Answers and Explanations

DISCRETE PRACTICE QUESTIONS

1 . D

This    is  a   racemic mixture of  2-butanol   because it  consists    of  equimolar   amounts of  (R)-2-butanol

and (S)-2-butanol.  The (R)-2-butanol   molecule    rotates the plane   of  polarized   light   in  one

direction,  and the (S)-2-butanol   rotates it  by  the same    angle   but in  the opposite    direction;  as  a

result, no  net rotation    of  polarized   light   is  observed.

2 . B
The maximum number of stereoisomers of a compound equals 2n, where n is the number of
chiral carbons in the compound. In this molecule, C-1 (the aldehydic carbon) is not chiral, nor is C-
5 (because it is attached to two hydrogen atoms). Therefore, with three chiral centers, there are
23 = 8 stereoisomers.

3 . C
This answer choice is an example of a meso compound—a compound that contains chiral centers
but has an internal plane of symmetry:

Owing   to  this    internal    plane   of  symmetry,   the molecule    is  achiral and,    hence,  optically   inactive.

Choices (A) and (B) are enantiomers of  each    other   and will    certainly   show    optical activity    on  their

own.    Choice  (D),    because it  contains    a   chiral  carbon  and no  internal    plane   of  symmetry,   is

optically   active  as  well.