Product distributions enthalpy changes

Because the free enthalpy change in this type of reaction is virtually zero, the result at equilibrium is a random distribution of the alkylidene groups. Thus, starting with methyl oleate, the equilibrium mixture consists of 50 mol% of the starting material and 25 mol% of each of the products 9-octadecene and dimethyl 9-octadecene-1,18-dioate. The cis/trans ratio of the reaction products is also in accordance with thermodynamics. This demonstrates that - in the presence of a suitable catalyst - the metathesis of unsaturated fatty acid esters provides a convenient and highly selective route to unsaturated diesters. Unsaturated diesters are important intermediates for the production of useful chemical products such as macrocyclic compounds. For instance, the diester obtained by metathesis of ethyl oleate has been subjected to a two-step reaction sequence, i.e. a... 

What could be the cause of such a large difference in thermodynamic stability After all, the number of Ni -N coordinate-covalent bonds is six in both the products of these two reactions, so the enthalpy changes (Ai/) involved when these bonds are formed should be fairly similar. That seems to leave entropy as the major explanation for the effect. Indeed, the rationale for the chelate effect can be understood in two ways, both related to the relative probabilities that the two reactions will occur. First, consider the number of reactants and products in the two cases. As written more explicitly in Equations (6.11) and (6.12), it is apparent that the number of ions and molecules scattered throughout the water structure in the first reaction stays the same (seven in both the reactants and the products). In the second reaction, however, three ethylenediamine molecules replace six water molecules in the coordination sphere, and the number of particles scattered at random throughout the aqueous solution increases from four to seven. The larger number of particles distributed randomly in the solution represents a state of higher probability or higher entropy for the products of the second reaction. Therefore, the second reaction is favored over the first due to this entropy effect. 

First of all the BDE itself may depend on the solvent as a result of contributions of solvation to the enthalpy of the bond dissociation process, i.e., differences between the solvation enthalpies of the reactant (L M-R) and products (L M. + R.) of Equation 1. There is not much direc information about tRis. Most of the applications of ghe kinetic method to dat have involved low spin octahedral d complexes (notably of Co ) where the product of homolysis is a 17 electron low spin five-coordinate d complex. Such complexes generally exhibit little tendency to add a sixth ligand so that solvent influences due to coordination are likely to be small. Support for this is provided by invariance of the Co-C BDE of PhCH(CH )-Co(DH)2Py in solvents of considerably different polarity and coordinating ability, e.g., toluene and acetone. Bond dissociation processes that are accompanied by substantial changes in charge distribution (unexpected for the homolytic dissociation of neutral radicals) are, of course, expected to exhibit significant medium effects. 

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