Substituent Effects on the Relative Stabilities of Tautomers of 2-Butanone in the Gas Phase: A Theoretical PM3 and DFT Study

Al Khansa Saeid; Gamal Aboulgasem; Osama Ali Kriewah; Aïcha Mbarek

doi:10.54361/ajmas.2698332

Authors

Al Khansa Saeid Department of Chemistry, Faculty of Education El Ajelat, University of Zawia, Libya https://orcid.org/0009-0005-4808-7402
Gamal Aboulgasem Department of Chemistry, Faculty of Science El Ajelat, University of Zawia, Libya https://orcid.org/0009-0002-1993-4578
Osama Ali Kriewah Department of Chemical Technology, Faculty of Science and Technology, Jadu, Libya https://orcid.org/0000-0002-9991-3517
Aïcha Mbarek Advanced Materials Laboratory, National School of Engineers of Sfax, University of Sfax, Tunisia https://orcid.org/0000-0002-9409-8613

DOI:

https://doi.org/10.54361/ajmas.2698332

Keywords:

Keto–enol Tautomerism, PM3, DFT, Electron-Donating Groups, Electron-Withdrawing Groups

Abstract

This work presents a comprehensive theoretical study of the effects of substituents (X = F, OH, NH₂, CH₃, CN, CF₃) on the keto–enol tautomerism of 2-butanone. Using semiempirical PM3 and DFT (B3LYP/6-31G(d,p)) methods, we evaluated the relative stabilities of 2-butanone and its enol form, 2-buten-2-ol, in the gas phase. Thermodynamic parameters (ΔH, ΔG), isodesmic reaction enthalpies, and frontier molecular orbital (FMO) analyses were employed to investigate the influence of electron-donating and electron-withdrawing substituents at the α-carbon position. Our results indicate that 2-butanone is thermodynamically more stable than 2-buten-2-ol across all substituents. Electron-donating groups (EDGs) such as CH₃ and NH₂ exert a minor destabilization of the keto form, while electron-withdrawing groups (EWGs) like CN and CF₃ significantly reduce keto stability, favoring enolization. FMO analysis revealed that EWGs lower the HOMO-LUMO gap, indicating increased reactivity and a tendency to stabilize the enol tautomer. These insights provide a valuable framework for understanding substituent effects on tautomerism in aliphatic ketones.