Organic Chemistry

890 CHAPTER 21 More About Amines • Heterocyclic Compounds

the -carbon bonded to three hydrogens, and the minor alkene product results from

removing a hydrogen from the -carbon bonded to two hydrogens.

In the next reaction, the major alkene product comes from removing a hydrogen from

the -carbon bonded to two hydrogens, because the other -carbon is bonded to only

one hydrogen.

PROBLEM 7

What are the minor products in the preceding Hofmann elimination reaction?

We saw that in an E2 reaction of an alkyl chloride, alkyl bromide, or alkyl iodide, a

hydrogen is removed from the -carbon bonded to the fewesthydrogens (Zaitsev’s

rule; Section 11.2). Now we see that in an E2 reaction of a quaternary ammonium ion,

the hydrogen is removed from the -carbon bonded to the mosthydrogens (anti-

Zaitsev elimination).

Why do alkyl halides follow Zaitsev’s rule, while quaternary amines violate the

rule? When hydroxide ion starts to remove a proton from the alkyl bromide, the bro-

mide ion immediately begins to depart and a transition state with an alkene-likestruc-

ture results. The proton is removed from the -carbon bonded to the fewest hydrogens

in order to achieve the most stable alkene-like transition state.

When, however, hydroxide ion starts to remove a proton from a quaternary ammonium

ion, the leaving group does not immediately begin to leave, because a tertiary amine is

not as good a leaving group as or As a result, a partial negative charge

builds up on the carbon from which the proton is being removed. This gives the transi-

tion state a carbanion-likestructure rather than an alkene-like structure. By removing

a proton from the -carbon bonded to the most hydrogens, the most stable carbanion-

like transition state is achieved. (Recall from Section 11.2 that primary carbanions are

more stable than secondary carbanions, which are more stable than tertiary carban-

ions.) Steric factors in the Hofmann reaction also favor anti-Zaitsev elimination.

b

Cl-,Br-, I-.

OH

CH 3 CH CHCH 3

more stable

H

Br

δ−

δ−

OH

CH 3 CH 2 CCH 2

less stable

Zaitsev elimination

alkene-like transition state

H

Br

δ−

δ−

OH

CH 2 CHCH 2 CH 2 CH 3

more stable

H

+N(CH

3 ) 3

δ−

δ−

OH

CH 3 CHCHCH 2 CH 3

H

+N(CH

3 ) 3

δ−

δ−

less stable

anti-Zaitsev elimination

carbanion-like transition state

b

b

b

propene

isobutyldimethylamine

CH 3

CH 3

CH 3 HO−

CH 3 CHCH 2 NCH+ 2 CH 2 CH 3 + CH 2 CHCH 3 + H 2 O

CH 3

CH 3

CH 3

∆ CH 3 CHCH 2 N

-carbon -carbon

b b

b

b

-carbon -carbon

1-pentene

major product

2-pentene

CH 3 NCH (^3) minor product
CH 3 HO−
CH 3 CHCH 2 CH 2 CH 3 CH 2 CHCH 2 CH 2 CH 3 + CH 3 CH CHCH 2 CH 3
trimethylamine
CH 3
++CH 3 NCH 3 H 2 O

• ∆