2.4- Dinitrophenyl acetate hydrolysis

Artificial catalysts have also been incorporated into amphiphilic structures (Guler and Stupp, 2007). These catalysts were imidazolyl-functiona-lized peptides, which demonstrate a greater rate of 2,4-dinitrophenyl acetate hydrolysis when immobilized on the peptide amphiphile than the rate observed when the same enzyme is present in solution. Although the density of the enzymes on the fiber surface has not been established, the authors attribute the increase in enzymatic activity to the likely concentration of enzyme along the fiber surface, and this study illustrates one of the advantages of enzyme immobilization. 

The kinetics of 2,4-dinitrophenyl-acetate hydrolysis catalyzed by polymers containing imidazole, carboxylic acid, oxidation groups and their complexes with surfactants, such as 1-cetylpyridinium chloride and cetylundecyldimethylammonium bromide, was determined by spectrophotometry [57]. Catalytic rate constants of the second-order-rate increase with a rise in the surfactant concentration until they reach a plateau at a polymer/surfactant ratio of 1 6. Anionic surfactant does not accelerate the polymer-catalyzed hydrolysis. The catalytic mechanism of a polymer/surfactant complex enables the penetration of the substrate into a pseudophase of a soluble complex. This leads to an increase of the ester concentration in the neighbourhood of a polymer imidazole fragment and accelerates the process. Such a pseudophase promotes the protonation of imidazole rings. 

The effect of mixtures of surfactants and polyelectrolytes on spontaneous, water-catalysed hydrolysis (Fadnavis and Engberts, 1982) was mentioned in Section 4, but mixtures of functionalized polyelectrolytes and cationic surfacants are effective deacylating agents (Visser et al., 1983). Polymerized isocyanides were functionalized with an imidazole group and the deacylation of 2,4-dinitrophenyl acetate in the polyelectrolyte was speeded by addition of single or twin chain quaternary ammonium ion surfactants, up to a plateau value. Anionic surfactants had essentially no effect. It is probable that the cationic surfactants accelerate the reaction by increasing the deprotonation of the imidazole groups. 

Reference reaction is attack on 2,4-dinitrophenyl acetate by RCOO- of pK, 2.4 (k2 = 3.3 x 10-3 s-1 based on a short extrapolation using P = 1.0 Jencks and Gilchrist, 1968). The reaction measured is the subsequent hydrolysis of the mixed anhydride the observed value thus sets only a lower limit for EM (Fersht and Kirby, 1967b, 1968a)... 

Fig. 3. A plot of the observed rate constant versus a for the hydrolysis of 3-nitro-4-acetoxy-benzene sulfonate in the presence of (1) 0.016 Af 4-methylpyridine (control) and (2) poly(4-vinyl-pyridine) with 0.01 M pyridine units. Line (3) is a calculated line projected from the pH dependence of the hydrolysis of a neutral substrate, dinitrophenyl acetate. From Letsinger and Savereide (55).

Consider the hydrolysis of 2,4-dinitrophenyl acetate (DNPA), a compound for which the acid-catalyzed reaction is unimportant at pH > 2 (see Fig. 13.8). In a laboratory class, the time course of the change in concentration of DNPA in homoge-... 

Buckingham, D. A., and C. R. Clark, Metal-hydroxyde-promoted hydrolysis of activated esters. Hydrolysis of 2,4-dinitrophenyl acetate and 4-nitrophenyl acetate , Aust. J. Chem., 35,431-436 (1982). 

Fig. 13. Approximate pH-rate profiles for hydrolysis of A, ethyl acetate B, phenyl acetate and C, 2,4-dinitrophenyl acetate, all at 25°C. Data from Skrabal188, Kirsch and Jencks300 and Tommila...

This sensitivity to substitution of neutral hydrolysis means that the pH-independent reaction gradually becomes more important than the hydroxide reaction at the high pH end of the region, and becomes much more rapidly more important than acid-catalyzed hydrolysis at low pH. Thus from Fig. 13, the acid-catalyzed reaction can be seen to be significant for the hydrolysis of ethyl acetate between pH 4 and 5, and for phenyl acetate about pH 2 but for 2,4-dinitrophenyl acetate the acid-catalyzed reaction is not detectable at pH 1, and is presumably important only in relatively strong acid. It seems certain that this fast neutral hydrolysis is at any rate a partial explanation for the low efficiency of acid catalysis in the hydrolysis of very weakly basic esters, such as the trifluoroacetates and oxalates, in moderately concentrated acid (see p. 145). 

A second approach demonstrating the existence of the intermediate anhydride is exemplified by the aniline trapping technique of Gold el al.2si. Low concentrations of added amines, particularly aniline, react much more rapidly with the anhydride than does the solvent. Thus, if the hydrolysis of, for example, 2,4-dinitrophenyl acetate catalyzed by acetate ion, is carried out in the presence of low concentrations of aniline, acetanilide is formed281, viz. 

PY-PY cooperation. In the study of the hydrolysis of the neutral substrate, 2,4-dinitrophenyl acetate, by poly(4-vinylpyridine), the hydrolysis rate profile shows an upward curvature as the fraction of the neutral pyridine residue increases (41). 

Second-Order Rate Coefficients" for Hydrolysis of 2,4-Dinitrophenyl Acetate in Water1 "... 

The [Co(NH3)50H] -promoted hydrolysis (and that promoted by other exchange-labile anc non-labile metal hydroxide species) of 2,4-dinitrophenyl acetate and 4-nitrophenyl acetate haf been studied in some detail." Kinetic studies (25 °C, / = 1.0 M) show that these reactions follov shallow Bronsted slopes (/3 = 0.33 and 0.40) for the two substrates which extend over a range o lO " in nucleophile basicity. A correlation is reported which allows the prediction of reactioi... 

Figure 3. Kinetic plots of log kohs versus pH for the base-catalyzed hydrolysis of p-nitrophenyl acetate (NPA), dinitrophenyl acetate (DNPA), and trinitrophenyl acetate (TNPA).

Figure 4. A linear free-energy relationship between the logarithm of rate constant for metal-catalyzed hydrolysis log fcMeOK) of p-nitrophenyl acetate (NPA), dinitrophenyl acetate (DNPA), and trinitrophenyl acetate (TNPA) and the hydrolysis constant (MeH2O MeOH + H+) for the formation of the corresponding hydroxy complex. This relationship provides support for the argument that the MeOH species is involved in a direct nucleophilic attack on the carbonyl carbon of the nitrophenyl acetate esters.

Bovine erythrocyte CA shows both oxonase and esterase activity with dimethyl 2,4-dinitrophenyl phosphate and 2,4-dinitrophenyl acetate respectively. Both processes show similar pH dependencies with pJTa values of 7.37 (oxonase) and 7.53 (esterase) and maximum activity in the basic form. Inhibition by acetazol-amide at pH 8.95 yields the same inhibition constant for both reactions, implying involvement of the active-site zinc to the same extent in both processes. Methyl and ethyl pyruvate are susceptible to hydrolysis,... 

The imidazole-catalysed hydrolysis of polar substituted 2,4-dinitrophenyl acetates (21 X = Cl, OMe) has been investigated at different temperatures. The observed rates correspond to the bimolecular nucleophilic addition of the imidazole at the carboxylic carbon atom followed by a very fast hydrolysis of the (V-acetylimidazole in water. The influence of polar substituents in the acid moiety of the ester molecule on the hydrolysis reaction can be described by an electrostatic dipole-dipole interaction in the same way as the neutral hydrolysis of polar substituted ethyl acetates. By the use of both quantum and classical dynamics, a study of the neutral hydrolysis of 4-methoxyphenyl dichloroacetate (22) in water concluded that the rate-determining step is a proton transfer concerted with formation of a C-O bond. ... 

Product analysis Product identity or distribution attributable only to reaction via intermediate formed by nucleophilic attack I "0]Acetate-catalyzed hydrolysis of 2,4-dinitrophenyl benzoate gives C 0]-benzoic acid 86... 

The reaction of oarboxylate with various substrates is an example of a system which necessitates a careful search for products. Early examples of these reactions demonstrated that direct nucleophilic attack takes place. For example, using a dilatometric method, formate was found to catalyze strongly the hydrolysis of acetic anhydride whereas propionate and butyrate slow down the reaction (Kilpatrick, 1928). These results are attributable to mixed anhydride formation with the total rate being determined by the reactivity of the mixed anhydride. The reaction of acetate with 2,4-dinitrophenyl benzoate results in the formation of an unstable mixed anhydride, as was shown in an experiment in which 0 labeled acetate was used the benzoic acid product contains 75% of the O label (Bender and Neveu, 1958). The other 25 % of the O label is presumed to be lost to acetate by solvent attack at the benzoyl carbon if direct nucleophilic interaction is the sole path. 

Reductive cleavage of 2,4-dinitrophenylhydrazones. Certain 2,4-dinitrophenyl> hydrazones of 3-ketosteroids can be cleaved under mild conditions if the nitro groups are reduced to amino groups by chromous chloride. Thus when the 2,4-DNP of 4,5a-dihydrocortisone 21-acetate in methylene chloride was shaken in a separatory funnel with chromous chloride and dilute hydrochloric acid for 15 min., workup of the organic layer afforded the ketone in 94% yield. Use of a water-immiscible solvent protects the released ketone an inert atmosphere if required can be provided with dry ice. Cortisone 2,4-DNP was cleaved in only 3-5 hrs. and the yield was 60%. DN P derivatives of A -diene-S-ones were completely resistant to hydrolysis. Contributed by Leon Mandell... 

Experimental evidence for such nucleophilic catalysis is sununarized in Figure 7.27. Hydrolysis of 2,4-dinitrophenyl benzoate catalyzed by acetate ion labeled with on both oxygen atoms led to product in which both benzoic acid and acetic acid were labeled with 0, but about 75% of the label derived from one 0 in the reactant was found in the benzoate ion. The data were interpreted as evidence for the intermediacy of the mixed anhydride CH3CO2COC6H5. 

It has to be pointed out that, with a few exceptions, the acceleration by polyelectrolytes was associated with decreases and Table IV gives the thermodynamic parameters for the aquation reactions of Co(NH3)5Br induced by Ag". Similar decreases in and AS were found for various reactions the Hg -induced aquation of Co(NH3)5Br , the SpjAr reaction of dinitrochlorobenzoic acid with OH [51], the hydrolysis of 2,4-dinitrophenyl phosphates [reaction (E)] [33], the outer-sphere electron-transfers between Co-complexes [Co(NH3)5N3, Co(NH3)5Br , Co(en)2Cl2 ] and Ru(NH3)6 or [8, 20] [en ethylenediamine], the polyvinyl-imidazole-accelerated solvolysis of p-nitrophenylacetate [52], the coupling reactions of dinitrofluorobenzene with aminoacids [53], dipeptides [53] and aniline [54], the lignin sulfonic acid-accelerated hydrolysis of methyl acetate [55], and the hydrolysis of nitrophenyl esters [37]. The opposite tendency (acceleration caused by increases... 

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