Catalysis, acid-base equation

Much of the study of kinetics constitutes a study of catalysis. The first goal is the determination of the rate equation, and examples have been given in Chapters 2 and 3, particularly Section 3.3, Model Building. The subsection following this one describes the dependence of rates on pH, and most of this dependence can be ascribed to acid—base catalysis. Here we treat a very simple but widely applicable method for the detection and measurement of general acid-base or nucleophilic catalysis. We consider aqueous solutions where the pH and p/f concepts are well understood, but similar methods can be applied in nonaqueous media. 

Bronsted acid/base catalysis (equation, a, p) 113, 210, 355ff.,360ff., 392 Buckminsterfullerene, reaction with ArN 188... 

Acid-base catalysis, 232-238 Brqnsted equation for, 233-236 general, 233, 237 mechanisms for, 237 specific, 232-233, 237 Activated complex (see Transition state) Activation enthalpy, 10, 156-160 for composite rate constants, 161-164 negative, 161 Activation parameters, 10 chemical interpretation of, 168-169 energy of activation, Ea, 10 enthalpy of activation (A// ), 10, 156-160... 

Combining equations 7.3.4 and 7.3.2 indicates that in aqueous media for specific acid-base catalysis,... 

Allyl sulphones can be converted to dienes by alkylation and elimination of sulphinic acid under basic conditions (equation 64)105. Several vitamin A related polyenes have been synthesized following this two-step protocol (Table 10)106. The poor leaving-group ability of the arylsulphonyl group requires treatment with strong base for elimination. However, elimination of the allylsulphonyl group takes place readily under palladium catalysis (equation 65)107. Vinyl sulphones can be converted to dienes via Michael addition, alkylation with allyl halides and elimination of sulphinic acid sequence (equation 66)108. 

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. 

Such behavior is more common than the full rate/pH profile of (1.207). Equation (1.208) is observed in acid catalysisand (1.209) in base catalysis. The rate constant for the reaction of only one of the two forms can be obtained directly, that is, in (1.208) and in (1.209). Ancillary information on is required to assess the rate constant of the acid-base partner. The absence of reliable data on A jj can pose a problem in assessing the missing rate constant. 

The /3 value is a measure of the charge formed in the transition state rather than of the extent of bond formation. However, in equations 2.60 and 2.61 and in other examples where there is no acid-base catalysis, charge and bond formation are linked, so /3 does also give a measure of the extent of bond formation. But when there is also acid-base catalysis partly neutralizing the charges formed in the transition state, there is no relation between (3 and the extent of bond formation.46... 

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. 

This raises another question. Why is the serine hydroxyl an effective nucleophile when water and other hydroxylic compounds clearly are not similarly effective Apparently, the nucleophilicity of the serine —CH2OH is enhanced by acid-base catalysis involving proton transfers between acidic and basic side-chain functions in the vicinity of the active site. The serine is believed to transfer its OH proton to an amphoteric10 site B—A—H on the enzyme at the same instant that the proton of B—A—H is transferred to another base B e (Equation 25-8). These proton transfers are, of course, reversible ... 

The Bronsted catalysis law states that the individual catalytic constants, kn, should be related to the equilibrium acidities by Equation 8.12, or, for a base-catalyzed process, by Equation 8.13. (See Section 3.3, p. 141, for further dis-... 

It is possible that catalysis involves general acid, general base and general acid-base catalysis all together. Since [B] =FB([HA] + [B]), Equation 11.2 may be transformed into... 

In enzymes, the active site may possess acid and base groups intimately associated with the conjugate base and acid functions, respectively, of the complexed substrate the push-pull mechanism is possible but might not be a driving force. The halogenation of acetone in the presence of aqueous solutions of carboxylic acid buffers exhibits the rate law of Equation 11.2 where the third-order term, although small, has been shown to be significant and due to bifunctional concerted acid-base catalysis (Scheme 11.13) ... 

Reactions were studied under the pseudo first-order condition of [substrate] much greater than [initial dihydroflavin]. Under these conditions, the reactions are characterized by a burst in the production of Flox followed by a much slower rate of Flox formation until completion of reaction. The initial burst is provided by the competition between parallel pseudo first-order Reactions a and b of Scheme 3. These convert dihydroflavin and carbonyl compound to an equilibrium mixture of carbinolamine and imine (Reaction a), and to Flox and alcohol (Reaction b), respectively. The slower production of Flox, following the initial burst, occurs by the conversion of carbinolamine back to reduced flavin and substrate and, more importantly, by the disproportionation of product Flox with carbinolamine (Reaction c followed by d). Reactions c and d constitute an autocatalysis by oxidized flavin of the conversion of carbinolamine back to starting dihydroflavin and substrate. In the course of these studies, the contribution of acid-base catalysis to the reactions of Scheme 3 were determined. The significant feature to be pointed out here is that carbinolamine does not undergo an elimination reaction to yield Flox and lactic acid (Equation 25). The carbinolamine (N(5)-covalent adduct) is formed in a... 

The view of generalized acid-base catalysis as a prototropic shift assisted by acids and bases raises, quite naturally, the question of the relationship between the catalytic power of the acid or base and its own ionization constant. It had early been recognized that there is a correlation between the two constants. Taylor proposed the first quantitative relation, that the acid-catalytic constant of an acid knA was proportional to XjaA, the square root of its ionization constant. For generalized acid-base catalysis, the Bronsted equation, proposed later,"" has gained wide empirical use ... 

Some experimental criteria are mainly concerned with a distinction between unimolecular and bimolecular mechanisms in acid—base catalysis. One of the most straightforward criteria is based on the determination of the volume change on activation, AV, from the pressure dependence of the rate coefficient, according to the equation... 

According to the theory of the acid-base catalysis given by Bell (3) the rate of a reaction in which the determining step is the interaction between a catalyst C and a substrate S, both being ionic, is given by the equation ... 

Preparation of imines and enamines from carbonyl compounds and amines can be achieved with a dehydrating agent under acid/base catalysis [563]. Basically, primary amines afford imines unless isomerization to an enamine is favored as a result of conjugation, etc (see Eq. 252), and secondary amines afford iminium salts or enamines. These transformations can be conducted efficiently with a catalytic or stoichiometric amount of a titanium salt such as TiCU or Ti(0-/-Pr)4. Equation (247) illustrates an advantageous feature of this method in the imination of a hindered ketone. f-Butyl propyl ketone resisted the formation of the imine even by some methods reported useful for sterically hindered ketones [564,565]. The TiCU-based method works well, however, for this compound, giving the desired imine in high yield within a relatively short reaction period [566]. Imine derivatives such as iV-sulfonylimines could be... 

Addition of HaS to nitriles (73) represents an important method for the synthesis of primary thioamides (74 equation 33). The reaction is carried out under base or acid catalysis. The base-catalyzed process is particularly useful in the preparation of aromatic thioamides, i.e. for R = aryl, hetaryl in (73) triethylamine in molar proportion or pyridine, which at the same time also serves as solvent, are employed most commonly. The approach tolerates the presence of various other functionalities such as an amide, hydrazone, enamine, a-oxo, a-acetoxy ° or a-amino group. Moreover, the reaction has been applied to the synthesis of thioasparagine as such or incorporated into a dipeptide, as shown... 

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