Catalysis by acids and bases

Other reactions show a more complex dependence on pH. In Chapter 14, we studied the reaction of various molecules, RNHj, with carbonyl compounds to give imines, hydrazones. 

FIGURE 23.2 Catalysis of ester hydrolysis by acids or bases. 

FIGURE 23.3 pH dependendance of the rate of formation of acetone oxime. Note Mechanism remains the same over pH range of interest but still has an upper limit. 

Write a mechanism for the mutarotation of glucose in acidic, basic, and neutral solution, showing carefully the function of the acid and base in catalysis. 

The reverse aldol reaction shown below gives a rate equation - Ar[starting material] [HO ]. Write a mechanism for this process that is consistent with the rate equation, remembering that proton transfers to or from oxygen and nitrogen atoms are rarely rate limiting. 

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Volume 20 Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27,1984... 

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. 

K.Tanabe, Solid Acids and Bases, Academic Press, New York, 1970 Catalysis by Acids and Bases(B.Imelik, C.Naccache, G.Couduier, YBen Taarit and J.C. Vedrine eds.), Elsevier, Amsterdam, 1984, p.l. 

Although the discussion to this point has been concerned with the explanation of the behavior of Bronsted acids as catalysts, there is an enormous range of reactions in which catalysis by acids and bases occurs. Many of the important types of organic reactions involve catalysis by acids or bases. In this section, several reactions will be mentioned, but the mechanistic details will not be presented in this book on inorganic chemistry. The discussion is intended to show the scope of catalysis by acids and bases. 

The reactions shown above are just a few of the enormous number of processes that involve catalysis by acids and bases. Some of the reactions are of economic importance and they certainly serve to illustrate that this type of chemistry is not solely the province of either the organic or inorganic chemist. 

At the end of the review there are some examples involving catalysis by acids and bases, metal ions, micelles, amylose, catalytic antibodies, and enzymes to give the reader a feeling for how Kurz s approach may be usefully applied to other catalysts. Very few of these examples, or those involving cyclodextrins, were discussed in the original literature in the same terms. It is hoped that the present treatment will stimulate further use and exploration of the Kurz approach to analysing transition state stabilization. 

Hydration and dehydration reactions have proved particularly convenient for studying catalysis by acids and bases over a wide range of structures and catalytic power. The results usually show a good correlation between acidic and basic catalytic constants and k ) and the of the catalyst, according to the usual Bronsted relations kjp = Ga qK /p) , ki, q = Gi, pjqKf, where p and q are statistical corrections. (These statistical corrections have not always been applied consistently, but the discrepancies thus introduced are not serious.) The information so far obtained is summarized in Table 3, and comments on the individual reactions follow. 

Finally, it may be noted that the reversible addition of water to 2-hydroxypteridine is subject to general catalysis by acids and bases... 

D. Plee, A. Shutz, 6. Poncelet and J.J. Fripiat, Catalysis by Acid and Bases (Stud. Surf. Sci. Catal., 20), Elsevier, Amsterdam, 1985, p.343. 

The other important mechanistic question is one of catalysis by acids and bases. The situation is complicated because of the several possible sites of protonation. The substrate may be protonated on carbonyl oxygen and also on X if that group has unshared pairs, as it does in all the common cases the nucleophile Y may be protonated or not the tetrahedral intermediate may be protonated in various possible ways and either of the two steps may be rate-determining. It is therefore not suprising that the details are still under investigation. 

The-Claisen rearrangement to the ortho position is a first-order reaction,67 68 and the process does not require catalysis by acids and bases. The rearrangement is intramolecular, since rearrangement of mixtures of ethers such as allyl /3-naphthyl ether and cinnamyl phenyl ether,60 or cinnamyl 4-methylphenyl ether and allyl 4-aminophenyl ether,68 yields none of the cross products which would result from an intermolecular reaction. The process is best represented by the cyclic mechanism, in which the following processes take place, with the electronic shifts during reaction indicated by the arrows.23 39 69... 

Tanabe, K. Catalysis by solid bases and related subjects. In Catalysis by Acids and Bases, B. Imelik et al. (eds). Elsevier, Amsterdam, 1985. 

Depending on solvent, temperature, and the nature of the groups Ri and R2, the equilibrium may lie to either side. Thus it has been found that in pure acetone at 25°C, there is about 2.5 X 10 per cent of eiiol. In contrast, pure acetyl acetone, CII3COCH2COCH3, exists as 80 per cent enol, but it is only 15 per cent enol in water solution. The transformation of ketones to enols involves a proton shift, and as we might expect, the rate is susceptible to catalysis by acids and bases. 

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