Catalysis specific-acid

Acid catalysis (specific or general) occurring in acidic solutions ... 

This is known as specific acid catalysis, specific in that H3O is the only acidic species that catalyses the reaction the reaction rate is found to be unaffected by the addition of other potential proton donors (acids) such as NH4 , provided that [H3O ], i.e. pH, is not changed, indirectly, by their addition. The mechanism of the above acetal hydrolysis is believed to be. 

Any process for which the rate of reaction is accelerated through the participation of an acid as a catalyst. See General Acid Catalysis Specific Acid Catalysis... 

GENERAL ACID CATALYSIS SPECIFIC ACID CATALYSIS CATALYSIS... 

There are two types of acid catalysis specific-acid catalysis and general-acid catalysis. In specific-acid catalysis, the proton is fully transferred to the reactant before the slow step of the reaction begins (Figure 24.3a). In general-acid catalysis, the proton is transferred to the reactant during the slow step of the reaction (Figure 24.3b). The mechanism for acid-catalyzed hydrolysis on p. 1003 shows that the slow steps of the reaction are specific-acid catalyzed. 

Furthermore, chiral bisphosphorylimides have been investigated within Brpnsted acid catalysis. Specifically, the... 

First of all, given the well recognised promoting effects of Lewis-acids and of aqueous solvents on Diels-Alder reactions, we wanted to know if these two effects could be combined. If this would be possible, dramatic improvements of rate and endo-exo selectivity were envisaged Studies on the Diels-Alder reaction of a dienophile, specifically designed for this purpose are described in Chapter 2. It is demonstrated that Lewis-acid catalysis in an aqueous medium is indeed feasible and, as anticipated, can result in impressive enhancements of both rate and endo-exo selectivity. However, the influences of the Lewis-acid catalyst and the aqueous medium are not fully additive. It seems as if water diminishes the catalytic potential of Lewis acids just as coordination of a Lewis acid diminishes the beneficial effects of water. Still, overall, the rate of the catalysed reaction... 

Pos twe-Tone Photoresists. The ester, carbonate, and ketal acidolysis reactions which form the basis of most positive tone CA resists are thought to proceed under specific acid catalysis (62). In this mechanism, illustrated in Figure 22 for the hydrolysis of tert-huty acetate (type A l) (63), the first step involves a rapid equihbrium where the proton is transferred between the photogenerated acid and the acid-labile protecting group ... 

The conversion of aromatic monomers relative to C-5—C-6 linear diolefins and olefins in cationic polymerizations may not be proportional to the feedblend composition, resulting in higher resin aromaticity as determined by nmr and ir measurements (43). This can be attributed to the differing reactivity ratios of aromatic and aHphatic monomers under specific Lewis acid catalysis. Intentional blocking of hydrocarbon resins into aromatic and aHphatic regions may be accomplished by sequential cationic polymerization employing multiple reactors and standard polymerization conditions (45). 

The original catalysts for this process were iodide-promoted cobalt catalysts, but high temperatures and high pressures (493 K and 48 MPa) were required to achieve yields of up to 60% (34,35). In contrast, the iodide-promoted, homogeneous rhodium catalyst operates at 448—468 K and pressures of 3 MPa. These conditions dramatically lower the specifications for pressure vessels. Yields of 99% acetic acid based on methanol are readily attained (see Acetic acid Catalysis). 

The sulfonated resin is a close analogue of -toluenesulfonic acid in terms of stmcture and catalyst performance. In the presence of excess water, the SO H groups are dissociated, and specific acid catalysis takes place in the swelled resin just as it takes place in an aqueous solution. When the catalyst is used with weakly polar reactants or with concentrations of polar reactants that are too low to cause dissociation of the acid groups, general acid catalysis prevails and water is a strong reaction inhibitor (63). 

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. 

Specific acid catalysis is observed when a reaction proceeds through a protonated intermediate that is in equilibrium with its conjugate base. Because the position of this equilibrium is a function of the concentration of solvated protons, only a single acid-dependent term appears in the kinetic expression. For example, in a two-step reaction involving rate-determining reaction of one reagent with the conjugate acid of a second, the kinetic expression will be as follows ... 

Notice that specific acid catalysis describes a situation in which the reactant is in equilibrium with regard to proton transfer, and proton transfer is not rate-determining. On the other hand, each case that leads to general acid catalysis involves proton transfer in the rate-determining step. Because of these differences, the study of rates as a function of pH and buffer concentrations can permit conclusions about the nature of proton-transfer processes and their relationship to the rate-determining step in a reaction. 

Both specific acid catalysis and general acid catalysis can be observed. (Review Section 4.8 for the discussion of specific and general acid catalysis.)... 

Specific acid catalysis is catalysis by the hydronium ion (in water) or the lyonium ion in general. Acid-catalyzed ester hydrolysis is an example. 

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. 

Condensation of methyl urethane, H2NC06CH3j with CH20 gives rise to the methylene diurethane (III) and this compd, under different specific conditions of refluxing, resulta in the various specific linear (IV V) and cyclic products (I II) as shown. The observations of relevance here are that the various products, both linear and cyclic, are interconvertible under acid catalysis including / into IF and conversely. The... 

It is important for acid-catalysed reactions to determine whether the reaction is specifically catalysed by hydrogen ions or whether general acid catalysis takes place. Specific acid catalysis has been conclusively demonstrated for the benzidine rearrangement by three different sorts of kinetic experiments. In the first, it has been shown41 by the standard test for general acid catalysis (by measuring the rate of reaction in a buffered solution at constant pH over a range of concentration... 

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