Ester hydrolysis acid-catalyse

Let us first remind ourselves of the two mechanisms for hydrolysis of an ester, namely acid-catalysed hydrolysis and base-catalysed hydrolysis (see Section 7.9.2). 

The first example is an esterification (or lactonization as the product is a cyclic ester or lactone cr the forwards direction and an ester hydrolysis in the backwards direction. Ester hydrolysis a catalysed by acid or base but esterification by acid only. In addition, even a weak base will be enough to turn the starting material into the carboxylate anion, which will not cyclize. The equilibrium is to the right in acid solution and to the left in basic solution. 

A. Pentoses.—t-Ascorbic acid 2- and 3-phosphates, together with their phosphate esters, give a characteristic colour with ferric chloride and this colour reaction has been used in a study of the hydrolysis of L-ascorbic acid 3-phosphate (58). The acid-catalysed, pseudo-firsi-order hydrolysis proceeds with P—O bond fission, as does the bromine oxidation of its phenyl ester. Both of these observations can be rationalized if (58) is... 

Considering first the simpler of the two cases, the straight line for the methyl esters (34a) which has a p value of -3-25. From this p value it is apparent that this reaction cannot be proceeding via the normal (Aac2) pathway (p. 241) for acid-catalysed ester hydrolysis which, as we know (reaction 6, p. 364), has a p value of +0-03. That value refers, however, to hydrolysis being carried out with dilute sulphuric acid, while here 99-9% sulphuric acid is being used one... 

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... 

If we now extend our consideration of base-catalysed (BAC2), and acid-catalysed (Aac2), hydrolysis to esters in general, including aliphatic ones (RC02Et), we see that there is a close similarity between the transition states (42b or 42a) for the rate-limiting step in each of the two pathways they are both tetrahedral and differ... 

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... 

Scheme 6 depicts three possible hydrolysis mechanisms.140 The first (pathway (i)) is normal acid-catalysed ester hydrolysis in which attack of solvent (H20) upon the protonated intermediate is rate determining. The second (pathway (ii)) is the... 

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... 

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. 

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)... 

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... 

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