Question

8. Indicate whether each of the following statements is true or false. If a statement is false, explain why a) Two structural isomers may be chiral. : True b) A compound must be chiral in order to have an enantiomer. True c) A chiral compound with an asymmetric carbon atom designated always has a positive optical rotation. False (R) as d) Mirror-image molecules are always enantiomers. False chiral. False e) All compounds with asymmetric carbon atoms are f)The relative configuration of (R)-3-chloro-2-hydroxypropanoic acid, CH(CI)CH(OH)COOH, is L. g) Some diastereomers have a mirror-image relationship. False h) A racemic mixture of an enantiomeric pair is not optically active.

Answer 1

a) True

b) True

Individual members of a pair of enantiomers have the property of chirality, which is simply the name we give to non-superposability on a mirror image. Members of a pair of diastereomers need not necessarily be chiral, but individual enantiomers always are.

c) False

There is no general correlation between the the rectus (R) and sinister (S) prefixes and dextro/levorotation. In some cases the (R)-enantiomer is the dextrorotary enantiomer (as in the case of D-alanine), and in other cases the (R)-enantiomer is the levorotary enantiomer (as in the case of L-cysteine). The relationship can only be determined on a case-by-case basis with experimental measurements or detailed computer modeling.

d) False

Enantiomers are pairs of stereoisomers that are chiral. A chiral molecule is non-superimposable on its mirror image, so that the mirror image is actually a different molecule. The two non-identical mirror images are a pair of enantiomers.

e) True

An asymmetric carbon atom (chiral carbon) is a carbon atom that is attached to four different types of atoms or groups of atoms.

f) False: Consider {Badly named, but illustrative} (R)-3-Hydroxy-2-hydroxypropanal aka D-glyceraldehyde

g) False

Diastereomers are NOT mirror inages.

h) True