Organic Chemistry

Section 5.19 Stereochemistry of Electrophilic Addition Reactions of Alkenes 223

2. Stereochemistry of Hydrogen Addition
   In a catalytic hydrogenation, the alkene sits on the surface of the metal catalyst and
   both hydrogen atoms add to the same side of the double bond (Section 4.11). There-
   fore, the addition of to an alkene is a syn addition reaction.

If addition of hydrogen to an alkene forms a product with two asymmetric carbons,
only two of the four possible stereoisomers are obtained because only syn addition can
occur. (The other two stereoisomers would have to come from anti addition.) One
stereoisomer results from addition of both hydrogens from above the plane of the dou-
ble bond, and the other stereoisomer results from addition of both hydrogens from
below the plane. The particular pair of stereoisomers that is formed depends on
whether the reactant is a cis-alkene or a trans-alkene. Syn addition of to a cis-
alkene forms only the erythro enantiomers. (In Section 5.8, we saw that the erythro
enantiomers are the ones with identical groups on the same side of the carbon chain in
the eclisped conformers.)

H 2

Pt

HH

C C

addition of H 2 is a syn addition

HH

CC

H 2

Fischer projections of the stereoisomers of the product

CH 2 CH 3

HH

Br Br

CH 3

CH 3

CH 2 CH 3

CH 2 CH 3

CH 3

CH 3

CH 2 CH 3

CH 2 CH 3

H CH 3

CH 3 Br

CH 2 CH 3

CH 2 CH 3

Br CH 3

CH 3 H

CH 2 CH 3

HBr CH 3 CH 2 CH CCH 2 CH 3

CH 3 CH 3

Br

H 2 O 2

+

CH 3 CH 2 CH 2 CH 3

H 3 C

cis-3,4-dimethyl-3-hexene 3-bromo-3,4-dimethylhexane

new asymmetric

carbons

CH 3

CC * *

Br

CH 3

C C

CH 2 CH 3

CH 3 CH 2 H

H

Br

CC

CH 3 CH 2

CH 3 CH 2 CH 3

H

CH 3

Br

C C

CH 2 CH 3

CH 3 CH 2 CH 3

H

CH 3

CH 3

CC

CH 3 CH 2

CH 3 CH 2 CH 3

H

Br

perspective formulas of the stereoisomers of the product

HH

CC H

+

+ =

H 3 C CH 2 CH 3 CH 2 CH 3

cis-2,3-dideuterio-

2-pentene

erythro enantiomers

Fischer projections

erythro enantiomers

perspective formulas

(eclipsed conformers)

erythro enantiomers

perspective formulas

(staggered conformers)

D D

Pt/C

H 2

CH 3

HD

HD

CH 2 CH 3

CH 3

DH

DH

CH 2 CH 3

H 3 C CH 2 CH 3

D D

CC

HH

C C

H 3 C

D D

CH 2 CH 3 H

+

H 3 C

D D

CC

HH

C C D

D CH 2 CH 3

H 3 C