In the case of cyclic compounds, the cis isomer will be the meso compound and the
trans isomer will exist as enantiomers.The preceding structure for cis-1,2-dibromocyclohexane suggests that the com-
pound has a plane of symmetry. Cyclohexane, however, is not a planar hexagon—it
exists preferentially in the chair conformation, and the chair conformer of
cis-1,2-dibromocyclohexane does not have a plane of symmetry. Only the much less
stable boat conformer of cis-1,2-dibromocyclohexane has a plane of symmetry. Then,
is cis-1,2-dibromocyclohexane a meso compound? The answer is yes. As long as any
one conformer of a compound has a plane of symmetry, the compound will be achiral,
and an achiral compound with two asymmetric carbons is a meso compound.This holds for acyclic compounds as well. We have just seen that 2,3-dibromo-
butane is an achiral meso compound because it has a plane of symmetry. To see that it
had a plane of symmetry, however, we had to look at a relatively unstable eclipsed
conformer. The more stable staggered conformer does not have a plane of symmetry.
2,3-Dibromobutane is still a meso compound, however, because it has a conformer
that has a plane of symmetry.PROBLEM-SOLVING STRATEGYWhich of the following compounds has a stereoisomer that is a meso compound?a. 2,3-dimethylbutane e. 1,4-dimethylcyclohexane
b. 3,4-dimethylhexane f. 1,2-dimethylcyclohexane
c. 2-bromo-3-methylpentane g. 3,4-diethylhexane
d. 1,3-dimethylcyclohexane h. 1-bromo-2-methylcyclohexaneeclipsed conformerCH 3plane of
symmetry
BrCH 3HHBrstaggered conformerCH 3no plane of
symmetry
BrCH 3HHBrHHBr Br BrBrchair conformer boat conformerno plane of symmetryplane of
symmetrycis-1,2-dibromocyclohexane trans-1,2-dibromocyclohexaneHCH 3HCH 3cis-1,3-dimethylcyclopentane trans-1,3-dimethylcyclopentanea meso compound a pair of enantiomersa meso compound a pair of enantiomersHBrHBrHBrBrHBrHHBrHCH 3CH 3HHCH 3CH 3H202 CHAPTER 5 Stereochemistry
3-D Molecules:
cis-1,3-Dimethylcyclopentane
and its mirror image