The indanyl cation: a real time observation

A stopped‐flow investigation by U.V. spectroscopy has been carried out using various reactions which yield the indanyl cation: polymerization of indene by trifluoromethane sulfonic acid (TfOH), ionization of 1‐chloroindane by antimony pentafluoride and protonation of a dimer of indene (2‐α‐indanyl indene) by TfOH, at variable temperature. The monomer and dimer cations present a main absorption at 318‐325 nm and the polyindene cation at 330 nm. A side reaction yields a derived cation, which absorbs at 519 nm. The molar absorbance of the indanyl cation has been estimated (ϵ=15 500 L.mol.−1 cm−1).