Enthalpy solvent effect

The stability of macrocyclic complexes has been shown to be greater than the comparable open chain complexes. Unfortunately, most quantitative data apply to N and S ligand systems. This increased stability is the net effect of a number of factors including changes in entropy, enthalpy, solvent effects, pH, ring size and conformation (equation 30). ... 

Unfortunately, the number of mechanistic studies in this field stands in no proportion to its versatility" . Thermodynamic analysis revealed that the beneficial effect of Lewis-acids on the rate of the Diels-Alder reaction can be primarily ascribed to a reduction of the enthalpy of activation ( AAH = 30-50 kJ/mole) leaving the activation entropy essentially unchanged (TAAS = 0-10 kJ/mol)" . Solvent effects on Lewis-acid catalysed Diels-Alder reactions have received very little attention. A change in solvent affects mainly the coordination step rather than the actual Diels-Alder reaction. Donating solvents severely impede catalysis . This observation justifies the widespread use of inert solvents such as dichloromethane and chloroform for synthetic applications of Lewis-acid catalysed Diels-Alder reactions. 

It was concluded that the variations in rate are due to variations in activation enthalpy rather than entropy, and since the rates of substitution rates at the para positions of toluene and /-butylbenzene varied by only 4 % for a change in reactivity of 6,430, it was concluded that the Baker-Nathan reactivity order does not arise from a solvent effect (c/. Table 57). 

The enthalpy values of the displacement in the above solvents were calculated to be — 2.0, — 2.0 and —2.1 kcal mol , i.e., practically identical within the experimental error. These observations verify the validity of the assumption for the cancellation of solvation effects in hydrogen bonds in non-polar solvents6 5b,c. Solvent effects on the hydrogen bond have been discussed by others66 - -80 82. 

Theoretically, the problem has been attacked by various approaches and on different levels. Simple derivations are connected with the theory of extrathermodynamic relationships and consider a single and simple mechanism of interaction to be a sufficient condition (2, 120). Alternative simple derivations depend on a plurality of mechanisms (4, 121, 122) or a complex mechanism of so called cooperative processes (113), or a particular form of temperature dependence (123). Fundamental studies in the framework of statistical mechanics have been done by Riietschi (96), Ritchie and Sager (124), and Thorn (125). Theories of more limited range of application have been advanced for heterogeneous catalysis (4, 5, 46-48, 122) and for solution enthalpies and entropies (126). However, most theories are concerned with reactions in the condensed phase (6, 127) and assume the controlling factors to be solvent effects (13, 21, 56, 109, 116, 128-130), hydrogen bonding (131), steric (13, 116, 132) and electrostatic (37, 133) effects, and the tunnel effect (4,... 

Solvent effects also play an important role in the theory separating enthalpy and entropy into external and internal parts (134-136) or, in other terms, into reaction and hydration contributions (79). This treatment has been widely used (71, 73, 78, 137-141). The most general thermodynamic treatment of intermolecular interaction was given by Rudakov (6) for various states of matter and for solution enthalpy and entropy as well as for kinetics. A particular case is hydrophobic interaction (6, 89, 90). 

In series with a constant enthalpy, controlled by entropy changes, steric effects (15), or more particularly, kinetic steric effects (13, 14) and solvent effects (14) may be decisive. 

In this chapter, we have collected and discussed the available data in both gas and liquid phases related to Scheme 2.1. Emphasis will be given to homolytic bond dissociation enthalpies of silanes. Generally, Df/values are extrapolated from the gas phase to solution without concerning solvent effects (particularly in the... 

The report of Gmndnes and Christian (14) presents another interesting complication from solvent effects. The reported enthalpy of formation of the (CHg)3N—SOg adduct in the gas phase is — 9.1 0.4 k. cal mole i, while that measured in heptane is — 11.03 0.3. The cause of this discrepancy is not understood and before drawing any definite conclusions about the nature of the solvent effects in this system, it would be informative to have a calorimetrically determined enthalpy in... 

Solvent effects and adduct formation of [VO(acac)2l and other [VO(j8-diketonato)2] complexes have been studied by several methods361,366,52 -532 and in coordinating solvents [VO(acac)2] is known to add a sixth ligand according to equation (36). Older reports include a spectrophotometric and calorimetric study of [VO(acac)2] and [VO(tfacac)2] adducts.521 With [VO(acac)2] in nitrobenzene, the enthalpy change for reaction (36) ranges from 44.3 kJ mol-1 for n-decylamine to 24.3 kJ mol-1 for methanol. Equilibrium constants K were between 1000 and —0.6. [VO(acac)2] is not a sensitive indicator of relative base strength.521 For [VO(acac)2], A0 decreases by —3 G (and go increases by —0.0004) as amine adducts are formed.522... 

Merenyi R, Janousek Z, Viehe HG (1986) Studies on the captodative effect. Entropy/enthalpy compensation as solvent effect in radical forming reactions. A relative radical stabilisation scale. In Viehe HG (ed) Substituent effects in radical chemistry. Reidel, Dordrecht, pp 301-324 Merga G, Schuchmann H-P, Rao BSM, von Sonntag C (1996) The oxidation of benzyl radicals by Fe(CN)63. J Chem Soc Perkin Trans 2 551-556... 

The enthalpy and entropy changes of micellization have been calculated for benzenesulfonate and alkylammonium salts in low-polar solvents suggesting that micellization is essentially an enthalpy-driven effect. The aggregation can take place at low concentrations of surfactant and can have different aggregation numbers. The absence of well-defined critical micelle concentrations (CMC) for some systems in the low-polar solvents was observed (Kertes and Gutman, 1976). 

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