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The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base-induced disproportionation of an aldehyde lacking a hydrogen atom in the alpha position. : Cannizzaro first accomplished this transformation in 1853, when he obtained benzyl alcohol and potassium benzoate from the treatment of benzaldehyde with potash (potassium carbonate). More typically, the reaction would be conducted with sodium or potassium hydroxide: :2 C6H5CHO + KOH → C6H5CH2OH + C6H5CO2K The oxidation product is a salt of a carboxylic acid and the reduction product is an alcohol.〔Geissman, T. A. "The Cannizzaro Reaction" ''Org. React.'' 1944, ''2'', 94. (Review)〕 ==Mechanism== The reaction involves a nucleophilic acyl substitution on an aldehyde, with the leaving group concurrently attacking another aldehyde in the second step. First, hydroxide attacks a carbonyl. The resulting tetrahedral intermediate then collapses, re-forming the carbonyl and transferring hydride to attack another carbonyl. In the final step of the reaction, the acid and alkoxide ions formed exchange a proton. In the presence of a very high concentration of base, the aldehyde first forms a doubly charged anion from which a hydride ion is transferred to the second molecule of aldehyde to form carboxylate and alkoxide ions. Subsequently, the alkoxide ion acquires a proton from the solvent. center Overall, the reaction follows third-order kinetics. It is second order in aldehyde and first order in base: ::: rate = k()2() At very high base a second path (k') becomes important that is second order in base: ::: rate = k()2() + k'()2()2 The k' pathway implicates a reaction between the doubly charged anion (RCHO22−) and the aldehyde. The direct transfer of hydride ion is evident from the observation that the recovered alcohol does not contain any deuterium attached to the α-carbon when the reaction is performed in the presence of D2O. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cannizzaro reaction」の詳細全文を読む スポンサード リンク
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