Competition between enthalpy and entropy

Recent reports emphasizing the role of 1,4-diradicals in triplet Paterno-Biichi reactions should also be mentioned. The first to be discussed concerns the chiral induction of photocycloadditions of various olefins to chiral phenylglyoxalates11 28,29, The high diastereoselectivity of these reactions often reaches de values of >96% and shows a characteristic temperature dependence with specific points of inversion. This behavior is a result of competition between enthalpy- and entropy-controlled partial selection steps. 

This effect will not be discussed in detail here, however, extensive investigations by Scharf and co-workers show that temperature is one of the controlling parameters11. Characteristic temperature dependencies and points of inversion indicate a competition between enthalpy- and entropy-controlled partial selection (see Section 1.6.1.4.3.1.). Details and instructions on how to utilize measurements of the temperature dependence in order to optimize a specific asymmetric reaction can be found in Scharf s review article11. 

Our intention has not been to write an exhaustive treatment of polymer surfaces and interfaces but rather to produce a compact, reasonably complete treatment of the subject discussed at the level of the molecular length scales associated with polymers. This seemed appropriate because our approach was based on the statistical mechanics of coarse-grained models a constant theme is the competition between enthalpy and entropy in polymers at or near... 

J/m/K. The agreement between the nmr estimates and those from equation (1) add weight to the estimates in Table III. In Figure 2 the variation of log i and log 0 as functions of pKa reflect the vital role of ligand basicity in the inner-outer sphere competition. These curves indicate that the cross-over from predominantly outer sphere to predominantly inner sphere occurs near pKa values of 2. However, since the enthalpy and entropy changes for inner sphere complexation are larger than for outer sphere formation, both AH and AS would still be endothermic (characteristic of inner sphere reaction). 

At intermediate temperatures between 0 and T, the competition between the enthalpy and entropy terms will result in a saddle in the free energy versus composition curve, as is shown in Figure 4.1. This curve has two minima, which approach each other with increasing temperature, until they merge at T. If we draw a tangent between the minima, we find that the free energy will be less if the liquid separates... 

As any ring-opening polymerization, the polymerization of lactams is characterized by competition between the intermolecular reaction resulting in a linear polyamide and the intramolecular reaction of cyclization. Thus, if thermodynamically feasible, as indicated by a negative value of the free-energy change of polymerization AGp, the conversion of lactam to the linear polyamide can be realized if an appropriate reaction path exists for a reasonable rate in a polymer-monomer equilibrium characterized by a preponderance of linear macromolecules. The corresponding equilibrium monomer concentration [M]g is related to the standard enthalpy and entropy ASp of polymerization and to... 

Now let s combine the enthalpy and entropy terms. The enthalpy term is negative and the entropy term is positive, so the sign of AG for an addition reaction will be determined by the competition between these two terms ... 

For the cases of HCl and HBr, the sign of AG is determined by a competition between the enthalpy and entropy terms. At high temperature, the entropy term will dominate, and AG will be positive. At low temperature, the enthalpy term will dominate, and AG will be negative. Therefore, the process will be thermodynamically favorable at low temperatures. 

Walker, F.A. and Benson, M., Entropy, enthalpy, and side arm porphyrins. 1. Thermodynamics of axial ligand competition between 3-picoline and a series of 3-pyridyl ligands covalently attached to zinc tetraphenylporphyrin. J. Am. Chem. Soc., 1980, 102, 5530-5538. 

More than 50 years ago, Flory and Huggins [13-17] formulated a lattice model which captures the essential features of this competition between configurational entropy of mixing and enthalpy contributions, and even today this extremely simplified model is the basic ground on which most of the discussion... 

For any spontaneous process to occur chemical thermodynamics tells us that there must be a lowering of the free energy, AG, of the system. The driving force for polymer adsorption is thus the competition between the net energy change on adsorption (enthalpy of adsorption), the loss of conformational entropy of the adsorbed polymer, and the gain in entropy of solvent molecules released from the surface and polymer upon adsorption. 

In order for AG to be n ative, the enthalpy term mnst be larger than the entropy term. This competition between the enthalpy term and the entropy term is temperatnre dependent. At low temperature, the entropy term is small, and the enthalpy term dominates. As a resnlt, AG will be negative, which means that prodncts are favored over reactants (the eqniUbrinm constant K will be greater than 1). In other words, addition reactions are thermodynamically favorable at low temperature. 

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