of the formula Ar--X, where Ar is as previously defined and X is a halogen atom of atomic number greater than 9, a diazonium group or a triflate group; with at least one olefinic compound of the formula ##STR7## where R1, R2, and R3 are as previously defined; in the presence of(A) a polar liquid reaction medium formed from (i) at least one liquid organic solvent/diluent, and (ii) a hydrogen halide