Equilibrium, acid-base tetrahedral mechanism

Anhydrides may be prepared by coupling two carboxylic acids under acidic conditions. If ethanoic acid (acetic acid, 21) is heated with HCl, protonation to give an oxocarbenium ion is followed by reaction with a second equivalent of acetic acid to give a tetrahedral intermediate. This reaction is the usual acid-catalyzed acyl addition mechanism. Protonation of the OH unit leads to loss of water and formation of the anhydride. Each step in this process is reversible and steps must be taken to drive the equilibrium (see Chapter 7, Section 7.10, for a discussion of equilibria) toward the anhydride product by removing the water by-product (see Chapter 18, Section 18.6.3). Remember that such techniques are an application of Le Chatelier s principle (discussed in Section 18.3). Even when this is done, isolation of pure anhydrides by this method can be difficult. Unreacted acid may contaminate the product and atternpts to remove the acid with aqueous base may induce hydrolysis of the anhydride. 

Tertiary benzoyloxymethylsulfonamides (17) undergo hydrolysis via pH-independent and acid- and base-catalysed processes. Reactions are also buffer catalysed for buffer species with pXa values > 10.5. For the pH-independent pathway, hydrolysis takes place via formation of an Al-sulfonyliminium ion (Scheme 2). The mechanism of the acid-catalysed process involves pre-equilibrium protonation of the substrate followed by iminium ion formation. The base-catalysed pathway involves the normal Bac2 mechanism of ester hydrolysis. The buffer-catalysed reaction gives rise to a curved Brpnsted plot, with values of 1.6 and 0.25 for nucleophiles with pXa values <12.5 and >13, respectively. This is indicative of nucleophilic catalysis associated with a change in rate-limiting step from formation of the tetrahedral intermediate for buffer species with pXa > 13 to decomposition of the tetrahedral intermediate for buffer species with pXa < 12.5. ... 

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