Effect of a Perfluoroalkyl Group on the Elimination and Substitution Reactions of Two Homologous Series of Perfluoroalkyl-Substituted Iodoalkanes

Article abstract of DOI:10.1021/jo00187a012

Substitution and elimination reactions of two homologous series of compounds, induced by strong bases, were studied in aqueous alcohol and anhydrous methanol solution.Series I compounds, R_F(CH2)nI having n = 2 (R_F = a perfluoroalkyl group, also named an F-alkyl group), gave only R_FCH=CH2 under all conditions.By contrast, R_F(CH2)3Igave 4-10 times as much substitution as elimination products.Isomerization of R_FCH2CH=CH2 (6) to R_FCH=CHCH3 (7) occurred; this result may account, in part, for the extremely high 7/6 alkene ratios (37-81/1) obtained from elimination reactions of R_FCH2CHICH3.All series II compounds, R_FCH2CHI(CH2) nCH3 (n = 0-5), gave entirely elimination, and principally toward the R_F group.E/Z isomer ratios varied from 2.65 to 5.These results were compared to those obtained from 1- and 2-iodooctane under the same conditions.A practical synthesis of CF3(CF2)5CH2CH2CH2I is described; the isomeric (F-alkyl)propenes 6 and 7 were also separately prepared.Rates of reactions under standard conditions for both series I and II compounds were measured.Kinetically, only second-order processes were observed, but a sharp break occurred in rate as the R_F group was separated more than two carbons from the departing iodine atom, in series I compounds.Series II compounds reacted about one-tenth as fast as series I compounds.These results are discussed in the context of previous work with F-alkyl-substituted compounds.