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Aromatic substitution by free benzyl cations from the nuclear decay of multi‐tritiated toluene. Competitive benzylation of benzene and toluene in the gas phase and in solution
Author(s) -
Giacomello Pierluigi,
Angelini Giancarlo,
Sparapani Cinzia,
Speranza Maurizio
Publication year - 1989
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610020605
Subject(s) - chemistry , toluene , transalkylation , benzene , electrophilic aromatic substitution , solvent , nitromethane , isomerization , photochemistry , inorganic chemistry , medicinal chemistry , organic chemistry , catalysis , alkylation
Benzyl cations, free of counter ions, have been generated from the nuclear decay of tritium atoms contained exclusively in the side‐chain of multi‐labelled toluene and allowed to react in competition experiments with benzene and toluene. The aromatic substitution was studied in both the gaseous and liquid phase, employing pure aromatics or aromatics dissolved in n‐hexane, carbon tetrachloride and nitromethane as the reaction medium. In the gaseous systems, at 20 Torr total pressure, High meta substitution of toluene indicates extensive isomerization of the excited intermediates, while the increase of the k T / k B value in the Presence of NH 3 suggests that selective transalkylation competes with proton transfer to ammonia from the benzyl cation adducts with benzene, but not with toluene. High positional and low substrate selectivities were measured in solution, the decrease in k T / k B (from 2·8 to 1·7) in the different solvents being accompanied by an increase in the o /2 p ratio. Competition experiments between benzene and methanol indicate that the reaction is not diffusion controlled. The influence of the reaction medium on the substrate selectivity is explained by postulating the formation of a cation–solvent adduct, where the solvent molecule undergoes displacement by the substrate. Positional selectivity would be controlled in a subsequent step.