Activating—deactivating effects theory

The ortho-para- versus meta-directing and activating versus deactivating effects of substituents can also be described in terms of PMO theory. The discussion can focus either on the structure of the cr complex or on the aromatic substrate. According to the Hammond postulate, it would be most appropriate to focus on the intermediate in the case of reactions which are relatively endothermic. The transition state should then resemble the a complex in reactions when the initial step has an appreciable activation energy. For more highly reactive electrophiles the transition state may be more reactant-like, in which case consideration of the reactant and application of frontier orbital theory would be more appropriate. Let us examine the effect of substituents from both perspectives. 

It has also been suggested that the molecular diameter of nitrogen pentoxide is effectively much greater for activating collisions, as well as for deactivating collisions, than that calculated in the ordinary way from the kinetic theory. The difficulty about this suggestion is that the... 

Metals and alloys. The principal industrial metallic catalysts ate found in periodic group VIII which are transition elements with almost completed 3d, 4d, and 5d electron orbits. According to one theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently will depend on the operating conditions. Platinum, palladium, and nickel, for example, form both hydrides and oxides they are effective in hydrogenation (vegetable oils, for instance) and oxidation (ammonia or sulfur dioxide, for instance). Alloys do not always have catalytic properties intermediate between those of the pure metals since the surface condition may be different from the bulk and the activity is a property of the surface. Addition of small amounts of rhenium to Pt/A12Q3 results in a smaller decline of activity with higher temperature and slower deactivation rate. The mechanism of catalysis by alloys is in many instances still controversial. 

Instead, they proposed a time on stream theory to model the catalyst deactivation. However, in an earlier work by Voorhies (2), a linear correlation between conversion and coke on catalyst for fixed-bed catalytic cracking was derived. Rudershausen and Watson (3) also observed the similar behavior. Coke on catalyst can reduce the activity by covering the active sites and blocking the pores. The effects of pore size on catalyst performance during hydrotreating coal oils in trickle-bed reactors have been studied experimentally by Ahmed and Crynes (4) and by Sooter (5). The pore size effects in other studies are also reported 7, 8). Prasher et al. (9) observed that the effective diffusivities of oils in aged catalysts were severely reduced by coke deposition. 

The effect that substituents already present have on electrophilic aromatic substitution reactions represents an area of structure-reactivity relationships that has been under study since about 1870. The classification of substituents as activating and ortfto-para-directing or deactivating and mete-directing has been known since those early studies. A basic understanding of these substituent effects became possible when ideas about electronic interactions and resonance theory were... 

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