Substituted diphenylmethyl cations. Part 2. Modes of decomposition of ions and a reinvestigation of the electronic absorption spectra

Abstract

The various modes of decomposition of diphenylmethyl cations, Ph₂C⁺–X, with different substituents X, have been examined. Substituents containing carbonyl groups cause the ions to cyclise rapidly to 9-substituted fluorenes. Trapping experiments have established the carboxy-, methoxycarbonyl-, and acetyl-substituted diphenylmethyl cations to be present in solution sufficiently long for reliable u.v. spectra to be recorded. The diphenyl(styryl)methyl cation immediately rearranges at room temperature to give the 1,3-diphenylindanyl cation. The diphenyl(phenylacetylenyl)methyl cation rapidly hydrates to form 1,3,3-triphenylprop-2-en-1-one in sulphuric acid solutions of intermediate strength, but in 96% H₂SO₄ the 1,1,3-triphenylprop-2-ynyl cation is stable at room temperature. The energy of the absorption maximum of the first band in the electronic spectra of the diphenylmethyl cations correlate with the Taft substituent constants, σ*.