Ch_322b_18.18

Ch_322b_18.18 - O X + RCH 2 C X O !-haloacid product acyl...

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! - HaloAcids: The Hell-Volhard-Zelinsky Reaction Carboxylic acid may be functionalized at the !" position by introducing a Cl or Br atom using a method developed in the 1880s. In the Hell-Volhard-Zelinsky reaction, the carboxylic acid is reacted with Cl 2 or Br 2 in the presence of a small amount of phosphorous to yield !" haloacids . Halogenation occurs only at the !" position . If excess X 2 is used, ! , ! -dihalo- and ! , ! , !" trihaloacids are formed. CH 3 CH 2 CH 2 COH O = butanoic acid Br 2 P CH 3 CH 2 CHCOH O Br 2-bromobutanoic acid (77%)
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A Mechanism for the HVZ Reaction formation of the active catalyst PX 3 2 P + 3 X 2 2 PX 3 Only a small amount of the catalyst is needed. formation of a small amount of the acyl halide RCH 2 COH O = + PX 3 RCH 2 C X O electrophilic attack on the enol RCH=C :OH X : X-X + RCH-C- X :OH X + + X - RCH-C- X O X + H X -H + enolization and halogenation via the enol RCH 2 C X O RCH=C OH X enol acyl group exchange via the mixed acid anhydride RCH-C- X O X + RCH 2 COH O RCH-C-O-CCH 2 R O X O -H X +H X mixed anhydride -H X +H X RCH-COH
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Unformatted text preview: O X + RCH 2 C X O !-haloacid product acyl chloride reformed Reactions of !" Haloacids !" Haloacids are reactive in nucleophilic substitution reactions, which allows other functional groups to be introduced into the carboxylic acid structure. RCHCOOH X + Z:-RCHCOOH Z + X-Examples Br CH 2 COOH + 2 NH 3 bromoacetic acid H 2 NCH 2 COOH + NH 4 Br aminoacetic acid (glycine) Cl CH 2 CO 2-+ :C N-chloroacetate ion N CCH 2 CO 2-H 3 O + N CCH 2 COOH cyanoacetic acid N CCH 2 COOH alkaline hydrolysis (i) HO-, H 2 O, heat (ii) H 3 O + HOOCCH 2 COOH malonic acid C 6 H 5 CH 2 COOH Br 2 P C 6 H 5 CHCOOH Br NaCN C 6 H 5 CHCOOH CN H 3 O + H 2 O C 6 H 5 CH COOH COOH Quiz 18.018 Design a synthesis of phenylmalonic acid from phenylacetic acid. C 6 H 5 CH 2 COOH C 6 H 5 CH COOH COOH phenylmalonic acid phenylacetic acid...
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This note was uploaded on 10/20/2009 for the course CHEM 322BL at USC.

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Ch_322b_18.18 - O X + RCH 2 C X O !-haloacid product acyl...

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