Butane-deuterium exchange

Recent work [log] casts some doubt on the generality of this interpretation. Evidence from deuterium-exchange experiments on butan-2-one indicates that the transition state for enolisation has little if any carbanion character that weak bases, like acids, favour formation of the more substituted enol, and that even the fairly strong base DO" caused deute-... 

Myers et al. (39) have studied the deuterium exchange between two paraffins, i.e., between deuterobutane and butane, on a variety of dualfunctional catalysts (various impregnated types of platinum-acidic oxides). They find that the hydrogen exchange correlated primarily with the (de-)hydrogenation activity of the catalyst, but in addition, an appreciable positive correlation with acidic activity was demonstrated, which led the authors to conclude that this may mean that acid sites and platinum sites coact to promote the latter effect. ... 

Alcohol (a) can be used to indicate the extent of syn- and anti-elimination (see Scheme 8) and (b), through a study of H/ H exchange can indicate whether the mechanism is El, E2, or ElcB. Thus for an El mechanism, deuterium exchange can occur in all positions in the butenes and loss of a- H from [2- Hi ]butan-2-ol provides strong evidence of this mechanism. 

In support of this hypothesis it has been observed that when aluminum bromide is treated with oxygen a reaction occurs which liberates bromine,48 and further, the halogen atoms rather than the hydroxyl group are responsible for the catalytic activity of HOAlBr2. Thus when the compound DOAlBr2 is prepared, the hydrogen-deuterium exchange with n-butane is not at all proportional to isomerization.4 ... 

It was found that by treating either n-butane or isobutane with 10 mole % deuterium bromide-aluminum bromide catalyst for 20 hours at 25°, no isomerization of the butanes occurred and only 6 and 9.5% of the deuterium exchanged with n-butane and isobutane, respectively. When, however, 0.1 mole % butenes was added to n-butane and the isomerization reaction was carried out under the same experimental conditions, over 40% of the butane isomerized to isobutane and 92% of the deuterium underwent an exchange reaction. These results indicate clearly that olefins take an active part in isomerization. The results obtained are in agreement with the proposed mechanism of isomerization. 

T. F. Narbeshuber, M. Stockenhuber, A. Brait, K. Seshan, J. A. Lercher, Hydrogen/deuterium exchange during-butane conversion on H-ZSM-5, J. Catal, 1996, 160, 183-189. 

Recently, Rooney (80) expressed the view that the a,j8 exchange process involved tt olefin complexes and asserts that this explains the pattern of exchange on a palladium film of deuterium with 1,1-dimethylcyclo-butane. Its failure to exhibit appreciable multiple isotopic exchange was attributed to the difficulty of forming a tt olefin complex because of the strain in cyclobutene. 

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 behavior of CeX in the presence of D2 and D2O is in marked contrast to that of NiX. The latter is believed (8) to operate via a radical mechanism when deuterium was present, appreciable hydrogenation to butane was observed and the rate of isomerization was markedly enhanced, whereas the rate of isomerization was not altered in the presence of D2O. Again in contrast to CeX, with NiX there was extensive exchange with deuterium but virtually no deuterium incorporation into the butenes when D2O was employed. 

The effect of temperature is peculiar (see Fig. 13). The deuterium content of the reactant olefin, in this case 1-butene, at about 10% conversion falls with increasing temperature, paralleling its behavior in isomerization. However, the deuterium content of the isomerized 2-butenes rises rapidly, as does also the deuterium number of the butane. This tells us (1) intramolecular hydrogen transfer becomes less important with rising temperature and is perhaps replaced by reactions involving adsorbed deuterium atoms (step 1. ii), and (2) butane is probably formed through the same intermediates as those which yield 2-butenes, viz., 2-butyI radicals. The same kind of behavior is shown by the other butenes. Partial pressure variation has no remarkable effect on the course of exchange and isomerization. 

The exchange reaction occurring in the presence of aluminum bromide between deuterium of the deuterium bromide and hydrogen of butanes, when small amounts of olefins are present, can be represented by the equations ... 

The process took place on Ni/Cr203 between 373 and 525 K and must require the dissociative chemisorption of the methanes followed by random recombination not surprisingly, deactivation was found at all temperatures." The reaction between normal and deuterated n-butane has also been studied on a number of supported platinum catalysts." Little or no exchange was seen between light methylcyclo-hexane and deuterated n-octane (or the reverse) over Pt/Si02 at 755 K either in the presence or absence of hydrogen or deuterium because of rapid conversion to other molecules reaction did however take place between them at lower temperatures, and on Pt/Al203 it occurred both on the metal and the support. ... 

The behaviour of Ir/Al203 resembled that of Pt/AbOs, but relative isomerisation and exchange rates were even lower at 253-293 No isomerisation of 1-butene was detected, and only 2.5% of 1-butene-rfi was detected mean deuterium numbers of the butane were close to two. 

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