Dehydration reactions enthalpy change

The data mentioned above indicate that the sequences of reactions in the individual systems are quite complex. Thermal analysis metliods have been found to be especially suitable for laboratory study of these reactions. The reactions involving a change in enthalpy are identified by means of differentia] thermal analysis reactions involving changes in weight (dehydration, decarbonization) may be characterized by means of gravimetric thermal analysis, and electroconductive thermal analysis indicates by abrupt changes the temperatures at which a melt is formed. 

The use of isotopes has found surprisingly few applications in the determination of mechanisms of dehydrations. Below the temperature of appreciable water evolution, DjO/HjO exchange could, in principal be used to measure, in partially decomposed salt, e quantities of water held at surface sites of different reactivities, i.e. water physically adsorbed, chemisorbed, within amorphous phases and at the reaction interface. Similarly, kinetic isotope effects in dehydration reactions have not been extensively investigated. If corresponds to the enthalpy of dissociation, the substitution of a deuterium bond for a hydrogen bond could result in a change of the magnitude of E. ... 

However, each of the individual reactions involves the formation of a compound from its elements or the decomposition of a compound into those elements. The standard enthalpy change of a reaction that involves the formation of a compound from its elements is referred to as the enthalpy (or heat) of formation of that compound and is denoted by the symbol AH. Thus, for the dehydration of n-propanol. 

The dehydration reaction of the complex involves only one step temperature of peak of dehydration reaction, T = 93.4 °C. Enthalpy change for dehydration reaction, AH = 582.8 kJ mol . 

The standard molar Gibbs energy of reaction (8.12) on the mole fraction scale when dodecane is the diluent of the TBP is AG° = -46 kJ mol", the reaction being dominated by its enthalpy change, AH° = -55 kJ mof as determined by Marcus [30]. The net enthalpy change arises from the amount invested in desolvation (dehydration) of the uranyl ion (aq) and the two nitrate ions N03 (aq) but regained... 

The enthalpy and entropy changes of the total complexation reaction, AH° and AS°, reflect, primarily, step 1 [Eq. (3.18a)], the dehydration snbreaction. 

Now, if we assume that the active sites of these enzymes have a hydrophobic pocket at Sj as well as discrete subsites for substrate amino acids, we can explain these results by assigning different levels of importance to these different modes of interaction for the two enzymes. To account for the Pi specificity of FKBP, we not only assume a more prominent role for Pi-Si interactions but also that these interactions are characterized by dehydration of the Michaelis complex, E S, as it proceeds to the transition state, [E S]t. What we are suggesting here is that in E S, the Pi residue is not yet buried in Si and that the active site and the substrate are still at least partially solvated. As E S proceeds to [E S], the Pi residue becomes buried in the Si pocket and the residual water of solvation is expelled from the active site. This scenario can reasonably account for the large values of A/ft and ASt that we observe for reactions of FKBP, since the formation of hydrophobic contacts between apolar groups in aqueous solution is known to be accompanied by positive enthalpy and entropy changes (Nemethy, 1967). Likewise, to account for the lack of Pi specificity for CyP, we assume that subsite interactions play a more prominent role than do Pi—Si interactions. Thus, the Pi-Si hydrophobic interactions that dominate the thermodynamic parameters for FKBP have a smaller role for this enzyme. 

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