Reactivity and orientation

For the reason given above and for other reasons, it is unlikely that the encounter complex is a tt complex, but just what kind of attraction exists between Y+ and ArH is not known, other than the presumption that they are together within a solvent cage (see also p. 520). There is evidence (from isomerizations occurring in the alkyl group, as well as other observations) that tt complexes are present on the pathway from substrate to arenium ion in the gas phase protonation of alkylbenzenes.28 

The SeI mechanism (substitution electrophilic unimolecular) is rare, being found only in certain cases in which carbon is the leaving atom (see 1-38, 1-39) or when a very strong base is present (see 1-1, 1-11, and 1-42).29 It consists of two steps with an intermediate carbanion. The IUPAC designation is DE + AE. 

Reactions 2-41, 2-45, and 2-46 also take place by this mechanism when applied to aryl substrates. 

The orientation and reactivity effects of each group are explained on the basis of resonance and field effects on the stability of the intermediate arenium ion. To understand why we can use this approach, it is necessary to know that in these reactions the product is usually kinetically and not thermodynamically controlled (see p. 214). Some of the reactions are irreversible and the others are usually stopped well before equilibrium is reached. Therefore, which of the three possible intermediates is formed is dependent not on the thermodynamic stability of the products but on the activation energy necessary to form each of the three 

These conclusions are correct as far as they go, but they do not lead to the proper results in all cases. In many cases there is resonance interaction between Z and the ring this also 

Clearly, therefore, there is a very wide spread in reactivity, as in electrophilic substitution in benzene derivatives, but here we have the contrasting feature that the orientation pattern is relatively insensitive to the substituent. Consequently, it is important to establish the nature of this unusual orientating influence arising from the five fluorine 

In the benzenoid system, therefore, the activating effects of fluorine vary in the order meta-F ortho-F S para-F, although this can also vary with the system (see Table 9.6). Clearly, the para-F is slightly deactivating but is not very different from H at the same position. 

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A familiar feature of the electronic theory is the classification of substituents, in terms of the inductive and conjugative or resonance effects, which it provides. Examples from substituents discussed in this book are given in table 7.2. The effects upon orientation and reactivity indicated are only the dominant ones, and one of our tasks is to examine in closer detail how descriptions of substituent effects of this kind meet the facts of nitration. In general, such descriptions find wide acceptance, the more so since they are now known to correspond to parallel descriptions in terms of molecular orbital theory ( 7.2.2, 7.2.3). Only in respect of the interpretation to be placed upon the inductive effect is there still serious disagreement. It will be seen that recent results of nitration studies have produced evidence on this point ( 9.1.1). 

It is the purpose of this and the following chapter to report the quantitative data concerning the relationship of structure to orientation and reactivity in aromatic nitration. Where data obtained by modern analytical methods are available they are usually quoted in preference to the results of older work. Many of the papers containing the latter are, however, noted in the brief discussion which is given of interpretations of the results. 

Resonance effects are the primary influence on orientation and reactivity in electrophilic substitution. The common activating groups in electrophilic aromatic substitution, in approximate order of decreasing effectiveness, are —NR2, —NHR, —NH2, —OH, —OR, —NO, —NHCOR, —OCOR, alkyls, —F, —Cl, —Br, —1, aryls, —CH2COOH, and —CH=CH—COOH. Activating groups are ortho- and para-directing. Mixtures of ortho- and para-isomers are frequently produced the exact proportions are usually a function of steric effects and reaction conditions. 

For a review of orientation and reactivity in benzene and other aromatic rings, see Hoggett, J.G. Moodie, R.B. Renton, J.R. Schofield, K. Nitration and Aromatic Reactivity Cambridge University Rress Cambridge, 1971, pp. 122, 163. 

The most extensive study in the field of host-guest reactions in clathrates has been that of Lahav, Leiserowitz, and co-workers (56,241,243) on the choleic acids. The results of these combined chemical and crystallographic investigations are of possible importance for stereoselective steroid functionalization. In these studies potentially reactive guests were activated thermally or photochemically to produce species that attacked the walls of the channel at specific sites determined by the proximity, orientation, and reactivity of the host molecules at the wall relative to the activated guest species. 

Motivating the research is the need for systematic, quantitative information about how different surfaces and solvents affect the structure, orientation, and reactivity of adsorbed solutes. In particular, the question of how the anisotropy imposed by surfaces alters solvent-solute interactions from their bulk solution limit will be explored. Answers to this question promise to affect our understanding of broad classes of interfacial phenomena including electron transfer, molecular recognition, and macromolecular self assembly. By combining surface sensitive, nonlinear optical techniques with methods developed for bulk solution studies, experiments will examine how the interfacial environment experienced by a solute changes as a function of solvent properties and surface composition. 

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