Tetramethylammonium fluoride

Tetramethyl&shy;ammonium fluoride is the quaternary ammonium salt with the formula (CH3)4NF. This hygroscopic white solid is a source of “naked fluoride": fluoride ions not complexed with a metal atom. Most other soluble salts of fluoride are in fact bifluorides, HF2–. Historically, there have been two main approaches to prepare tetramethyl&shy;ammonium fluoride: hydrofluoric acid neutralization of tetramethyl&shy;ammonium hydroxide, and salt metathesis between different ammonium salts and inorganic fluoride sources, such as KF or CsF. Because the fluoride anion is extremely basic, the salt slowly reacts with acetonitrile, inducing dimerization to CH3C(NH2)=CHCN, which co-crystallizes.

Related salts

 * Tetramethyl&shy;phosphonium fluoride (CH3)4PF forms stable acetonitrile solutions. It is prepared from the ylide and potassium bifluoride:
 * (CH3)3P=CH2 +  KHF2   →   (CH3)4PF  +  KF
 * Gaseous tetramethylphosphonium fluoride exists as the phosphorane but autoionizes in acetonitrile solution. A more elaborate phosphazenium salt ([(CH3)2N)3P]2N+F−) is also known.


 * Anhydrous Tetrabutylammonium fluoride has been prepared by the reaction of hexafluorobenzene and tetrabutylammonium cyanide.