Ch_322b_18.20

Ch_322b_18.20 - Chapter 18.20 lecture note

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Decarboxylation of Carboxylic Acids The loss of CO 2 from a carboxylic acid is called decarboxylation . Although the loss of CO 2 is usually exothermic (because of the stability of CO 2 ), the reaction is kinetically slow . However, certain structural features promote decarboxylation. ! -Keto acids decarboxylate rapidly when heated to 100-150 o C. The decarboxylation reaction occurs with either the acid or its carboxylate salt. R-C-O-H O = R-H + CO 2 ! -Keto Acids, - C CCOOH O R-C-CH 2 -C-OH O O ! " 100-150 o C RCCH 3 + CO 2 O ketone
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Reaction of the acid involves a six-membered cyclic transition state that initially produces the enol of the ketone product. Decarboxylation of the Acid RCCH 2 COH O = O ! -keto acid C CH 2 C=O O O H required geometry R C CH 2 C=O O O H six-membered cyclic TS R C=CH 2 O H R enol + CO 2 C=CH 2 O H R enol rapid tautomerization (isomerization) RCCH 3 O ketone This decarboxylation reaction proceeds well because of a reasonable mechanism that avoids any high energy intermediates. It is believed that bond-making and bond-breaking in the cyclic
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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_18.20 - Chapter 18.20 lecture note

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