• Vangalur S Srinivasan

Articles written in Journal of Chemical Sciences

• Kinetics of Tl(III) oxidation of hydroxylamine hydrochloride in aqueous sulphuric acid

The kinetics of Tl(III) oxidation of hydroxylamine hydrochloride in 1·0 M H2SO4 at a fixed chloride concentration has been investigated to make a formal comparison of the observed rate with that of hydrazine sulphate. The reaction exhibits total second order kinetics—first order in each reactant. The rate of the reaction depends inversely on the first power of [H+] and [Cl] suggesting the possible reactive species as TlOH2+. To account for the stoichiometry of the reaction [Tl(III)]: [H2NOH·HCl)=1·5∶1 and the products of the reaction, HNO2 and N2O, two reaction schemes have been proposed.

• Tl(III) acetate oxidation of cyclanols and bicyclo(2,2,1) heptan-2-ols

Tl(III) acetate oxidation of cyclohexanol, cyclopentanol, cycloheptanol,trans-2-chlorocyclohexanol,cis-4-t-butylcyclohexanol,trans-4-t-butylcyclohexanol, borneol, isoborneol, exo (β) norborneol and endo (α) norborneol has been studied in the presence of 0·90 M H2SO4. The observed reactivity pattern among the cyclanols is cyclopentanol &lt; cyclohexanol &gt; cycloheptanol &lt; cyclooctanol which is the same as the one noted in V(V) oxidation of the substrates under similar conditions—an order contrary to the I-strain concept. In both cases Mn(II) catalysis and acrylonitrile polymerisation have been observed in cyclohexanol oxidation alone. The kinetic isotope effect in the Tl(III) oxidation of cyclohexanol is 2.82 as against a value of 6·4 obtained for Tl (III) oxidation of benzhydrol. The kinetic observations are explained on the basis of a radical mechanism operating in the case of cyclohexanol, as it is a strainless ring system, with the intermediacy of Tl (II).Trans-2-chloro andtrans-2-phenyl groups, due-I effect, retard the rate of the reaction.Cis-4-t-butylcyclohexanol reacts faster than the trans compound due to relief of strain in the transition state. The reactivity pattern among the bicyclo (2,2,1) heptan-2-ols is, isoborneol &gt; borneol &gt; exo (β)-norborneol &gt; cyclopentanol &gt;endo-(α)-norborneol. This is consistent with the relief of strain in the transition state due to the hybridisation change from sp3 to sp2 and lessening of torsional interaction. This can also be due to the formation of less-strained products.

• Facile chlorination of acetophenone by chloramine-T in the cationic micellar phase

The chlorination of acetophenone by chloramine-T (cat) has been catalysed by added detergents, sodium laurylsulphate (NaLS) and cetyltrimethylammonium bromide (ctab). Phenacyl chloride is the exclusive product of the reaction and the % yield of the product is greater in the cationic micellar phase indicating facile chlorination in this media (even in the absence of the mineral acid). Using Piszkiewicz’s treatment, the positive cooperativity for the micellar catalysed reactions has been calculated. At the cationic micellar phase ofctab, the interaction between ammonium ion and phenyl and methylene groups probably leads to greater stabilisation than the simple hydrophobic association encountered in the micelle interior of NaLS.

• Micellar inhibited hydrolysis of esters—evaluation of binding constant and cooperativity index

The inhibition in the rate of hydrolysis of four esters, by the anionic micelles of sodium laurylsulphate has been explained by a binding model. It provides the critical micelle concentration under reaction conditions. Kinetic data suggest that more than 90% of the substrate is micelle-bound at 0.010 M concentration of the anionic micelle and the binding constants obtained agree with those of other systems studied elsewhere. The marked inhibition in the comicellar phase and the cationic micellar phase of sodium Iaurylsulphate, cetyldimethylbenzylammonium chloride or cetyltrimethylammonium bromide, has been explained by using the same pseudo model and the binding constant,Ks and the fraction of substrate held at the comicellar surface,Fsm, indicates greater binding at the comicellar surface. The cooperativity treatment has been extended to micelle-inhibited reactions and the proposed model has been tested with literature data as well as in the present work. The cooperativity index value ranges from 0.59 to 1.51, and a value less than unity indicates negative cooperativity.

• Influence of imidazole and bifunctional nucleophiles on the micellar catalysed hydrolysis and hydroxylaminolysis of esters

Though 2- and 3-hydroxypyridines, structurally resemble imidazole, 2-hydroxypyridine seems to function as a nucleophile in the hydrolysis ofp-nitrophenylbenzoate as it can acquire a pyridine form (Mosher 1959) while the latter cannot. The bifunctional activity of benzamidine has also been enhanced by anionic micelles of sodium laurylsulphate. The anionic micelle formed by sodium laurylsulphate retards the rate of hydroxylaminolysis ofp-nitrophenylbenzoate, while the cationic micelle formed from cetyltrimethylammonium bromide enhances the rate in H2NOH−H2NOH·HCl buffer at pH=6·14. Such behaviour is in favour of the anion, H2NO, acting as a nucleophile to some extent. On the contrary, hydroxylaminolysis ofp-nitrophenylphenylmethane sulphonate (PMS) proceeds at a slower rate and imidazole catalysis is observed as these esters at pH=6·14 possibly prefer aBAc 2 mechanism, which is absent at pH=9·2 as the same reactions proceed by aE1cB path.

• Kinetic model for micellar catalysed hydrolysis of esters-biomolecular reactions

The cationic micelles of cetyltrimethylammonium bromide, cetyldibenzylammonium chloride and cetylpyridinium chloride stabilize the tetrahedral intermediate formed in the hydrolysis of carboxylic esters (e.g.p-nitrophenylbenzoate) to a greater extent, preferring a$$B_{Ac^2 }$$ mechnism, than the anionic intermediate formed in the hydrolysis ofm-nitrophenyl-N-N-diphenylphosphorodiamidate, which prefers aE1cB mechanism. The co-operative index,n1, calculated for these reactions is greater than 1 indicating that the substrate induced micellation is responsible for the observed catalysis. Based on the present kinetic model for a bimolecular reaction the fraction of substrate and nucleophile bound to the micelle have been calculated. The above results suggest that reaction occurs between the substrate solubilised into the micelle and the nucleophile residing at the Stern layer rather than at the micelle-water interface. The equilibrium constant, and critical micelle concentration evaluated using the present model are in agreement with the values obtained by using earlier models, suggesting a method of evaluating these parameters from kinetic data only.

• # Journal of Chemical Sciences

Volume 134, 2022
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