19F N.M.R. chemical shifts in substituted benzotrifluorides and benzal fluorides

Abstract

19F substituent chemical shifts (scs) for a series of meta and para benzotrifluorides (p-trifluoromethyl-benzenes) are reported and are found to follow the reverse scs trend previously noted for benzyl fluorides (fluoromethylbenzenes), i.e. donors cause downfield shifts and acceptors upfield ones. Specific deviations for electron- withdrawing substituents make DSP correlations for the benzo- trifluorides less precise than for benzyl fluorides. A series of p- benzal fluorides (p-difluoromethyl-benzenes) has also been synthesized and the scs values are also found to follow the reverse trend. The magnitude of scs values decreases in the sequence benzyl fluorides > benzal fluorides > benzo-trifluorides.