Organic Chemistry

less hindered alkyl group should be provided by the alkyl halide and the more hin-

dered alkyl group should come from the alkoxide.

PROBLEM 21

What other organic product will be formed when the alkyl halide used in the synthesis of

butyl propyl ether is

a. propyl bromide?

b. butyl bromide?

PROBLEM 22

How could the following ethers be prepared, using an alkyl halide and an alcohol?

a. c.

b. d.

In Section 6.10, you saw that alkynes can be synthesized by the reaction of an

acetylide anion with an alkyl halide.

Now that you know that this is an reaction (the alkyl halide reacts with a high con-

centration of a good nucleophile), you can understand why it is best to use primary

alkyl halides and methyl halides in the reaction. These alkyl halides are the only ones

that form primarily the desired substitution product. A tertiary alkyl halide would form

only the elimination product, and a secondary alkyl halide would form mainly the

elimination product because the acetylide ion is a very strong base.

If you want to synthesize an alkene, you should choose the most hindered alkyl

halide in order to maximize the elimination product and minimize the substitution

product. For example, 2-bromopropane would be a better starting material than

1-bromopropane for the synthesis of propene because the secondary alkyl halide

would give a higher yield of the desired elimination product and a lower yield of the

competing substitution product. The percentage of alkene could be further increased

by using a sterically hindered base such as tert-butoxide ion instead of hydroxide ion.

2-bromopropane major product minor product

CH 3 CHCH 3 HO−

Br OH

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

1-bromopropane minor product major product

CH 3 CH 2 CH 2 Br + HO− CH 3 CH CH 2 +++CH 3 CH 2 CH 2 OH H 2 OBr−

SN 2

CH 3 CH 2 CC− + CH 3 CH 2 CH 2 Br CH 3 CH 2 C CCH 2 CH 2 CH 3 + Br−

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

CH 3

OCCH 3

CH 3

CH 3

CH 3 CH 2 CHOCH 2 CH 2 CH 3 CH 2 O

CH 3

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

In synthesizing an ether, the less

hindered group should be provided

by the alkyl halide.

ethyl bromide

tert-butoxide ion tert-butyl ethyl ether

ethene

CH 3 CH 2 Br CH 3 CO−

CH 3

CH 3

CH 3 CH 2 OCCH 3

CH 3

CH 3

CH 3

CH 3

+ + CH 2 CH 2 ++CH 3 COH Br−

ethoxide ion

tert-butyl bromide

2-methylpropene

CH 3 CH 2 O− CH 3 CBr

CH 3

CH 3

CH 3

+ CH 2 CCH 3 ++CH 3 CH 2 OH Br−

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