Organic Chemistry

200 CHAPTER 5 Stereochemistry

PROBLEM 22

Draw all possible stereoisomers for each of the following compounds:

a. 2-chloro-3-hexanol c. 2,3-dichloropentane

b. 2-bromo-4-chlorohexane d. 1,3-dibromopentane

PROBLEM 23

Draw the stereoisomers of 1-bromo-3-chlorocyclohexane.

PROBLEM 24

Of all the possible cyclooctanes that have one chloro substituent and one methyl sub-

stituent, which ones do not have any asymmetric carbons?

PROBLEM 25

Draw a diastereomer for each of the following.

a. c.

b. d.

5.10 Meso Compounds

In the examples we have just seen, each compound with two asymmetric carbons has

four stereoisomers. However, some compounds with two asymmetric carbons have

only three stereoisomers. This is why we emphasized in Section 5.9 that the maximum

number of stereoisomers a compound with nasymmetric carbons can have (provided

it doesn’t have any other stereocenters) is instead of stating that a compound with

nasymmetric carbons has stereoisomers.

An example of a compound with two asymmetric carbons that has only three

stereoisomers is 2,3-dibromobutane.

The “missing”isomer is the mirror image of 1 because 1 and its mirror image are the

same molecule. This can be seen more clearly if you look either at the perspective for-

mulas drawn in their eclisped conformations or at the Fischer projections.

H 3 CCH 3

H

Br

H

Br

C C

H 3 CCH 3

H

Br

Br

H

C C

H 3 CCH 3

Br

H

H

Br

C C

perspective formulas of the stereoisomers of 2,3-dibromobutane (eclipsed)

123

H 3 CBr

H

Br

H

CH 3

C

H 3 CH

H

Br

Br

CH 3

C

HCH 3

Br

H 3 C

H

Br

C C C C

perspective formulas of the stereoisomers of 2,3-dibromobutane (staggered)

123

CH 3 CHCHCH 3

Br

2,3-dibromobutane

Br

2 n

2 n,

H H

HO CH 3

Cl

H

CH 3 CH 2

Cl

H

CH 3

C C

C C

H 3 C CH 3

HH

CH 3

CH 3

OH

H OH

H