Polar Group Enhanced Gas-Phase Acidities of Carboxylic Acids: An Investigation of Intramolecular Electrostatic Interaction’ Kinetic Method

Authors

Jianhua Ren, University of the PacificFollow

Document Type

Article

Publication Title

Journal of Physical Chemistry A

Department

Chemistry

ISSN

1089-5639

Volume

110

Issue

50

DOI

10.1021/jp0624855

First Page

13405

Last Page

13411

Publication Date

11-23-2006

Abstract

We studied the effects of polar groups on the gas-phase acidities of carboxylic acids experimentally and computationally. In this connection, the gas-phase acidities (ΔH_acid, the enthalpy of deprotonation, and ΔG_acid, the deprotonation free energy) of borane-complexed methylaminoacetic acid ((CH₃)₂N(BH₃)CH₂CO₂H) and methylthioacetic acid (CH₃S(BH₃)CH₂CO₂H) were measured using the kinetic method in a flowing afterglow-triple quadrupole mass spectrometer. The values of ΔH_acid and ΔG_acid of (CH₃)₂N(BH₃)CH₂CO₂H were determined to be 328.8 ± 1.9 and 322.1 ± 1.9 kcal/mol, and those of CH₃S(BH₃)CH₂CO₂H were determined to be 325.8 ± 1.9 and 319.2 ± 1.9 kcal/mol, respectively. The theoretical enthalpies of deprotonation of (CH₃)₂N(BH₃)CH₂CO₂H (329.2 kcal/mol) and CH₃S(BH₃)CH₂CO₂H (325.5 kcal/mol) were calculated at the B3LYP/6-31+G(d) level of theory. The calculated enthalpies of deprotonation of N-oxide−acetic acid (CH₃NOCH₂CO₂H, 329.4 kcal/mol) and S-oxide−acetic acid (CH₃SOCH₂CO₂H, 328.6 kcal/mol) are comparable to the experimental results for borane-complexed methylamino- and methylthioacetic acids. The enthalpy of deprotonation of sulfone−acetic acid (CH₃SO₂CH₂CO₂H, 326.1 kcal/mol) is about 2 kcal/mol lower than the S-oxide−acetic acid. The calculated enthalpy of deprotonation of sulfoniumacetic acid, (CH₃)₂S⁺CH₂CO₂H, is 243.0 kcal/mol. Compared to the corresponding reference molecules, CH₃NHCH₂CO₂H and CH₃SCH₂CO₂H, the dipolar group and the monopolar group substituted carboxylic acids are stronger acids by 11−14 and 97 kcal/mol, respectively. We correlated the changes of the acidity upon a polar group substitution to the electrostatic free energy within the carboxylate anion. The acidity enhancements in polar group substituted carboxylic acids are the results of the favorable electrostatic interactions between the polar group and the developing charge at the carboxyl group.

Recommended Citation

Ren, J. (2006). Polar Group Enhanced Gas-Phase Acidities of Carboxylic Acids: An Investigation of Intramolecular Electrostatic Interaction’ Kinetic Method. Journal of Physical Chemistry A, 110(50), 13405–13411. DOI: 10.1021/jp0624855
https://scholarlycommons.pacific.edu/cop-facarticles/668