Organic Chemistry

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

Table 11.6 Summary of the Products Expected in Substitution and Elimination Reactions

Class of alkyl halide versus E2 versus E1

Primary alkyl halide Primarily substitution, unless there is steric hindrance Cannot undergo reactions

in the alkyl halide or nucleophile, in which case

elimination is favored

Secondary alkyl halide Both substitution and elimination; the stronger and bulkier Both substitution and elimination

the base, the greater is the percentage of elimination

Tertiary alkyl halide Only elimination Both substitution and elimination

SN 1 >E1

SN 2 SN 1

PROBLEM 17

How would you expect the ratio of substitution product to elimination product formed

from the reaction of propyl bromide with in methanol to change when the nucleo-

phile is changed to

PROBLEM 18

Only a substitution product is obtained when the following compound is treated with sodi-

um methoxide:

Explain why an elimination product is not obtained.

Conditions

Now let’s look at what happens when conditions favor reactions (a poor

nucleophile/weak base). In reactions, the alkyl halide dissociates to form a

carbocation, which can then either combine with the nucleophile to form the substitu-

tion product or lose a proton to form the elimination product.

Alkyl halides have the same order of reactivity in reactions as they do in E1 re-

actions because both reactions have the same rate-determining step—dissociation of

the alkyl halide (Table 11.5). This means that all alkyl halides that react under

conditions will give both substitution and elimination products. Remember that prima-

ry alkyl halides do not undergo reactions because primary carbocations are

too unstable to be formed.

Table 11.6 summarizes the products obtained when alkyl halides react with nucleo-

philes/bases under and conditions.SN 2 >E2 SN 1 >E1

SN 1 >E1

SN 1 >E1

SN 1

C

H

CH 3 O+

CC

C

H

C+

OH

C

H

CBr

+ Br− + H^3 O

+

+

H 2 O

substitution

H 2 O

elimination

SN 1 >E1

SN 1 >E1

SN 1 /E1

CH 3

CH 3

Br

CH 3 S-?

CH 3 O-

2-bromo-2-methyl-

propane

2-methylpropene

CH 3 CBr CH 3 CH 2 O− CH 3 CCH 2

CH 3 CH 3

CH 3

+ CH 3 CH 2 OH + CH 3 CH 2 OH + Br−

a tertiary alkyl

halide

3-D Molecule:

2-Methyl-1-propene

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