Equilibrium constant deuterium exchange reaction between

Problem Determine the equilibrium constant of the homogeneous gaseous isotopic exchange reaction between hydrogen and deuterium iodide, via.,... 

We can use the ratio of fractionation factor products to predict relative equilibrium constants for reactions in H2O or D2O. Consider the dissociation of an acid in H2O or D2O. Eq. 8.11 tells us that the ratio of equilibrium constants for a reaction in pure D2O (Kq) to that in pure H2O (Kh) is the ratio of the mathematical products of fractionation factors for the products (<, P) and reactants (0yO- Letters / and j are running tags of each exchangeable site in the product and reactant, respectively. In essence, the ratio between the equilibrium constants in the.se two solvents reflects the relative ability of the products and reactants to accept deuterium over hydrogen. We ask you to derive Eq. 8.11 in Exercise 4 at the end of the chapter. 

Similar results have been obtained for methane 12) and for ethane 19). The values quoted in Table II also illustrate the point that the distribution of deuterium between hydrogen and propane differs from the value expected for a random distribution. With the ratio of pressures used, the expected percentage for the mean deuterium content of the hydrocarbon would be 33.3, which is substantially less than the experimental value of 40.9 %. This type of deviation is also found with other hydrocarbons, but it does not affect the validity of using classical theory for the calculation of the interconversion equilibrium constants in studies of mechanism of exchange reactions. More accurate values for these equilibrium constants are necessary, however, if one is interested in the separation of isotopes by chemical processes. 

The rate of hydrogen exchange depends on the protolytic properties of both the solvent and the substrate. In fact there is a correspondence between the magnitude of the rate constants for deuterium exchange with ND3 and the conventional ionization constants of hydrocarbons which were used by Conant and Wheland (1932) and by McEwen (1936) to obtain the first quantitative estimates of the acidity of hydrocarbons. To do this, they determined the equilibrium of metallation of hydrocarbons by organo-alkali metal compounds. This reaction was described by Shorygin (1910) and is represented by the equation... 

This reaction between s-trinitrotoluene and ethoxide ion, elegantly studied by Caldin and Long (47), is a reversible one to form a purple solution. By spectroscopic techniques it is possible to measure the forward rate, the reverse rate and the equilibrium constant. Although the form of the kinetic equations does not permit a differentiatimi between a proton transfer process and an addition process, the authors favor the former explanation, attributing the purple color to formation of the anion XII, and citing as evidence the deuterium exchange experiment and the fact that trinitrotoluene, in the presence of ethoxide ion or pyridine, acts as a nucleophile toward benzaldehyde. The purple solution is decolorized by a series of weak acids at rates which are related to the dissociation constants of the acids and measurable at temperatures from — 80° to -f-20°. 

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