8Chapter 04

Wolfs chm 201 202 4 71 potential energy diagram

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Unformatted text preview: rdinate Dr. Wolf's CHM 201 & 202 4- 70 4.9 Potential Energy Diagrams for Potential Multistep Reactions: Multistep The SN1 Mechanism Dr. Wolf's CHM 201 & 202 4- 71 Potential Energy Diagram-Overall Potential Energy Diagram-Overall The potential energy diagram for a The multistep mechanism is simply a collection of the potential energy diagrams for the individual steps. potential Consider the three-step mechanism for the reaction of tert-butyl alcohol with HCl. tert (CH3)3COH + HCl Dr. Wolf's CHM 201 & 202 25°C (CH3)3CCl + H2O 4- 72 carbocation formation carbocation capture R+ proton transfer ROH Dr. Wolf's CHM 201 & 202 + ROH2 RX 4- 73 carbocation formation (CH3)3C δ+ O H δ– Cl carbocation capture R+ H proton transfer ROH Dr. Wolf's CHM 201 & 202 + ROH2 RX 4- 74 carbocation formation carbocation capture H δ+ (CH3)3C O δ+ H R+ proton transfer ROH Dr. Wolf's CHM 201 & 202 + ROH2 RX 4- 75 carbocation formation carbocation capture δ+ (CH3)3C R+ δ– Cl proton transfer ROH Dr. Wolf's CHM 201 & 202 + ROH2 RX 4- 76 Mechanistic notation Mechanistic notation The mechanism just described is an The example of an SN1 process. example SN1 stands for substitution-nucleophilicunimolecular. unimolecular. The molecularity of the rate-determining step defines the molecularity of the step overall reaction. Dr. Wolf's CHM 201 & 202 4- 77 Mechanistic notation Mechanistic notation The molecularity of the rate-determining The step defines the molecularity of the step overall reaction. H δ+ (CH3)3C O δ+ H Rate-determining step is unimolecular dissociation of alkyloxonium ion. Dr. Wolf's CHM 201 & 202 4- 78 4.10 Structure, Bonding, and Stability of Carbocations Dr. Wolf's CHM 201 & 202 4- 79 Carbocations Carbocations R + C R R Most carbocations are too unstable to be isolated, but occur as reactive intermediates in a number of reactions. When R is an alkyl group, the carbocation is stabilized compared to R = H. Dr. Wolf's CHM 201 & 202 4- 80 Carbocations Carbocations H + C H H Methyl cation least stable Dr. Wolf's CHM 201 & 202 4- 81 Carbocations Carbocations H3C + C H H Ethyl cation (a primary carbocation) is more stable than CH3+ Dr. Wolf's CHM 201 & 202 4- 82 Carbocations Carbocations H3C + C CH3 H Isopropyl cation (a secondary carbocation) is more stable than CH3CH2+ Dr. Wolf's CHM 201 & 202 4- 83 Carbocations Carbocations H3C + C CH3 CH3 tert-Butyl cation (a tertiary carbocation) is more stable than (CH3)2CH+ Dr. Wolf's CHM 201 & 202 4- 84 Figure 4.15 Stabilization of carbocations Figure 4.15 Stabilization of carbocations Figure Figure via the inductive effect via the inductive effect via via + Dr. Wolf's CHM 201 & 202 positively charged carbon pulls electrons in σ bonds electrons closer to itself closer 4- 85 Figure 4.15 Stabilization of carbocations Figure 4.15 Stabilization of carbocations Figure Figure via the inductive effect via the inductive effect via via δ+ Dr. Wolf's CHM 201 & 202 δ+ δ+ δ+ positive charge is "dispersed ", i.e., shared by carbon and the three atoms attached to it 4- 86 Figure 4.15 Stabilization of carbocations Figure 4.15 Stabilization of carbocations Figure Figure via the inductive effect via the inductive effect via via δ+ δ+ δ+ δ+ electrons in C—C bonds are more bonds polarizable than those polarizable iin C—H bonds; n there...
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This document was uploaded on 01/03/2012.

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