Solvolysis
Tertiary alkyl halides are very unreactive in
substitutions that proceed by the SN2 mechanism.
The SN1 Mechanism
of
Nucleophilic Substitution
But they are highly reactive in solvolysis
reactions.
Hydrolysis of tert-butyl
tert-butyl bromide.
CH3
CH3
C
..
Br
.. :
H
+
CH3
: O:
H
CH3
Hydrolysis of tert-butyl
tert-butyl bromide.
CH3
+
H
O:
C
CH3
C
H
CH3
CH3
..
Br
.. :
H
+
: O:
CH3
H
CH3
CH3
C
..
O
.. H
+
H
H
O:
C
H
+
.. –
: Br :
..
..
Br :
..
+
CH3
+
CH3
CH3
.. –
: Br :
..
This is the nucleophilic substitution
stage of the reaction; the one with
which we are concerned.
CH3
Hydrolysis of tert-butyl
tert-butyl bromide.
CH3
CH3
C
CH3
..
Br
.. :
H
+
: O:
CH3
H
The reaction rate is independent
of the concentration of the nucleophile
and follows a first-order rate law.
rate = k[(CH3)3CBr]
Hydrolysis of tert-butyl
tert-butyl bromide.
CH3
+
H
O:
C
CH3
C
H
CH3
CH3
CH3
..
Br
.. :
H
+
: O:
CH3
H
CH3
+
O:
C
H
CH3
+
+
.. –
: Br :
..
.. –
: Br :
..
The mechanism of this step is
not SN2. It is called SN1 and
begins with ionization of (CH3)3CBr.
H
Kinetics and Mechanism
CH3
CH3
C
rate = k[alkyl halide]
First-order kinetics implies a unimolecular
rate-determining step.
Proposed mechanism is called SN1, which
stands for
substitution nucleophilic unimolecular
Mechanism
..
Br
.. :
CH3
unimolecular
slow
H 3C
CH3
+
C
+
.. –
: Br :
..
CH3
Mechanism
H 3C
CH3
+
C
H
carbocation
formation
: O:
CH3
R+
carbocation
capture
H
proton
transfer
bimolecular
fast
CH3
CH3
C
CH3
+
H
O:
H
RX
+
ROH2
Characteristics of the SN1 mechanism
first order kinetics: rate = k[RX]
unimolecular rate-determining step
carbocation intermediate
rate follows carbocation stability
rearrangements sometimes observed
reaction is not stereospecific
racemization occurs in reactions of
optically active alkyl halides
8.7
Carbocation Stability and SN 1
Reaction Rates
ROH
Table 8.5 Reactivity toward substitution by the
SN1 mechanism
Electronic Effects Govern SN1 Rates
RBr solvolysis in aqueous formic acid
The rate of nucleophilic substitution
by the SN1 mechanism is governed
by electronic effects.
Carbocation formation is rate-determining.
The more stable the carbocation,
carbocation, the faster
its rate of formation, and the greater the
rate of unimolecular nucleophilic substitution.
Alkyl bromide
Class
Relative rate
CH3Br
Methyl
1
CH3CH2Br
Primary
2
(CH3)2CHBr
Secondary
(CH3)3CBr
Tertiary
43
100,000,000
Decreasing SN1 Reactivity
8.8
Stereochemistry of SN1 Reactions
(CH3)3CBr
(CH3)2CHBr
CH3CH2Br
CH3Br
Stereochemistry of an SN1 Reaction
Generalization
Nucleophilic substitutions that exhibit
first-order kinetic behavior are
not stereospecific.
stereospecific.
H
CH3
C
R-(–
-(–)-2-Bromooctane
)-2-Bromooctane
Br
CH3(CH2)5
H
HO
CH3
C
(CH2)5CH3
(S)-(+)-2-Octanol
)-(+)-2-Octanol (83%)
CH3 H
H 2O
C
OH
CH3(CH2)5
(R)-(–
)-(– )-2-Octanol
)-2-Octanol (17%)
Figure 8.5
Ionization step
gives carbocation;
carbocation; three
+
bonds to chirality
center become coplanar
+
Leaving group shields
one face of carbocation;
carbocation;
nucleophile attacks
faster at opposite face.
More than 50%
Less than 50%
Because...
8.9
Carbocation Rearrangements
in SN1 Reactions
carbocations are intermediates
in SN1 reactions, rearrangements
are possible.
Example
CH3
CH3
C
CHCH3
H
Br
CH3
H 2O
CH3
C
CH2CH3
OH
(93%)
H 2O
CH3
CH3
C
H
CH3
CHCH3
+
CH3
C
+
CHCH3
H
8.10
Effect of Solvent
on the
Rate of Nucleophilic Substitution
Table 8.6
SN1 Reactivity versus Solvent Polarity
In general...
Solvent
SN1 Reaction Rates Increase
in Polar Solvents
Dielectric
constant
Relative
rate
6
33
58
78
1
4
5,000
150,000
acetic acid
methanol
formic acid
water
Most polar
transition
state
stabilized by
polar solvent
δ+ R
X δ−
transition
state
stabilized by
polar solvent
δ+ R
Fastest rate
X δ−
activation energy
decreases;
rate increases
R+
R+
energy of RX
not much
affected by
polarity of
solvent
energy of RX
not much
affected by
polarity of
solvent
RX
RX
Table 8.7
SN2 Reactivity versus Type of Solvent
In general...
CH3CH2CH2CH2Br + N3–
SN2 Reaction Rates Increase in
Polar Aprotic Solvents
An aprotic solvent is one that does
not have an —OH group.
Solvent
Type
Relative
rate
CH3OH
polar protic
1
H 2O
polar protic
7
DMSO
polar aprotic
1300
DMF
polar aprotic
2800
Acetonitrile
polar aprotic
5000
When...
primary alkyl halides undergo nucleophilic
substitution, they always react by the SN2
mechanism
Mechanism Summary
SN1 and SN2
tertiary alkyl halides undergo nucleophilic
substitution, they always react by the SN1
mechanism
secondary alkyl halides undergo nucleophilic
substitution, they react by the
SN1 mechanism in the presence of a weak
nucleophile (solvolysis)
solvolysis)
SN2 mechanism in the presence of a good
nucleophile