MECHANISMS OF COMPETING SOLVOLYTIC ELIMINATION AND SUBSTITUTION-REACTIONS - THE ROLE OF ION-PAIR INTERMEDIATES IN AQUEOUS SOLVENTS

Solvolysis of 9-(2-bromo-2-propyl)fluorene 1-Br in mixtures of water w ith methanol or acetonitrile at 25-degrees-C provides the elimination product 9-isopropenylfluorene 2, and the substitution products 9-(2-hy droxy-2-propyl)fluorene 1-OH and 9-(2-methoxy-2-propyl)fluorene 1-OMe. The Grunwald-Winstein parameter was measured in methanol-water mixtur es as m(obs) = 0.70, which is composed of the parameter for the elimin ation reaction, m(E) = 0.65, and the substitution reaction, m(s) = 0.8 3. The parameters for the corresponding chloride 1 -Cl were measured a s m(obs) = 0.82, m(E) = 0.76 and m(s) = 0.97. The kinetic deuterium is otope effects for the reactions of the hexadeuteriated analogue 9-(2-b romo[H-2(6)]-2-propyl)fluorene ([H-2(6)]-1- Br) were measured as (k(E) H + k(S)H)/(k(E)D6 + k(S)D6) = 2.3 +/- 0.1 for the disappearance of th e substrate in 70 vol% methanol in water, and k(E)H/k(E)D6 = 3.1 +/- 0 .1 and k(S)H/k(S)D6 = 1.4 +/- 0.1 for the elimination and substitution , respectively. In pure acetonitrile the alkene 2 is the sole product and the isotope effect was found to be k(E)H/k(E)D6 = 2.3 +/- 0.1. The se results strongly indicate a branched mechanism involving rate-limit ing formation of a common contact ion pair which either undergoes nucl eophilic attack by the solvent or is dehydronated. The intermediate sh ows a very small discrimination between different nucleophiles. Thiocy anate ion and azide ion, which are assumed to react with diffusion-con trolled rates with the ion pair, are only a few times more reactive th an a water molecule in 70 vol% methanol in water, k(SCN)/K(HOH) = 3 an d k(N3)/k(HOH) = 4, ratios of second-order rate constants. Methanol wa s found to be slightly less reactive than water, k(MeOH)/k(HOH) = 0.7.