Compounds of the formula ##STR1## wherein X.sup.1 represents fluorine or chlorine; X.sup.2 denotes hydrogen, fluorine or chlorine; and R.sup.1 signifies an alkyl group in which optionally a >CH--CH< group is replaced by >C.dbd.C< and/or a methylene group is replaced by oxygen, their manufacture, liquid crystalline mixtures which contain such compounds and their use for electro-optical purposes.