Competing base-promoted E2 and E1 reactions of an acidic tertiary substrate

Abstract

The solvolysis of 4-chloro-4-(4′-nitrophenyl)pentan-2-one (1-Cl) in aqueous acetonitrile yields the alcohol 4-hydroxy-4-(4′-nitrophenyl)pentan-2-one (1-OH) and the elimination products 4-(4′-nitrophenyl)-2-oxopent-4-ene (2), (E)-4-(4′-nitrophenyl)-2-oxopent-3-ene (3), and (Z)-4-(4′-nitrophenyl)-2-oxopent-3-ene (4). Addition of thiocyanate ion does not increase the total reaction rate in 25 vol% acetonitrile in water, but gives rise to the substitution products 1-SCN and 1-NCS, at the expense of 1-OH, and a small increase in the fraction of alkene 3. Addition of acetate ion and other weak bases gives rise to base-promoted E2 (or E1cB) to give alkene 3 (and 4). The Brønsted parameter for this reaction was measured as β = 0.76. Water-promoted E2 (or E1cB) reaction was not observed.