Replacement of OH in the COOH group by fluorine

Carboxylic acid fluorides are not of particular preparative importance since the reactions that can be carried out with them proceed analogously with the cheaper carboxylic acid chlorides. These fluorides are very stable, even towards hydrolysis. Formyl fluoride can be kept at room temperature for several hours before it decomposes into CO and HF. The preparation of carboxylic acid fluorides from the corresponding chlorides and KF was mentioned on page 204. If butyric anhydride is heated gradually to 180° with KHF2, butyryl fluoride, b.p. 67° 1167 mm, distils off, and benzoyl fluoride, b.p. 155-156°, can be obtained analogously from benzoic anhydride at 190-240°. KF can also be used in place of KHF2. Yields are around 80-90%.1079 

This procedure requires that the acid anhydrides be prepared as a preliminary, but merely heating lower fatty acids, or mono- or tri-chloroacetic acid, with benzoyl fluoride gives the corresponding acid fluorides provided that these are distilled off continuously so that they are removed from the equilibrium mixture1080 (cf. page 247). 

This reaction can be combined with halogen exchange by heating together a carboxylic acid, benzoyl chloride and KF805 or dry KHF2.821 

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