Trends in Alkyl Substitution and Elimination Reactions Nucleophile/Base
Weak Bases/ Strong
Nucleophiles
Uncharged
Nucleophiles
Strong Bases
(pKas > 15)
(-9 < pKa <10)
-
-
-
-
(-2 < pKa < 15)
-
(I , Br , SCN , N3 , CH3COO ,
RS , CN , etc)
Alkyl Chain
Mainly SN2 reactions.
Neighboring carbon atoms
(on the β-carbon) reduce the
rate of substitution, i.e.,
neopentyl chloride does not
undergo SN2 reactions.
Primary
R-CH2-X
Secondary
(
H3C C C
-­‐
-­‐
-­‐
,CH3O , OH , NH2 , -­‐
(C3H7)2NLi, (CH3)3CO )
(H2O, ROH, RSH, R3N, NH3)
Mostly SN2 but some E2
products. E2 reactions
predominate when
conditions like heat are used
or with highly substituted βcarbon atoms. Elimination
reactions are favored by the
use of strong bulky bases
like, i.e. tert-butoxy.
Mainly E2. Use of strong
bulky bases like, i.e., tertbutoxy favor the formation of
the least substituted alkene
– Hoffman rule.
SN2 reactions. (Nucleophilic
activity order: N ≈ S >> O)
Almost no substitution
reactions. Mainly E2 even
with weak bases.
Mainly E2. Only traces of
substitution, if any.
SN1 in polar protic solvents –
solvolysis (H2O, CH3OH). E2
with nucleophiles that have
nitrogen. Heat favors E1
reactions.
Expect SN2 reactions with
ease in primary benzylic
compounds. Expect also
some SN1.
SN2 expected in primary
benzylic compounds. E2 in
secondary benzylic
compounds with β-hydrogen
atoms. In polar protic
solvents expect SN1 with
primary benzylic
compounds.
SN2 reactions with ease with
primary allylic compounds.
Expect also some SN1.
SN2 reaction in primary
allylic compounds. E2 in
secondary allylic with βhydrogen atoms. In polar
protic solvents expect SN1
with primary allylic
compounds.
Expect SN2 for primary
benzylic compounds and
nucleophiles with nitrogen or
sulfur. Solvolysis (SN1)
mainly occurs in polar protic
solvents and primary
benzylic compounds.
Secondary benzylic halides
give primarily E2, some E1
also expected.
Expect SN2 for primary allylic
compounds and nucleophiles
with N or S. Solvolysis (SN1)
in polar protic solvents and
primary allylic compounds.
Secondary allylic halides
give mainly E2 products,
some E1 expected.
SN2 & E2. Bases weaker
than acetate (pKa < 4.8) give
higher substitution rates.
Highly branched alkyl
compounds provide greater
elimination rates.
Mostly SN2. Polar protic
solvents (H2O, CH3OH, etc.)
favor SN1 & E1. Polar
aprotic solvents (acetone,
DMSO, etc.) favor SN2 & E2.
Heat favors E2.
Tertiary
Benzylic
Allylic
Prepared by José Laboy