12 . D
Remember that the equation for specific rotation is . In this example, αobs is +12°
(remember that dextrorotatory, or clockwise, rotation is considered positive), c = 0.5 , l = 1 cm
= 0.1 dm. Remember that path length is always measured in decimeters when calculating specific
rotation. Therefore, the specific rotation can be calculated as:13 . B
Racemic mixtures like omeprazole contain equimolar amounts of two enantiomers and thus
have no observed optical activity. Each of the two enantiomers causes rotation in opposite
directions, so their effects cancel out. Esomeprazole only contains one of the two enantiomers
and thus should cause rotation of plane-polarized light.14 . C
Draw out these structures. The two names describe the same molecule, which also happens to be
a meso compound because it contains a plane of symmetry. These compounds are not
enantiomers because they are superimposable mirror images of one another, not
nonsuperimposable mirror images. These compounds are better termed meso-2,3-
dihydroxybutanedioic acid:
15 . C
In butane, the position at which the two methyl groups are 120° apart is an eclipsed
conformation. This has a moderate amount of energy, although not as high as a totally eclipsed
conformation in which the two methyl groups are 0° apart.