Catalysis, general base specificity

The rate of a general acid-catalyzed reaction is equal to 2ha ha[HA] multiplied by some function of the substrate concentration(s) where [HA] is the general acid concentration and A ha is the corresponding catalytic rate constant. Experimentally, general acid catalysis can be distinguished from specihc acid catalysis by analysis of the effect of buffer concentration on the overall reaction rate. See also Specific Acid Catalysis Catalysis General Base Catalysis... 

The acceleration of a reaction by a unique catalyst rather than a family of related substances or materials. The term is commonly used with respect to H+ or OH catalysis (thus, specific acid and specific base catalysis). See General Acid Catalysis General Base Catalysis... 

GENERAL BASE CATALYSIS GENERAL BASE CATALYSIS SPECIFIC BASE CATALYSIS CATALYSIS... 

GENERAL ACID CATALYSIS GENERAL BASE CATALYSIS SPECIFICITY... 

FER CATALYSIS GENERAL ACID CATALYSIS GENERAL BASE CATALYSIS HOMOGENEOUS CATALYST HETEROGENEOUS CATALYST INTRAMOLECULAR CATALYSIS MICELLAR CATALYSIS Mich ALT IS-MeNTEN KINETICS PHASE-TRANSFER CATALYSIS PSEUDOCATALYSIS RATE OF REACTION SPECIFIC CATALYSIS. 

The role that acid and base catalysts play can be quantitatively studied by kinetic techniques. It is possible to recognize several distinct types of catalysis by acids and bases. The term specie acid catalysis is used when the reaction rate is dependent on the equilibrium for protonation of the reactant. This type of catalysis is independent of the concentration and specific structure of the various proton donors present in solution. Specific acid catalysis is governed by the hydrogen-ion concentration (pH) of the solution. For example, for a series of reactions in an aqueous buffer system, flie rate of flie reaction would be a fimetion of the pH, but not of the concentration or identity of the acidic and basic components of the buffer. The kinetic expression for any such reaction will include a term for hydrogen-ion concentration, [H+]. The term general acid catalysis is used when the nature and concentration of proton donors present in solution affect the reaction rate. The kinetic expression for such a reaction will include a term for each of the potential proton donors that acts as a catalyst. The terms specific base catalysis and general base catalysis apply in the same way to base-catalyzed reactions. 

The experimental detection of general acid catafysis is done by rate measurements at constant pH but differing buffer concentration. Because under these circumstances [H+] is constant but the weak acid component(s) of the buffer (HA, HA, etc.) changes, the observation of a change in rate is evidence of general acid catalysis. If the rate remains constant, the reaction exhibits specific acid catalysis. Similarly, general base-catalyzed reactions show a dependence of the rate on the concentration and identity of the basic constituents of the buffer system. 

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. 

Base catalysis. Develop chemical equations and rate laws that illustrate specific and general base catalysis. Equations (10-37)—(10-44) can be used as a model. 

Reactions can be catalyzed by acid or base in two different ways, called general and specific catalysis. If the rate of an acid-catalyzed reaction run in a solvent S is... 

Mechanism Kinetic" Order P-Hydrogen Exchange Faster Than Elimination General or Specific Base Catalysis hAd Electron Withdrawal atCp Electron Release at C Leaving- Group Isotope Effect or Element Effect... 

The ionizable functional groups of aminoacyl side chains and (where present) of prosthetic groups contribute to catalysis by acting as acids or bases. Acid-base catalysis can be eithet specific ot general. By specific we mean only protons (HjO ) or OH ions. In specific acid or specific base catalysis, the rate of reaction is sensitive to changes in the concenttation of protons but... 

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

H30+ and OH-. Reactions that are dependent on the concentrations of HXt and X- are categorized as involving general acid and general base catalysis. Table 7.1, adapted from Ashmore (30), indicates a number of catalytic reactions of the specific and general acid-base types in order to provide some orientation as to the types of reactions in the various categories. A thorough discussion of these reactions is obviously... 

Since specific acid and specific base catalysis and generalized acid and generalized base catalysis by cations and anions all involve ionic species, these processes are influenced by the... 

The systematic variation of cH+, cOH-, etc. allows the experimental determination of each rate constant. If the terms in the first summation on the right of equation 8.2-9 predominate, we have general acid catalysis if those in the second summation do so, we have general base catalysis otherwise, the terminology for specific acid-base catalysis applies, as in the previous section. 

Fig. 7.2. a) The most common mechanism of base-catalyzed ester hydrolysis, namely specific base catalysis (HCT catalysis) with tetrahedral intermediate and acyl cleavage. Not shown here are an W mechanism with alkyl cleavage observed with some tertiary alkyl esters, and an 5n2 mechanism with alkyl cleavage sometimes observed with primary alkyl esters, particularly methyl esters, b) Schematic mechanism of general base catalysis in ester hydrolysis. Intermolecular catalysis (bl) and intramolecular catalysis (b2). c) The base-catalyzed hydrolysis of esters is but a particular case of nucleophilic attack. Intermolecular (cl) and intramolecular (c2). d) Spontaneous (uncatalyzed) hydrolysis. This becomes possible when the R moiety is... 

The value of k has been determined for a very large number of bases. There is a reasonable correlation of log/ B S P-A iBH+ agreement with (2.162). The value of p is — 0.25, which indicates that general base catalysis holds. For specific base catalysis, P would equal 1, while for reactions in which solvent catalysis predominates p would be zero, since only the A , [H2O] term would then be important. 

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