Base catalysis, decomposition

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

Apart from acid-base catalysis, homogeneous catalysis occurs for other liquid-phase reactions. An example is the decomposition of H202 in aqueous solution catalyzed by iodide ion (II). The overall reaction is... 

Furthermore, although the intercepts k kiK/k- ) and the slope (kikjK/k-i) are equally influenced by the dimerization constant K in equation 28, this does not imply that they should show the same effect on changing the solvent. According to the dimer mechanism , it could be expected that the base catalysed decomposition of the transition state SB2, measured by Ag, should be more depressed by small additions of protic solvents than the spontaneous decomposition measured by Ag. Indeed, the overwhelming evidence on the classical base catalysis by amines shows that usually Ag is more important in aprotic than in protic solvents1. 

PVA Formation Reaction. Poly(vinyl alcohol) is itself a modified polymer being made by the alcoholysis of poly(vinyl acetate) under acid or base catalysis as shown in Equation 1 (6.7). This polymer cannot be made by a direct polymerization because the vinyl alcohol monomer only exists in the tautomeric form of acetaldehyde. This saponification reaction can also be run on vinyl acetate copolymers and this affords a means of making vinyl alcohol copolymers. The homopolymer is water soluble and softens with decomposition at about 200°C while the properties of the copolymers would vary widely. Poly(vinyl alcohol) has been widely utilized in polymer modification because ... 

Fig. 8.20. Two-step activation ofN-[(acyloxy)methyl] prodrugs, a) Cleavage of the ester bond, which may be enzymatic and/or nonenzymatic, is followed by decomposition of the N-(hy-droxymethyl) intermediate, b) For (V-(hydroxymethyl) derivatives of amides and imides, the decomposition is base-catalyzed, c) For N-(hydroxymethyl) derivatives of amines, the decomposition can be uncatalyzed or undergo acid or base catalysis (modified from [214]).

Belke et al. (1971) reported general base and general acid catalysis in cyclization of 2-hydroxymethylbenzamide [equation (18)]. However, with 2-hydroxymethyl-6-aminobenzamide strict general base catalysis by buffer bases is observed with a Bronsted coefficient of O 39 (Fife and Benjamin, unpublished data). In contrast with the unsubstituted amide, the Bronsted plot is nicely linear. An amino-group in the 6-position might assist decomposition of a tetrahedral intermediate as in [37a, b] or a kinetic equivalent. The pH-rate constant profile for spontaneous cyclization at zero buffer concentra-... 

This is a further example of a carbonyl-electrophile complex, and equivalent to the conjugate acid, so that the subsequent nucleophilic addition reaction parallels that in hemiacetal formation. Loss of the leaving group occurs first in an SNl-like process with the cation stabilized by the neighbouring oxygen an SN2-like process would be inhibited sterically. It is also possible to rationalize why base catalysis does not work. Base would simply remove a proton from the hydroxyl to initiate hemiacetal decomposition back to the aldehyde - what is needed is to transform the hydroxyl into a leaving group (see Section 6.1.4), hence the requirement for protonation. 

An important point to note in figure 8.1 is that the same general acid or base that catalyzes the formation of the tetrahedral intermediate also can participate in the decomposition of the intermediate. When a general acid (HA) donates a proton to the ester oxygen it becomes a base (A ), which can retrieve the proton as the intermediate breaks down. When a general base (B ) removes a proton from water it becomes an acid (BH), which can provide a proton to the alcohol. Note also that general-acid and general-base catalysis are not mutually exclusive they can both occur in a concerted manner in the same step of a reaction. 

The base-catalysed decomposition of nitramide (3 in Scheme 1.4) is of special historical importance as it was the reaction used to establish the Bronsted catalysis law. The reaction has been studied over manyyears and considerable evidence indicates that the decomposition... 

It is reported that an industrial explosion was initiated by charging potassium hydroxide in place of potassium carbonate to the chloro-nitro compound in the sulfoxide [1], Dry potassium carbonate is a useful base for nucleophilic displacement of chlorine in such systems, reaction being controlled by addition of the nucleophile. The carbonate is not soluble in DMSO and possesses no significant nucleophilic activity itself. Hydroxides have, to create phenoxide salts as the first product. These are better nucleophiles than their progenitor, and also base-destabilised nitro compounds. Result heat and probable loss of control. As it nears its boiling point DMSO also becomes susceptible to exothermic breakdown, initially to methanethiol and formaldehyde. Methanethiolate is an even better nucleophile than a phenoxide and also a fairly proficient reducer of nitro-groups, while formaldehyde condenses with phenols under base catalysis in a reaction which has itself caused many an industrial runaway and explosion. There is thus a choice of routes to disaster. Industrial scale nucleophilic substitution on chloro-nitroaromatics has previously demonstrated considerable hazard in presence of water or hydroxide, even in solvents not themselves prone to exothermic decomposition [2],... 

Experimental evidence that supports HNO as the sole initial reactive species from decomposition of donors under biological conditions include the pH independence of dihydrorhodamine (DHR) oxidation (fluorescent assay) and ferricytochrome c (ferricyt c) reduction (colorimetric assay) (147, 167). Oxidant formation or substrate reduction would be expected to be mediated by NO-rather than HNO due to the rapid autoxidation of NO- to ONOO- (Eq. 9) and the relative reduction potentials. The slow deprotonation of HNO to NO- has been shown to proceed through base catalysis [5 x 104AT1 s-1[OH-] (106)],... 

Index Entries Reaction kinetics glucose decomposition dilute acid hydrolysis kinetic modeling acid-soluble lignin acid-base catalysis rules. 

The most widely accepted mechanism for electrophilic aromatic substitution involves a change from sp2 to sps hybridization of the carbon under attack, with formation of a species (the Wheland or a complex) which is a real intermediate, i.e., a minimum in the energy-reaction coordinate diagram. In most of cases the rate-determining step is the formation of the a intermediate in other cases, depending on the structure of the substrate, the nature of the electrophile, and the reaction conditions, the decomposition of such an intermediate is kinetically significant. In such cases a positive primary kinetic isotope effect and a base catalysis are expected (as Melander43 first pointed out). 

The dependence of the rate upon the inverse of the hydrogen ion concentration (base-catalysis) is reasonably attributed to the necessity for the coordinated water molecule to lose a proton. The resulting ethyl-ene-hydroxypalladium species (the cis isomer), I, is then believed to undergo an internal addition reaction of the hydroxyl group to the coordinated ethylene to form the dichloro-2-hydroxyethylpalladium anion, II. The final step is a decomposition of the last compound into acetaldehyde, palladium metal, hydrogen ion and chloride anions. 

In conclusion, it now appears that the cause of base catalysis in the aminolysis of aromatic substrates may not be of common origin. Further investigations of the mode of decomposition of these zwitterionic intermediates and the analogous o-complexes are clearly desirable. DMSO will remain central to these studies (Fendler et al., 1975b), due to the possibility of isolation of such complexes in this medium. It now appears that DMSO may also be valuable for studies of electrophilic catalysis, since its use tends to avoid ion pairing effects which complicate kinetic studies in aprotic media of lower dielectric constant. The problem of ion pairing is considered further in the following section. 

The mechanism of the formation of compound 1137 appears to be two sequential [4+2] cycloadditions between the exocyclic diene of compounds 1139 and 1141 and a dienophile (Scheme 223). The 2,3 d ethylenepyrrole required for the Diels-Alder reaction can be generated by the thermal elimination of acetic acid to form compound 1139, which is observed by mass spectroscopy. There are two possible pathway by which diene 1139 can proceed to tricycle 1137. The first is the elimination of a second molecule of acetic acid from diene 1139 to form 5-benzyl-aza[5]radialene 1140, which is also observed by mass spectroscopy. Attempts to improve the yield of compound 1137 by accelerating the elimination of acetic acid by acid or base catalysis failed, resulting in the decomposition of compound 1136 < 20000L73>. 

Fia. XVI. 1. Bronsted plot for the base-catalyzed decomposition of nitramide. (Data from compilation by R. P. Bell, Acid-Base Catalysis, Oxford University Press, New York, 1941.) + Doubly charged bases, B++, mostly aquo-iona, [M(H20) (0I1)]++. o Doubly charged bases, B". o- Uncharged bases, B° all substituted anilines. 

Exchange of hydrogen in alkaline solution was reported by Franke and M5nch and subsequently confirmed but attributed to exchange of phosphite formed by alkaline decomposition. A study by proton nmr methods reports simple first-order dependence on the concentrations of HjPOJ and OD. At 25°C and H = 2.0, the rate parameters are k = (6.0+0.7)10 l.mole . sec ( a = 18.9 0.5 kcal.mole ). Reaction of D2POJ and OH has k = (1.92 0.1)10 l.mole sec ( a = 18.7 kcal.mole ). It is suggested that there may be general base catalysis, from qualitative observations of the effect of phosphate. 

Many investigations of the decomposition of hydrogen peroxide involving base catalysis have been made these have been reviewed . The extent to which the surface of glass vessels can control such rates has been emphasised . The rates of the homogeneous reaction have been measured , after scrupulous attention to purification. Then the rate equation is... 

For ethane-1,2-diol the diester C2 is in equilibrium with the reactants, and its decomposition to the reaction products is rate limiting and not subject to acid-base catalysis. When the concentrations employed are such that C2 is present in appreciable concentration, the mixed-order kinetics described in section 1.3.2 are observed. Second-order kinetics (for the overall reaction) can arise in three ways (a) C2 in equilibrium, but its concentration negligible, (b) formation of C2 from Ci rate limiting, and the latter in equilibrium with the reactants but present in very low concentration, and (c) formation of Q rate-limiting. For pinacol in the range pH 2-10 alternative (a) cannot be operative because general acid-base catalysis is observed. The most likely step to be subject to catalysis is the formation of C2 from Cl, i.e. alternative (b), because this is a cyclisation and a base (B) could well facilitate reaction by removal of the C-OH proton, viz. 

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