Substitution and Elimination
as Competing Reactions
Alkyl halides can react with Lewis bases by
nucleophilic substitution and/or elimination.
-elimination
C
H
C
C
X
+ :Y
C + H Y + :X
–
H
C
C
+ :X
–
Y
nucleophilic substitution
–
Nucleophilicity vs. Basicity
All Nucleophiles act like Bases and vice-versa.
Base; Ability to accept proton
measured by thermodynamics
Nucleophilicity; ability to cause substitution
reaction (depends on reaction conditions)
measured by kinetics
Guidelines for Determining Basicity
and Nucleophilicity
1. When the same nucleophilic atom is
compared; the anion is both more basic and
nucleophilic with respect to the neutral
molecule.
OH- More Basic & Nucleophilic
Than H2O
Table 8.4 Nucleophilicity
Rank
Nucleophile
Relative
rate
good
HO–, RO–
104
RCO2–
103
H2O, ROH
1
fair
weak
When the attacking atom is the same (oxygen
in this case), nucleophilicity increases with
increasing basicity.
Guidelines for Determining
Basicity and Nucleophilicity
2. If the comparison is made in the same
period; both nucleophilicity and basicity
increase going to the left.
NH2- > OH- >FMore Basic and Nucleophilic
Guidelines for Determining
Basicity and Nucleophilicity
3. If the comparison is made going down a
group; basicity increases going up while
nucleophilicity increases going down.
F-
Cl-
Br-
I-
Increasing Nucleophilicity
Increasing Basicity
Basicity Trend Down a Group
Acid
Conjugate Base
HF; weak acid
F-; strong conj. base
HCl; strong acid
Cl-; weak conj. base
HBr; strong acid
Br-; weak conj. base
HI; strong acid
I-; weak conj. base
Nucleophilicity Trend Down a Group
Solvation
– small negative ions are highly
solvated in protic solvents
– large negative ions are less solvated
– As solvation increases; nucleophile
strength decreases
Table 8.4 Nucleophilicity
Rank
Nucleophile
Relative
rate
strong
I-
>105
good
Br-
104
fair
Cl-, F-
103
A tight solvent shell around an ion makes it
less reactive. Larger ions are less solvated than
smaller ones and are more nucleophilic.
Guidelines for Determining Basicity
and Nucleophilicity
4. Steric hinderance decreases nucleophile strength
and increases base strength.
H
H
H
C
H
O
H
H H
C
C
C
O
H
C
H
H
H
-
H
Substitution and Elimination
as Competing Reactions
Alkyl halides can react with Lewis bases by
nucleophilic substitution and/or elimination.
-elimination
C
H
C
C
X
+ :Y
C + H Y + :X
–
H
C
C
+ :X
–
Y
nucleophilic substitution
–
Substitution vs. Elimination
Effect of Alkyl Halide
1° RX
Formation of carbocation will not happen; rules
out E1 or SN1
• Since primary alky halides are not sterically
hindered; substitution likely
• Bulky Base favors elimination over substitution
Uncrowded Alkyl Halides
Decreased crowding at carbon that bears the leaving
group increases substitution relative to elimination.
primary alkyl halide
CH3CH2CH2Br
NaOCH2CH3
ethanol, 55°C
CH3CH2CH2OCH2CH3 +
(91%)
CH3CH=CH2
(9%)
But a crowded alkoxide base can favor elimination
even with a primary alkyl halide.
primary alkyl halide + bulky base
CH3(CH2)15CH2CH2Br
KOC(CH3)3
tert-butyl alcohol, 40°C
CH3(CH2)15CH2CH2OC(CH3)3 + CH3(CH2)15CH=CH2
(13%)
(87%)
Substitution vs. Elimination
Effect of Alkyl Halide
3° RX
•
•
SN2 is unlikely due to steric hinderance
Weak Nucleophile/ Polar Protic Solvent ->
E1/SN1
– Steric hinderance of 3° RX make
substitution unlikely except in solvolysis
reactions
• Strong Base/ Bulky Base -> E2
(CH3)2CCH2CH3
Example
Br
CH3
CH3CCH2CH3
CH3
CH3
+
CH2=CCH2CH3 +
CH3C=CHCH3
OCH2CH3
ethanol, 25°C
64%
36%
2M sodium ethoxide in ethanol, 25°C
99%
1%
Substitution vs. Elimination
Effect of Alkyl Halide
2° RX
All mechanisms are possible
• Strong Nucleophile but Weak Base -> SN2
• Weak Nucleophile/Weak Base -> SN1/E1
• Strong Base/ Bulky Base -> E2
Elimination vs. Substitution
Nucleophilicity vs. Basicity
Strong Nucleophiles
SN2
Strong Bases
E2
Weak Nucleophiles/ Weak
Base
E1 or SN1
Sterically Hindered Base or Elimination over
Nucleophile
Substitution
Substitution vs. Elimination
Solvent Effect
• Protic Polar -> E1, SN1
• Aprotic Polar -> E2 / SN2
Effect of Nucleophile/Base Concentration
• High Concentration -> SN2/ E2
• Low Concentration -> SN1/E1