Organic Chemistry

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

Sawhorse projections reveal another reason that anti elimination is favored. In syn

elimination, the electrons of the departing hydrogen move to the frontside of the carbon

bonded to X, whereas in anti elimination, the electrons move to the backside of the car-

bon bonded to X. We have seen that displacement reactions involve back-side attack be-

cause the best overlap of the interacting orbitals is achieved through back-side attack

(Section 10.2). Finally, anti elimination avoids the repulsion the electron-rich base experi-

ences when it is on the same side of the molecule as the electron-rich departing halide ion.

In Section 11.1, we saw that an E2 reaction is regioselective,which means that

more of one constitutional isomer is formed than the other. For example, the major

product formed from the E2 elimination of 2-bromopentane is 2-pentene.

The E2 reaction is also stereoselective,which means that more of one stereoisomer

is formed than the other. For example, the 2-pentene obtained as the major product

from the elimination reaction of 2-bromopentane can exist as a pair of stereoisomers,

and more (E)-2-pentene is formed than (Z)-2-pentene.

We can make the following general statement about the stereoselectivity of E2 re-

actions: If the reactant has two hydrogens bonded to the carbon from which a hydro-

gen is to be removed, both the Eand Zproducts will be formed because there are two

conformers in which the groups to be eliminated are anti. The alkene with the bulkiest

groups on opposite sides of the double bondwill be formed in greater yield because it

is the more stable alkene.

Br and H

are anti

CH 3 CH 2

CH 3 CH 2

CH 2 CH 3

H

H

H

H

CH 3

CH 3

Br

H

H

H CH 3

Br

H

C

C

CH 2 CH 3

H

H

CH 3

C

C

(E)-2-pentene

more stable

(Z)-2-pentene

less stable

(E)-2-pentene

41%

CC

HCH 2 CH 3

H 3 C H

(Z)-2-pentene

14%

CC

HH

H 3 C CH 2 CH 3

CH 3 CH 2 OH

CH 3 CH 2 O−

CH 3 CH 2 CH 2 CHCH 3 CH 3 CH 2 CH CHCH 3

Br

2-pentene

major product

(mixture of E and Z )

+ CH 3 CH 2 CH 2 CH CH 2

1-pentene

minor product

2-bromopentane

X

anti elimination

back- side attack

H base

X

syn elimination

front-side attack

base H

When two hydrogens are bonded to the

-carbon, the major product of an E2

reaction is the alkene with the bulkiest

substituents on opposite sides of the

double bond.

B

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