Organic Chemistry

CH 3

C

O

CH 2 CH 3

the Si face

the Re face

1

3 2

Section 18.11 Stereochemistry of Nucleophilic Addition Reactions: Reand SiFaces 765

3-D Molecules:

2-Butanone;

(S)-2-Butanol;

(R)-2-Butanol

18.11 Stereochemistry of Nucleophilic

Addition Reactions: Reand SiFaces

A carbonyl carbon bonded to two different substituents is a prochiral carbonyl carbon
because it will become a chirality center (asymmetric carbon) if it adds a group unlike ei-
ther of the groups already bonded to it. The addition product will be a pair of enantiomers.

The carbonyl carbon and the three atoms attached to it define a plane. The nucleo-
phile can approach either side of the plane. One side of the carbonyl compound is
called the Re(pronounced “ree”) face, and the other side is called the Si(pronounced
“sigh”) face; Reis for rectusand Siis for sinister—similar to Rand S. To distinguish
between the ReandSifaces, the three groups attached to the carbonyl carbon are as-
signed priorities using the Cahn–Ingold–Prelog system of priorities used in E,Zand
R,Snomenclature (Sections 3.5 and 5.6 respectively). The Reface is the face closest
to the observer when decreasing priorities are in a clockwise direction,
and the Siface is the opposite face—the one closest to the observer when decreasing
priorities are in a counterclockwise direction.
Attack by a nucleophile on the Reface forms one enantiomer, whereas attack on the
Siface forms the other enantiomer. For example, attack by hydride ion on the Reface
of butanone forms (S)-2-butanol, and attack on the Siface forms (R)-2-butanol.

Whether attack on the Reface forms the Ror Senantiomer depends on the priority
of the attacking nucleophile relative to the priorities of the groups attached to the car-
bonyl carbon. For example, we just saw that attack by hydride ion on the Reface of bu-
tanone forms (S)-2-butanol, but attack by a methyl Grignard reagent on the Reface of
propanal forms (R)-2-butanol.

O

C

H

CH 3 MgBr attack on the Re face

followed by H+

the Si face is closest

to the observer

CH 3 CH 2

C

CH 3 CH 2

C

CH 3 CH 2

OH

H

CH 3

CH 3

OH

H

(S)-2-butanol

(R)-2-butanol

1

2 3

CH 3 MgBr attack on the Si face

followed by H+

O

C

H− attack on the Re face

followed by H+

H− attack on the Si face

followed by H+

the Re face is closest

to the observer

CH 3 CH 2 CH 3

C

CH 3

OH

CH 2 CH 3

H

C

OH

CH (^3) CH 2 CH 3
H
(S)-2-butanol
(R)-2-butanol
1
3 2
1172732
a pair of enantiomers
O
C
HZ
X Y
a prochiral
carbonyl carbon
Y
Z
X
OH
C
Y
Z
X
OH
C