Hydrolysis acid-catalysed

A further observation is the fact that differences in rates of nitration between the reagents prepared at different temperatures tended to zero as the water concentration of the added nitric acid was decreased to zero73. It has been argued that, since the acid-catalysed hydrolysis of acetic anhydride must be very rapid at 25 °C and removes water which initially competes with acetic anhydride and acetyl nitrate for protons, this removal permits equilibria (30) and (31) to be displaced towards products. The more anhydrous the nitric acid, the less important is this initial hydrolysis of the acetic anhydride and so the difference in the nitrating power of the differently prepared mixtures becomes less. When reagents are mixed at low temperatures, the hydrolysis of the anhydride is very slow, but once this is accomplished, formation of the protonated acetyl nitrate and subsequent nitration is rapid as observed73. 

Polyamide 6 depolymerisation was performed using microwaves as the energy souree for the acid catalysed hydrolysis, with phosphorie aeid as the catalyst. The product mixture was analysed by ehromatographie and spectroscopic methods. 19 refs. 

The chromic acid oxidation of dithionic acid is independent of oxidant concentration and its rate is equal to that of the acid-catalysed hydrolysis to sulphite and sulphate, which must therefore constitute the rate-determining process . ... 

The basicities of some phosphinamides (84) have been measured and the acid-catalysed hydrolysis studied. Unsubstituted and A -alkyl derivatives follow an A2 mechanism of reversible protonation followed by ratedetermining water attack. However, the rates for the A -aryl derivatives follow Hq (but with a slope of 0.5), and an A mechanism was suggested as most consistent with this fact and the solvent isotope effect. The anomalous dependence on Ho, together with the large negative value of A5, while not necessarily excluding an ionization mechanism, leaves the question in some doubt. 

The p value for base-catalysed hydrolysis (+2-51) is +ve and quite large, reflecting the development of not inconsiderable -ve charge at the reaction centre in the rate-limiting step—attack on this centre by OH (step CD in the BAC2 pathway). By contrast, the p value for acid-catalysed hydrolysis (+0-03) is very nearly zero which means, of course, that the rate of this hydrolysis does not vary significantly from one ester to another, no matter what the m- or... 

T.S. for base-catalysed T.S. for acid-catalysed hydrolysis (BAC2) hydrolysis (Aac2)... 

Thus we have already seen (p. 384) that the acid-catalysed hydrolysis of m- and p-substituted benzoate esters (42) is (with a p value of 0 03) essentially uninfluenced by any polar effect exerted by the substituent, X and this substituent is sufficiently far removed from the reaction centre to be clearly incapable of exerting any... 

Now that steric parameters have been introduced in this way, the treatment can be extended to include the reactions of o-substituted benzene derivatives as well. Thus for the acid-catalysed hydrolysis of o-substituted benzamides (43), the value of 8 is found to be 0-81 so this reaction is apparently slightly less susceptible to the steric... 

The principal reaction hazard attached to use of acetic anhydride is the possibility of rapid and exothermic acid-catalysed hydrolysis unless the conditions prevailing (temperature, agitation, order of mixing, proportion of water) are such as to promote smooth and progressive hydrolysis with adequate heat removal. The examples below illustrate these factors. 

Aryloxy-l,2-diarylethanones can be cyclodehydrated to diarylbenzofurans by heating with sodium acetate and acetic anhydride in polyphosphoric acid. Quenching the hot reaction mixture with water leads to initially violent acid-catalysed hydrolysis of the excess anhydride. 

Accidental slow addition of water to a mixture of the anhydride and acetic acid (85 15) led to a violent, large scale explosion. This was simulated closely in the laboratory, again in the absence of mineral-acid catalyst [1]. If unmoderated, the rate of acid-catalysed hydrolysis of (water insoluble) acetic anhydride can accelerate to explosive boiling [2], Essentially the same accident, fortunately with no injuries or fatalities this time, was repeated in 1990. 

Further evidence for the Aa11 mechanism was obtained from a solvent kinetic isotope study. The theoretical kinetic isotope effects for intermediates in the three reaction pathways as derived from fractionation factors are indicated in parentheses in Scheme 6.143,144 For the Aa11 mechanism (pathway (iii)) a solvent KIE (/ch2o A d2o) between 0.48 and 0.33 is predicted while both bimolecular processes (pathways (i) and (ii)) would have greater values of between 0.48 and 0.69. Acid-catalysed hydrolysis of ethylene oxide derivatives and acetals, which follow an A1 mechanism, display KIEs in the region of 0.5 or less while normal acid-catalysed ester hydrolyses (AAc2 mechanism) have values between 0.6 and 0.7.145,146... 

The source of alcohol 50 is most probably acid-catalysed hydrolysis of 49 to the nitrosocarbonylbenzene intermediates 51, which, like acid chlorides, react with water to give benzoic acids 52 (Scheme 1 pathway (i)).159 Acylnitroso intermediates 51 were trapped as the Diels Alder adducts 53 in reactions in CH3CN/H20 and in the presence of cyclopentadiene. In CH3CN/10% H280, 53 was enriched in lsO... 

Figure 5.16 Formation of emulsion droplets, (a) Aqueous MPS solution after acid-catalysed hydrolysis and condensation, (b) Micrometre-sized emulsion droplets are rapidly formed upon addition of the base catalyst triethanolamine. (Reproduced from ref. 28, with permission.)... 

The microspheres—synthesised via a two-step process (acid-catalysed hydrolysis and condensation of 3-mercaptopropyltrimethoxysilane (MPS) in aqueous solution, followed by condensation catalysed by triethanolamine)—have a narrow size distribution (Figure 5.16) and are considerably more stable than polystyrene divinylbenzene microspheres as shown in phosphoramidite oligonucleotide synthesis by the excellent retention of fluorescence intensity in each of the reagent steps involved in phosphoramidite DNA synthesis (Figure 5.17, in which the organo-silica microsphere free thiol groups are derivatized with ATTO 550 maleimide coupled to the entrapped dye). 

When a product, formed in a second order reaction, acts as a catalyst or effects the rate of reaction, the reaction is known as autocatalytic reaction. For example, the acid catalysed hydrolysis of various esters and similar compounds and various biochemical processes. 

Smith, 1976. Reference reaction is acetic acid catalysed hydrolysis of benzimidazole. Smith calculates EM = 8 x 104 from this comparison. Correction for the (estimated) lower pAT. of the neighbouring carboxyl using p= 1.7 (Oakenfull and Jencks, 1971) raises this to 4 x 10 M... 

Buffet and Lamaty, 1976. The pA. -values quoted are for the corresponding methoxymethoxy-benzoic acids. The reference intramolecular reaction is the acetic acid catalysed hydrolysis of the methyl ester in each case corrected for differences in pKt using a= 0.5 (Capon and Nimmo, 1975)... 

Gold and Sghibartz (1978) have studied the effect of cations on the acid-catalysed hydrolysis of crown ether acetals such as [205]. For small ring... 

The acid-catalysed hydrolysis of phosphinic esters (158) is relatively insensitive to the substituent (R = Me, Et, Pr1, or Bu1) attached to phosphorus this contrasts with the base-catalysed process.126 The acid hydrolysis of the phosphinic esters (159) (see Organophosphorus Chemistry , Vol. 7, p. 129 for the base hydrolysis) takes place with alkyl-oxygen fission in an SnI process.127... 

Acid catalysed hydrolysis of isocyanides ( carbylamines ) to primary amines and formic acid is very rapid, sometimes explosively so. 

Trimethylsilylation of enolizable carbonyl compounds and alcohols has also been accomplished by the fluoride ion promoted reaction with hexamethyldisilane and ethyl trimethylsilylacetate [48, 49], with high stereospecificity giving Z-enol ethers from ketones [50]. l-Trimethylsilyl-(l-trimethylsilyloxy)alkanes, produced from the reaction of aldehydes with hexamethyldisilane, undergo acid-catalysed hydrolysis during work up to yield the trimethylsilylcarbinols [51]. In the case of aryl aldehydes, the initially formed trimethylsiloxy carbanion produces the pinacol (Scheme 3.1). 

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