Organic Chemistry

224 CHAPTER 5 Stereochemistry

If each of the two asymmetric carbons is bonded to the same four substituents, a

meso compound will be obtained instead of the erythro enantiomers.

In contrast, syn addition of to a trans-alkene forms only the threo enantiomers.

Thus the addition of hydrogen is a stereospecific reaction—the product obtained from

addition to the cis isomer is different from the product obtained from addition to the

trans isomer. It is also a stereoselective reaction because all possible isomers are not

formed; for example, only the threo enantiomers are formed in the following reaction.

Cyclic alkenes with fewer than eight carbons in the ring, such as cyclopentene and

cyclohexene, can exist only in the cis configuration because they do not have enough

carbons to incorporate a trans double bond. Therefore, it is not necessary to use the cis

designation with their names. Both cis and trans isomers are possible for rings con-

taining eight or more carbons, however, so the configuration of the compound must be

specified in its name.

Therefore, the addition of to a cyclic alkene with fewer than eight ring atoms forms

only the cis enantiomers.

Each of the two asymmetric carbons in the product of the following reaction is bonded to

the same four substituents. Therefore, syn addition forms a meso compound.

D D

+ H 2

1,2-dideuterio-

cyclopentene

Pt/C

DD

HH

H 3 C CH(CH 3 ) 2

++H 2

1-isopropyl-2-methyl-

cyclopentene

Pt/C

H 3 C CH(CH 3 ) 2

H 3 C CH(CH 3 ) 2

HH

H H

H 2

cis-cyclooctene trans-cyclooctene

H 2

CH 2 CH 3 CH 2 CH 3 HH CH^2 CH^3

CC

cis-2,3-dideuterio-

2-pentene

meso compound

D D

Pt/C

H (^2) = H D
H D
CH 2 CH 3
CH 3 CH 2 CH 2 CH 3
D D
CC
If you have trouble determining the con-
figuration of a product, make a model.
H H
H 3 C D HH
CC D H

• 
  


• =
  H 3 C
  CH 2 CH 3
  CH 2 CH 3
  D
  trans-2,3-dideuterio-
  2-pentene
  threo enantiomers
  Fischer projections
  threo enantiomers
  perspective formulas
  (eclipsed conformers)
  threo enantiomers
  perspective formulas
  (staggered conformers)
  D
  Pt/C
  H 2
  CH 3
  HD
  DH
  CH 2 CH 3
  CH 2 CH (^3) CH
  2 CH 3
  CH 3
  DH
  HD
  CH 2 CH 3
  H 3 C D
  D H H
  D D H
  CH 2 CH 3 +
  H 3 C
  D
  D
  H 3 C
  C C C C C C C C