Hydrolysis of phenyl acetate

Michaelis-Menten kinetics. Consider the hydrolysis of phenyl acetate catalyzed by acetyl cholinesterase,... 

Photolysis of 4- and 3-nitrophenyl acetates (176 —> 177 178 —> 179) in neutral aqueous solution leads to the corresponding phenols with quantum yields 0.002 and O.OO6105 (equation 84). A greater difference in the photoreactivity (quantum yields of 0.002 and 0.129, respectively) is shown between 2-mcthoxy-4-nitrophenyl acetate 180 and 2-methoxy-5-nitrophenyl acetate 182. The nitro substituent clearly exhibits a meta-activating effect in the hydrolysis of phenyl acetates. 

For example, the hydrolysis of phenyl acetate (7.15) by carboxylesterase isozymes was investigated over a broad pH range, allowing many insights into their catalytic mechanisms [30] (see Chapt. 3). This substrate was also used together with various inhibitors to characterize esterases in human and rat tissues [31], Thus, the approximate values of Km (in pM) and Vmax (in pmol min 1 (g tissue)-1) in human tissues were 300 and 60 in liver micro-somes, 200 and 40 in liver cytosol, and 1500 and 250 in plasma, respective-... 

Catalytic Rate Constants (kc) and Association Constants for Hydrolysis of Phenyl Acetate ... 

Aryl esters of 4-hydroxybutyric acid, 5-hydroxyvaleric acid, 2-hydroxyphenylacetic acid, and 3-(2-hydroxyphenyl)propionic acid lactonize with rate constants proportional to iopH picw (Capon et al., 1973). The second-order rate constant at 30° for lactonization of phenyl 4-hydroxybutyrate is ca. 3000 times greater than kOH for hydrolysis of phenyl acetate at 25°. Lactonization of phenyl 4-hydroxybutyrate is catalysed by acetate and phosphate buffers in... 

In other cases, the concentrations of the intermediates are much smaller. The intermediate in the acetate ion catalyzed hydrolysis of phenyl acetates is acetic anhydride. It can be chemically trapped by addition of aniline to the reacting solution, for acetanilide is formed much faster than acetic acid even at low aniline concentrations [264]. Nucleophilic catalysis is highly effective in these examples because phenoxy, substituted phenoxy and ethylthio are very good leaving groups and the intermediates are more reactive with respect to hydrolysis than the substrates. 

Figure 4.47 Energy well positions for the basic hydrolysis of phenyl acetate.

Figure 4.49 The pATgbH of the most basic species in each step of the basic hydrolysis of phenyl acetate versus reaction coordinate. The pifabH must be estimated for the tetrahedral intermediate.

The rate of hydrolysis of phenyl acetate is increased about(l 50-fold >t neutral pH by the presence of a carboxylate ion in the ortho position. The ort/io-carboxyl-substituted ester is commonly known as aspirin (Section 19.9). In the following reactions, the reactants and products are shown in the form that predominates at physiological pH (7.3). 

A simple mechanistic use of the a" correlation is the diagnosis that addition of hydroxide to the ester is the rate-limiting step in the alkaline hydrolysis of phenyl acetates. The rate constant koH correlates much better with a than with a ... 

Assume that the hydrolysis of phenyl acetate by carbonic anhydrase proceeds by a mechanism analogous to that for CO2 hydration. Describe the sequence of reactions that would be involved in the ester hydrolysis. 

Imidazole, acting as a nucleophile, catalyzes the hydrolysis of phenyl acetate by attack on the carbonyl carbon atom of the ester. The imidazole displaces the phenoxide anion and forms acetyl imidazole. In turn, the acetyl imidazole is quite unstable in water and hydrolyzes to form acetic acid, and regenerates the imidazole molecule. Write a suitable mechanism outlining these steps. 

Table 2 Constants for the hydrolysis of phenyl acetates in the presence of a- and S-CDs. ...

K. Fujita, A. Shinoda, and T. Imoto (1980), Hydrolysis of phenyl acetates with capped j5-cyclodextrins Reversion from meta to para selectivity. J. Amer. Chem. Soc. 102, 1161-1163. 

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