Solvent and substrate isotope effects on the enolization and carbon-acid ionization of isobutyrophenone

• Published in: Canadian journal of chemistry 1996-12, Vol.74 (12), p.2481-2486
• Main Authors: Keeffe, J.R, Kresge, A.J
• Format: Article
• Language: English
• ISSN: 0008-4042; 1480-3291
• DOI: 10.1139/v96-278

Bromine scavenging was used to measure rates of acid-catalyzed enolization of isobutyrophenone in H 2 O and in D 2 O solution and of isobutyrophenone-α-d in D 2 O solution. The results provide the solvent isotope effect k H  +/k D  + = 0.56 and the substrate isotope effect k H /k D  = 6.2 on the enolization reaction, both of which are consistent with the generally accepted mechanism for this process. The present results in combination with literature information also provide the solvent isotope effect on the enolization equilibrium, K E (H 2 O)/K E (D 2 O) = 0.92, and the solvent isotope effect on the ionization of isobutyrophenone as a carbon acid, k a K (H 2 O)/k a K (D 2 O) = 5.4, as well as the product of isotopic fractionation factor and medium effect, , for isobutyrophenone enol and the medium effect, Φ = 0.47, for its enolate ion. The isotope effect on K E is the first ever determined for the keto-enol equilibrium of a simple aldehyde or ketone; its near-unit value is consistent with expectation on the basis of fractionation factors for the species involved. Key words: isobutyrophenone, keto-enol equilibrium, carbon-acid ionization, solvent isotope effects, isotopic fractionation factors.