Intermediate Compound Formation Theory

The two main theories of catalysis are (i) intermediate compound formation theory and (ii) adsorption theory. 

The reasoning which led the author to make this first shot in the dark regarding the usefulness of combinations of solid compounds as ammonia catalysts was as follows If we assume that a labile iron nitride is an interminate in the catalytic ammonia synthesis, every addition to the iron which favors the formation of the iron nitride ought to be of advantage. In other words, the hypothesis was used that surface catalysis acts via the formation of intermediate compounds between the catalyst and one or more of the reactants. An experimental support for this theory was the fact that a stepwise synthesis via the formation and successive hydrogen reduction of nitrides had been carried out with calcium nitrides (Haber), and cerium nitrides (Lipski). Later, the author found molybdenum nitride as being the best intermediate for such a stepwise synthesis. 

Theory of the Action in the Lead Chambers.—The actual mechanism of the chemical process in the lead chambers has for years been a matter of much conjecture and controversy. The formation of unstable intermediate compounds which subsequently undergo decomposition with production of sulphuric acid is almost universally accepted, but unfortunately there is no general agreement as to the identity of the intermediate compound or compounds. 

Another school of thought attempts to define the nature of such addition products somewhat more exactly by supposing that they represent perfectly definite though very unstable chemical compounds which result from the interaction of the catalyst with one or more of the components of the reacting system and which later decompose with the liberation of the catalyst and the formation of the end products of the reaction. This theory, based as it is upon the assumption of the formation of intermediate compounds, is primarily concerned in defining and demonstrating the actual existence of such substances. 

In trying to confirm their theory in regard to the intermediate formation of oxygenated products during combustion, Bone 9 and his co-workers carried out extensive researches and from their work has come the present hydroxylation theory. According to Bone the oxidation of methane takes place in steps, methanol, formaldehyde, formic acid, and carbonic acid being formed in the order named. These various steps are indicated below. The double arrows point out the mam course of reaction, while the single arrows show how the intermediate compounds may decompose. 

The formation of water and carbon in rapid hydrocarbon combustion, as in explosions, represents the best experimental evidence in support of this mechanism."1 The failure of Bone to detect methanol among the oxidation products in his experiments weakens this theory considerably, however. The only intermediate compound that could be identified was an aldehyde which together with water usually appeared as the early product. This is true of the higher paraffins as well as of methane. 

They assumed the formation of the hypothetical intermediate compound AI2CI5C6H5 in this reaction and of similar compounds in other reactions. This theory has never been demonstrated experimentally to be true and has, therefore, been superseded by other theories. 

The other theories claim the formation of intermediate compounds between the catalyst and the substances upon which the catalyst exercises an effect. This view is based moreover on experimentation and in a large number of cases it has been possible to isolate the intermediate substance, which is formed and which is decomposed, in turn, to cause the reaction under... 

Since Ostwald had no theory of catalysis he proposed superficial analogies a catalyst acts like oil on machinery, or a whip on a tired horse. The formation of intermediate compounds was made probable when J. Wagner showed in the oxidation of ferrous iron by permanganate in presence of chlorides, and J. Brode in the oxidation of hydriodic acid by hydrogen peroxide, that the products depended on the catalyst used. 

Sabatier postulated the formation of different intermediate compounds, each with its own mode of decomposition, and he recognised that some organic reactions are reversible, and where intermediate compounds cannot be isolated, there may be a production of surface compounds (chemisorption), thus linking the two theories of catalysis, the physical and chemical. Recent work has largely confirmed his views. 

The most widely accepted theory of enzyme action is based on the formation of an intermediate compound or adsorption complex between enzyme and substrate (Brown, 1902 Henri, 1903). Since both conceptions of the nature of the enzyme-substrate complex can lead to the same kinetic equations, the distinction seems unimportant at present. In the following development of the kinetic equations, the original scheme of Michaelis and Menten (1913) will be followed and compound formation will be considered to take place. However, in later discussions, the process will be considered as a type of adsorption. 

Chance (16a) recently demonstrated the formation of an intermediate compound of catalases and hydrogen peroxide. The compound had many properties in common with the intermediate compound of hydrogen peroxide and peroxidase. The spectrum could be determined in the region of the Soret band showing a small shift of the band toward the red. The rate of formation of this compound was 3 X lO liter mole sec, exceeding the value required by the Michaelis theory for catalase activity. Without the addition of acceptor the compound decomposed slowly at about the same rate as the peroxidase-peroxide compound. The catalase activity increases the equilibrium constant to 1 X 10 mole liter h The inter-... 

The theory of sublimation, t.e. the direct conversion from the vapour to the sohd state without the intermediate formation of the liquid state, has been discussed in Section 1,19. The number of compounds which can be purified by sublimation under normal pressure is comparatively small (these include naphthalene, anthracene, benzoic acid, hexachloroethane, camphor, and the quinones). The process does, in general, yield products of high purity, but considerable loss of product may occur. 

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

The mechanism involved in the formation of maltol (XXXIX) from the streptose residue of streptomycin, a mechanism which also applies to 5-hydroxystreptose, has been outlined by Lemieux and Wolfrom.1 An alternative theory has been suggested63 this is based on the supposed formation of an intermediate acyloin compound, as shown by the following formulas. However, this theory is unacceptable since one of the require-... 

The peroxide theory of Bach [20] and Engler [23] fixed the phenomenon of peroxide formation as the primary product of hydrocarbon oxidation by dioxygen. However, the problem of the mechanism of peroxide formation remained unsolved. The new stage of successful study of organic compound oxidation began after the discovery of free radicals as active intermediates of many chemical processes. 

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