Entropy-driven reactions

The thermochemistry was finally established in 1993, when Gable and Phan reported that norbornene reacted with Cp Re03 to give an equilibrium mixture of reactants and diolates.79 Using the observed equilibrium constant, they calculated that extrusion of unstrained alkenes was an entropy-driven reaction. 

The TLM model has been applied not only to inorganic anions but also to organic species. In a study of 2,4-dichlorophenol, 2,4,6-trichlorophenol, andpentachlorophe-nol sorption in an allophanic soil (Cea et al. 2010), the isotherm experiments were well described (as shown in Figure 12.4) by the TLM in which monodentate outer and inner sphere complexes were considered to form between deprotonated organic molecules and active sites on the variable-charge soil. The calculated thermodynamic parameters suggest that chlorophenol sorption is a spontaneous (AG < 0), endothermic (AH > 0), and entropy-driven reaction (AS > 0). 

D. Y. Zhang et al.. Engineering entropy-driven reactions and networks catalyzed by DNA. Science i 8 (2007), 1121-5. 

Reactions that have positive AH° and positive A S ° are favored by entropy but dis-favored by enthalpy. Such reactions are spontaneous at high temperature, where the T AS° term dominates A G °, because matter becomes dispersed during the reaction. A reaction is entropy-driven under these conditions. These reactions are nonspontaneous at low temperature, where the A iiT ° term dominates A G °. 

The interaction between 4-(4-hydroxybut-2-ynyloxy)-3-(phenylsulfonyl)-l,2,5-oxadiazole-2-oxide 16 and bovine serum albumin (BSA) was studied by spectroscopic methods including fluorescence and UV-Vis absorption spectroscopy. The results indicate that molecules 16 bind with BSA forming 1 1 complex. Thermodynamic parameters, such as AH, AG, and A.Y, were calculated. The results indicate that the binding reaction is mainly entropy driven and hydrophobic forces play a major role in this reaction <2006CHJ1050>. 

As noted in table 11.1, the ability of THFTCA to separate LJO from trivalent lanthanide ions is mainly of enthalpic origin. Reaction 11.33 has a considerably more unfavorable enthalpic contribution than reaction 11.32. The complexation is, however, predominantly entropy driven because the T ArS° term dominates the ArH° contribution for all systems. The large positive entropy changes observed for reactions 11.32 and 11.33 result from the release of water molecules coordinated to the metal on complexation with the tridentate THFTCA2- ligand. Note that a negative entropy contribution would be expected if these reactions were truly 2 particle = 1 particle reactions [226]. 

The thennodynainics of complexation between hard cations and hard (O, N donor) hgands often are characterized by positive values of both the enthalpy and entropy changes. A positive AH value indicates that the products are more stable than the reactants, i.e., destabilizes the reaction, while a positive entropy favors it. If TAS > AH°, AG° will be negative and thus log(3 positive, i.e., the reaction occurs spontaneously. Such reactions are termed entropy driven since the favorable entropy overcomes the unfavorable enthalpy. 

From the temperature dependence of the equilibrium constant for proton exchange between some deuterated and undeuterated primary and secondary amines, monitored by high-pressure mass spectrometry, the reaction enthalpy, or difference in proton affinity, could be measured.101 Protonation of the deuterated amine is favored by 0.2kcalmol-1, varying with structure by 0.1 kcal mol-1 but with no obvious pattern. However, the equilibrium, at least for CH3CD2NHCH3, appears to be entropy driven, not enthalpy. 

That the entropy change is unfavourable could be confidently predicted, given the presence of two moles of gas on the left-hand side. As the temperature is increased, the TAS" term becomes more important neglecting the small temperature dependence of AH° and A5°, it can be easily shown that AG° will become zero at about 850 K, at which temperature the decomposition of the complex should be complete. Such decomposition can be achieved at lower temperatures if the partial pressure of ammonia is kept low, by pumping. Most thermal decompositions-which are often the reverse of acid-base reactions (see Section 9.2) - are entropy-driven. All substances containing chemical bonds can be decomposed by heating to a sufficiently high temperature. 

Usually, there is a compensation effect, that is AHd —TAS11 so that AGd 0. Experimentally, AS1 is positive (especially for macrocyclic ligands), and so is very often A Hr (which means that usually, the Ln-L bonds are weaker than the Ln-OH2 ones) so that complexation reactions in water are entropy driven and, moreover, a linear relationship between AH and A. S 1 holds for the Ln(III) series of cations. One has, however, to be cautious when this approach is applied to polydentate ligands. The thermodynamic parameters may also reflect other factors such as the formation of stable 5-membered chelate rings. When another solvent is considered, the solvation enthalpy is much smaller than in water and the above considerations may no more hold. 

Few measurements of the so-called driving force,the Gibbs free energy, AG°, have become available as yet however, for a number of reactions in organic nonaqueous solutions it is entropy driven, that is, AG°is driven to a negative value over the positive (endothermic) AZ/ by a positive AS° and its influence as TA5" in the basic thermodynamic equation AG = AFP - TAS°. 

We also have used C fixation to measure equilibria and rates of dissociation as a function of temperature. The conclusions reached from these studies have been reported. The dependence of the dissociation equilibria on pH was consistent with dissociation reactions involving the addition to two protons per subunit, a pH-independent dissociation, and a dissociation upon the loss of one proton per subunit. The rate constants for dissociation were consistent with terms first order in hydrogen and hydroxide ions and a pH-independent path. The equilibrium constants in the range 3-35° at pH 7.2 exhibited no dependence on temperature the association reaction was entropy-driven with A5 = 68 cal moL The rate constants for the pH-independent dissociation followed A// = 6 kcal mol The order of effectiveness of concentrated salts in promoting dena-turation was correlated with their effect on the activity coefficient of ace-tyltetraglycine ethyl ester and suggested that peptide groups became more exposed upon dissociation. 

Reaction Heat. A reaction can only proceed if AG < 0. In relation to Eq. (4.6), it was mentioned that for most reactions —AH is larger than TAS. This implies that during the reaction heat is produced (the amount of reaction heat can be measured by calorimetry). The reaction then is said to be exothermic and enthalpy driven. There are also endothermic reactions, where heat is consumed in other words, AH > 0. Because AG must be negative for the reaction to proceed, this implies that TAS > AH, and the reaction is said to be entropy driven. 

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