Esters acid-catalysed

CF3CO2H. Colourless liquid, b.p. 72-5 C, fumes in air. Trifluoroacetic acid is the most important halogen-substituted acetic acid. It is a very strong acid (pK = o y) and used extensively for acid catalysed reactions, especially ester cleavage in peptide synthesis. 

The kinetics of the sulphuric acid-catalysed decarboxylation of a range of alkyl substituted benzoic acids have been measured by Schubert et a/.634,635. The variation of rate coefficient with temperature for mesitoic acid is given in Table 206 and the value for the methyl ester shows that, at this acid concentration, the... 

Carbene complexes which have an all-carbon tether between the diene and the dienophile react via intramolecular Diels-Alder reaction to give the corresponding bicyclic compound. The stereoselectivities of these reactions are comparable to those observed for the Lewis acid-catalysed reactions of the corresponding methyl esters and much higher than those of the thermal reactions of the methyl esters which are completely unselective. Moreover, the ris-sub-stituted complexes undergo endo-selective reactions where the corresponding reaction of the ester fails [109] (Scheme 61). 

The reagent (1) is easily made from thiol (3) and ethylene oxide or 2-chloroethanol (2). The acid RCOgH is then protected by acid-catalysed ester (4) formation. 

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... 

Wittig olefin syntheses have also been used in syntheses of ( I-A -IS-dehydroprostaglandin Ej and of ( )-prostaglandin Ej. In the course of the latter, coupling of the salt (142) with methyl 6-formylheptanoate followed by acid-catalysed isomerization gave the ester (143). 

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... 

There is indeed yet another pathway for acid-catalysed ester... 

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... 

The residue from a large scale atmospheric pressure distillation of trimethyl phosphate exploded violently. This was attributed to rapid decomposition of the ester, catalysed by the acidic degradation products, with evolution of gaseous hydrocarbons. It is recommended that only small batches of alkyl phosphates should be vacuum distilled and in presence of magnesium oxide to neutralise any acid by-products, and to suppress the acid catalysed reaction. 

The ester is liable to explode during distillation unless thoroughly dry allyl alcohol is used for the preparation, and if more than 60% of the product is distilled over. Acid-catalysed polymerisation is probably involved. 

The octalinyl ester 166 is produced in excellent yield when the butenylcyclohexenol 165 is treated with formic acid at room temperature (equation 80)89. The dimethyl analogue 167 reacts similarly to give 168 (equation 81)90. The trifluoroacetic acid-catalysed ring-closure of the ketene thioacetal 169 to give a 1 2 mixture of the cis- and trans-ketones 170 and 171 (equation 82) has been reported91. 

Example The addition of a few drops of concentrated sulphuric acid catalyses the reaction between an alcohol and a carboxylic acid to form an ester... 

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... 

Aldehyde 54 and the hydroxamic acids 55 were generated together in an acid-catalysed elimination reaction (Scheme 7 pathway (ii)). A crossover experiment indicated that esters are formed in a concerted rearrangement concomitant with the likely formation of the hydroxynitrene 57 (Scheme 7 pathway (iii)) while there is no evidence to date for the formation of hydroxynitrene, joint solvolysis of equimolar quantities of /V-acetoxy-/V-butoxy-/>-chlorobenzamide 26e and N- acetoxy-/V-benzyloxybenzamide 27a afforded significant quantities of butyl p-chlorobenzo-ate (36%) and benzyl benzoate (54%) as the only esters. This is an example of a HERON reaction, which has been identified in these laboratories as a characteristic rearrangement of bisheteroatom-substituted amides.32,33,42 43 155 158 Since ester formation was shown to prevail in neutral or low acid concentrations, it could involve the conjugate anion of the hydroxamic acid (vide infra).158... 

In reactivity, the same series of mutagens exhibited positive Hammett correlations for AaiI acid catalysed solvolysis (p — + 0.32) and SN2 reactivity with A-methyl-aniline (p — + 1.7), hydroxide (p — +0.55) and L-cysteine ethyl ester (p — + 1.1), all reactions in which the benzoyloxyl group leaves with electrons and which are therefore accelerated by electron-withdrawing groups. The negative Hammet p-value for... 

The final part of the ionogenic reaction is of first order with respect to the ionogenic species (Figure 7) and the rate constant kx is proportional to [HClO4]0 (Figure 8). This may indicate that when the concentration of ester becomes small, and that of free acid large, the dominant ionogenic reaction is the acid-catalysed ionisation of the ester. 

Autocatalysis happens when a reaction product, formed during reaction, acts as a catalyst which accelerates the progress of the reaction even at constant temperature. An example is the acid-catalysed saponification of various esters and related compounds. Autocatalytic reactions can be easily experimentally identified by means of differential thermal analysis methods. 

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)... 

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