Hexafluoroacetone (HFA) is a chemical compound with the formula (CF3)2CO. It is structurally similar to acetone; however, its reactivity is markedly different. It a colourless, hygroscopic, nonflammable, highly reactive gas characterized by a musty odour. According to electron diffraction, HFA and acetone adopt very similar structures, the C-O distance being only longer in the fluorinated compound (124.6 vs 121.0 pm), possibly due to steric effects. The term "hexafluoroacetone" can refer to the sesquihydrate (1.5 H2O), which is a hemihydrate of hexafluoropropane-2,2-diol, a geminal diol. Hydrated HFA behaves differently from the anhydrous material.

Synthesis

The industrial route to HFA involves treatment of hexachloroacetone with HF (a Finkelstein reaction):

Laboratory methods

Hydrated HFA can be converted to HFA by treatment with hot sulfuric acid. It has also be prepared from hexafluoropropylene oxide, which will rearrange to give HFA when heated in the in the presence of a Lewis acid such as AlCl3. The Lewis acid catalysed oxidation of hexafluoropropylene will also produce HFA, via a similar mechanism. Although it is commercially available, HFA can be prepared on the laboratory-scale from hexafluoropropylene. In the first step KF catalyzes the reaction of the alkene with elemental sulfur to give the 1,3-dithietane dimer of hexafluorothioacetone. This species is then oxidized by potassium iodate to give HFA.

Uses

Hexafluoroacetone is used in the production of hexafluoroisopropanol: It is also used as a precursor to hexafluoroisobutylene, a monomer used in polymer chemistry, and as a building block in the synthesis of midaflur, bisphenol AF, 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, and alitame.

Reactivity

With water, hexafluoroacetone converts to the hydrate. The equilibrium constant (Keq) for the formation of this geminal diol is 106 M−1. The analogous equilibrium for acetone is an unfavorable 10−3 M−1. Hexafluoroacetone-hydrates are acidic. In an analogous reaction, ammonia adds to hexafluoroacetone to give the hemiaminal (CF3)2C(OH)(NH2) which can be dehydrated with phosphoryl chloride to give the imine (CF3)2CNH. Nucleophiles attack occurs at the carbonyl carbon of Hexafluoroacetone, as illustrated above. Thus, HFA readily forms lactones when treated with hydroxy- and amine-substituted carboxylic acids. In such reactions, HFA serves both as electrophile and dehydrating agent: