title: Ketonization of enols in aqueous solution: is carbon protonation always rate-determining?
abstract: The pH-rate profiles for the ketonization of the (E)- and (Z)-photoenols of o-methylacetophenone (MA) in aqueous solution were determined by nanosecond laser flash photolysis. Carbon protonation of the enol anions of MA by solvent water is exceptionally fast, k(0)'(K)≈ 2.0 × 10(7) s(-1), too fast to permit establishment of the acid-base equilibrium on the enol oxygen prior to ketonization. Analysis of the pH-rate profile of the (E)-enol using the common assumption of rate-determining carbon protonation would lead to an erroneous value for the acidity constant of that enol, pK(a,c)(E) = 11.3, which is too high by about two pK units.
authors.affiliation: Department of Chemistry, University of Basel, CH-4056 Basel, Switzerland. anna.pelliccioli@novartis.com
authors.forename: Anna Paola
authors.initials: AP
authors.lastname: Pelliccioli
authors.forename: Peter
authors.initials: P
authors.lastname: Šebej
authors.forename: Jakob
authors.initials: J
authors.lastname: Wirz
mesh_terms: D000098:Acetophenones / Q000737:chemistry; D000136:Acid-Base Equilibrium; D002244:Carbon / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007659:Ketones / Q000737:chemistry*; D007834:Lasers; D010100:Oxygen / Q000737:chemistry; D010782:Photolysis; D011522:Protons; D013237:Stereoisomerism; D014867:Water / Q000737:chemistry
chemical_list: D000098:Acetophenones; D007659:Ketones; D011522:Protons; D014867:Water; D002244:Carbon; C038699:acetophenone; D010100:Oxygen