Selectivity, intermolecular

It can be concluded, as already stated above, that the diminution in intermolecular selectivity observed in these nitrations with nitronium salts in organic solvents does not of itself require any special mechanistic considerations as regards the process of substitution. 

With the more concentrated solution the results, as regards loss of intermolecular selectivity, were similar to those obtained with nitronium salts (table 4.1, column a), whilst with the more dilute solution a more usual situation was revealed. The significance of the former observations is again open to doubt because of the likelihood that mixing was relatively slow, and also because reaction upon encounter is here a serious probability. 

In other systems electrophiles other than the nitronium ion are involved with activated substrates (in these cases intermolecular selectivity is high, whereas with nitronium salts it is low). 

Although the intermolecular selectivity of the nitration of alkylbenzenes by nitric acid in trifluoroacetic acid is controlled by both electronic and steric factors, it is argued that intramolecular selectivity is controlled by steric effects on transition state solvation. 

Second, a tunable laser radiation can excite atoms or molecules of a certain species (or even of a certain isotopic composition) in a mixture, that is, can ensure the intermolecular selectivity of the subsequent photochemical processes [3]. 

Olah s original experiments, in which the intermolecular selectivities were determined by direct competition for the electrophile by toluene and benzene, have given rise to controversy and criticism.174 Schofield and Moodie suggested... 

About (i) and (2) there can be no dispute, but (3) must be rejected. The implication that the nitronium ion, effectively freed from a close association with another entity, is not the nitrating agent in those reactions of benzene and its homologues, under conditions in which substantial intermolecular selectivity is observed, conflicts with previous evidence ( 3.2). Thus, in nitration in organic solvents and in aqueous nitric acids, the observation of kinetically zeroth-order nitration, and the effect of added nitrate on this rate, is compelling evidence for the operation of the nitronium ion. The nitric acidium ion is not the electrophile under these conditions, and it is difficult to envisage how a species in which the water is loosened but not yet completely eliminated could be formed in a slow step independent of the aromatic and be capable of a separate existence. It is implicit that this species should be appreciably different from the nitronium ion in its electrophilic properties. There is no support to be found for the participation of the aromatic in the formation of the electrophile. 

A more recently recognized kinetic complication is that, for all aromatics more reactive than toluene, Eq. (3.3) is encounter limited, so that all these substrates react at the same rate [68JCS(B)800] in 68.3 wt% H2S04 this is —40 times that of benzene (67CC352). A problem here is that although there is no intermolecular selectivity under these conditions, the... 

As far as the tertiary benzylic solvolyses are concerned, any structural and mechanistic perturbations are reflected only in the variation of the p parameter. The p value for a reaction series is a parameter of intermolecular selectivity and can change sensitively with the reactivity (or the stability of transition state). This behaviour is often referred to as adherence to the reactivity-selectivity relationship (RSR), where the selectivity (5) may vary inversely with the intrinsic reactivity of members of a reaction series, as formulated in equation (3),... 

CHDF end group, no selective reaction would occur on time scales above 10 s, figure B2.518. In contrast to IVR processes, which can be very fast, the wtemiolecular energy transfer processes, which may reduce intermolecular selectivity, are generally much slower, since they proceed via bimolecular energy exchange, which is limited by the collision frequency (see chapter A3.13). 

Figure B2.5.18. General scheme for inter- and zwframolecular selectivity in laser chemistry. Intermolecular selectivity a laser with frequency v selectively excites molecules A, which subsequently react, in a mixture of A and B molecules, /zztromolecular selectivity a laser with frequency Vj (V2) selectively excites the chromophore Chr j (Chr2) of a molecule which preferentially follows reaction 1 (2) at this position (after...

Strategies for achieving intra- and intermolecular selectivity are the subject of a very active freld of current research with many open questions. Under the label coherent control it includes approaches that exploit the coherence properties of laser radiation to control chemical reactions. Figure B2.5.18 summarizes the different schemes of intra- and intermolecular selectivity. 

Some recent observations on the intramolecular and intermolecular selectivity of some nitrating agents are presented in summary form in this paper. Attention will be concentrated on nitration by nitric acid in aqueous sulphuric acid, and by way of introduction reference will be made both to our own earlier studies and also to those of other workers in this field. It will become apparent that a situation of increasing complexity is being gradually revealed. 

A most reasonable interpretation of these results would be that the reactive entity is some species, perhaps radical in nature, which shows a very different intramolecular selectivity from that of the nitronium ion. The efficacy of nitrogen dioxide itself appears ruled out, however, by the low conversions. Study of the intermolecular selectivity of the reagent by competition (Table III) indicates that the reagent is about as selective as the nitronium ion between alkylbenzenes. The relative rates observed are compared with two systems (, , 11,22) where the nitronium ion is well established as the electrophilic species. For toluene the unusual isomer ratios were observed in these competition runs. 

We are therefore faced with an anomalous situation. A gross change of the reactive species from the nitronium ion is indicated by the results of study of intramolecular selectivity but this change would have gone unsuspected if reliance had been placed solely on the evidence of intermolecular selectivity. 

In determining selectivity in ligogenic processes (Chapter 9), the center of attention is usually on either whole molecules Sj vs. S2, or, molecular sites tj vs. t2. In the former instance, one deals with morphoselectivity - selectivity based on morphic relationships between molecules in contrast, in the latter instance, one invokes situselectivity - selectivity based on topic relationships between molecular sites. In this chapter we deal with morphoselectivity situselectivity is treated in the next chapter. The concept of morphoselectivity is synonymous with substrate selectivitystructural selectivity," intermolecular selectivity," shape selectivityand, enzyme substrate selectivity it is also implicit in intermolecular chemoselectivity. This chapter deals with the concept of morphoselectivity, and establishes the commonality of all the above literature terms. 

D,D% F, F faces - as in discussions of intermolecular selectivity (Figure D.4). The literature a/p system is inapplicable to these acyclic systems. Furthermore, the literature Re/Si (relsi), B/N (b/n) systems do not differentiate between enantiotopic faces, on the one hand, and diastereotopic faces, on the other. Thus, whether it is the enantiotopic faces of iii, or the diastereotopic faces of iv and v, the faces are described by the same Re/Si and B/N descriptors. In contrast, in the novel HED system, iii has enantiotopic faces E / 3, iv possesses diastereotopic faces D/F, and v incorporates chirodiastereotopic faces D /F. Thus, the HED system (a) identifies the relative stereotopicity of the paired faces, including homotopic ones, (b) differentiates between enantiotopic and diastereotopic faces, and (c) reveals the chirality of the molecular field - e.g. homotopic vs. chirohomotopic (i vs. ii), and, diastereotopic vs. chirodiastereotopic (iv vs. v). 

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