A compound with two chiral centers that is meso will always have opposite absolute configurations at the two chiral centers. That is, a meso compound will never be (R,R) or (S,S); instead, it will be (R,S). Explain why this must be true.

For each structure,

1. star (*) any asymmetric carbon atoms.

2. label each asymmetric carbon as (R) or (S).

3. draw any internal mirror planes of symmetry.

4. label the structure as chiral or achiral.

5. label any meso structures.

(e)

Which of the following compounds are meso?

(f)

The original definition of meso is 'an achiral compound that has chiral diastereomers.' Our working definition of meso is 'an achiral compound that has chiral centers (usually asymmetric carbon atoms).' The working definition is much easier to apply, because we don't have to envision all possible chiral diastereomers of the compound. Still, the working definition is not quite as complete as the original definition.

a. Show how cis-cyclooctene is defined as a meso compound under the original definition, but not under our working definition. (Review Figure 5-19)

Draw three-dimensional representations of the following compounds. Which have asymmetric carbon atoms? Which have no asymmetric carbons but are chiral anyway? Use your models for parts (a) through (d) and any others that seem unclear.

(g)

Which of the following compounds has a stereoisomer that is a meso compound?

c. 2,4-dimethylpentane

d. 1,3-dichlorocyclohexane

Which of the following compounds has a stereoisomer that is a meso compound?

a. 2,4-dibromohexane

b. 2,4-dibromopentane

Which of the following has an achiral stereoisomer?

a. 2,3-dichlorobutane

b. 2,3-dichloropentane

Which of the following compounds are chiral? Draw each compound in its most symmetric conformation, star (*) any asymmetric carbon atoms, and draw any mirror planes. Label any meso compounds. You may use Fischer projections if you prefer.

(c) (2R,3S)-2-bromo-3-chlorobutane

(d) (2R,3S)-2,3-dibromobutane