Choice (B) is incorrect because diastereomers are stereoisomers that are not mirror images of
each other. Choice (C) is incorrect because meso compounds must contain a plane of symmetry,
which neither of these molecules has. Choice (D) is incorrect because structural isomers are
compounds with the same molecular formula but different atomic connections. The connectivity
in these two molecules is the same, which means that they are stereoisomers, not structural
isomers.8 . A
(+)-glyceraldehyde and (–)-glyceraldehyde, or (R)- and (S)-2,3-dihydroxypropanol, are
enantiomers. Enantiomers are nonsuperimposable mirror images. Each has only one chiral
center (C-2), which has opposite absolute configuration in these two molecules.9 . C
(E)-2-butene can also be called trans-2-butene; (Z)-2-butene can also be called cis-2-butene. As
such, they are cis–trans isomers. Remember that cis–trans isomers are a subtype of
diastereomers in which the position of substituents differs about an immovable bond.
Diastereomers are molecules that are non-mirror-image stereoisomers (molecules with the same
atomic connectivity). These are not enantiomers because they are not mirror images of each
other.10 . C
Because they have the same molecular formula but different atomic connectivity, 3-
methylpentane and hexane are constitutional isomers.
11 . B
These two molecules are stereoisomers of one another, but are not nonsuperimposable mirror
images. Therefore, they are diastereomers. Note that these molecules differ by at least one, but
not all, chiral carbons.