Specific base, general acid catalysis

I, pp. 162-8 jencks PP- uses the selectivity—reactivity relationship between Br nsted slopes and nucleophilic reactivity to distinguish between general acid catalysis and specific acid—general base catalysis. 

General Acid Catalysis Versus Specific Acid-General Base Catalysis ... 

Specific acid-general base catalysis Rate =/f [B][HNu][C= 0][H30 ... 

Specific and general acid catalysis, p. 74 3.3.2 Specific and general base catalysis, p. 75. 

The pH-rate profile for reaction of nitrosobenzene with A-methylhydroxylamine (to form only 1-methyl 2-phenyldiazene-2-oxide) has been found to exhibit a negative break between pH 0.5 and 3.0. This has been ascribed to a change in ratedetermining step from nucleophilic attack on nitrosobenzene at low pH to dehydration of the A,iV -dihydroxy intermediate at higher pH the dehydration is subject to general-acid catalysis a = 0.34) and specific and general-base catalysis (f) = 0.20). The pH-rate profile is similar to that for reaction of A-methylhydroxylamine with... 

The acid-base catalysis illustrated in these equations is termed general to distinguish it from specific acid or base catalysis in which the catalyst is the proton or hydroxide ion. 

In acid-base catalysis, both an acid (or base) and its conjugate base (or acid) take part in different reaction steps and are eventually restored. Such reactions are first order in acid (or base) if the link-up with that species controls the rate, or first order in H+ (or OH") if a subsequent step involving the conjugate base (or acid) does so. Traditionally, the first alternative is called "general" acid or base catalysis the second, "specific" acid or base catalysis. However, this distinction is not always applicable as there may be no clear-cut rate-controlling step, and reversibility of later steps may produce a more complex behavior. 

In this equation, is the experimentally determined hydrolytic rate constant, /Cq h the uncatalysed or solvent catalysed rate constant, and /CgH- te the specific acid- and base-catalysis rate constants respectively, ttd ky - are the general acid- and base-catalysis rate constants respectively, and [HX] and [X ] denote the concentrations of protonated and unprotonated forms of the buffer. 

In the case of apparent general acid catalysis of acetylimidazole hydrolysis, the mechanism can be defined as a specific acid-general base process by comparison with the general base catalysis of N-methyl,N -acetylimidazolium ion. The rate of disappearance of N-methyl,N -acetylimidazolium ion in water at 25° is proportional to the concentration of the basic form of buffer components such as acetate, phosphate, N-methylimidazole, etc., (equation 30) (Wolfenden and Jencks, 1961). The buffer terms show a 1 1 correlation with the general acid-catalyzed rate of acetylimidazole disappearance (Jencks and Carriuolo, 1959) in water at 25°, when the rate expression for the latter reaction is written in terms of equation (32) rather than equation (31), that is, in terms of a general base-catalyzed hydration of protonated acetylimidazole (pX= 3-6). 

There are many chemical reactions that are catalyzed by acids or bases, or by both. The most common acid catalyst in water solution is the hydronium ion and the most common base is hydroxyl ion. However, some reactions are catalyzed by any acid or by any base. If any acid catalyzes the reaction, the reaction is said to be subject to general acid catalysis. Similarly, general base catalysis refers to catalysis by any base. If only hydronium or hydroxyl ions are effective, the phenomenon is called specific acid or base catalysis. 

We have indicated how to determine the various kinetic constants appearing in the expression for specific acid and base catalysis. Let us now consider how to evaluate the various contributions to the rate constant in the case of general acid-base catalysis. For reactions of this type in a solution of a weak acid or base and its corresponding salt, the possible catalysts indicated by equation (7.3.3) are the hydro-nium ion, the hydroxide ion, the undissociated weak acid (or base), and the conjugate base (or acid), In the case of acetic acid the general acid would be the neutral CHjCOOH species and the conjugate base would be the acetate ion (CH3COO"). In this case the apparent rate constant can be written as... 

We should not expect that a given reaction will exhibit only general acid (or base) catalysis or specific acid (or base) catalysis. In principle, reactions may be subject to more than one kind of catalysis. For example, the catalytic rate expression for the reaction of iodine with acetone in buffer solutions was determined to be... 

Water is an amphoteric compound, and thus it can lend all types of assistance, whichever is required in a given process, or even a combination of several. It may itself be a general acid or base catalyst, or, in the presence of other acids and bases, serve as an environment deploying proton or hydroxide ion for specific acid or base catalysis. 

The mechanism of each of the four possible combinations may involve an Arrhenius or a van t Hoff intermediate. This leads to eight possible mechanisms, schematically presented in Table 13.2. Each of these mechanisms can be developed using either the preequilibrium or the steady-state approximation to arrive at the corresponding rate law. The lessons that can be learnt from the treatment of these mechanisms are also indicated in Table 13.2 some mechanisms lead exclusively to specific acid or base catalysis, while others lead to general acid or base catalysis. Furthermore, the specific catalysis is associated with the existence of a limiting rate, that is the rate that will not increase indefinitely with the H+, or OH , concentration, but attain a limiting valne eqnal to 2[S]o-... 

Nucleophilic Electrophilic Specific base Specific acid General base General acid FIGURE 2.3 Representation of varions types of catalysis and snbcatalysis. 

Blagoeva, I.B., Toteva, M.M., Ouarti, N., Rnasse, M.-F. Changes in the relative contribution of specific and general base catalysis in cationic micelles the cyclization of substituted ethyl hydantoates. J. Org. Chem. 2001, 66(6), 2123-2130. Takag, Y., Warashina, M., Stec, W.J., Yoshinari, K., Taira, K. Recent advances in the elucidation of the mechanisms of action of ribozymes. Nucl. Acid. Res. 2001, 29(9), 1815-1834. 

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