. . . . "ving from 4 to 20, preferably 6-16, C-atoms, andR.sub.1 and R.sub.2 may additionally represent a halogen such as fluorine, chlorine or bromine, andx and y have the same meaning as indicated above.The groups R.sub.1 and R.sub.2 may be derived preferably from, for example, hydrogen, fluorine, chlorine, bromine, methane, ethane, hexane, cyclohexane, propene, toluene or benzene, or together they may f" .