Orientation Effects in Substituted Benzenes

It was in March of 1939 when I met Henry at Princeton University where I was a visiting scientist from Kyoto University. We started to work together in September of that year. At first we studied the orientation effect of substituents for benzene. Our first paper on this subject may have been the first to calculate successfully the charge distribution in substituted benzenes. 

A more subtle application of the same technique involves isotopic substitution. Because most C02 frequencies are reproducible from crystal to crystal to within 0.07 cm ", it is possible to demonstrate small, but significant shifts of v3 when certain hydrogen atoms are replaced by deuterium. Analogous orientation effects have been reported for mixed crystals of isotopically substituted benzene, where isotopic differences in adjacent molecules gave rise to splittings as large as 4.7 cm "1 [91],... 

Moreover, product formation was not the same. Dibenzyl ethylsulfonium salt 72 gave only benzyl ethyl sulfide (73) and benzyl thiocyanate 74, but no dibenzyl sulfide or ethyl thiocyanate. By contrast, the cyclic sulfonium salt 75 gave a mixture containing chiefly (80%) dihydroisothianaphthene (76) with an equivalent amount of ethyl thiocyanate (77) and a minor amount (20%) of the sulfide-thiocyanate 78. This work represents the first direct experimental information on the rate of bimolecular nucleophilic substitution and relative orientation of the benzene ring and the leaving group and it demonstrates clearly the importance of stereoelectronic effects in these reactions. 

To determine the effect of a group on orientation is, in principle, quite simple the compound containing this group attached to benzene is allowed to undergo substitution and the product is analyzed for the proportions of the three isomers. Identification of each isomer as ortho, meta, or para generally involves comparison with an authentic sample of that isomer prepared by some other method from a compound whose structure is known. In the last analysis, of course, all these identifications go back to absolute determinations of the Korner type (Problem 10.8, p. 332). 

Resonance and induction have a marked effect on orientation in aromatic substitution. One would therefore expect some correlation between orientation in substitution and the electronic absorption spectra of benzene derivatives. In Table 5.4 the position of the 200 mp benzene band10 and the spectroscopic moments14 have been compared with the percentage of the meta isomer produced in nitration. A fair parallelism is apparent in the three columns. 

The previous sections leave no doubts that aromatic compounds, react with positively charged electrophiles to form a-complexes-arenium ions. But are they the primary intermediates It is not by accident that the problem of preliminary formation of radical cations has arisen. Its statement is an attempts to explain the orientational peculiarities of electrophilic aromatic substitution of hydrogen. The widespread view that the orientation in the reactions of aromatic compounds with electrophiles is dictated by the relative stabilities of the cr-complexes explains but a part of the accumulated material. In the first place this refers to the meta- and para-orienting effects of electron-releasing substituents in benzene in terms of the QCT -approach and to that of the relative reactivity of various aromatic substrates... 

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