Catalysis-based mechanisms

Hydrolysis and subsequent condensation can occur in two ways depending upon the strength and concentration of acid or base catalyst used. It has been shown that condensation reactions are acid and base specific [35]. In addition, more basic conditions tend to increase the gelling time [36] and generally the condensation reactions continue, but gelation does not occur. 

Under acidic conditions, rapid first step takes place with protonation of alkoxide group, resulting in the withdrawal of electron density from the silicon atom, rendering it more susceptible to attack from water (Fig. 18.7). The water displaces a protonated alkojq group(s) [37]. The mainly used acids in acid-catalyzed synthesis are HCl, ammonia, and acetic acid. 

Under basic condition, first step takes place with dissociation of water to produce hydroxyl anions followed by the attack of these hydroxyl anions on the silicon atom (Fig. 18.8). It has been reported that -0 H displaces -0 Rby attack from the opposite side, which is similar to acid catalysis [37], Potassium hydroxide, HF, and amines are mainly used in acid-catalyzed mechanisms. 

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Hydrolysis of aspirin in H2 0 leads to no incorporation of into the product salicylic acid, ruling out the anhydride as an intermediate and thereby excluding mechanism 1. The general acid catalysis of mechanism III can be ruled out on the basis of failure of other nucleophiles to show evidence for general acid catalysis by the neighboring carboxylic acid group. Because there is no reason to believe hydroxide should be special in this way, mechanism III is eliminated. Thus, mechanism II, general base catalysis of hydroxide-ion attack, is believed to be the correct description of the hydrolysis of aspirin. 

Lewis acid-base mechanism, 233, 234 Lewis acid catalysts, 13, 546 Lewis-acid-catalyzed ammonolysis, of nylon, 571 Lewis acids, 224 catalysis by, 68-69 Lewis bases, 338... 

The same framework of eight possible mechanisms that was discussed for ester hydrolysis can also be applied to amide hydrolysis. Both the acid- and base-catalyzed hydrolyses are essentially irreversible, since salts are formed in both cases. For basic catalysis the mechanism is Bac2-... 

It is not the aldehyde or ketone itself that is halogenated, but the corresponding enol or enolate ion. The purpose of the catalyst is to provide a small amount of enol or enolate. The reaction is often done without addition of acid or base, but traces of acid or base are always present, and these are enough to catalyze formation of the enol or enolate. With acid catalysis the mechanism is... 

The above achievements depend highly on both the recent advances in rational catalyst design with the aid of computational science represented by DFT calculations and the wide range of catalyst design possibilities that are afforded by FI catalysts. These possibilities are derived from the readily varied steric and electronic properties of the phenoxy-imine ligands. It is expected that future research on FI catalysts will provide opportunities to produce additional polyolefin-based materials with unique microstructures and a chance to study catalysis and mechanisms for olefin polymerization. 

Typical examples referring to titanium derivatives are alkoxides with TBHP and titanosilicate (in particular TS-1) in the presence of H202. Based on this latter system, ENICHEM" commercialized a procedure for hydroxylation of phenol to cathecol and hydroquinone. Other activated arenes are also hydroxylated by TS-1 and hydrogen peroxide". Interestingly, for TS-1 catalysis a mechanism similar to that proposed... 

The third test of the conjugate base mechanism that we put forward was based on the idea that the first step should be written as an equilibrium, and the reaction rate should show specific hydroxide ion catalysis. If this is indeed in equilibrium, and deuterium exchange studies say that it must be, then the rate of the reaction must depend on the hydroxide ion concentration, and on nothing else. 

A new approach to develop a molecular mechanism for Fischer-Tropsch catalysis based on the use of [Fe2Co(CN)6] and [Fe(HCN)2]3 precursor complexes has been disclosed.509 The former produced mainly liquid aliphatic hydrocarbons, whereas the latter gave waxy aliphatic products. Results acquired by various techniques were interpreted to imply that chain growth proceeds via the insertion of CO into an established metal-carbon bond, that is, a C, catalytic insertion mechanism is operative. It follows that C2 insertion is an unlikely possibility. 

An interesting feature of the general acid and general base mechanisms shown is the catalysis of both steps (see figure lOd and e). For example, in the first step of general... 

Fig. 22. Proposed mechanism for Tyr catalysis based on the reaction chemistry of ix-rj1 t)2 peroxo complexes.

Prins summarizes advances in understanding of the reactions in catalytic hydrodenitrogenation (HDN), which is important in hydroprocessing of fossil fuels. Hydroprocessing is the largest application in industrial catalysis based on the amount of material processed. The chapter addresses the structures of the oxide precursors and the active sulfided forms of catalysts such as Ni-promoted Mo or W on alumina as well as the catalytically active sites. Reaction networks, kinetics, and mechanisms (particularly of C-N bond rupture) in HDN of aliphatic, aromatic, and polycyclic compounds are considered, with an evaluation of the effects of competitive adsorption in mixtures. Phosphate and fluorine promotion enhance the HDN activity of catalysts explanations for the effect of phosphate are summarized, but the function of fluorine remains to be understood. An account of HDN on various metal sulfides and on metals, metal carbides, and metal nitrides concludes this chapter. 

At low concentrations of BH+ such that k2 > k j [BH+], the observed first-order rate coefficient for conditions where buffer is present in excess over reactant is kx [B]. The rate of reaction is determined by a slow proton removal by base from carbon. At high concentrations of BH+, the observed first-order rate coefficient is (k1fe2/k-i)[B]/[BH+]. In this case, if the reaction is carried out in aqueous solution, the rate of reaction depends upon the hydroxide ion concentration and is independent of the buffer concentration at a fixed buffer ratio (specific base catalysis). The mechanism under these conditions consists of rapid pre-equilibrium formation of a carbanion followed by a slow step. Over the whole range of buffer concentration the first-order rate coefficient (M,hs) measured at fixed buffer ratio first increases (/ bs = kl [B]) with buffer concentration but reaches a limiting value (kohs = (ki k2 /k-i) [B] /[BH+]). This change in mechanism has been observed for a limited number of reactions [58]. Reactions (38) [58(a)] and (39) [58(b)] occurring in ethanol and reaction (40) [58(c)] in aqueous... 

Herschlag D. Ribonuclease revisited. Catalysis via the classical general acid-base mechanism or a triester-like mechanism. J. Am. Chem. Soc. 1994 116 11631-11635. 

The intrinsic consequences of such strong activation of the carboxy group toward aminolysis are (1) the increased acidity of the C -proton which favors enolization giving 16 and (2) the facile ring closure of the carboxy-activated amino acid or peptide component to oxazol-5(4//)-ones 17 by base catalysis. Both mechanisms lead to loss of stereochemical integrity, i.e. racemization or epimerization as illustrated in Schemes 6 and (for the correct use of the terms racemization or epimerization see Vol. E 22b, Section 7.4). 

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