Potential Energy Diagrams for Multistep Reactions: The SN1 Mechanism 6

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

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

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

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

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

carbocation formation carbocation capture proton transfer ROH2 + ROH RX 21

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

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

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