Effects of alkyl chain length on the thermodynamics of proton ionization from arsonic and arsinic acids

Abstract

ΔG°, ΔH°, and ΔS° values are reported for the two proton ionizations from several n-alkylarsonic acids (methyl, ethyl, propyl, butyl, pentyl, and hexyl) and from several protonated di-n-alkylarsinic acids (methyl, ethyl, propyl, butyl, and pentyl) in aqueous solution at 25°. The ΔG° values were determined from pH measurements, and the ΔH° values were determined calorimetrically. The variations in ΔG°, ΔH°, and ΔS° with alkyl chain length found for both the arsonic and arisinic acid ionizations differ from hydrocarbon chain lengthening effects on the thermodynamics of proton ionization previously reported for protonated amines, protonated amino-acids, and carboxylic acids.