Acids specific

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 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.)... 

The proton could be completely transferred and then the departing alcohol molecule would leave to form a carbocation in a distinct second step. This is the specific acid-catalyzed mechansism. 

Carboxylic A term describing a specific acidic group (COOH) that contrib- utes cation-exchange ability to some resins. 

Sulfonic A specific acidic group (sosh) on which depends the exchange activity of certain cation adsorbents. 

Specific acid and base Hydrolysis of esters Ri COOR. + H.O = Ri COOH + R.OH... 

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. 

FIGURE 16.11 Specific and general acid-base catalysis of simple reactions in solution may be distinguished by determining the dependence of observed reaction rate constants (/sobs) pH and buffer concentration, (a) In specific acid-base catalysis, or OH concentration affects the reaction rate, is pH-dependent, but buffers (which accept or donate H /OH ) have no effect, (b) In general acid-base catalysis, in which an ionizable buffer may donate or accept a proton in the transition state, is dependent on buffer concentration. 

Kresge and Chiang480 measured the rate coefficients for detritiation of [1-3H]-2,4,6-trimethoxybenzene in acetate buffers and found the first-order rate coefficient (lO7 ) to increase from 2.5 at 0.01 M acetic acid to 8.3 at 0.1 M acetic acid, whereas if the reaction was specific acid-catalysed no change in rate should have been observed. A similar technique to that described above for separation of the rate coefficients due to hydronium ions and other acids was used, the values for the former being obtained using dilute hydrochloric acid at which acidities no undissociated acid was present (Table 131). Rate coefficients were then measured... 

From the shapes of the rate versus acid strength graphs obtained for mesit-aldehyde and triisopropylbenzaldehyde it was concluded that although neither compound was specific acid-catalysed, the latter compound showed the nearer tendency to this catalysis at the higher acid concentrations again this may be a manifestation of the greater steric hindrance to protonation by H3 S04 than by H30+. 

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... 

The kinetic data based on the demonstration of specific acid catalysis in buffers, solvent isotope effects and acidity functions all support mechanisms where the proton-transfers are fast. It is possible to write equations which accommodate these facts together with the first-order dependence on hydrazo-compound and the concurrent first and second-order dependence on acidity. These are... 

This scheme requires a rate-determining (second) proton-transfer, against which there is considerable experimental evidence in the form of specific-acid catalysis, the solvent isotope effect and the hg dependence discussed earlier. Further, application of the steady-state principle to the 7i-complex mechanism results in a rate equation of the form... 

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