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Chapter 16 (Lecture #13)

Chapter 16 (Lecture #13) - Addition of Hydrogen Cyanide...

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Addition of Hydrogen Cyanide: Cyanohydrins Hydrogen cyanide (HCN) adds to aldehydes and unhindered ketones to produce cyanohydrins . RCH O = + H-C N RC-C N OH H RCR' O = + H-C N RC-C N OH R' unhindered cyanohydrins The reaction is carried out by adding a mineral acid to a mixture of the carbonyl compound and sodium cyanide. Too much acid retards the reaction by transforming all of the cyanide into HCN, a weak acid, and non-nucleophilic. The following sequence of steps describes the mechanism of cyanohydrin formation. RCR' O = + :C N - cyanide ion (a good nucleophile) slow RC-C N O R' - H-C N RC-C N OH R' :C N - +
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Synthetic Uses of Cyanohydrins Cyanohydrins are useful chemical intermediates for synthesizing other compounds. Hydrolysis of the nitrile function gives ! -hydroxyacids or ! , " -unsaturated acids. CH 3 CH 2 CCH 3 O = 2-butanone NaCN, H + CH 3 CH 2 CCN CH 3 OH 2-hydroxy-2-methylbutanenitrile CH 3 CH 2 CCN CH 3 OH HCl H 2 O, heat 95% H 2 SO 4 heat (-H 2 O) hydrolysis CH 3 CH 2 CCOOH CH 3 OH 2-hydroxy-2-methylbutanoic acid an ! -hydroxyacid CH 3 CH=C-COOH CH 3 2-methyl-2-butenoic acid an ! , " -unsaturated acid
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The Addition of Ylides: The Wittig Reaction C = O R R' aldehyde or ketone + (C 6 H 5 ) 3 P C R'' R''' + - a phosphorous ylide R R' C = C R'' R''' alkene + (C 6 H 5 ) 3 P=O triphenylphosphine oxide The classic reaction involves addition of a
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