Benzamide oxygen exchange with

The formation of the tetracovalent intermediate appears to be the rate-determining step. On the other hand the hydroxide ion-catalysed hydrolysis of benzamide (Bender and Ginger, 1955) is accompanied by oxygen exchange with solvent (see Table 6). In basic solution, the reaction proceeds via the symmetrical intermediate. 

The exchange of carbonyl oxygens with heteroatoms has provided useful routes for the introduction of S and Se atoms into pentalene systems. A mixture of (4-cyano-5-methylthio-l,2-dithiole-3-ylidine)benzamide (89) and 2,4-bis(phenylthio)-l,3-dithia-2,4-diphosphetane 2,4-disulfide (90) upon reflux, give l,6,6aA -trithia-3-azapentalene derivative (91) shown in Scheme 25. If the benza-mide (89) is refluxed with PjSes instead, the l-selena-6,6aA -dithia analogue (92) is formed as a result <85CC1704>. 

The pi e-equilibrium proton transfer is demonstrated by the HjO-DjO solvent effect on the rate of hydrolysis of acetamide in dilute acid (Reitz, 1939), and the slow bimolecular attack of water is demonstrated by the effect of substituents on the rate in dilute acid (see Ingold, 1953, p. 784 for review Leisten, 1959). Hydrolyzing benzamide does not exchange its oxygen with the solvent (Bender and Ginger, 1955 Bender et al., 1954). This observation gives no indication as to whether the attacking water molecule adds or substitutes, but only shows that if it adds then the decomposition of the adduct is fast and does not influence the measured rate. 

O vs. N Protonation.—Controversy over the site of protonation of amides has been raised anew by Liler. Using u.v. spectroscopy, she has suggested that benzamide is 50 50 O N protonated in 80 % sulphuric acid. However, a considerable weight of evidence has been produced to indicate that protonation or Lewis acid complexation of amides consistently occurs by co-ordination with oxygen. Studies have included the acidity-dependent changes in the tt-tt and n-it u.v. absorption bands of aliphatic amides kinetic evidence based on rate data for acid-catalysed amide hydrolysis in both dilute and concentrated acid an n.m.r. study of proton exchange rates, where for A -methylacetamide the molar ratio of 0 N protonated species exceeds 10 a n.m.r. study of adducts of boron trifluoride and antimony pentachloride with N-labelled ureas, where the hybridization-dependent —H coupling constants were inconsistent with N-co-ordination ... 

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