Ethyl nitrobenzoates, hydrolysis

These are pseudo-first-order rate constants for the alkaline hydrolysis of ethyl / -nitrobenzoate at 25°C. 

The results of the alkaline hydrolysis of ethyl nitrobenzoate at various times are in the table. Check first and second order mechanisms. 

Hammett (1937) observed that the value of the constant op for a nitro group as obtained from the acidity constant of 4-nitrobenzoic acid (crp = 0.78) was applicable, for example, to the rate of hydrolysis of ethyl 4-nitrobenzoate, but not to the acidity constants of 4-nitrophenol and 4-nitroanilinium ion. These two acidities were compatible only with significantly higher constants denoted by op (1.21 and 1.27, respectively). This was (correctly) interpreted as resulting from the donor-acceptor interaction represented by the mesomeric structures 7.7. 

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]

Esters show similar behaviour. Hantzsch found the /-factor for ethyl acetate to be close to 2, and accurate determinations by Leisten also gave values close to 2 for ethyl and methyl benzoate and p-nitrobenzoate. In a solvent containing sufficient water, hydrolysis of the ester occurs. The reaction of methyl benzoate has a time of half-change of a few hours at 25°C in sulphuric acid containing about 0.07 M water, and Leisten actually used the increase in the / -factor, from 2 to 3, to follow the hydrolysis reaction, viz. 

This behaviour is that expected if the positive effect of the dipolar aprotic solvent on the activity of the hydroxide ion is superimposed on the normal negative effect of an organic solvent, and should be observed also for other dipolar aprotic solvents. In fact this type of behaviour is found for the alkaline hydrolysis of ethyl p-nitrobenzoate in mixtures of water and acetone, which is less polar than dimethyl sulphoxide. Other esters show intermediate behaviour in this solvent259 (Fig. 16). It should be stressed that this represents special... 

Fig. 16. Effects of added acetone on the alkaline hydrolysis of substituted ethyl benzoates. Data for ethyl p nitrobenzoate ( ) and benzoate ( ) at 25°C (from ref. 259). The central curve...

If we compare the acid strengths Ka) of a series of substituted benzoic acids with the strength of benzoic acid itself (Table 26-4), we see that there are considerable variations with the nature of the substituent and its ring position, ortho, meta, or para. Thus all three nitrobenzoic acids are appreciably stronger than benzoic acid in the order ortho para > meta. A methoxy substituent in the ortho or meta position has a smaller acid-strengthening effect, and in the para position decreases the acid strength relative to benzoic acid. Rate effects also are produced by different substituents, as is evident from the data in Table 26-5 for basic hydrolysis of some substituted ethyl benzoates. A nitro substituent increases the rate, whereas methyl and methoxy substituents decrease the rate relative to that of the unsubstituted ester. 

In contrast, the slower rate of alkaline hydrolysis of ethyl 2-nitrobenzoate than of its 3- and 4-isomers is more likely due to a steric hindrance effect of the 2-nitro group (see Table 26-5) ... 

Interest in the kinetics of alkaline hydrolysis of esters in DMSO + water mixtures was stimulated by the observation that the rate constant often increased gradually as x2 increased. This is observed, for example, in the alkaline hydrolysis of ethyl acetate. For higher esters, e.g. ethyl p-nitrobenzoate, the rate constant drops slightly at low x2 but then rises again until k/k x2 = 0) > 1 (Tommila, 1964). The rate of alkaline hydrolysis of esters of benzoic acid is accelerated when DMSO is added (Tommila and Palenius, 1963), as also is the rate of alkaline hydrolysis of 2,4-dinitrofluorobenzene. In the latter case the effect is less dramatic because the rate constant for spontaneous hydrolysis also increases (Murto and Hiiro, 1964). The rate constants also increase when DMSO is added to aqueous solution for reactions between hydroxide ions and benzyl chloride (Tommila... 

Connors KA, Bender ML. Kinetics of alkaline hydrolysis and 39. N-Butylaminolysis of Ethyl p-Nitrobenzoate and Ethyl p-Nitro-thiolbenzoate. J. Org. Chem. 1961 26 2498-2504. 

The solid-liquid alkaline hydrolysis of esters such as phenyl benzoate, ethyl p-nitrobenzoate and 2,4-dichlorophenylbenzoate has been studied in the presence of sodium p-toluene sulfonate, sodium xylene sulfonate and the sodium salts of phenol, p-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol. The solubilities of the esters in water is very low, but in the presence of the hydrotropes considerably higher reaction rates are obtained due to higher solubilities (30). The same behaviour was observed by using urea in the solid-liquid oximation of cyclododecanone where the reaction rate was increased. From studies of these types of reactions, the following observations were made (31) ... 

Figure 11.4 The effect of CTAB on the hydrolysis of four esters expressed as a ratio ( / o) of the first order rate constants obtained in the presence (/c ) and absence (/cq) of the surfactant. , EPAB A, PAPA , PNPA B, EPNB all in Delory and King s buffer O, EPAB in Sorensen s glycine buffer. The esters used were ethyl p-aminobenzoate (EPAB), p-nitrophenyl acetate (PNPA), ethyl p-nitrobenzoate (EPNB) and p-aminophenyl acetate (PAPA). From Meakin et al. [28].

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