Study of the Redox Potentials of Benzoquinone and Its Derivatives by Combining Electrochemistry and Computational Chemistry

Abstract

As simple and ubiquitous redox-active organic molecules, quinones participate in diverse electron transfer processes in chemistry and biological systems for energy transformation and signal transduction. We introduce here a practical exercise to study the redox potentials of benzoquinone and its two derivatives by combining the electrochemistry method with quantum chemistry computation. The practical reduction potentials of three quinones were measured by cyclic voltammetry experiment. Quantum chemistry computation, on the other hand, provided theoretical Gibbs free energy change and reduction potentials of the three quinones, which were found to be in line with the experimental results. Detailed thermodynamic energy analysis based on the computation results revealed that the reduction Gibbs free energy changes of the three quinones were mainly contributed from the electronic energies change of the molecules, while thermal energy and entropy played a relatively minor role. The energy levels of the lowest unoccupied molecular orbital (LUMO) of the three quinones, which were modulated by substituent groups and conjugation structures, were further identified as the main origin of the reduction potential. This experiment provides practical and theoretical training on the fundamental ideas involved in major courses like thermodynamics, quantum chemistry, electrochemistry, and organic chemistry, and promotes students to find the tie between macroscopic redox properties of chemicals and their microscopic molecular structures, a key topic in chemistry education.

First and Second Reductions in an Aprotic Solvent: Comparing Computational and Experimental One-electron Reduction Potentials for 345 Quinones

“…The semiquinone can undergo further one-electron reduction to an aromatic dianion (Q 2-). [23][24][25][26] In aqueous or protic media, the same reduction is often coupled to a proton transfer resulting in neutral semiquinone (QH ) or hydroquinone (QH 2 ) species. 17,19,24 Due to their redox characteristics, quinones and their derivatives play a critical role in biological processes such as respiration and photosynthesis.…”

“…17,19,24 Due to their redox characteristics, quinones and their derivatives play a critical role in biological processes such as respiration and photosynthesis. 23,27,28 They serve as important scaffolds for synthesis of clinically approved drugs for treating cancer (e.g., Adriamacyin and Cerubidine), [29][30][31][32] diabetes, 33 and cardiovascular diseases, 34 among others. 35 Quinones have also been used in applications ranging from energy storage, 36,37 natural dyes (e.g., 2hydroxy-1,4-naphthoquinone, or Henna), 29,[38][39][40] and synthetic dyes (e.g., Disperse Red 9, or 1-(methylamino)-anthraquinone, used in bank security dye packs that get activated upon bank robberies).…”

First and Second Reductions in an Aprotic Solvent: Comparing Computational and Experimental One-Electron Reduction Potentials for 345 Quinones

“…Quinones are a class of conjugated redox-active molecules derived from aromatic compounds and containing two carbonyls in a cyclic arrangement . Due to their redox characteristics, quinones and their derivatives play a critical role in a range of applications including catalysis, energy storage, , and biological processes such as respiration and photosynthesis. − In aprotic solvent, neutral quinones can undergo a one-electron reduction to a radical anionic semiquinone (Q •– ). The semiquinone can undergo further one-electron reduction to an aromatic dianion (Q 2– ), as illustrated in Scheme . ,− In aqueous or protic media, the same reduction is often coupled to a proton transfer, resulting in neutral semiquinone (QH • ) or hydroquinone (QH 2 ) species. ,, …”

Application of Electrochemical Chlorination Reaction in Organic Laboratory Course

“…However, in these reports, a Pt cathode is required to facilitate cathodic hydrogen evolution, which would be a major cost for an experiment, especially for undergraduate training. Recently, there has been progress in electrochemistry in chemical education. − In particular, the undergraduate laboratory course focusing on electrochemical synthesis has received increasing attention − since it can convey the classic and state-of-the-art concept of green chemistry driven by electrons to undergraduate students directly. With these efforts, however, electrochemical chlorination is still unexplored in chemical education.…”