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MCAT Question A Day - 4/25/13 - Answer!

A racemic mixture of 2-bromobutane contains

A.  the R enantiomer only.
B.  the S enantiomer only.
C.  equal amounts of the (1) and (2) optical isomers.
D.  only diastereomers.

The correct answer is (C). A racemic mixture does not rotate the plane of polarized light because it contains equal amounts of enantiomers. One enantiomer rotates polarized light clockwise (dextrorotary), while its nonsuperimposable mirror image rotates the plane of polarized light in the opposite direction (levorotary) by the same angle.