Process for the nuclear chlorination of toluene

Para-chlorotoluene is prepared selectively at high yield by nuclear chlorination of toluene, in the presence of a Lewis acid catalyst, and a co-catalyst comprising at least one compound selected from the group consisting of phenoxthine derivatives and highly chlorinated phenoxthine derivatives obtained by chlorinating said phenoxthine derivatives at higher degree, said phenoxthine derivatives being represented by the general formula of: (I) wherein R represents methyl group or groups at the position-1 and/or position-3, m is zero or an integer of 1 or 2, X is hydrogen or chlorine atom, and Y is chlorine atom or methyl group; and wherein m is an integer of 1 or 2 and X is hydrogen or chlorine atom when Y is chlorine atom, X is chlorine atom when Y is methyl group and m is zero, and X is hydrogen atom when Y is methyl group and m is an integer of 1 or 2.