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