Organic Chemistry

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

In an E2 reaction of an alkyl halide, a base removes a proton from a carbon that is

adjacent to the carbon bonded to the halogen. As the proton is removed, the electrons

that the hydrogen shared with carbon move toward the carbon bonded to the halogen.

As these electrons move toward the carbon, the halogen leaves, taking its bonding

electrons with it. The electrons that were bonded to the hydrogen in the reactant have

formed a bond in the product. Removal of a proton and a halide ion is called

dehydrohalogenation.

The carbon to which the halogen is attached is called the -carbon. A carbon adja-

cent to the -carbon is called a -carbon. Because the elimination reaction is initiated

by removing a proton from a -carbon, an E2 reaction is sometimes called a

reaction. It is also called a 1,2-elimination reactionbecause the

atoms being removed are on adjacent carbons. ( is any base.)

In a series of alkyl halides with the same alkyl group, alkyl iodides are the most re-

active and alkyl fluorides the least reactive in E2 reactions because weaker bases are

better leaving groups (Section 10.3).

11.2 The Regioselectivity of the E2 Reaction

An alkyl halide such as 2-bromopropane has two -carbons from which a proton can

be removed in an E2 reaction. Because the two -carbons are identical, the proton can

be removed with equal ease from either one. The product of this elimination reaction

is propene.

In contrast, 2-bromobutane has two structurally different -carbons from which a

proton can be removed. So when 2-bromobutane reacts with a base, two elimination

products are formed: 2-butene and 1-butene. This E2 reaction is regioselectivebecause

more of one constitutional isomer is formed than the other.

What is the regioselectivity of an E2 reaction? In other words, what are the factors

that dictate which of the two elimination products will be formed in greater yield?

b

CH 3 CHCH 3 + CH 3 O− CH 3 CH CH 2 + CH 3 OH + Br−

Br

2-bromopropane

propene

-carbons

b

b

relative reactivities of alkyl halides in an E2 reaction

most reactive RI > RBr > RCl > RF least reactive

B≠-

B-elimination

b

a b

a

RCH CHR RCH CHR + BH + Br−

B− H

Br

-carbon

-carbon

a base

p

The weaker the base, the better it is

as a leaving group.

CH 3 CHCH 2 CH 3 CH 3 O− CH

3 OH

+ CH 3 CH CHCH 3 + CH 2 CHCH 2 CH 3 + CH 3 OH + Br−

Br

2-bromobutane

2-butene

80%

(mixture of E and Z)

1-butene

20%

-carbons

Movie:

E2 Dehydrohalogenation

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