M. Niyaz Khan

The mechanistic diagnosis of induced catalysis in the aqueous cleavage of phenyl salicylate in the presence of borate buffer

Abstract In a borate buffer, the rate of hydrolysis of phenyl salicylate has been found to be increased 10 6 -fold compared to the rate of hydrolysis of phenyl benzoate under essentially similar conditions. This rate enhancement is attributed to the intramolecular nucleophilic attack by hydroxyl group of the complex formed between boric acid and phenyl salicylate. The proposed stepwise mechanism involves the nucleophilic attack by the hydroxyl group of the boric acid-ester complex at the ester carbonyl carbon as the rate-determining step. An alternative mechanism, involving nucleophilic attack by hydroxide ion on carbonyl carbon of the monoester formed from boric acid and phenyl salicylate, has been ruled out on the basis of the absence of enhanced nucleophilic reactivity of tertiary and secondary amines toward phenyl salicylate in the presence of the borate buffer. Significantly low reactivity of phenyl salicylate toward tertiary amines compared to secondary amines has been proposed as the consequence of the presence of labile proton on the nitrogen atom of the attacking amine.

Aqueous degradation of N-(hydroxymethyl)-phthalimide in the presence of specific and general bases. Kinetic assessment of N-hydroxymethyl derivatives of nitrogen heterocycles as possible prodrugs

The conversion of N-(hydroxymethyl)phthalimide (NHPH) to phthalimide could not be detected within 300 s at pH 9.0, whereas in 0.18 M NaOH complete conversion of NHPH to phthalimide was observed within 50 s. In the presence of 0.2-0.4 M 1,4-diazabicyclo[2.2.2]octane buffer solutions (pH 9.30-9.54), 40-60% conversion of NHPH to phthalimide occurred within 90-120 s. The initial concentration of NHPH affected the extent of conversion of NHPH to phthalimide.

The kinetics and mechanism of hydrolytic cleavage of phenyl salicylate under highly alkaline medium

Abstract The kinetics of hydrolytic cleavage of phenyl salicylate within the hydroxide ion concentration range of 0.05–2.0M at 35° indicate rate constants are almost independent of [OH - ] within the range of 0.05 M to 0.1 M and vary linearly, within the range of 0.1–2.0 M with rates attributable to intramolecular general base catalyzed neutral hydrolysis. The hydroxide ion catalyzed hydrolytic cleavage is

Kinetic studies on general base‐catalyzed cleavage of phthalimide (PTH) in 2‐hydroxyethylamine and 2‐methoxyethylamine buffers

10 4 times slower than the respective reaction of phenyl benzoate. The rate enhancement of pH independent rate of hydrolysis has been found to be solely due to favourable ΔS * .

Spectrophotometric determination of alkyl salicylate from the mixture of phenyl and alkyl salicylates by the use of the selective reactivity of phenyl salicylate toward secondary amines

Kinetic studies on the nucleophilic cleavage of phthalimide (PTH) in buffers of 2-hydroxyethylamine and 2-methoxyethylamine reveal nonlinear plots of kn vs. [Buf]T (at constant pH) where kn and [Buf]T represent apparent second-order rate constant and total amine buffer concentration, respectively. The nonlinear variation of kn against [Buf]T is attributed to the occurrence of a stepwise mechanism in the aminolysis of PTH. Intermolecular general base catalysis is detected in the reactions of both amines with nonionized PTH (SH) only within the pH range of the present study.

Kinetics and mechanism of general acid-catalysed thiolytic cleavage of 9-anilinoacridine

Salicylate esters have been used as topical analgesics where an alcoholic component acts as the solvent. Phenyl salicylate, for example, is used in salol aqueous cream which contains a cream base with 5% propylene glycol. Recently, Irwin et al. [1-3] have shown the occurrence of transesterification of salicylate esters in alkaline alcoholic medium which could lead to serious problems in such pharmaceutical formulations. The kinetic studies on these transesterification reactions carried out by Irwin et al. [1-3] involve the high-performance liquid chromatographic (HPLC) technique. Recently, the author observed that piperidine is highly reactive toward phenyl salicylate and completely nonreactive toward methyl and ethyl salicylates [4, 5]. This selective reactivity of piperidine toward phenyl salicylate could be used to quantitatively estimate the alkyi salicylate (formed when phenyi salicylate is mixed with alkanol) by using an alternative spectrophotometric technique. The details of the method, and its use in the estimation of the 2-hydroxyethyl salicylate formed in the reaction of phenyl salicylate with 1,2ethanediol, are described in this paper.

Kinetics and Mechanism of the Alkaline Hydrolysis of Maleimide

The rates of the reactions of 2-mercaptoethanol (2-ME) with 9-anilinoacridine (9-ANA) have been studied in the buffer solutions of 2-ME, hydroxylamine, phosphate and morpholine. Both ionised and non-ionised forms of 2-ME and free hydroxylamine show nucleophilic reactivity toward protonated 9-ANA. The rate constants for general acid-catalysed thiolytic cleavage of protonated 9-ANA reveal a Brønsted plot of slope (alpha) of 0.93 which indicates that probably the rate-determining step involves proton transfer in a thermodynamically unfavourable direction. A stepwise mechanism for thiolysis has been suggested. General acid catalysis could be detected for thiolysis of non-protonated 9-ANA only in the buffer solutions of phosphate and morpholine. General acid catalysis seems to be unimportant when the nucleophile is non-ionised 2-ME which is attributed to the probable occurrence of intramolecular general acid catalysis.