title: Solvent effects on the rate of the keto-enol interconversion of 2-nitrocyclohexanone.
abstract: The rates of tautomerization of 2-nitrocyclohexanone (2-NCH) have been measured spectrophotometrically at 25.0 +/- 0.1 degrees C in several organic aprotic solvents and their binary mixtures. In cyclohexane the reaction is effectively catalyzed by bases and inhibited by acids while the so-called "spontaneous reaction" appears essentially due to autocatalysis. Apparent second order rate constants (k(app)(B)) for the reaction catalyzed by triethylamine (TEA) and pyridine (Pyr) have been obtained. From the experimental k(app)(B) values rate constants for the enolization (k(1)(B)) and ketonization (k(-1)(B)) reactions have been calculated. A Kamlet-Taft type linear solvation energy relationship (LSER) adequately accounts for the observed solvent effects. Activation parameters for both reactions show that solvent effects are mainly entropic in origin and that there is a shift of the transition state from a ketone-like to an enol-like structure on passing from less to more polar solvents.
authors.affiliation: Dipartimento di Scienze del Farmaco, Università G. d'Annunzio Via dei Vestini 31, Chieti, Italy. siani@unich.it
authors.forename: Gabriella
authors.initials: G
authors.lastname: Siani
authors.forename: Guido
authors.initials: G
authors.lastname: Angelini
authors.forename: Paolo
authors.initials: P
authors.lastname: De Maria
authors.forename: Antonella
authors.initials: A
authors.lastname: Fontana
authors.forename: Marco
authors.initials: M
authors.lastname: Pierini
mesh_terms: D000042:Absorption; D000475:Alkenes / Q000737:chemistry*; D002384:Catalysis; D003510:Cyclohexanes / Q000737:chemistry; D003512:Cyclohexanones / Q000737:chemistry*; D007536:Isomerism; D007659:Ketones / Q000737:chemistry*; D007700:Kinetics; D011522:Protons; D012997:Solvents / Q000737:chemistry*; D013053:Spectrophotometry; D013696:Temperature
chemical_list: C533398:2-nitrocyclohexanone; D000475:Alkenes; D003510:Cyclohexanes; D003512:Cyclohexanones; D007659:Ketones; D011522:Protons; D012997:Solvents; C506365:Cyclohexane