HF adds to perfluorinated benzocyclobutene, 1-methyl-, and 1-ethylbenzocyclobutenes in the presence of SbF₅ to give o-H-perfluoroethyl, o-H-perfluoroisopropyl-, and o-H-perfluoro-sec-butylbenzenes, respectively. The ease of this reaction decreases in the same order. Perfluoro-1,1- and perfluoro-1,2-diethylbenzocyclobutenes, perfluoroindane, and perfluorotetralin are virtually unaltered under analogous conditions. The difference in the behavior of benzocyclobutenes may be related to the stability of the carbocations, which are intermediates or models of the transition state. This is in accord with the results of MNDO calculations with complete geometrical optimization for the relative energies of the starting compounds and intermediate ions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2618–2622, November, 1991.