If the phenyl group or the hetaryl group is substituted, the substituent is preferably a halogen atom, for example a fluorine, bromine, chlorine or iodine atom, preferably afluorine or chlorine atom, or a straight chained C.sub.1 -C.sub.3 -alkyl group, for example a methyl or ethyl group, or a straight chained C.sub.1 -C.sub.3 -alkoxy group, for example a methoxy group.