4 . C
To be considered a chiral center, a carbon must have four different substituents. There are eight
stereocenters in this molecule, which are marked below with asterisks.The other carbons are not chiral for various reasons. Many are bonded to two hydrogens; others
participate in double bonds, which count as two bonds to the same atom.5 . B
This molecule is a chair conformation in which the two equatorial methyl groups are trans to
each other. Because the axial methyl hydrogens do not compete for the same space as the
hydrogens attached to the ring, this conformation ensures the least amount of steric strain.
Choice (A) would be less stable because the diaxial methyl group hydrogens are closer to the
hydrogens on the ring, causing greater steric strain. Choice (C) is incorrect because it is in the
more unstable boat conformation.
6 . C
The relative configuration is retained because the bonds of the stereocenter are not broken; thus
the positions of groups around the chiral carbon are maintained. The absolute configuration is
also retained because both the reactant and product are (R).
7 . A
These compounds are nonsuperimposable mirror images. To make analysis a bit easier, we can
rotate structure II 180° to look like structure III. Structures I and III more clearly have opposite
stereochemistry at every chiral center, meaning that they are enantiomers.