 ⁄ 10 ) dm × 3 g 100 mL−1 ) = +1.49°.
    • Optical purity, OP = αmix ⁄ α+ = 1.49 ⁄ 14.2 = 0.105
    • The S isomer is the (+) isomer, as indicated by the negative rotation of R alanine.
    • Proportion of (+) isomer = ( OP + 1 ) ⁄ 2 = ( 0.105 + 1 ) ⁄ 2 = 0.55
    • The ratio of (R)(−) isomer to (S)(+) isomer in the mixture is 0.45 : 0.55
  6. Naturally occuring D-glucose is the (+) isomer. A racemic mixture of glucose exposed to bacteria would be slowly enriched in the (-) isomer as the (+) isomer is metabolised.
  7. Alanine is an amine, and a racemic mixture could be reacted with a chiral acid chloride to form two diastereomeric amides, which could be resolved by differential crystallisation and hydrolysed to regenerate the amines. Even more simply, you could use preparative normal phase HPLC with a chiral column to separate the two enantiomers directly, if only small quantities were required.