Extended kinetic method study of the effect of ortho‐, meta‐, and para‐chlorination on the proton affinity of phenylalanine with full entropy analysis

Abstract

Extended kinetic method study of the effect of ortho-, meta-, and para-chlorination on the proton affinity of phenylalanine with full entropy analysis Joong-Won ShinAbstract Absolute proton affinities (PAs) of phenylalanine, ortho-chlorophenylalanine, meta-chlorophenylalanine, and para-chlorophenylalanine were determined using the extended kinetic method with full entropy analysis to investigate the effect of chlorination on the PA of phenylalanine. The measured absolute PAs of the molecules are 907.9 ± 2.3, 901.9 ± 2.2, 889.0 ± 2.8, and 904.9 ± 3.1 kJ·mol −1 , respectively. The decrease and variations in the PAs upon chlorination are investigated based on the electronegativity of chlorine, polarizability, dipole moment, and the substituent effect. The effect of electronegativity of chlorine fails to explain differences in the PAs of the chlorine derivatives, and the ordering of the PAs is also not in line with polarizability.However, the ordering of dipole moments and substituent effects are, in part, consistent with that of the measured PAs except for the para and ortho derivatives, respectively. Thus, while no one molecular property fully explains the decrease in the absolute PA upon chlorination and its variation depending on the chlorination site, differences in dipole moments and the field effect provide the most plausible explanation for the observed changes.