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Unformatted text preview: Chapter 9: Fu rther Reactions of Alcohols and the Chemistry of Ethers 9.1. Reactions of Alcohols with Base: Preparation of Alkoxides Strong bases are needed to deprotonate alcohols completely o LDA, butyllithium, alkali metal hydrides (KH) are strong bases that deprotonate alcohols to form alkoxides with lithium or potassium Alkali metals also deprotonate alcoholsbut by reduction of H + o Alcohols react with metals to form metal alkoxide and H 2 ; methanol most reactive and tertiary least reactive Strong bases covert alcohols into alkoxide by acid-base reaction; stronger base = more alkoxide Alkali metals react with alcohols by reduction to generate hydrogen gas an alkoxide; steric hindrance applies 9.2. 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 in S N 2 reactions o Protonate oxygen to form alkyloxonium ion with a positive charge on the oxygen and water which is a good leaving group o Alkyloxonium ion good for nucleophilic attack o Good with HBr and HI, but not so much HCl because Cl is a weak nucleophile Secondary and tertiary alcohols and HX: water can be a leaving group to form carbocations in S N 1 and E1 reactions o Good nucleophiles & lower temperatures: S N 1 reaction o E1 reaction is called dehydration because of loss of water molecule; nonnucleophilic acids such as H 3 PO 4 or H 2 SO 4 are used 9.3. Carbocation Rear rangements Hydride shifts give new S N 1 products o Carbocations can undergo rearrangement by hydride shifts in which hydrogen moves with both electrons from its original position to neighboring carbon atom o This occurs when a secondary carbocation is transformed into a more stable tertiary carbocation o The hydrogen and positive charge exchange places o Very fast, faster than S N 1 or E1 reactions because of hyperconjugation o Primary carbocations are too unstable to be formed by rearrangement o Secondary-secondary and tertiary-tertiary equilibrate readily so different products are seen Carbocation rearrangements also give new E1 products Other carbocation rearrangements are due to alkyl shifts o Carbocations that lack suitable hydrogens next to positively charged carbon undergo alkyl group migration or alkyl shift o The migrating group takes it electron pair with it to form a bond to neighboring carbocation: moving group and positive charge formally exchange places o Rates of alkyl and hydride shifts are comparable when it leads to similar stability o Shifts are faster if tertiary carbocation is formed Primary alcohols may undergo rearrangement o I t reduces steric hindrance for S N 2 reaction to take place o Requires elevated temperatures and long reaction times 9.4. Organic and I norganic Esters from Alcohols Organic esters are also called carboxylates or alkanoates ; formed from organic (carboxylic) acids by replacement of acidic hydrogen with alkyl group...
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