The SN mechanism in aromatic compounds. XXXV. Comparative reactivity of pentachlorofluorobenzene, hexafluorobenzene, and fluorobenzene

Abstract

In a recent paper1 dealing with S , reactions of perhalogenobenzenes it was predicted that pentachlorofluorobenzene would be more reactive, replacing fluorine, than hexafluorobenzene, and this is now confirmed. Hammett substituent constants (a-) for pentachlorophenyl and pentafluorophenyl groups are derived and compared with values for ortho-and para-nitrophenyl groups. Experimental MaterialsThe solvent and reagent were prepared as described.' 2,3,4,5,6-Pentachloro-I-fluorobenzene was prepared according to the procedure of Finger and Kruse2 from the commercially available pentachloronitrobenzene, using anhydrous potassium fluoride in dimethylformamide. The product, recrystallized from 95% ethanol, was obtained as long white needles of m.p. 137-138' (lit.2 137-138'). Kinetic ProcedureGenerally as described,' but runs were followed by estimating the concentration of reagent (0Me-) by potentiometric back-titration ~f excess quenching acid. A check by estimation of fluoride confirmed the reaction as simple replacement of F by 0Me. Good second-order plots were obtained, more precise by the acid-base method. Experimental kinetic data were measured over a range of about 30". Values of rate constants 103k2 (1. mole-I see-l) and temperature are: 3.30, 49.9'; 3.38, 50.1"; 5.53, 59.9"; 13.9, 65.1'; 21.6, 70.1"; 37.3 and 39.6, 80.1°. The Arrhenius parameters and other derived values are included in Table 1. The error in AEf computed by least squares is less than the estimated error in AEf of iO.5 kcal mole-'. Estimated error in logl,B is about i0.3. ProductThe product, pentachloroanisole, was isolated from a reaction mixture allowed to proceed to "infinity", and was obtained as fine white needles, m.p. 108-109" (lit.3 108-110').

Organic Fluorine Chemistry

“…On the other hand, these olefins react readily with nucleophiles such as marcaptans, thiophenols, phenols, alcohols, amines, and even fluoride ions (generated from potassium or cesium fluorides) [256). CFaCHFCFa [256] Similarly, fluorine atoms in poly-and perfluoroaromatics drain electrons from the nucleus and make it susceptible to nucleophilic attacks by hydroxyl, alkoxyl, aryloxyl, sulfhydryl, and amino groups [257,258). (123) Sufficiently electron-poor systems such as perfluoropyridine or perfluoroacyl fluorides may even be attacked by carbanions produced, for example, by the reaction of perfluoro-olefins with potassium or cesium fluoride [259,260).…”

“…(258) 19O"C CaHsOH + CICOCaF7 ~ CaHsOCOCaF7 52.5% [431] Phenol and trifluoroacetic anhydride react to give phenyl trifluoroacetate, a very good acylating agent [432]. The compound can also be prepared by decomposition of a mixed anhydride obtained from trifluoroacetic acid and phenyl chloroformate [433].…”