Aerosol Direct Fluorination: Alkyl Halides. 2. Chlorine Shift and the Stability of Radicals

Article abstract of DOI:10.1021/jo00189a014

Unlike alkyl bromides and iodides, alkyl chlorides are shown to be stable to direct fluorination, even under ultraviolett irradiation, at temperatures of 30 deg C and below.Although less reactive than the bromides and iodides, F-alkyl chlorides may be derivatized, presenting another example of direct fluorination-survivable functionality.High (63 percent) to moderate (32 percent) isolated yields of the analogous perfluororalkyl chlorides can be synthesized by aerosol direct fluorination of 1-chloropropane, 1-chlorobutane, 1-chloro-2-methylpropane, 1-chloro-3-methylbutane, 1-chlo-ro-2-methylbutane, and chlorocyclopentane with generally less than 20 percent C-C bond cleavage.Tertiary alkyl chlorides generally undergo intramolecular 1,2-chloride shift in the earliest stages of reaction in a manner characteristic of β-chloro radicals forming principally primary F-alkyl chlorides.Thus 2-chloro-2-methylpropane produces 1-chloro-F-2-methylpropane (47 percent), and 2-chloro-2-methylbutane produces a 16:6.3:1 ratio of 1-chloro-F-2-methylbutane, 1-chloro-F-3-methylbutane, and 2-chloro-F-3-methylbutane, respectively, in 32 percent combined yield.Secondary alkyl chlorides undergo a similar but incomplete rearrangement producing mixtures of primary and secondary F-alkyl chlorides.Thus 2-chloropropane produces a 2:1 mixture of 2-chloro-F-propane and 1-chloro-F-propane in 50 percent combined yield; 2-chlorobutane produces a 1:1.5 mixture of 2-chloro-F-butane and 1-chloro-F-butane in 34 percent combined yield, and 3-chloropentane produces a 2:3:1 mixture of 3-chloro-F-pentane, 2-chloro-F-pentane, and 1-chloro-F-pentane, respectively, in a combined yield of 30 percent.Because secondary alkyl chlorides partially rearrange but primary alkyl chlorides donot rearrange at all on fluorination, doubt is cast on the postulate that the intermediate radicals are equilibrating.