Ch_322a_7.09

Ch_322a_7.09 - parallel the energies of the product states...

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Carbocation Stability and the Transition State The order of carbocation stability is C R R R + > C R R + H > C R H + H > C H H + H tertiary secondary primary methyl The rate-determining step in the dehydration of alcohols is bond heterolysis to form a carbocation intermediate. Since this step is highly endergonic , according to the Hammond-Leffler Postulate, the transition state has considerable carbocation character . C OH 2 + oxonium ion bond heterolysis RDS C OH 2 ! + ! + TS C + + H 2 O carbocation The reactivity of alcohols in the dehydration reaction is determined by the stabilities of the carbocation intermediates that strongly influence the energy of the transition state in the rate-determining step.
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Energetics of the Rate-Determining Step Relative Free Energy oxonium ions normalized to the same energy C-OH 2 R R R + C-OH 2 H R R + C-OH 2 H R H + C R R R + + H 2 O TS C R R + H + H 2 O TS C R H + H + H 2 O TS ! G 3 o ! G 2 o ! G 1 o
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The Reactivity Order of Alcohols The energies of the transition states
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Unformatted text preview: parallel the energies of the product states ( carbocation states ) leading to the free energies of activation for the rate-determining step: ! G 3 o ! G 2 o ! G 1 o < < and the observed reactivity order of alcohols in the dehydration reaction: C-OH R R R C-OH H R R C-OH H R H > > most reactive Stability Order of Carbocations The stability order of the carbocations is due to better dispersal of the positive charge as electron-releasing alkyl groups replace hydrogens at the carbocation center. C R R R + ! + ! + ! + C R R + ! + ! + C R + ! + H H H tertiary secondary primary Quiz 7.09 What is the expected reactivity order (most to least) of the following alcohols in the acid-catalyzed dehydration reaction? H CH 2 OH OH CH 3 OH CH 3 H H I II III reactivity order: II > III > I...
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This note was uploaded on 07/22/2009 for the course CHEM 322AL taught by Professor Jung during the Summer '07 term at USC.

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Ch_322a_7.09 - parallel the energies of the product states...

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