Haloformates Fluoroformates

In the same way as a,a,(Z-trihalogenated carbonyl compounds are cleaved hydrolytically to produce haloforms, the fluorinated analogs also react by release of trifluoromethane. The fluoroform reaction has been described with aldehydes such as trifluoroacetaldehyde, alkyl trifluoromethyl ketones, and aryl trifluoromethyl ketones, e.g. l-(2,4-dihydroxyphcnyl)-... 

Attempts to apply the thermal decarboxylation reaction in the liquid phase to aromatic halofor-mic acid esters have shown that their reaction is different from that ol the aliphatic haloformates. It was found that evolution of carbon dioxide occurs, but only high boiling products could be isolated.136 On heating in the presence of aromatics and Lewis acids, aryl chloroformates do not react to give chlorinated aromatics with concomitant decarboxylation, but undergo a Friedel-Crafts reaction to give phenyl benzoates.137 Under similar conditions phenyl fluoroformate undergoes only polymerization and carbonate formation.137... 

Study carefully the pKgS for the haloform series, CHX3— they may not do what you think they should Chloroform is much more acidic than fluoroform even though fluorine is more electronegative (likewise with bromoform and chloroform). The anion CFamust be slightly destabilized because of some backdonation of electrons. The anion from chloroform and bromoform may also be stabilized by some interaction with the d orbitals (there aren t any on fluorine). The conjugate base anion of bromoform is relatively stable— you will meet this again in the bromoform/iodoform reaction (Chapter 21). 

Haloform test. A modification of a test devised by Fujiwara permits detection of fluoroform, chloroform, bromoform, or iodoform or of any precursor of a haloform or of a dihalocarbene. A few drops of a mixture suspected of containing a haloform or a precursor is added to a mixture of 3 ml. of 10% sodium hydroxide and 2 ml. of pyridine. If the mixture turns pink to bright blue-red within one or two minutes of vigorous shaking at room temperature, a haloform is present. If the color change occurs not at room temperature but when the solution is heated on the steam bath for one minute, a precursor of a haloform or a dichlorocarbene is indicated. The intensity of the color is proportional to the concentration of haloform or of precursor. 

Table 2. Experimental shifts of the C-H/D) (in cm ) in the CI3C-H B, CI3C-D B, and BrsC-D B H-bonded complexes [8. Note that the H) of haloforms appears to be in Fermi resonance with the overtone of the C-H bending vibrational modes. The eorresponding corrections for Fermi resonance have been recently made in Ref. [78a] for fluoroform dissolved in liquefied Ar, N2, CO, and CO2 and as shown therein, their effeet is rather insignificant...

Fluoroform has proved resistant to photochemical nitrosation with nitrosyl chloride, but the haloforms CHFaCl and CHFClj have been converted by this technique into chlorodifluoronitrosomethane and dichloro-fluoronitrosomethane in low yield. The same method has been used to convert chloroform into trichloronitrosomethane in low (ca. 15%) yield at 40 °C this nitroso-compound reacts with silver monofluoride to give trifluoronitrosomethane(<2%) and 0-nitroso-AW-bistrifluoromethylhydrox-ylamine (ca. 20%), with silver monofluoride in dimethylformamide to yield trifluoronitrosomethane (<2%), hexafluoroazoxymethane (10-5%), and... 

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