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

The acid-catalysed solvolysis of 9(2-phenoxy-2-propyl)fluorene in mixt ures of water with acetonitrile or methanol at 25 degrees C provides 9 -(2-hydroxy-2-propyl)fluorene, 9-(2-propenyl)fluorene, and 9-(2-acetam ido-2-propyl)fluorene or 9-(2-methoxy-2-propyl)fluorene, respectively. The overall kinetic deuterium isotope effects for the reactions of th e kexadeuterated analogue 1,1,1,3,3,3-H-2(6))-9-(2-phenoxy-2-propyl)fl uorene in 90 vol.% 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 th e isotope effect k(S)(H)/k(S)(D6) = 1.4 +/- 0.1 for formation of the s ubstitution 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 measu red in other solvent mixtures. The results strongly indicate a branche d mechanism involving rate-limiting formation of a common carbocation- molecule pair (with a secondary isotope effect of ca 1), or is dehydro nated (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 w ater molecule as a nucleophile towards the ion-molecule pair, k(MeCN)/ k(HOH) greater than or equal to 1 (ratio of second-order rate constant s). The discrimination between methanol and water is anomalously small , k(MeOH)/k(HOH) = 0.7.