Ester hydrolysis base catalysed

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. 

Reactivity of esters towards base-catalysed hydrolysis was studied by Stirling and coworkers. They have shown that well-packed monolayers of aliphatic esters with the carbonyl group buried deeply below the surface [HS(CH2)ioOCO(CH2)8CH3] are very resistant towards hydrolysis. At the same time, esters which have the carbonyl function close to the monolayer surface [HS(CH2)ioOCOCH3] hydrolyse more rapidly400. In such... 

Another early example of Hammett s is shown in Fig. 13.2, which represents a plot of log k for base-catalysed hydrolysis of a group of ethyl esters (3) against log K for ionisation in water of the corresponding carboxylic acids (2). Judged... 

A reason for such non-conformity on the part of o-substituted benzene, and of aliphatic, derivatives is not far to seek. Thus for the base-catalysed hydrolysis (p. 238) of the esters (3) in Figs 13.2 and... 

This is borne out by a comparison of the rates of base-catalysed hydrolysis (cf. p. 238) of m-N02 (5), and of m-Me (6), substituted ethyl benzoates with that of the unsubstituted ester a reaction in which the slow, and hence rate-limiting, step is initial attack on the ester by eOH (p. 239) ... 

The simplest possible use that can be made of the Hammett equation is to calculate k or K for a specific reaction of a specific compound, where this information is not available in the literature, or indeed where the actual compound has not even been prepared yet. Thus it is known that the base-catalysed hydrolysis of ethyl m -nitrobenzoate is 63-5 times as fast as the hydrolysis of the corresponding unsubstituted ester under parallel conditions what then will be the comparable rate for base-catalysed hydrolysis of ethyl p-methoxybenzoate under the same conditions Looking at the table of [Pg.374]

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... 

Intramolecular general base catalysed reactions (Section II, Tables E-G) present less difficulty. A classification similar to that of Table I is used, but since the electrophilic centre of interest is always a proton substantial differences between different general bases are not expected. This section (unlike Section I, which contains exclusively unimolecular reactions) contains mostly bimolecular reactions (e.g. the hydrolysis of aspirin [4]). Where these are hydrolysis reactions, calculation of the EM still involves comparison of a first order with a second order rate constant, because the order with respect to solvent is not measurable. The intermolecular processes involved are in fact termolecular reactions (e.g. [5]), and in those cases where solvent is not involved directly in the reaction, as in the general base catalysed aminolysis of esters, the calculation of the EM requires the comparison of second and third order rate constants. 

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... 

As indicated above, the traditional base-catalysed hydrolysis of 0,5-dialkyl thio-carbonates for the synthesis of thiols is generally unsatisfactory, as oxidation leads to the formation of disulphides. Under phase-transfer conditions, the procedure produces thioethers to the virtual exclusion of the thiols, as a result of the slow release of the thiolate anions in the presence of the electrophilic ester. However, a simple modification of the reaction conditions provides an efficient one-pot reaction [50] from haloalkanes (Table 4.15) via the intermediate formation of the thermally labile (9-/ert-butyl-5-alkyl dithiocarbonates (Scheme 4.8). 

Regio-control of the formation of benzoic esters of mixed benzoin derivatives results from the initial formation of the cyanhydrin benzoate (see Chapter 3). Subsequent base-catalysed hydrolysis of the esters produces the mixed benzoins (Scheme 6.20) [66]. 

Neighbouring-group participation in the hydrolysis of esters and amides has been reviewed. The effects of urea, Na+ and Li+ on the intramolecular general-base-catalysed glycolysis of phenyl salicylate (217) in glycol-acetonitrile solvent at constant water concentration have been reported. ... 

A typical role for the histidine imidazole ring is shown below, in the enzyme mechanism for a general base-catalysed hydrolysis of an ester. The imidazole nitrogen acts as a base to remove a proton from water, generating hydroxide that attacks the carbonyl. Subsequently, the alkoxide leaving group is reprotonated by the imidazolium... 

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