Ionization and tautomerization of 2-nitrocyclohexanone in aqueous solution.

Area of Science:

• Physical Organic Chemistry
• Alicyclic Chemistry
• Tautomerism Studies

Background:

• Keto-enol tautomerism is a fundamental chemical process.
• Understanding substituent effects on tautomeric equilibria is crucial for organic chemistry.
• Alicyclic ketones present unique structural and reactivity considerations.

Purpose of the Study:

• To investigate the keto-enol tautomerism of 2-nitrocyclohexanone (2-NCH) in aqueous solution.
• To determine the acid dissociation constants (pKa) of the keto and enol forms of 2-NCH.
• To quantify the tautomerization equilibrium constant (pKT) and compare it with unsubstituted cyclohexanone.

Main Methods:

• Spectrophotometric measurements at 25°C and ionic strength of 0.4 mol dm-3 (NaCl).
• Ketonization rate constants determined using NaHSO3 as a keto form scavenger.
• UV-vis spectroscopy employed to record spectra at various pH values.

Main Results:

• pKa of the enol form (EH) determined as 4.78 from the rate-pH profile.
• pKa of the keto form (KH) determined as 5.97 from UV-vis spectra.
• Tautomerization equilibrium constant (pKT) calculated as 1.19.

Conclusions:

• The alpha-nitro substituent dramatically alters the acidities and tautomerization constant of alicyclic ketones compared to cyclohexanone.
• Steric and inductive effects dominate over resonance contributions in nitroketones.
• The study provides insights into reaction mechanisms using Brønsted equation, nonperfect synchronization, and Marcus theory.