Acetal 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. 

This is known as specific add catalysis, specific in that H30 is the only acidic species that catalyses the reaction the reaction rate is found to be unaffected by the addition of other potential proton donors (acids) such as NH4 , provided that [H3Offi], i.e. pH, is not changed, indirectly, by their addition. The mechanism of the above acetal hydrolysis is believed to be,... 

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. 

Addition of acetic anhydride to a solution of chromium trioxide in water caused violent boiling [1], due to the acid-catalysed exothermic hydrolysis of the anhydride [2],... 

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... 

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... 

Prior to 1967 acetal hydrolysis had been found to be a specific-acid catalysed reaction with the accepted mechanism [equation (46)] involving fast pre-equilibrium protonation of the acetal by hydronium ion, followed by unimolecular rate-determining decomposition of the protonated intermediate to an alcohol and a resonance stabilized carbonium ion (Cordes, 1967). An A-1 mechanism was supported by an extremely large body of evidence, but it appeared unlikely that such a mechanism could expledn the... 

Figure 4. Free energy-reaction co-ordinate diagrams for acid-catalysed hydrolysis of simple acetals...

The main cyanogenic glycoside in laurel is prunasin, the P-o-glucoside of benzaldehyde cyanohydrin. The enzymic hydrolysis of prunasin may be visualized as an acid-catalysed process, first of all hydrolysing the acetal linkage to produce glucose and the cyanohydrin. Further hydrolysis results in reversal of cyanohydrin formation, giving HCN and benzaldehyde. 

It should also be noted that hydrolysis of glycosides (acetals or ketals) will occur under acid-catalysed conditions if we have an excess of water present. This is a reversal of the process for glycoside... 

Specific rate coefficients (related to unit amount of acid centres) were approximately the same for solid catalysts as well as for HC1 [474]. However, when a montmorillonite clay activated by adsorption of protons on its surface was used as the catalyst in ethyl acetate hydrolysis [475], a higher specific rate coefficient (about 1.8 times at 25°C) was found for the reaction catalysed by adsorbed protons than by dissolved acid, this result being explained by the authors by an increase of activation entropy in the former case. 

A re-investigation of the mechanism of the hydrolysis of acetic anhydrides in water was made by Bunton and Fendler4. They found that the hydrolysis of trimethylacetic anhydride follows an A-2 mechanism both in water and in aqueous dioxan and is slower than that of acetic anhydride. If acetic anhydride follows an A-l mechanism in water, both anhydrides should have the same mechanism and both should have similar reactivities in aqueous acids. If the A-2 mechanism is followed, steric effects should make trimethylacetic anhydride the less reactive compound. The entropy of activation of the hydrolysis catalysed by perchloric acid was re-estimated, taking into account the electrolyte effect of the perchlorate ions upon the rate of spontaneous hydrolysis, and a new value for A5 of—35 eu (for the spontaneous hydrolysis) and — 10 eu at 2 M HC104 for the acid-catalysed hydrolysis was obtained, compared with the value of + 2.2eu obtained previously. The new value is in the order of magnitude of A 5 for A-2 reactions but smaller than that observed for trimethylacetic anhydride under similar acidic conditions (— 26 eu). Plots... 

A quantitative assessment of the effects of head group bulk on, S k2 and E2 reactions in cationic micelles has been made.148 The kinetics of the acid-catalysed hydrolysis of methyl acetate in the presence of cationic, anionic, and non-ionic surfactants has been reported on.149 The alkaline hydrolysis of -butyl acetate with cetyltrimethylammonium bromide has also been investigated.150 The alkaline hydrolysis of aromatic and aliphatic ethyl esters in anionic and non-ionic surfactants has been studied.151 Specific salting-in effects that lead to striking substrate selectivity were observed for the hydrolysis of /j-nitrophenyl alkanoates (185 n = 2-16) catalysed by the 4-(dialkylamino)pyridine-fimctionalized polymer (186) in aqueous Tris buffer solution at pH 8 and 30 °C. The formation of a reactive catalyst-substrate complex, (185)-(186), seems to be promoted by the presence of tris(hydroxymethyl)methylammonium ion.152... 

Scheme 1.9 Acid-catalysed hydrolysis of acetic anhydride.

Formulating credible mechanisms, i.e. accounting for all the electron movements and bonding changes in an acid- or base-catalysed reaction, is a difficult task although the framework on which the final mechanism is based is relatively easy to write. For this reason, and in the absence of evidence suggesting otherwise, a mechanism is conventionally illustrated as a series of steps as this enables the electron accounting to be most easily determined. This is illustrated by the mechanism for acid-catalysed acetal hydrolysis (Scheme 11.1a) shown... 

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