Specific base-general acid

II. SYSTEMS SHOWING CLASSICAL KINETICS A. The Specific Base-General Acid (SB-GA) Mechanism... 

For reactions in which the decomposition of the zwitterionic intermediate, ZH, is, at least partially, rate-limiting, two major mechanisms are now widely accepted. These are known as the specific base-general acid (SB-GA) and the rate-limiting proton transfer (RLPT) mechanisms and are shown in Scheme lla. 

The observation of general base-catalysis in the hydration of the carbonyl group can be explained by one of two kinetically indistinguishable mechanisms. These are (a) one-step general base catalysis represented in formula (18), or (b) specific base-general acid catalysis, (19). 

As you have seen throughout this book, acids and bases provide the most widely used ways of speeding up reactions. If you want to make an ester—add some acid. If you want to hydrolyse an ester—add some base. We explained in Chapter 12 the ways in which acid and base catalysts help reactions along, and we introduced you to the terms specific acid and specific base, general acid and general base. We will now look in a little more detail at these types of catalysis and give some pointers as to how to establish which of them, if any, is operative in any given reaction. 

Kinetic studies have been reported for the reactions of some 5-nitro-2-(4-nitrophenoxy)-3-X-thiophenes (10) with aniline and 4-methoxyaniline in methanol, which yield the corresponding 2-anilino derivatives. Base catalysis was interpreted by the SB-GA (specific base-general acid) mechanism. The reactions of 5-bromothiophene-2-carboxaldehyde (11) with secondary amines in water have been shown to produce the corresponding 5-aminated derivatives by the SnAt mechanism. ... 

Homogenous catalysts may be further subdivided into specific-acid, specific-base, general-acid, general-base, nucleophilic, and electrophilic catalysts. A specific acid is merely a proton, a specific base is a hydroxyl ion. General-acids and -bases are any other acidic or basic species, respectively. The bromination of acetone illustrates these different catalysts. It may proceed by specific-acid catalyzed enolization (52) ... 

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