ydrogen atom, a C.sub.1 -C.sub.4 alkyl radical, a C.sub.1 -C.sub.4 monohydroxyalkyl radical, a C.sub.2 -C.sub.4 polyhydroxyalkyl radical or a 4'-aminophenyl radical, R.sub.2represents a hydrogen atom or a C.sub.1 -C.sub.4 alkyl radical, a C.sub.1 -C.sub.4 monohydroxyalkyl radical or a C.sub.2 -C.sub.4 polyhydroxyalkyl radical, R.sub.3 represents a hydrogen atom, a halogen atom such as a chlorine a