Catalysis base-catalyzed reactions

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. 

In agreement with expectation for a rate-determining proton transfer, the reaction shows general acid catalysis. Base-catalyzed ketonization occurs by C-protonation of the enolate. 

We divided the topic into five main sections. There are no purely acid- and base-catalyzed reactions, however, at least when metals are involved in the reaction. Metals should activate electrophiles as a Lewis acid. Thus, base catalysis discussed... 

Hydrolysis can proceed through numerous mechanisms via attack by H20 (neutral hydrolysis) or by acid (H+) or base (OH ) catalysis. Acid and base catalyzed reactions proceed via alternative mechanisms that require less energy than neutral hydrolysis. The combined hydrolysis rate term is a sum of these three constituent reactions and is given by... 

A material such as Na°/NaY catalyzes the aldol condensation of acetone, to form mesityl oxide and eventually isophorone. Another strong base catalyzed reaction is the side chain alkylation of toluene with ethylene. In contrast with acid catalysis, side chain reaction is strongly preferred over ring alkylation. With a Na°/NaX in the gas phase at 473 K, toluene reacts to give n-propylbenzene (66%) and the dialkylated product, 3-phenylpentane (32%) (41). 

Survey of mechanisms of acid—base catalyzed reactions 5.1 MECHANISMS OF ACID CATALYSIS... 

The base-catalyzed reaction in Equations (2a) and (2b) is very similar to the corresponding alcohol reaction with this difference. Since the mer-captide ion is more nucleophilic than the alkoxide ion one will expect that smaller differences in the charge density around the carbonyl carbon of the isocyanate will affect the reactivity of the mercaptides than that of the alkoxides. This would explain the higher sensitivity of thiols to base-catalysis and the higher activity of bases for the catalysis of the thiol reaction. 

Examples of acid- and base-catalyzed reactions are common in organic chemistry e.g. ester hydrolysis (Fig. 4.72). Since ester hydrolysis can be catalyzed by either acids or bases, clearly such a reaction would be even faster if both modes of catalysis could be employed simultaneously. Although this is not generally possible in solution, the complexity of the structure of an enzyme allows it to position both proton donors and acceptors properly to utilize both modes of catalysis simultaneously. 

Some insight into the nature of these catalyzed cyclizations is provided by the synthesis of benzobicyclo[3.1.0]hexene 17. Intramolecular cycloaddition under base catalysis (from the tosylhydrazone 14) or by heating compound 18 gave the [3.3.0] bicyclic product 16 but, in the boron trifluoride catalyzed reaction, the [3.2.1] bicyclic product 13 was formed. In the base-catalyzed reaction the intermediate 4,5-dihydro-3//-pyrazole 16 suffered tautomerization (to give 19). The diazo compound 13 cannot tautomerize, and gave a quantitative yield of benzobicyclohexene 17 when heated. These boron trifluoride catalyzed addition reactions do not involve the free diazo compound and in this particular case the cyclic A -tosyl intermediate 12 could be isolated from the reaction mixture. ... 

General add-base catalysis is typically inefficient, compared with nucleophilic catalysis, and this is particularly well documented for intramolecular reactions, as discussed above (Section 2.3.1). The reasons for this disparity have been discussed in terms of broad generalizations, citing most often the looseness, and thus relatively low entropy, of the transition state for a general acid-base catalyzed reaction compared with a cyclization process in which ring formation is complete apart from one partial covalent bond. (Compare, for example, the observed general base catalyzed hydrolysis 5.1 and the abortive but lO times faster nucleophilic reaction... 

The surprising inertness of the glycosylamines to mutarotation by a base-catalyzed mechanism arises from the small tendency of an amino nitrogen-atom to release a proton. At high alkalinities, the base-catalyzed reaction takes place to a small degree with glycosylamines of primary and secondary amines.277 With those of tertiary amines, there is no proton to be released, and the mutarotation reaction must take place by acid catalysis (see Fig. 16). The high sensitivity of the... 

These early views envisaged reactions which could take place in the absence of a catalyst, but which were facilitated by its presence. Evidence gradually accumulated to show that many of the reactions subject to acid-base catalysis could not take place at all in the complete absence of catalysts, apparently spontaneous reactions being often due to catalysis by acidic or basic solvent molecules, or by some adventitious acidic or basic impurity. This seemed to indicate that the catalyst took a fundamental part in the reaction, possibly in a chemical sense. It was soon realized that the essential property of acids and bases was their power respectively to lose and to add on a proton, and enquiry also showed that substrates involved in acid catalysis could always be supposed to have some basic properties, while those in base-catalyzed reactions could always in principle act as acids, though the acid-base properties of the substrates were often so weak as to elude detection by ordinary means. This led to the suggestion that acid-base catalysis always involves an acid-base reaction between the catalyst and the substrate. Such a reaction is also often termed a protolytic reaction, since it involves the transfer of a proton between the two reacting species. 

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