Ethane-deuterium exchange

A mixture of Pt(ll) and metallic Pt in an aqueous medium was shown to oxidize ethane to yield acetic and glycolic acids. A series of deuterium-exchange processes enabled a complex mechanism to be elucidated metallic platinum catalyzes the oxidation of intermediate alcohols to acid products, whereas the Pt(ll) salt activates the initial alkene (Scheme 7X29... 

The kinetics and equilibrium of protium-deuterium exchange between pentafluoro-ethane and aqueous hydroxide ion have been studied.125... 

The data on hydrogen/deuterium exchange in paraffins on oxides indicated that a negatively charged intermediate is formed (26,27). Also the studies of adsorption of ethane and propane on oxide catalysts by IR spectroscopy showed that heterolytic dissociation of the C-H bond takes place with the formation of negatively... 

Figure 63. Compensation plot of Arrhenius parameters for ethane total exchange with deuterium (see Table 6.2) O foils, films, Pt(l 11) blacks, powders A supported Pt. The enclosed points are fa- Pt and Pd (see text).

Studies in deuterated water have shown that the hydroxyl proton does not end up in the ethanal formed. The decomposition of the 2-hydroxyethyl is not a simple P-elimination to palladium hydride and vinyl alcohol, which then isomerises to ethanal. Instead, the four protons stemming from ethene are all present in the initial ethanal product [6] (measured at 5 °C in order to suppress deuterium/hydrogen exchange in the product) and most authors have therefore accepted an intramolecular hydride shift as the key-step of the mechanism (see Figure 15.2). There remains some doubt as to how the hydride shift takes place. 

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. 

Fig. 1. The percentages of the various ethanes observed during the exchange of an 8 1 deuterium ethane mixture on 3.0 mg. of rhodium at 42.7° C. , CjHe A, j C2HsDs j Cl, CjH2ll4 O, C2Hr)j j C2r)a. Xhe percentage of C2H6D was too small to be measured accurately.

It is important to understand why this apparent first-order behavior is found for the course of an exchange reaction with time whatever the true kinetics of the reaction. A failure to understand this feature of exchange reactions has sometimes led to unjustifiable statements about the ratedetermining step in such reactions. It is convenient to discuss a specific example—the exchange of ethane with deuterium. Suppose that the only adsorbed species taking part in the reaction are (a) physically adsorbed... 

We shall now consider in outline a general method devised by Anderson and Kemball (19) for the interpretation of the initial distributions of products obtained in multiple-exchange processes. The method was devised, in the first instance, to apply to the exchange of ethane and deuterium, but it can be extended quite simply to cover other types of molecules. It involves the adoption of a specific mechanism from which calculated distributions are then obtained for comparison with observed distributions. The method will be illustrated for the exchange of ethane. The mechanism adopted is as follows. 

One of the earliest studies of the reaction of C2H4 with D2, in which a full mass spectrometric analysis of the products was performed, used a nickel wire as catalyst [115,116]. Some typical results are shown in Fig. 11. These results showed that ethylene exchange was rapid and the deutero-ethylenes are probably formed in a stepwise process in which only one deuterium atom is introduced during each residence of the ethylene molecule on the surface, that is there is a high probability of ethylene desorption from the surface. From Fig. 11(a) it can also be seen that the major initial products are ethane-d0 and ethane-d,. This is consistent with a mechanism in which hydrogen transfer occurs by the reaction... 

When ethene reacts with deuterium on a metal catalyst, the products typically consist of (i) deuterated ethenes containing from one to four deuterium atoms, formed by alkene exchange, (ii) deuterated ethanes containing from zero to six deuterium atoms formed by addition and (iii) hydrogen... 

Similar results were obtained with ethylene but not with ethane. Apparently the catalyst tends to produce deuterium atoms and to open up the double bond so that a deuterium atom can attach to one of the carbon atoms. Either exchange or addition may follow. 

---