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

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CHAPTER 9 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 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 reduction of H + . Vigorous: Less Vigorous: Relative reactivities: Uses for alkoxides: Hindered alkoxides E 2 reactions with haloalkanes to form alkenes. 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 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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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. Secondary and tertiary alcohols undergo carbocation reactions with acids: S N 1 and E1.
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E1 reactions of alcohols (dehydrations) result in the formation of 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. Dehydrations of tertiary alcohols often occur just above room temperature.
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Summary
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Carbocation Rearrangements Hydride shifts give new S N 1 products. Treatment of substituted secondary alcohols produces unexpected results:
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Rearrangement of an initial secondary carbocation to the more stable tertiary carbocation by a hydride shift results in a rearranged product.
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Hydride shifts are very fast (faster than S N 1 or E1) which is partially due to hyperconjugation in the carbocation weakening the C-H bond):
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Primary carbocations are too unstable to be formed by rearrangement. Secondary or tertiary carbocations equilibrate readily, leading to a mixture of products when trapped by a nucleophile.
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Carbocation rearrangement takes place regardless of the precursor leading to the carbocation.
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Carbocation rearrangements also give new E1 products. Under conditions favoring elimination (elevated temperatures and nonnucleophilic media), carbocations can also rearrange to give rearranged products.
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Alkyl shifts, rather than hydride shifts, can occur when a carbocation lacks a
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CHAPTER+9 - CHAPTER 9 Further Reactions of Alcohols and the...

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