Acyl fluorides

Acyl fluorides are organofluorine compounds with the formula RC(O)F (R = alkyl, aryl). In terms of structure, they resemble the more common acyl chlorides, i.e., an acyl group bonded to a halide. The synthesis and properties of acyl fluorides are somewhat distinctive however.

Preparation

The several routes to acyl fluorides can be roughly subdivided into three approaches.

Halide exchange

Acyl chlorides react with hydrogen fluoride to give acyl fluorides. This reaction is relevant to the use of electrofluorination route to perfluorinated carboxylic acids, such as PFOA.

S- and P-based reagents

The OH group of carboxylic acids can be converted to fluoride using various oxophilic reagents. Aromatic (as well as aliphatic) acyl fluorides can be prepared from carboxylic acids with PPh₃ and fluoride sources. Cyanuric fluoride as well as sulfur tetrafluoride also convert carboxylic acids to acyl fluorides.

SF₄ + RCO₂H → SOF₂ + RC(O)F + HF

Fluorophosgene

Salts of trifluoromethoxide react with carboxylic acids to give acyl fluorides. The process entails formation of difluorophosgene. Solutions of trifluoromethoxide are prepared by displacement from aryl trifluoromethyl ethers.

Reactions

A common reaction of acyl fluorides is hydrolysis to the carboxylic acid.

Perfluoroacyl fluorides react with fluoride salts to perfluoroalkoxides:

C₃F₇COF + F⁻ → C₄F₉O⁻

Utility

Acyl fluorides are intermediates in the electrochemical fluorination (ECF) route to perfluorocarboxylic acids, such as perfluorooctanoic acid).