Acid strength, catalysis

As might be expected intuitively, there is a relationship between the effectiveness of general acid catalysts and the acid strength of a proton donor as measured by its acic dissociation constant K. This relationship is expressed by the following equation, which is known as the Brensted catalysis law ... 

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+. 

Bifunctional catalysis in nucleophilic aromatic substitution was first observed by Bitter and Zollinger34, who studied the reaction of cyanuric chloride with aniline in benzene. This reaction was not accelerated by phenols or y-pyridone but was catalyzed by triethylamine and pyridine and by bifunctional catalysts such as a-pyridone and carboxylic acids. The carboxylic acids did not function as purely electrophilic reagents, since there was no relationship between catalytic efficiency and acid strength, acetic acid being more effective than chloracetic acid, which in turn was a more efficient catalyst than trichloroacetic acid. For catalysis by the carboxylic acids Bitter and Zollinger proposed the transition state depicted by H. 

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. 

In general acid catalysis, the rate is increased not only by an increase in [SH ] but also by an increase in the concentration of other acids (e.g., in water by phenols or carboxylic acids). These other acids increase the rate even when [SH ] is held constant. In this type of catalysis the strongest acids catalyze best, so that, in the example given, an increase in the phenol concentration catalyzes the reaction much less than a similar increase in [H30 ]. This relationship between acid strength of the catalyst and its catalytic ability can be expressed by the Breasted catalysis equation ... 

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

Circulation flow system, measurement of reaction rate, 28 175-178 Clausius-Clapeyron equation, 38 171 Clay see also specific types color tests, 27 101 compensation behavior, 26 304-307 minerals, ship-in-bottle synthesis, metal clusters, 38 368-379 organic syntheses on, 38 264-279 active sites on montmorillonite for aldol reaction, 38 268-269 aldol condensation of enolsilanes with aldehydes and acetals, 38 265-273 Al-Mont acid strength, 38 270-271, 273 comparison of catalysis between Al-Mont and trifluorometfaanesulfonic acid, 38 269-270... 

It appears that all these possibilities can be excluded. If reactions (a) or (gf) were rate-limiting the reaction velocity would be independent of the concentration of the substrate, while reaction (e) (identical with (Z)) would predict no catalysis by acids or bases. If reactions (b), (d) or (h) determined the rate the reaction would show specific catalysis by hydrogen or hydroxide ions, in place of the general acid-base catalysis actually observed. Reactions (c), (f) and (m) are unacceptable as rate-limiting processes, since they involve simple proton transfers to and from oxygen. Reactions (j) and (k) might well be slow, but their rates would depend upon the nucleophilic reactivity of the catalyst towards carbon rather than on its basic strength towards a proton as shown in Section IV,D it is the latter quantity which correlates closely with the observed rates. 

While average deprotonation energy is a good measure of the intrinsic Bronsted acid strength of a zeoHte, it is the extrinsic acidity, also impacted by the chemical interaction between the protonated basic probe molecule and the deprotonated zeoHte, that really counts for catalysis. 

The acid strength of protons in the crystalline molecular sieve structure plays a key role in of MTO catalysis. The acid sites of silicoalumina-based zeolites... 

The several attempts, published in the literature, to describe the kinetics of vapour phase olefin (mostly ethylene) hydration can be classified into two groups according to the basic model used. One model, for reactions catalysed by phosphoric acid supported on solids, treats the kinetics as if the process were homogeneous acid catalysis and takes into account the acid strength of the supported acid. Thus, a semiempirical equation for the initial reaction rate [288]... 

Gibbs energy of dissociation, table 293 pKa values of, table 293 strengths of 95-96 Acid -base catalysis 469,486 - 491 concerted 490 of mutarotation 487 Acid - base chemistry... 

In conclusion, we have shown that due to the diffusional problems, monodirectional large pore zeolites are very inefficient to perform bimolecular reactions involving carbonylic reagents in the liquid phase. By contrast, catalysis by tridirectional Y and fl zeolites show similar features, with a high catalytic activity which increases with the acid strength of the acid sites. 

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