Chapter 9

Chapter 9 - CHAPTER 9 Further Reactions of Alcohols and the...

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CHAPTER 9 urther Reactions of Alcohols and Further Reactions of Alcohols and the Chemistry of Ethers
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A variety of reaction modes are available to alcohols.
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Reactions of Alcohols with Base: Preparation of Alkoxides 9-1 Strong bases are needed to deprotonate alcohols completely. Base strength must be stronger than that of the alkoxide.
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Alkali metals also deprotonate alcohols – but by eduction of H + reduction of H . Vigorous: Less Vigorous: Relative reactivities:
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Uses for alkoxides: indered alkoxides: Hindered alkoxides: E 2 reactions with haloalkanes to form alkenes. ess hindered alkoxides: Less hindered alkoxides: S N2 reactions with haloalkanes to form ethers.
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Reactions of Alcohols with Strong Acids: Alkyloxonium Ions in Substitution and 9-2 Elimination Reactions of Alcohols ater has a high pK 5 7) which means that its conjugate base Water has a high pK a (15.7) which means that its conjugate base, OH - is an exceedingly poor leaving group. The –OH group of an alcohol must be converted into a better leaving group for alcohols to participate in substitution or elimination reactions.
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Reactions of Alcohols with Strong Acids: Alkyloxonium Ions in Substitution and 9-2 Elimination Reactions of Alcohols Haloalkanes from primary alcohols and HX: water can be a leaving group. Protonation of the hydroxy substituent of an alcohol to form an alkyloxonium ion converts the –OH from the poor leaving group, OH - , to the good leaving group, H 2 O.
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Primary bromoalkanes and iodoalkanes can be prepared by the reaction with HBr and HI. Chloroalkanes cannot be prepared by this method because Cl - is too poor a nucleophile.
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econdary and tertiary alcohols undergo Secondary and tertiary alcohols undergo carbocation reactions with acids: S N 1 and E1. Primary alkyloxonium ions undergo only S N 2 reactions with acid. Their carbocation transition state energies are too high to allow and E1 reactions under ordinary laboratory conditions S N 1 and E1 reactions under ordinary laboratory conditions. Secondary and tertiary alkyloxonium ions lose water when treated with acid to form a carbocation.
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When good nucleophiles are present, the S N 1 mechanism predominates. Here the tertiary carbocation is generated at a relatively low temperature, which prevents the competing E 1 reaction. At higher temperatures, or in the absence of good nucleophiles, elimination becomes dominant. p,
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Secondary alcohols show complex behavior when treated with HX, following S N 2, S N 1, and E1 pathways. Relatively hindered (compared to primary alcohols): Retarded S N 2 reactivity Slow to form carbocations (compared to tertiary alcohols) Retarded S N 1 reactivity.
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E1 reactions of alcohols (dehydrations) result in the formation of lkenes. Non- ucleophilic acids, such as H O r H O are alkenes. Non nucleophilic acids, such as H 3 PO 4 or H 2 SO 4 , are used in this case, rather than the nucleophilic acids, HBr and HI.
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This note was uploaded on 07/25/2009 for the course CHEM 140A taught by Professor Whiteshell during the Spring '04 term at UCSD.

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Chapter 9 - CHAPTER 9 Further Reactions of Alcohols and the...

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