1. Potential Energy Diagrams for Multistep Reactions: The SN1 Mechanism6

2. the potential energy diagram for proton transfer from HBr to water
Recall...
the potential energy diagram for proton transfer from HBr to water  
H2O H Br
Potential energy
H2O + H—Br
H2O—H + Br – +
Reaction coordinate 22

3. 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
(CH3)3CCl + H2O
25°C 9

4. Step 1: Proton transfer from HCl to tert-butyl alcohol
Mechanism
Step 1: Proton transfer from HCl to tert-butyl alcohol
(CH3)3C .. H Cl : .. + O : H
fast, bimolecular H O : +
(CH3)3C Cl .. –
tert-butyloxonium ion 9

5. Step 2: Dissociation of tert-butyloxonium ion
Mechanism
Step 2: Dissociation of tert-butyloxonium ion
(CH3)3C O H : +
slow, unimolecular O H : + +
(CH3)3C
tert-Butyl cation 9

6. Step 3: Capture of tert-butyl cation by chloride ion.
Mechanism
Step 3: Capture of tert-butyl cation by chloride ion. Cl : .. – +
(CH3)3C +
fast, bimolecular
(CH3)3C Cl : ..
tert-Butyl chloride 9

7. carbocation formation carbocation capture
proton transfer
ROH2 +
ROH RX 21

8. carbocation formation carbocation capture
(CH3)3C O H Cl + – R+
ROH2 +
ROH RX 21

9. (CH3)3C + O H
carbocation capture R+
proton transfer
ROH2 +
ROH RX 21

10. carbocation formation
(CH3)3C + Cl –
carbocation formation R+
proton transfer
ROH2 +
ROH RX 21