Potential Energy Diagrams for
Multistep Reactions:
The SN1 Mechanism
Recall...
the potential energy diagram for proton transfer
from HBr to water

H2O

H
Br
Potential
energy
H2O + H—Br
Reaction coordinate
+
H2O—H + Br –
Extension
The potential energy diagram for a multistep
mechanism is simply a collection of the
potential energy diagrams for the individual
steps.
Consider the mechanism for the reaction of
tert-butyl alcohol with HCl.
(CH3)3COH + HCl
25°C
(CH3)3CCl + H2O
Mechanism
Step 1:
Proton transfer from HCl to tert-butyl alcohol
.. (CH3)3C ..
O : + H Cl:
..
H
fast, bimolecular
H
+
(CH3)3C O :
+
H
tert-butyloxonium ion
.. –
: Cl:
..
Mechanism
Step 2:
Dissociation of tert-butyloxonium ion
H
+
(CH3)3C O :
H
slow, unimolecular
H
+
(CH3)3C
+
tert-Butyl cation
:O:
H
Mechanism
Step 3:
Capture of tert-butyl cation by chloride ion.
(CH3)3C
+
+
.. –
: Cl:
..
fast, bimolecular
(CH3)3C
..
Cl
.. :
tert-Butyl chloride
carbocation
formation
carbocation
capture
R+
proton
transfer
ROH
+
ROH2
RX
carbocation
formation
–
+
(CH3)3C
O
carbocation
capture
H
Cl
R+
H
ROH
+
ROH2
RX
H
+
(CH3)3C
O
+
carbocation
capture
H
R+
proton
transfer
ROH
+
ROH2
RX
+
carbocation
formation
(CH3)3C
R+
proton
transfer
ROH
+
ROH2
RX
–
Cl