Propinquity

Catalysis occurs because the catalyst in some manner increases the probability of reaction. This may result from the reactants being brought closer together [catalysis by approximation, or the propinquity effect ], or somehow assisted to achieve the necessary relative orientation for reaction. Noncovalent interactions may be responsible for the effect. Covalent bond changes may also take place in catalysis. In a formal way, the manner in which catalysis occurs can be described by schemes such as Schemes I and II. 

The proximity effect. This is the simple idea that in an intramolecular reaction the substrate function may be exposed to a larger local concentration of the reagent than in an intermolecular reaction, because the two functions are covalently constrained to occupy adjacent space. This effect has been called the approximation or propinquity effect. The proximity effect certainly seems physically reasonable and is likely to make some contribution to intramolecular reactivity, but it cannot be a major contributor when EM is large, because EM is itself a measure of a presumed local concentration, and the observed large EM values are physically impossible concentrations. The magnitude of rate enhancement achievable by prox-... 

The normal substances, however, really exhibit small deviations which are all the greater the more complex is the molecule of the substance. The theory of van der Waals, or in fact any hypothesis from which a theorem of corresponding states could be derived, assumes however that the transition from the gaseous to the liquid state, as well as the changes of density in either state, result from alterations in the propinquity of molecules which otherwise remain unaltered. Any association or dissociation of the substance would therefore give rise to abnormalities, and in fact the substances which deviate most from the normal relations (e.g.l water, acetic acid) are those which appear, on other grounds, to be associated in the liquid state. In the case of acetic acid the commencement of polymerisation, even in the state of vapour, is evident from the abnormal densities. 

Abnormally low atomic heats were explained by Richarz on the assumption of a diminution of freedom of oscillation consequent on a closer approximation of the atoms, which may even give rise to the formation of complexes. This is in agreement with the small atomic volume of such elements, and with the increase of atomic heat with rise of temperature to a limiting value 6 4, and the effect of propinquity is seen in the fact that the molecular heat of a solid compound is usually slightly less than the sum of the atomic heats of the elements, and the increase of specific heat with the specific volume when an element exists in different allotropic forms. 

Koshland (1962) has calculated, however, that such a propinquity effect will not explain the large rate enhancements observed with enzymes unless there are more than two functional groups involved with utilization of five functional groups (2 substrates and 3 catalytic groups) a rate increase of 10 would be possible. Such multifunctional catalysis would, of course, be impossible to demonstrate... 

Amide hydrolysis is not only subject to acid and base catalysis, but may be also brought about by a neighbouring group in the amide, the so-called propinquity catalysis effect. Wolfrom et a/.238 were among the first to suggest this possibility, in order to explain the observed hydrolysis of aldonamides, viz. 

This effect is also called the propinquity effect and means that the rate of a reaction between two molecules is enhanced if they are abstracted from dilute solution and held in close proximity to each other in the enzyme s active site this raises the effective concentration of the reactants. 

Propinquity (proximity) effects are important in reaction rate enhancement. In the case of the following compounds, anhydrides (products formed on removal of water) form at different rates. Arrange the compounds in order of their rates of anhydride formation and explain the reasons for the ordering. 

Propagation of errors, 40, 48, 248 Propinquity effect, 263, 365 Protol5Tsis, 147, 148 Proton inventory technique, 302 Proton transfer, 166 direct, 148 extent of, 346 fast, 97, 146, 173 isotope effect in, 296 partial, 395 Proximity effect, 365 Pseudo-first-order rate constant, 23 Pseudo-first-order reaction, 61 Pseudo-order rate constant, 23 Pseudo-order reaction, 23 Pseudo-order technique, 26, 78 Pulse NMR, 170... 

A general classification of most of the metal-free batteries is based on the electrode categories defined in Section 1.2 along with Eqs. (14) and (15). The terminology donor , acceptor (D, A) is used again, in spite of its shortcomings [10]. The other nomenclature, e.g. p-type instead of A-type and n-type instead of D-type, is omitted, however, for it suggests a propinquity to semiconductor physics which does not exist. 

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