Intrinsic reactivity scale

Topics of the general area of nucleophilic reactions of anions with neutral substrates in the gas phase included in this chapter are (1) the nucleophilicity of 02, in SN2 reactions, (2) development of an intrinsic reactivity scale for nucleophilic reactions with organic carbonyl-containing molecules, and (3) investigations of (CH30)2P0 in SN2 reactions with CH3X reactants and electron-transfer processes with XCF3 molecules. 

Intrinsic Reactivity Scale for Nucleophilic Addition Reactions at Carbonyl Centers... 

The intrinsic reactivity scale is a topic that led us to begin studies of gas-phase reactions in 1978. However, the problem of reversibility of the nucleophilic addition from the tetrahedral intermediate is even more severe in the gas-phase pressure regimes than it is in solution. 

As remarked already, kc/ku measures the maximal acceleration at levels of the CD sufficient to saturate complexation of the substrate. By looking carefully at the variations of this ratio with structure one may obtain insights into the mode of transition state binding (VanEtten et al., 1967a,b Bender and Komiyama, 1978). More useful is the ratio k2/ku (=kc/K ku) because it takes into account the effect of substrate binding and it scales the reactivity of S towards the CD to its intrinsic reactivity in the absence of CD. 

The calculation for dibenzylphosphoenolpyruvate assumes that the enol dibenzyl phosphate will have the same reactivity towards bimolecular attack by RCOOH as the dialkyl phosphate group of A.5.5. The comparison between A.5.3 and A.5.4 shows that the substitution of an alkoxy group by phenyl increases the reactivity by an order of magnitude towards COOH, and this factor allows us to put the phosphonate A.5.1 on the scale. The intrinsic reactivity of A.5.1 and A.5.2 are assumed the same b Gordon el al., 1964 Blackburn and Brown, 1969 c van Holst el al., 1974... 

The purification of the exhaust gas of PTA plants is one application where halohydrocarbon destruction catalysts have found use at a global scale (12, 13, 17, 18). Typically the untreated exhaust contains a mixture of volatile organic components including methyl bromide, carbon monoxide, hydrocarbons, methyl acetate, and organic acids. The presence of the methyl bromide sets forth the requirement that a catalyst such as the HDC be used. Additionally, the catalyst must be able to effectively destroy all the other organic components (with their widely different intrinsic reactivities toward air oxidation) of the mixture at reasonably low temperatures. Currently most PTA offgas remediation catalysts are used at an inlet temperature higher than 350°C. An improvement of catalyst activity is desired to... 

When the platinum nucleophilicity scale was first proposed it was implied that one np, scale was applicable to all substrates and that plots of logk2 against npt°(Y) were linear, taking the form log 10 2 = S np, (Y) + C, where S is termed the nucleophilic discrimination factor of the substrate and C its intrinsic reactivity. Discussions of mechanism based on a comparison of nucleophilic discrimination factors are frequently encountered. Nucleophiles that do not retain their positions in the nucleophilicity scale, e.g. NO2", SeCN and SC(NH2)2, were termed biphilic by Cattahni since their behaviour could be explained by their n-acceptor properties. When the Pt reaction centre had a greater n-basicity than the standard complex (for example a smaller effective nuclear charge) the substrate was more reactive than predicted and vice versa. This concept had been deduced some years earlier by Bosnich from his work with octahedral Ru complexes... 

Figure 6.9.22 shows the temperature profiles in the case where the real values of the intrinsic rate constant (MIRO refinery) deviate by factors of 2 and 0.5 from the values obtained from laboratory-scale experiments with a different catalyst. The real intrinsic reactivity seems to be slightly lower (better agreement in this case by a factor of 0.5). 

An intrinsic reactivity index (IRI) has been developed, with a view to capturing electro-and nucleophilicity on a single scale, and using frontier molecular orbital data to access values. A correlation of IRI with Mayr s E and N parameters is also described. 

On the other hand, there is considerable interest to quantify the similarities between different molecules, in particular, in pharmacology [7], For instance, the search for a new drug may include a comparative analysis of an active molecule with a large molecular library by using combinatorial chemistry. A computational comparison based on the similarity of empirical data (structural parameters, molecular surfaces, thermodynamical data, etc.) is often used as a prescreening. Because the DFT reactivity descriptors measure intrinsic properties of a molecular moiety, they are in fact chemical fingerprints of molecules. These descriptors establish a useful scale of similarity between the members of a large molecular family (see in particular Chapter 15) [18-21],... 

In order to think constructively about these problems we need some scale on which we can measure that intrinsic property of the ions which determines (in part) both stability and reactivity. In practice, we have to content ourselves, as far as carbenium ions are concerned, with the gas-phase standard enthalpy of formation, AHf, of a series of ions, and make estimates of the effects of solvation energies. In this way we can go quite a long way towards rationalising the reactivity of initiating ions with various monomers. 

The generally observed identity of the r value for solvolysis reactivity and gas-phase stability AAG(c+> of the corresponding carbocation leads to an important prediction concerning the solvolysis transition state. In a typical (limiting) two-step SnI mechanism with a single dominant transition state, the r values of transition states for the various nucleophile-cation reactions should be essentially controlled by the intrinsic resonance demand of the intermediate cation the substituent effect should be described by a single scale of substituent constants (a) with an r value characteristic of this cation. In a recent laser flash-photolysis study (Das, 1993) on the recombination of stable trityl and benzhydryl cations with nucleophiles and solvents, McClelland et al. (1986, 1989) have treated the substituent effects on solvent-recombination processes by (2). 

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