Rate constants acid-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. 

A reaction with a rate constant that conforms to Eq. (10-21)—particularly to the feature that the catalysts are H+ and OH-, and not weak acids and bases—is said to show specific acid-base catalysis. This phenomenon is illustrated by the kinetic data for the hydrolysis of methyl o-carboxyphenyl acetate16 (the methyl ester of aspirin— compare with Section 6.6) ... 

The rate of a reaction that shows specific acid (or base, or acid-base) catalysis does not depend on the buffer chosen to adjust the pH. Of course, an inert salt must be used to maintain constant ionic strength so that kinetic salt effects do not distort the pH profile. 

One can test for general acid-base catalysis by varying [BH+] and [B] at constant pH. An easy test is to dilute the buffer progressively at a constant ratio of [BH+]/[B], making up any ionic strength change so as not to introduce a salt effect. If the rate is invariant with this procedure, then general acid-base catalysis is absent under the circumstances chosen. 

Acid-base catalysis. Interpret the finding that a particular rate constant remains constant when the different values of [OAc"] and [HO Ac] are used such that [OAc" J/[HOAc] remains constant, whereas the rate constant increases with [HOAc] in solutions to which OAc" was not added. The ionic strength was held constant. 

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... 

General acid/base catalysis is less significant in natural fresh waters, although probably of some importance in special situations. This phenomenon can be described fairly well via the Bronsted law (relating rate constants to pKa and/or pKb of general acids and bases). Maximum rates of general acid/base catalysis can be deduced from a compound s specific acid/base hydrolysis behavior, and actual rates can be determined from relatively simple laboratory experiments (34). 

In specific acid-base catalysis in aqueous systems, the observed rate constant, kobs, depends on cH+ and/or on c0H-, but not on the concentrations of other acids or bases present ... 

In general acid-base catalysis, the observed rate constant depends on the concentrations of all acids and bases present. That is, in aqueous systems,... 

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. 

Besides the effect of solvent polarity, the C=C rotation in many push-pull ethylenes is sensitive to acid catalysis (143). This is probably explained by protonation of the acceptor groups, for example, the oxygen atoms in C=0 groups (16), which increases their acceptor capacity. Small amounts of acids in halogenated solvents, or acidic impurities, may have drastic effects on the barriers, and it is advisable to add a small quantity of a base such as 2,4-lutidine to obtain reliable rate constants (81). Basic catalysis is also possible, but it has only been observed in compounds containing secondary amino groups (38). 

It has been shown by H naff (1963) that the rate of reaction of several carbonyl reagents (bisulphite, hydrazine, phenylhydrazine, semi-carbazide and hydroxylamine) with aqueous formaldehyde solutions is independent of the nature and concentration of the reagent, and is therefore determined by the rate of dehydration of methylene glycol. He obtained catalytic constants for hydrogen and hydroxide ions, and a detailed study of acid-base catalysis has been made by the same method by Bell and Evans (1966). 

Examples of possible intramolecular general acid-base catalysis were reported by Kupchan et al. (1962). The methanolysis of coprostanol acetate and coprostane 3/3, 5/3-diol 3-monoacetate [12] in aqueous methanol was conducted in triethylamine-triethyl-ammonium acetate buffer. The rates of methanolysis at constant... 

The intermolecular general-base catalysis of the hydrolysis may also be measured. Comparing the rate constants for this with those of the intramolecular reaction shows that a 13-M solution of an external base is required to give the same first-order rate as the intramolecular reaction has.12 The effective concentration of the carboxylate ion in aspirin is therefore 13 M. This is a typical value for intramolecular general-acid-base catalysis. 

Effective concentration 65-72 entropy and 68-72 in general-acid-base catalysis 66 in nucleophilic catalysis 66 Elastase 26-30, 40 acylenzyme 27, 40 binding energies of subsites 356, 357 binding site 26-30 kinetic constants for peptide hydrolysis 357 specificity 27 Electrophiles 276 Electrophilic catalysis 61 metal ions 74-77 pyridoxal phosphate 79-82 Schiff bases 77-82 thiamine pyrophosphate 82-84 Electrostatic catalysis 61, 73, 74,498 Electrostatic effects on enzyme-substrate association rates 159-161... 

Acid-base catalysis is seen to be an effective way of catalyzing reactions. We should now like to know the contribution of this to enzyme catalysis, but there is a fundamental problem in directly applying the results of the last section to an enzyme. The crux of the matter is that the rate constants for the solution catalysis are second-order, the rate increasing with increasing concentration of... 

In all of these reactions the species involved in the reversible reactions are progressively used up with time. However, there are reactions such as acid-base catalysis and enzyme catalysis where one of the species in the reversible reaction is the catalyst, and as such is regenerated so that the total catalyst concentration remains constant throughout the whole of the reaction. In such cases it is often essential to use initial rates or rates at various stages during reaction for analysis. 

Data for aliphatic aldehyde enolisation are very scarce, probably because the enolisation process is often complicated by oxidation and hydration. Nevertheless, the rate constants for base- and acid-catalysed iodination of R R2CHCHO were determined in aqueous chloroacetic acid-chloroacetate ion buffers (Talvik and Hiidmaa, 1968). The results in Table 4 show that alkyl groups R1 and R2 increase the acid-catalysed reactivity in agreement with hyperconjugative and/or inductive effects. This contrasts with aliphatic ketones for which steric interactions are important and even sometimes dominant. Data for base-catalysis are more difficult to interpret since a second a methyl group, from propionaldehyde to isobutyraldehyde, increases the chloroacetate-catalysed rate constant. This might result from a decrease of the a(C—H) bond-promoted hyperconjugative stabilisation of the carbonyl compound... 

Many hydrolysis reactions are pH dependent with base catalysis predominant. The acid-base catalysis is especially significant with carboxylic acid esters. The pseudo-first-order hydrolysis rate constant it/,(time" ) can be expressed as... 

The hydrolysis rate of dmgs in liquid dosage forms is strongly influenced by the pH of the solution and can be catalysed not only by H+ and OH ions (specific acid-base catalysis) but also by the components of the buffer used (general acid-base catalysis). We have looked at the ways in which the effect of the buffer components can be removed so that the pH of maximum stability of the solution can be determined from the pH-rate profile and the rate constants for specific acid-base catalysis can be calculated. 

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