Competing solvolytic substitution and elimination reactions via a common irreversibly formed ion–molecule pair intermediate

Abstract

AbstractThe acid‐catalysed solvolysis of 9‐(2‐phenoxy‐2‐propyl)fluorene in mixtures of water with acetonitrile or methanol at 25°C provides 9‐(2‐hydroxy‐2‐propyl)fluorene, 9‐(2‐propenyl)fluorene, and 9‐(2‐acetamido‐2‐propyl)fluorene or 9‐(2‐methoxy‐2‐propyl)fluorene, respectively. The overall kinetic deuterium isotope effects for the reactions of the hexadeuterated analogue 9‐(1,1,1,3,3,3‐2H6)‐9‐(2‐phenoxy‐2‐propyl)fluorene in 90 vol.% acetonitrile in water were measured as (k + k)/(k + k) = 1·54 ± 0·05, which is composed of the isotope effect k/k = 1·4 ± 0·1 for formation of the substitution products and k/k = 4·0 ± 0·2 for production of 9‐(2‐methoxy‐2‐propyl)fluorene. Similar isotope effects were measured in other solvent mixtures. The results strongly indicate a branched mechanism involving rate‐limiting formation of a common carbocation–molecule pair (with a secondary isotope effect of 1·54), which either undergoes nucleophilic attack by a solvent molecule (with a secondary isotope effect of ca 1), or is dehydronated (isotope effect ca 2·8) by the leaving group or by the solvent. The ion‐molecule pair shows very low selectivities. Thus, in 50 vol.% acetonitrile in water, an acetonitrile molecule is as efficient as a water molecule as a nucleophile towards the ion–molecule pair, kMeCN/kHOH ≥ 1 (ratio of second‐order rate constants). The discrimination between methanol and water is anomalously small, kMeOH/kHOH = 0·7.