Chapter 7 - Further Reactions of Haloalkanes

Chapter 7 - Further Reactions of Haloalkanes - Chapter 7:...

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Chapter 7: Further Reactions of Haloalkanes Unimolecular Substitution and Pathways of Elimination 7.1. Solvolysis of Tertiary and Secondary Haloalkanes Solvolysis is when a substrate undergoes substitution by solvent molecules Hydrolysis is solvolysis when water is the solvent This process occurs faster with tertiary than secondary and methyl and primary haloalkanes are unreactive in solvolysis Solvolysis also occurs in alcohol solvents Process produces products expected from S N 2 reaction but order of reactivity is reversed 7.2. Unimolecular Nucleophilic Substitution First order-rate law; not stereospecific; opposite order of reactivity of S N 2 Solvolysis follows first-order kinetics o Rate of hydrolysis of bromide is proportional to only concentration of starting halide, not water o The observed rate is rate of the slowest step in sequence: rate-determining step The mechanism of solvolysis includes carbocation formation o Unimolecular nucleophilic substitution (S N 1 ): only one molecule (haloalkane) participates in rate determining step (this molecule is the only one in rate law); it does not depend on nucleophile concentration o 1) Rate-determining step is dissociation of haloalkane to an alkyl cation and bromide; heterolytic cleavage to produce positively charged central carbon atom attached to three other groups and bearing an electron sextet, called a carbocation o 2) Carbocation is an electrophile (electron-deficient) that is attacked by nucleophile (solvent) to produce an alkyloxonium ion , the conjugate acid of an alcohol o 3) Like hydronium, alkloxonium ions are strong acids that readily deprotonate in water to furnish a final alcohol and hydronium ion o All steps for mechanism of solvolysis are reversible and equilibrium can be driven in
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This note was uploaded on 09/28/2009 for the course CHEM 3A taught by Professor Fretchet during the Spring '08 term at University of California, Berkeley.

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Chapter 7 - Further Reactions of Haloalkanes - Chapter 7:...

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