Siloxane basicity toward strong acid in nonpolar solution

Article abstract of DOI:10.1021/ja00015a010

The relative basicities of ten siloxanes and an ether were studied in benzene by determining from visible spectroscopic measurements the thermodynamic constants for the competition between the substrate and a reference base (4-chloro-2-nitroaniline) for acid (trifluoromethanesulfonic acid): RB-HA + S ? RB + S·HA. The K_eq values were considered as a measure of basicity with the following order established and explained by inductive effects in the protonated species: permethyl linear siloxanes (Me₃Si(OSiMe₂)_nOSiMe₃, n = 0-3) > dibutyl ether > HMe₂SiOSiMe₂H > branched siloxanes ((Me₃SiO)₃SiMe, (Me₃SiO)₄Si) > cyclic siloxanes ((Me₂SiO)_n, n = 3-5). The ΔH and ΔS values were positive and increased with higher basicity; this behavior was attributed to differential solvation of ion pairs.