Isolation of the Stable Boron Trifluoride – Hydrogen Fluoride Complexes of the Methyl-benzenes ; the Onium Salt (or σ-Complex) Structure of the Friedel-Crafts Complexes


THE alkylation of aromatic hydrocarbons catalysed by aluminium chloride and similar Friedel-Crafts catalysts is ordinarily accompanied by the formation of deeply coloured oily liquids of high density. The structure of these ‘Friedel-Crafts complexes’ has been investigated by a number of workers1. H. C. Brown and H. W. Pearsal2 have proved the formation of a molecular complex from toluene and aluminium chloride – hydrogen chloride at low temperature. In further work, H. C. Brown and J. D. Brady3 have established the structure of the hydrogen chloride – aluminium chloride complexes of the methylbenzenes as σ-complexes in contrast to the proposed hydrogen chloride complexes which have a π-complex structure (after the representation of Dewar4). The formation of the onium or σ-complexes was proved by Brown by the hydrogen chloride technique by measuring the solubility of hydrogen chloride in the presence of aluminium chloride in the isomeric methylbenzenes.

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  1. 1

    A summary of earlier work is contained in Thomas, C. A., “Anhydrous Aluminium Chloride in Organic Chemistry”, Amer. Chem. Soc. Mon. No. 87, pp. 48–54 (Reinhold Pub. Corp., New York, N.Y., 1941)

  2. 2

    Brown, H. C., and Pearsal, H. W., J. Amer. Chem. Soc., 74, 191 (1952).

  3. 3

    Brown, H. C., and Brady, J. D., J. Amer. Chem. Soc., 74, 3570 (1952).

  4. 4

    Dewar, M. J. S., J. Chem. Soc., 406 (1946).

  5. 5

    Norris, J. F., and Ingraham, N. J., J. Amer. Chem. Soc., 62, 1298 (1940).

  6. 6

    Norris, J. F., and Wood, J. E., J. Amer. Chem. Soc., 62, 1428 (1940).

  7. 7

    Brown, H. C., and Wallace, W. J., J. Amer. Chem. Soc., 75, 6268 (1953).

  8. 8

    McCaulay, D. A., Shoemaker, B. H., and Lien, A. P., Indust. Eng. Chem., 42, 2103 (1950). See also McCaulay, D. A., and Lien, A. P., J. Amer. Chem. Soc., 73, 2013 (1951); 74, 6246 (1952).

  9. 9

    Klatt, W., Z. anorg. allg. Chem., 234, 189 (1937).

  10. 10

    Oláh, G., and Kuhn, S., Z. anorg. allgem. Chem. (in the press).

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