Mechanisms of competing solvolytic elimination and substitution reactions. The role of ion-pair intermediates in aqueous solvents

Abstract

Solvolysis of 9-(2-bromo-2-propyl)fluorene 1-Br in mixtures of water with methanol or acetonitrile at 25 °C provides the elimination product 9-isopropenylfluorene 2, and the substitution products 9-(2-hydroxy-2-propyl)fluorene 1-OH and 9-(2-methoxy-2-propyl)fluorene 1-OMe. The Grünwald–Winstein parameter was measured in methanol–water mixtures as m_obs= 0.70, which is composed of the parameter for the elimination reaction, m_E= 0.65, and the substitution reaction, m_s= 0.83. The parameters for the corresponding chloride 1-Cl were measured as m_obs= 0.82, m_E= 0.76 and m_s= 0.97. The kinetic deuterium isotope effects for the reactions of the hexadeuteriated analogue 9-(2-bromo[²H₆]-2-propyl)fluorene ([²H₆]-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 the 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. These results strongly indicate a branched mechanism involving rate-limiting formation of a common contact ion pair which either undergoes nucleophilic attack by the solvent or is dehydronated. The intermediate shows a very small discrimination between different nucleophiles. Thiocyanate ion and azide ion, which are assumed to react with diffusion-controlled rates with the ion pair, are only a few times more reactive than a water molecule in 70 vol% methanol in water, k_SCN/k_HOH= 3 and k_N3/k_HOH= 4, ratios of second-order rate constants. Methanol was found to be slightly less reactive than water, k_MeOH/k_HOH= 0.7.