Alkanes, exchange with deuterium

The alkanes produced in this interaction are certainly formed as a result of a hydrolytic cleavage of alkyl platinum derivatives. The close agreemmt betweai the ratio of methancs-d formed in platinum(ll) methylation and in direct hydrogen-deuterium exchange with alkanes [12] is a sound support for the formation of alkyl platinum derivatives in the homogeneous activation of alkanes. 

Hydrogen-deuterium exchange in alkanes shows the same dependence on the composition of solvent, and concentration of mineral acid, of platinum( 11) and of chloride ions as does the exchange in aromatic hydrocarbons. The dependence of the exchange rate on solvent composition has been discussed in Section III,B,1. The rate is independent of mineral acid concentration, increases nonlinearly with increase in the concentration of platinum 11), and decreases nonlinearly with increase in the concentration of chloride ions (55). The nonlinear dependences are due to the solvolysis of the PtCL,2- ion ... 

Alkanes reduce platinum(IV) to platinum(II) under conditions similar to those of hydrogen-deuterium exchange with the solvent (at 100-120 °C),... 

When isoalkanes are contacted with D20-exchanged solid acids such as zeolites, sulfated zirconias, or heteropoly acids, proton-deuterium exchange takes place slowly even at room temperature but above 100°C the catalysts are rapidly depleted of their deuterons, which are recovered in the alkane.54,55 This exchange process is... 

To the best of our knowledge, there are no published data on the reactions of alkanes with H2 over palladium single crystals. Deuterium exchange reactions with various alkanes over palladium films were extensively investigated by Kemball et al. (39-42). Because those results were already discussed in several excellent review articles (43-46), we do not intend to dwell upon analyzing all the data at greater length. We only specify here the basic characteristics of Pd films exhibited in D2/alkane reactions ... 

The following basic facts have been established from exchange with deuterium of numerous alkanes and polycycloalkanes especially on Pd catalysts. 

Very recent work (60b) has confirmed that Ir films do not isomerize neopentane most of the transition metals as well as palladium (60c) rearrange isobutane to k-butane but are also inactive for the former conversion. This clearly indicates that isomerization of neopentane on Pt is mechanistically rather special and, in view of the known propensity of Pt to promote ay exchange with deuterium of paraffins (5,49), refocuses attention on the ay species diadsorbed on one metal atom as the precursor for bond shift in simple alkanes. The following mechanism for neopentane isomerization on Pt is feasible, where the shifting... 

The early use of deuterium in place of hydrogen in the study of catalytic hydrogenation led to the recognition that the process was not simply the addition of H2 to the double bond. Horiuti and Polanyi proposed that both H2 and alkene (1) are bound to the catalyst surface and transformed to products by a sequence of elementary steps, which they represented as shown in Scheme 1, where an asterisk ( ) represents a vacant site on the catalyst.The last step, (d), is virtually irreversible under the usual hydrogenation conditions, but can be observed in the exchange reactions of D2 with alkanes. The mechanism accounts for the isomerization of an alkene if the reversal of step (c), which involves the formation of the alkyl intermediate (3), involves the abstraction of a hydrogen atom other than the one first added, and is coupled with the desorption of the alkene, (2) - (1). At present, the bond between the alkene and the metal often is represented as a ir-complex (4), as in equation (7). ... 

E2S.15 Two observations must be explained in this catalytic deuteration of C-H bonds. The first is that ethyl groups are deuterated before methyl groups. The second observation is that a given ethyl group is completely deuterated before another one incorporates any deuterium. Let s consider the first observation first. The mechanism of deuterium exchange is probably related to the reverse of the last two reactions in Figure 26.20, which shows a schematic mechanism for the hydrogenation of an olefin by D2. The steps necessary for deuterium substitution into an alkane are shown below and include the dissociative chemisorption of an R-H bond, the dissociative chemisorption of D2, and the dissociation of R-D. This can occur many times with the same alkane molecule to effect complete deuteration. 

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