Hydrogenation multiplet theory

A. A. Balandin, Multiplet Theory of Catalysis Theory of Hydrogenation, Moscow State Univ., Moscow, 1970, in Russian. 

Dehydrogenation of the cyclohexanes over platinum, palladium, and nickel is selective in the sense that it affects only those compounds which can form aromatics by this reaction. Six-membered rings with substituent groups in positions such that elimination of hydrogen cannot form a benzene ring, for instance, 1,1-dimethylcyclohexane, undergo dehydrogenation at 300° with difficulty (442). The multiplet theory explains... 

The first mechanism proposed for the newly discovered hydrogenolysis of cyclopentane was the doublet mechanism (104), in line with the general multiplet theory of Balandin (105). According to this mechanism, cyclopentane is physically adsorbed on two metal atoms, and stands perpendicular to the surface in the vicinity of physically adsorbed hydrogen atoms (Fig. la). This mechanism, however, does not explain why cyclopentanes are hydro-genolyzed on platinum while cyclohexanes and paraffins are not. 

Indeed, as evidenced by experimentation, in accordance with the multiplet theory the activated adsorption of hydrogen poisons catalytic hydrogenation 41). The atoms of hydrogen held by activated adsorption are in dynamic equilibrium with other forms of adsorbed hydrogen and with the hydrogen in the gaseous phase. Therefore, the hydrogen atoms leave the valleys V from time to time. 

The application of the theory of absolute reaction rates (36) to catalysis turns out to be closest to the multiplet theory. The former was applied for the first time by Temkin (58) with a simplifying assumption that the sum of the partition functions of the particles on the surface equals unity. Let us note the results (36) that are near to the multiplet theory. The theory of absolute reaction rates, based on quantum mechanics and statistics, proved that in the case of adsorption, the attraction of the two-atom molecules (of hydrogen) to two atoms of the catalyst (carbon or nickel) is energetically more favorable than to one atom. It demonstrates that on solid surfaces the true energy of activation must be small and that for the endothermic process it must be nearly equal to the heat of the latter. As in the multiplet theory, the theory considers the new bonds as beginning to be formed before the old ones are broken. The theory deals with the real arrangement of atoms and with the mutual energy of their valence electrons. 

Few applications of the theory of absolute reaction rates to particular catalytic reactions have been made so far. Among them hydrogenation of alcohols on copper [59] can be pointed out. As has been shown by the author for reactions of the type mentioned, the application of the theory of absolute reaction rates to the model given by the multiplet theory leads to such results that are in accordance with experiment. 

In Eqs. (1.16) a complete break of bonds is assumed, which is an extreme case, however. The getting of index atoms into a deeper valley L (Fig. 6) corresponds to activated adsorption. The model of Fig. 6 satisfies the princijile of the conservation of valence angle (see p. 31). It permits one to employ a computation according to the theory of absolute reaction rates and conveys well enough different features of the theory of hydrogenation, based on the multiplet theory (78-83). 

Thus the alloy 74.9% Co -(-21.7% Fewith the latticeAl is still active, and the alloys 50% Co + 50% Fe and 24.2% Co + 75.8% Fe with a lattice A2 are not. Copper seems to be an exception (see above). Rie-niicker and Unger (207) carried out a similar very accurate study applying X-ray analysis and adsorption measurements of the surfaee the results supported the requirements of the multiplet theory. Recently, Rieniicker (208) found that for the hydrogenation of propylene (which proceeds according to the model of Fig. 6) binary alloys of Ni and Fe of diffei ent composition are also active only until they have a nickel structure, i.e., Al, which is in conformity with the multiplet theory. 

While hydrogenating compounds with a triple bond on Ft and other metals cis-olefins are formed in an overwhelming quantity over trans-olefins. This happens in spite of the fact that the /raws-olefins are more stable thermodynamically. The reason for such behavior of substituted acetylenes, according to the multiplet theory, is that the surface in the case of the two-point adsorption of a triple bond hinders the substituents and makes both of them move off the surface. For the same reason the cis forms of olefins hydrogenate more easily than the trans forms. This phenomenon is well known in catalytic synthesis. 

The multiplet theory explains the results of the extensive stereochemical studies of Linstead et al. (261, 262) on the hydrogenation on platinum of the derivatives of octahydrophenantrenes (1.35)... 

Almost exclusively cis-syn-cis forms are produced—the H-atoms add only on one side of the ring [for more detail, see Balandin 10). Lin-stead draws the following conclusions from his work (1) hydrogen is joined from the side of the catalyst, and not from the liquid or gaseous phase, (2) at hydrogenation on Pt the aromatic rings are situated flatly, and (3) steric hindrances between the catalyst and the substrate play a considerable part in hydrogenation. One can easily see that these results are in full conformity with the multiplet theory. 

According to the multiplet theory, cis forms must result on hydrogenation of disubstituted benzenes. Investigations on isomeric xylenes and on eight isomers of dimethylcyclohexane showed that under mild conditions this really occurs (264). 

The multiplet theory permits the building of stereochemical models of active complexes of hydrogenation of the compounds (VII) and (VIII). Since the molecules of triptycene possess a rigid structure, except for flattening or inversion according to the Sn2 mechanism, the molecule cannot accommodate an index group on an even surface. Therefore, one should assume the existence of elevations on the surface of the catalyst. As the Cn=Ci6 bond is internal, the molecule must superimpose on the elevation that carries the (111) facet (see above). 

Thus relative values of the rates of catalytic hydrogenation evidence specific properties of silica gel. In terms of the multiplet theory this means that there exists in this case a structural correspondence between the extra-index substituents and the carrier. 

We have already pointed out (see Section I) the similarity of the multiplet theory and the theory of catalysis by Polanyi. The equations of the multiplet theory [(1I.9)-(1I.11)] were given by the author as early as in 1929 Polanyi suggested similar equations in 1934 (35), how ever, for one reaction only—the para-ortho-conversion of hydrogen. In 1957 Temkin (366) developed the Polanyi theory and also obtained Eqs. (II.9) and (II.10), the same as in the multiplet theory. 

The calculation according to the multiplet theory showed that compound (II) and not (III) has to be formed as an intermediate one. Indeed the intermediate hydrogenation product separated at the moment of the breaking of the kinetic curve of hydrogenation has proved to be compound (II), expected according to the theory, but not (III). 

The multiplet theory has also proved useful for investigating the hydrogenation of monosaccharides (423). From the calculation of E according to Table XI the reaction sequence shown in Table XXII is expected. Reaction (1) is the hydrogenation of a monosaccharide into a polyalcohol (2) is the reduction of the hydroxyl group (3) is the Cannizzaro-Tishchenko reaction, i.e., the formation of gluconic acids ... 

Thus, according to Sabatier the hydrogenations of olefins and ketones are the same type of reaction, while according to the multiplet classification these two reactions are of different types, and indeed, the two reaction types require two different catalysts. Naturally, in the application of a given classification the thermodynamic nature of the reactions should be taken into account as well as their structural aspects. This classification and thermodynamic requirements do not yet deal with the kinetics of processes. The latter is involved in the principles of structural and energetic correspondence of the multiplet theory. 

Balandin (2) shortly afterwards began the publication of a series of papers developed from the theory that the catalytic decomposition of a relatively large molecule could only take place by simultaneous adsorption at several points this was known as the Multiplet Theory. With cyclohexane, for example, attachment at six centers was supposed to be necessary before benzene could be formed by the loss of three hydrogen molecules. Calculations along these lines for metallic catalysts, whose lattice dimen-... 

Balandin, A.A. (1970) Multiplet theory of catalysis. Part III. Theory of hydrogenation. Classification of organic reactions. Theory of complicate reactions. Structural algebra in chemistry, (russ., Klabimovskii, E.I., ed.), Publ. Moscow University, 475 pp. 

The multiplet theory does not therefore take account of tWe extremely active and unsaturated nature of catalyst surfaces, which in the above example serves to provide the requisite source of hydrogen atoms for reaction to proceed. As an extension of this, it may be stated that the rate-determining step of catalsdiic reaction may be one of three types ... 

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