How do I know if my unknown contains a fluorine atom(s)?

April 22, 2008

by Arvin Moser, Team Manager, Application Scientists, ACD/Labs

Ideally, identifying whether a fluorine atom(s) is present is as simple as acquiring a 19F NMR. However, running ‘uncommon’ experiments is not usually my first step in an elucidation (MDE). Imagine a scenario where the elucidator is unsure whether fluorine is present. He/she acquires a 19F NMR and sees no 19F resonances. Therefore the extra experiment can be considered a waste of time especially when other experiments may provide the clues needed.

I resort to routine experiments such as MS, 1H and 13C NMR, and HSQC to gather information for the presence of fluorine. Although a mass spectrum can indicate such losses as CF3, it is usually not the best approach for inferring the presence of a fluorine atom(s). A 1H NMR spectrum may provide some clues for fluorine but on a complicated spectrum, it may not be as obvious that fluorine is present.

Shown below is a portion of a 1H NMR spectrum for a steroid-type compound with a single fluorine atom. Only the vicinal protons to the fluorine atom show any 1H-19F splitting. However, the splitting is not conclusive evidence for the presence of a fluorine. The subsequent blog will illustrate some better clues into ascertaining the presence of fluorine.

Further complicating the matter, the multiplet at 2.4 pppm (CH group) is overlapping with another multiplet and the multiplet at 4.1 ppm (CHOH group) exhibits some hard to discern couplings.