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Kinetics of hydrolysis reactions in water-glycerol mixtures

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Title:Kinetics of hydrolysis reactions in water-glycerol mixtures
Authors:Richards, Charles Norman
Keywords:Chemical kinetics
Date Issued:1968
Abstract:The second-order rate constants for the acid-catalyzed hydrolysis of 2,2-dimethyl-l,3-dioxolane (DMD), 2-isopropyl-2-methyl-1,3- dioxolane (IMD), and 2,2-dimethyl-4-hydroxymethyl-l,3-dioxolane (DHD) in aqueous glycerol mixtures ranging from 0-40 wt. % glycerol have been determined spectrophotometrically. Values of wg, defined as the slopes of plots of log kobs/H+ vs. log aH2O have been deter- mined for IMD at 200 , 2So, 300 , 3So, 400 , and 4SoC. Values of wg for IMD and DHD have been determined at 25° and 45°C. The wg-values at 25°C for DMD, IMD, and DHD are -3.54, -1.81, and -2.78 respectively. These values seem to be a rough measure of the hydration change between reactants and the transition state. wg-Values are dependent upon the structure of the compound and on temperature. Methyl groups at the 2 position promote hydrophobic hydration in the ground state. This hydration is disrupted by the positive charge of the transition state. A group such as isopropyl insulates this hydration from the effect of the positive charge. The temperature dependence of these values, δ Twg ( °C^-1), for DMD, IMD, and DHD are 0.124, 0.071, and 0.028 respectively. These values give an indication of how much hydrophobic hydration is involved with each compound. The thermodynamic parameters of activation for these reactions in water and the various water-glycerol mixtures have been evaluated. The enthalpies and entropies of activation for DMD and IMD decrease with increasing glycerol content. The ΔH* and ΔS* for DHD are approximately independent of glycerol content. The ΔH* for DMD ranges from 20. 7 kca1/m in water to 17.6 kca1/m in 40% glycerol. The ΔH* for IMD ranges from 20.0 kcal/m in water to 17.6 kcal/m in 40% glycerol. The ΔH* for DHD is 19.3 kcal/m in water. The ΔS* for DMD ranges from +7.1 e.u. in water to - 2.4 e.u. in 40% glycerol. The ΔS* for IMD ranges from +4.8 e.u. in water to -2.5 e.u. in 40% glycerol. The ΔS* for DHD is -0.1 e. u. in water. The entropy of activation appears to be dependent upon the number of methyl groups that promote hydrophobic hydration in the ground state. When this hydration is disrupted in the transition state it is reflected in a more positive ΔS*. The ΔH* and ΔS* for the hydrolysis of DHD in 50% (v/v) dioxane have also been evaluated. The ΔH* is 18.2 kca1/m and the ΔS* is -6.1 e.u. The standard partial molar quantities for the transfer of DMD from water to water-glycerol solutions have been evaluated. The ΔH° and ΔS° generally increase with increasing glycerol content. This indicates that the lower energy requirements for the reaction in water-glycerol mixtures is primarily due to a ground state effect. The values of ΔH° and ΔS° for the transfer of DMD from water to 40% glycerol are 6.7 kca1/m and +21.1 e.u. respectively. The results are discussed in terms of solvent-solute interactions and the differences in ground state and transition state hydration.
Thesis (Ph. D.)--University of Hawaii, 1968.
Bibliography: leaves [70]-73.
vii, 73 l graphs, tables
Rights:All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections: Ph.D. - Chemistry

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