Organic Chemistry

404 CHAPTER 11 Elimination Reactions of Alkyl Halides • Competition Between Substitution and Elimination

Alexander M. Zaitsev (1841–1910)

was born in Kazan, Russia. The

German transliteration, Saytzeff, is

sometimes used for his family name.

He received a Ph.D. from the

University of Leipzig in 1866.

He was a professor of chemistry first

at the University of Kazan and

later at the University of Kiev.

The most stable alkene is generally (but

not always) the most substituted

alkene.

to two hydrogens. According to Zaitsev’s rule, the more substituted alkene will be

the one formed by removing a proton from the -carbon that is bonded to two hydro-

gens, rather than from the -carbon that is bonded to three hydrogens. Therefore,

2-pentene (a disubstituted alkene) is the major product, and 1-pentene (a monosubsti-

tuted alkene) is the minor product.

Because elimination from a tertiary alkyl halide typically leads to a more substituted

alkene than does elimination from a secondary alkyl halide, and elimination from a sec-

ondary alkyl halide generally leads to a more substituted alkene than does elimination

from a primary alkyl halide, the relative reactivities of alkyl halides in an E2 reaction

are as follows:

Keep in mind that the major product of an E2 reaction is the more stable alkene,and

Zaitsev’s rule is just a shortcut to determine which of the possible alkene products is

the more substituted alkene. The more substituted alkene is not, however, always the

more stable alkene. In the following reactions, the conjugated alkene is the more sta-

ble alkene even though it is not the most substituted alkene. The major product of each

reaction is, therefore, the conjugated alkene because, being more stable, it is more

easily formed.

three alkyl substituents two alkyl substituents one alkyl substituent

RCH RCH CHR RCH CH 2

RCH 2 CR

CR

R

R

Br

RCH 2 CHR

Br

RCH 2 CH 2 Br

tertiary alkyl halide > secondary alkyl halide > primary alkyl halide

relative reactivities of alkyl halides in an E2 reaction

b

b

Zaitsev’s rule leads to the most

substituted alkene.

CH 3 CH 2 CH 2 CHCH 3 + HO− CH 3 CH 2 CH CHCH 3 + CH 3 CH 2 CH 2 CH CH 2

Cl

2 -hydrogens

2-pentene

67%

(mixture of E and Z)

2-chloropentane 1-pentene

33%

3 -hydrogens disubstituted monosubstituted

CHCH 2 CHCHCH 3

HO−

CH 2 CH 2 CHCH CHCHCH 3 + CH 2 CHCH 2 CH CCH 3 + H 2 O + Cl−

Cl

CH 3 CH 3 CH 3

4-chloro-5-methyl-1-hexene

5-methyl-1,3-hexadiene

a conjugated diene

major product

5-methyl-1,4-hexadiene

an isolated diene

minor product

CH 2 CHCHCH 3

Br

2-bromo-3-methyl-

1-phenylbutane

CH 3

CH CHCHCH 3 +

3-methyl-1-phenyl-1-butene

the double bond is

conjugated with the

benzene ring

major product

CH 3

CH 2 CH CCH 3 + H 2 O + Br−

3-methyl-1-phenyl-2-butene

the double bond is

not conjugated with the

benzene ring

minor product

CH 3

HO−

Tutorial:

E2 Elimination regiochemistry

BRUI11-400_436r3 26-03-2003 10:20 AM Page 404