Organic Chemistry

Section 5.17 Nitrogen and Phosphorus Chirality Centers 217

Unlike enantiotopic hydrogens, diastereotopic hydrogens do not have the same reac-
tivity with achiral reagents. For example, in Chapter 11 we will see that because
trans-2-butene is more stable than cis-2-butene (Section 4.11), removal of and Br
to form trans-2-butene occurs more rapidly than removal of and Br to form
cis-2-butene.

PROBLEM 39

Tell whether the and hydrogens in each of the following compounds are enan-

tiotopic, diastereotopic, or neither.

a. c.

b. d.

5.17 Nitrogen and Phosphorus Chirality Centers

Atoms other than asymmetric carbons can be chirality centers. When an atom such as
nitrogen or phosphorus has four different groups or atoms attached to it and it has a
tetrahedral geometry, it is a chirality center. A compound with a chirality center can
exist as enantiomers, and the enantiomers can be separated.

If one of the four “groups”attached to nitrogen is a lone pair, the enantiomers can-
not be separated because they interconvert rapidly at room temperature. This is called
amine inversion(Section 21.2). One way to picture amine inversion is to compare it
to an umbrella that turns inside out in a windstorm.

R^2

R^1 N

transition state

R^3

N

R^2

R^3

R^1

N

R^2

R^1 R^3

sp^3

sp^3 sp^2

p orbital

‡

C

H 3 C

H

C

Ha

Hb

H

Br Hb

Ha

HaHb

CH 3 CH 2 CCH 3

Hb

Ha

Ha Hb

cis-2-butene

−HaBr

slower

CH 3

CH 3

H Br

H

H 3 C

H 3 C

C

C

−HbBr

faster

H 3 C H

C

CH 3

Ha Hb

Hb Ha

C

trans-2-butene

Ha

Hb

enantiomers

CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH 2 CH 3

H H

+NN+

CH 3 CH 3

enantiomers

CH 3 CH 2 O OCH 3 CH 3 O OCH 2 CH 3

PPH H

O O

Br− Br−