The Hydrolysis of t -Butyl Chloride in Aquo-Organic Mixtures: Heat Capacity of Activation and Solvent Structure

~A Y A T I DATTA and KIRON K. KUNDU. Can. J . Chem. 59. 3141 (1981). Autoionization constants (K,) of aqueous mixtures of tetrahydrofuran (THF) and 1.2-dimethoxyethane (DME) containing 10,30, and 50wt.% cosolvent in each case have been determined from emf measurements of the cell: Pt, H, (g, 1 atm)/KOH ( m , ) , KBr (tn,), solventlAgBr, Ag at seven equidistant temperatures ranging from 5 to 35°C. The standard free energies (AGO), entropies (AS0), and enthalpies (AH0) of autoionization of the solvents were also evaluated from these data. Relative free energy data, 6hG0(= ,AGO -, THF > D and their "acidity" in the reverse order, as expected from structural and electronic consideration of these cosolvent molecules. Analysis of the relative entropic contributions, T6 (AS0) ( r T (,AS0 -,%AS0), for the autoionization of these aqueous cosolvents and in particular AS,O(H,O) values derived therefrom, suggests that while THF promotes three dimenstional (3D) ice-like water stluctures at initial compositions and D induces breakdown of the 3D structures right from the beginning, DME breaks down water structures at initial compositions. but induces some order around 4-14 mol% DME by forming the possible H-bonded bidentate DME-water complexes. And beyond certain compositions, depending upon the relative size and shape, all the cosolvents break down water structure due to packing imbalance. Le DME au contraire rompt les structures de I'eau au depart mais favorise un agencement ordonne pour une composition allant de 4 a 14mol% de DME en formant possible~nent les co~nplexes bidentates de DME-eau lies par des liaisons hydrogene. Au-dela de certaines compositions compte tenu de la taille relative et de la forme, tous les cosolvants rompent la structure de I'eau par suite d'un entassement desordonnee.[Traduit par le journal] Introduction The autoionization constants (K,) of aquoorganic solvents at different temperatures and the related thermodynamic parameters not only help give the thermodynamic measures of acidity but also help throw some important reflections on their structural features (1). The basis of these structural reflections stems from the fact that while the ionization of water accompanying the production of order-producing H,O+ and OH-ions imparts negative magnitudes to TASo(ASO = entropy of ionization), the required configurational changes,

Section: Entropy Changessupporting

confidence: 93%

Thermodynamics of autoionization of aqueous tetrahydrofuran and 1,2-dimethoxyethane and the structuredness of solvents

Datta¹,

Kundu²

1981

“…In alcohol-water binary solvent systems, an extremum in the activation parameters towards the pure water end of the composition scale has been observed (5-13). The extremum behaviour seems to reflect a common solvent behaviour (12)(13)(14) and has been related to the accompanying solvent reorganization attending the activation process in binary alcohol-water systems (13). In the present investigation, we report the results of the study of the kinetics of the alkaline solvolysis of dichloroacetate ion in watermethanol solutions.…”

Section: Introductionmentioning

confidence: 73%

Solvolysis rates in aqueous–organic mixed solvents. IX. Solvolysis of dichloroacetate ion in water–methanol solutions

Diefallah¹,

Ashy²,

Baghlaf³

1981

. Can. J. Chem. 59, 1208 (1981). The kinetics of the alkaline solvolysis of dichloroacetate ion in water-methanol solutions have been studied in the temperature range of 50.0 to 65.0°C and the influence of solvent variation on reaction rate has been examined in terms of changes in the activation parameters. The activation parameters AH* and AS+ for the solvolysis reaction showed a minimum at about 0.8 water mole fraction. The significance of the results was discussed in view of the electrostatic theory and the changing of solvent structure. On a etudie la cinetique de la solvolyse alcaline de I'ion dichloroacetate dans des solutions de methanol-eau dans I'intervalle de temperature allant de 50,O a 65,0°C et on a examine I'effet de la variation du solvant sur la vitesse de reaction en fonction des variations des parambres d'activation. Les paramktres d'activation AH+ et AS* de la reaction de solvolyse atteignent un minimum lorsque la fraction molaire de I'eau est d'environ 0,8. On discute de la signification de ce resultat par rapport a la theorie electrostatique et au changement de la structure du solvant.[Traduit par le journal] Introduction Mixed solvents, in which water was frequently one component, provided a graded series of solvolytic media in which to explore the relation between rates, mechanism, and the effect of changing solvent properties (1-4). In alcohol-water binary solvent systems, an extremum in the activation parameters towards the pure water end of the composition scale has been observed (5-13). The extremum behaviour seems to reflect a common solvent behaviour (12-14) and has been related to the accompanying solvent reorganization attending the activation process in binary alcohol-water systems (13). In the present investigation, we report the results of the study of the kinetics of the alkaline solvolysis of dichloroacetate ion in watermethanol solutions. Studies of the alkaline solvolysis of mono-and trichloroacetate ions in binary alcohol-water systems were recently published (5-8). Comparisons between the mono-, di-, and trichloroacetates have shown that there are large differences in order, specific rate constant, and mechanisms of the solvolysis reactions of these ions.

“…The extremum behaviour of the activation enthalpy and entropy has been observed for many solvolytic reactions (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) and under a variety of conditions, and seems to reflect a common solvent behaviour (7-10). The appearance of the minimum in aqueous-organic solvent mixtures was explained by Arnett and co-workers (7) on the basis of the behaviour of heats of solution in aqueous alcohols.…”

Section: Resultsmentioning

confidence: 99%

“…For many solvolysis reactions carried out in aqueous-organic mixed solvents, the activation enthalpy and entropy pass through an extremum as the solvent composition is varied (1)(2)(3)(4)(5)(6)(7)(8)(9)(10). The position and depth of the extrema are dependent upon the substrate and the nonaqueous component of the solvent mixture.…”

Section: Introductionmentioning

confidence: 99%

Solvolysis rates in aqueous–organic mixed solvents. III. Kinetics of the alkaline solvolysis of monochloroacetate ion in water – tert-butyl alcohol solutions

Diefallah¹

1976

The temperature dependence of the rate of alkaline solvolysis of monochloroacetate ion in water – tert-bulyl alcohol solutions has been determined. The rates of reaction were first order with respect to both the chloroacetate ion and to the total solvoxide ion concentrations. The reactivity is enhanced, first slowly, and then more rapidly, by increasing the concentration of tert-bulyl alcohol in the solvent mixtures. The rate of reaction increases with the reciprocal of the dielectric constant of the medium and the plot of log kvs. 1/D can best be represented by two lines intersecting at a point corresponding to 0.9 water mole fraction. The activation parameters ΔH*and ΔS* for the solvolysis reaction showed a minimum at about 0.9 water mole fraction. The significance of these results from the viewpoint of the electrostatic theory and the changing of solvent structure in such mixtures is discussed.