Transacylation Interconversion of Acid Derivatives

Problem 16.28 (a) Give the mechanism for the reaction of RCOCI with Nu . (6) Compare, and explain the difference in the reactivities of RCl and RCOCI with Nu . (c) What is the essential difference between nucleophilic attack on C=0 of a ketone or aldehyde and of an acid derivative  

Problem 16.29 Account for the relative reactivities of RCOG with nucleophiles RCOX (RC02)20 RCOOR RCONH2.  

Often this order of reactivity is related to the order of leavability of G , which is the reverse of its order of basicity Cl is the best leaving group and the weakest base. 

Although this rationale gives the correct answer, it overlooks an important feature of the mechanism—the step eliminating G cannot influence the reaction rate because it is faster than the addition step. The first step, the slow nucleophilic addition, determines the reaction rate, and here resonance stabilization of the —C—G group is important II 

The greater the degree of resonance stabilization, the less reactive is RCOG. Thus, NH2 has the greatest degree and RCONH2 is the least reactive, while X has the smallest degree and RCOX is the most reactive. It so happens that the order of resonance stabilization correlates directly with the order of basicity e.g. —N of amide is the best resonance- 

Since HN, is only slightly more acidic than CH,COOH, Nj is slightly less basic and is a slightly better leaving group than CH,COO. RCON, is less reactive than RCOCI, but a little more reactive than the anhydride R—C—O—C—R. 

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