COMPETING SOLVOLYTIC SUBSTITUTION AND ELIMINATION REACTIONS VIA A COMMON IRREVERSIBLY FORMED ION-MOLECULE PAIR INTERMEDIATE

Article abstract of DOI:10.1002/poc.610071009

The acid-catalysed solvolysis of 9-(2-phenoxy-2-propyl)fluorene in mixtures of water with acetonitrile or methanol at 25 deg 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.percent acetonitrile in water were measured as (k_E^H+k_S^H)/(k_E^D6+k_S^D6)=1.54+/-0.05, which is composed of the isotope effect k_S^H/k_S^D6=1.4+/-0.1 for formation of the substitution products and k_E^H/k_E^D6=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.percent acetonitrile in water, an acetonitrile molecule is as efficient as a water molecule as a nucleophile towards the ion-molecule pair, k_MeCN/k_HOH>/=1 (ratio of second-order rate constants).The discrimination between methanol and water is anomalously small, k_MeOH/k_HOH=0.7.