Organic Chemistry

Section 11.8 Competition Between Substitution and Elimination 423

A bulky base encourages elimination

over substitution.

A weak base encourages substitution

over elimination.

However, if either the primary alkyl halide or the nucleophile/base is sterically hin-

dered, the nucleophile will have difficulty getting to the back side of the -carbon.

As a result, elimination will win the competition, so the elimination product will

predominate.

A secondaryalkyl halide can form both substitution and elimination products under

conditions. The relative amounts of the two products depend on the base

strength and the bulk of the nucleophile/base. The stronger and bulkier the base, the

greater the percentage of the elimination product. For example, acetic acid is a

stronger acid than ethanol which means that acetate ion

is a weaker base than ethoxide ion. The elimination product is the main product

formed from the reaction of 2-chloropropane with the strongly basic ethoxide ion,

whereas no elimination product is formed with the weakly basic acetate ion. The

percentage of elimination product produced would be increased further if the bulky

tert-butoxide ion were used instead of ethoxide ion (Section 10.3).

A tertiaryalkyl halide is the least reactive of the alkyl halides in an reaction

and the most reactive in an E2 reaction (Table 11.5). Consequently, only the elimina-

tion product is formed when a tertiary alkyl halide reacts with a nucleophile/base

under conditions.SN 2 >E2

SN 2

1 pKa=4.76 2 1 pKa =15.9 2 ,

SN 2 >E2

a

a primary

alkyl halide

CH 3 OH

propyl bromide methyl propyl ether

90%

propene

10%

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

CH 3 OH

the primary alkyl halide is sterically hindered

CH 3 CH 3 CH 3

1-bromo-2-methyl-

propane

isobutyl methyl ether

40%

2-methylpropene

60%

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

(CH 3 ) 3 COH

the nucleophile is sterically hindered

CH 3

CH 3

CH 3

CH 3

1-bromopentane

tert-butyl pentyl ether

15%

1-pentene

85%

CH 3 CH 2 CH 2 CH 2 CH 2 Br CH 3 CO−

CH 3

CH 3

+ CH 3 CH 2 CH 2 CH 2 CH 2 OCCH 3 + CH 3 CH 2 CH 2 CH CH 2 +CH 3 COH + Br−

a secondary

alkyl halide

a strong base favors

the elimination product

a weak base favors the

substitution product

2-chloropropane ethoxide ion 2-ethoxypropane

25%

propene

75%

CH 3 CHCH 3 CH 3 CH 2 O− CH 3 CHCH 3

Cl OCH 2 CH 3

O

+ CH + CH 3 CH CH 2 + CH 3 CH 2 OH + Cl−

3 CH 2 OH

2-chloropropane acetate ion isopropyl acetate

100%

CH 3 CHCH 3 CH 3 CO− CH 3 CHCH 3

Cl O OCCH 3

+ acetic acid + Cl−

BRUI11-400_436r3 26-03-2003 10:20 AM Page 423