The kinetics and mechanism of the Cr(VI) oxidation of D-fructose in the presence and absence of picolinic acid (PA) in aqueous acid media were studied under the conditions [D-fructose] T )[Cr(VI)] T at different temperatures. Under the kinetic conditions, the monomeric species of Cr(VI) was found to be kinetically active in the absence of PA whereas in the PA-catalysed path, the Cr(VI)-PA complex was considered to be the active oxidant. In this path, the Cr(VI)-PA complex undergoes a nucleophilic attack by the substrate to form a ternary complex which subsequently experiences a redox decomposition through glycol splitting leading to the lactone of C 5 -aldonic acid along with formaldehyde and the Cr(IV)-PA complex. The primary product formaldehyde undergoes further oxidation (in part) to form formic acid. Then the Cr(IV)-PA complex participates further in the oxidation of Dfructose and ultimately is converted into the inert Cr(III)-PA complex. In the uncatalysed path, the Cr(VI)-substrate ester experiences an acid-catalysed redox decomposition (2e transfer) in the rate-determining step giving rise to the products. The uncatalysed path shows a second-order dependence on [H ] whereas the PA catalysed path shows a fractional order in [H ]. Both paths show a first-order dependence on [D-fructose] T and [Cr(VI)] T . The PA-catalysed path is first order in [PA] T . All these patterns remain unaltered in the presence of externally added surfactants. The effects of a cationic surfactant, N-cetylpyridinium chloride (CPC), and an anionic surfactant, sodium dodecyl sulfate (SDS), on both the uncatalysed and PA-catalysed paths were studied. CPC inhibits both the uncatalysed and PAcatalysed paths whereas SDS catalyses the reactions. The observed micellar effects are explained by considering a distribution pattern of the reactants between the micellar and aqueous phases. The applicability of different kinetic models, e.g. the pseudo-phase ion-exchange model, the Menger-Portnoy model and the Piszkiewicz cooperative model, was tested to explain the observed micellar effects. The effect of [surfactant] T on the activation parameters was explored to rationalize the micellar effect.

The kinetics and mechanism of chromic acid oxidation of L-sorbose in the presence and absence of picolinic acid (PA) have been studied under the conditions, [L-sorbose]T >> [PA]T >> [Cr(VI)]T, at different temperatures. In the absence of PA, the monomeric chromic acid undergoes esterification with the substrate followed by the acid catalysed redox decomposition of the Cr(VI)-substrate ester through glycol splitting to formaldehyde and the lactone of C5-aldonic acid and Cr(IV) which subsequently participates in the faster reactions. In the presence of PA, the Cr(VI)-PA complex produced in a pre-equilibrium step experiences a nucleophilic attack by the substrate to produce a ternary complex which decomposes through glycol splitting giving rise to the organic products and Cr ( . In the presence of the surfactants like N-cetylpyridinium chloride (CPC, a cationic surfactant) and sodium dodecyl sulfate (SDS, an anionic sulfate), the reaction orders remain unchanged. CPC has been found to inhibit both the uncatalysed and PA-catalysed paths while SDS shows the rate accelerating effect for both the uncatalysed and PA-catalysed paths. The observed micellar effects have been rationalised by considering the distribution of the reactants between the micellar and aqueous phases in terms of the proposed reaction mechanism.

Integral heats of solution of Bu4NBr in aqueous binary solvent mixtures (containing 0.0 to 0.30 mol fraction of cosolvent) of t-butanol, acetone, dioxan and ethylene glycol were determined at 15,25 and 35°C. Integral heats of solution of NaBPh4 in the same aqueous binary solvents containing 20 % by volume (about 0.05 rnol fraction) of cosolvent were also determined at 15, 25 and 35°C.Corresponding excess partial molal heat capacities AC; were derived at 20 and 30°C. The partial molal enthalpies AZi of Bu4NBr increased (sharply in aqueous t-butanol up to 0.06 mol fraction) with the addition of increasing amount of cosolvent to water till the maximum value was attained at 0.1-0.3 mol fraction of cosolvent. The endothermicity maxima AZimax were in the decreasing order of t-butanol >acetone >ethylene glycol >dioxan. AH, . , , , decreased with increasing temperatures in all aqueous binary solvents. A e results indicate that addition of small amounts of t-butanol and acetone stabilize the water structure, the maximum stabilization occurring around 0.04 mol fraction of acetone and t-butanol. Further addition of t-butanol to 0.06mol fraction results in complete collapse of the water structure, whereas further addition of acetone results in gradual breakdown of the water structure which continues up to 0.3 mol fraction of acetone. Dioxan and ethylene glycol seem to have net structure breaking influence on the water structure.

. Can. J. Chem. 57, 1098 (1 979).Pseudo-first-order rate constants for the decarboxylation of 3-hydroxy-and 3-aminopicolinic acids in aqueous solution at 150°C were determined and plotted as a function of acidity. Each rate profile has a maximum at an acidity well above the isoelectric point and this is attr~buted to decarboxylation of an intermediate protonated at the 2-position, analogous to the intermediates involved in the decarboxylation of salicylic and anthranilic acids. There is also a shoulder on each rate profile at a lower acidity corresponding to the isoelectric point where the Hammick ylide mechanism has a rate maximum in most picolinic acid decarboxylations. It is concluded that 3-hydroxy-and 3-aminopicolinic acids decarboxylate by the ylide mechanism at low acidity and by the protonation mechanism at higher acidities. In agreement with this interpretation, the 13C kinetic isotope effect in 3-hydroxypicolinic acid decarboxylation is 2.0% on the ylide part of the curve, 1.3% where decarboxylation of the protonated intermediate is rate determining, and drops to 0.4% in the intermediate region.Comparison of the rate constants for ylide decarboxylation with those for other 3-substituted picolinic acids shows that 3-hydroxy and 3-amino substituents facilitate decarboxylation, probably by their inductive and field effects on the developing negative charge at the 2-position of the transition state. GERALD E. DUNN, HARALD F. THIMM et RAJANI K. MOHANTY. Can. J. Chem. 57, 1098 (1979.On a determine les constantes de vitesse de pseudo-premier ordre pour la decarboxylation des acides hydroxy-3 et amino-3 picoliniques en solution aqueuse a 150°C et on a trace la courbe qui les relie a l'aciditk. Pour chaque profil de vitesse, il existe un maximum a une acidit6 bien au dessus du point isoelectrique et on I'attribue a la decarboxylation d'un intermediaire protone en position 2, analogue aux intermediaires impliques dans la decarboxylation des acides salicyclique et anthranilique. On note aussi un epaulement sur chaque profil de vitesse, a une acidlte plus basse que celle du point isoelectrique, ou la vitesse pour le mecanisme ylide de Hammick est maximale pour les decarboxylations de la plupart des acides picoliniques. On en conclut que les acides hydroxy-3 et amino-3 picoliniques se decarboxylent par un mecanisme ylide a faible acidite et par un mkcanisme de protonation a des acidites plus 6levCes. En accord avec cette interpretation, l'effet isotopique cinetique 13C observe lors de la decarboxylat~on de l'acide hydroxy-3 picolinique est &gal a 2% dans la portion ylide de la courbe, a 1.3% lorsque la decarboxylation implique un intermediaire dans 1'Ctape qui determine la vitesse et tombe a 0.4% dans la region intermediaire. Une comparaison des constantes de vitesse pour la dtcarboxylation par I'intermediaire d'ylides avec celles d'autres acides picoliniques substitues en position 3 montre que les substituants hydroxy-3 et amino-3 facilitent la decarboxylation, probablement grdce a leurs effets inductifs et de c...

ACi values of tetra-alkylammonium halide salts Me4NBr, Me4NT, Et,NBr, Et4NI, Pr4NBr, Pr4NI and Bu4N13r in aqueous solutions at 30°C have been determined by measuring integral heats of solution at 25 and 35°C AC; values which vary from -16.5 to 179 cal deg.-' mole-' are in the increasing order of Me,NI< Me,NBr< Et4NBr 5 Et,NI< Pr4NBr % Pr4NI< Bu4NBr. Results are discussed in terms of the influence of tetra-allcylammonium halide salts on the water structure. Me4NBr and Me,NI break water structure, while all other tetra-alkylammonium bromide and iodide salts enhance or promote water structure. The influence of %N+ salts in promoting water structure increases greatly with increase in the size of the alkyl group R. The influence of anions (bromide and iodide) does not appear to be significant in modifying the structure of water. Results from this work support the concept of " cation-cation interactions " in explaining the nature of interaction of tetra-alkylammonium halide salts with water.