Preparation of aliphatically substituted fluorobenzenes

Aliphatically substituted fluorobenzenes of the general formula I ##STR1## where R is an aliphatic or cycloaliphatic radical, n is 2, 3 or 4 and X is hydrogen, fluorine, chlorine or bromine, are prepared by diazotization of the corresponding aniline derivative of the general formula II ##STR2## in the presence of tetrafluoboric acid and decomposition of the resulting diazonium tetrafluoborate of the general formula III ##STR3## by a process in which the decomposition reaction is carried out simultaneously with the diazotization reaction, in the presence of elemental copper and/or copper(I) and/or copper(II) salts and at from -15.degree. to 80.degree. C.