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Charge Distribution Differences in Benzenium and Nitrobenzenium Ions Based on 13CNuclear Magnetic Resonance Studies and Their Relevance to the Isomer Distribution in Electrophilic Aromatic Substitu5ons' George A. Olah,* Henry C. Lin,2 and David A. Forsyth2
Contribution f r o m the Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106. Received April 12, 1974 Abstract: Several stable benzenium and nitrobenzenium ions were prepared in solution by protonation and nitration of hexasubstituted benzenes. Carbon-13 and fluorine-19 nmr spectra of the two types of ions indicate significant differences in the pattern of charge distribution. The charge distribution differences are discussed in relation to observed differences in isomer ratios and to the potential importance in interpretation of substituent effects in electrophilic aromatic substitution.
T
he existence of arenium ions or u complexes, considered t o be intermediates in electrophilic aromatic substitution reactions, has been established in many studies by direct observation of the species as stable ions. Most investigations have dealt with cr complexes resulting from the alkylation or protonation of aromatic systems. Recently, the direct observation of long-lived u complexes in the nitration4 and ~ h l o r i n a t i o n of ~~ hexa- and pentasubstituted benzenes was reported. We now report an investigation of the electronic differences between nitrobenzenium ions and benzenium ions formed in protonation. The observation of significant differences in electron distributions in the cr complexes produced by various electrophiles could have important consequences regarding the interpretation of kinetic substituent effect data and isomer ratios in electrophilic aromatic substitution. The distribution of charge in ion-like transition states has been shown t o be of importance in determining the balance between resonance and fieldinductive effects of substituents,5 as well as the sensitivity to substituent influence, when the balance of contributing effects is constant.6 Thus, if charge distributions differ in o complexes, then electrophilic reactions for which u complex formation is rate-determining could display different responses t o substituent influences, even in the case that bond formation ([.e., progress along the reaction coordinate) is equal for the different reactions. Benzenium ions have been investigated by a variety ( 1 ) Stable Carbocations. CLXXIV. Part CLXXIII: G. A. Olah. R. J. Spear, P . W. Westerman, and J. M. Denis, J . Amer. Chem. Soc., 96. 5 8 5 5 (1974) i 2 ) Postdoc;oral Research Associate. (3) For a review, see D . M. Brouwer, E. L. Mackor, and C . MacLean in "Carbonium Ions," Vol. 11, G . A. Olah and P. v. R . Schleyer, Ed., Wiley-Interscience, New York, N.Y., 1970. (4) (a) G. A. Olah, H. C. Lin, and Y . I
