Kinetics of styrylquinolinc formation

Abstract

A comprehensive kinetic investigation of reactions occurring in the formation of styryl‐quinolines has been conducted. Specific rate data such as rate equations, rate constants, and thermodynamic activation values have been determined and utilized in a study of which factors are of greatest importance in the reactions forming 2‐styrylquinolines. A mechanism has been proposed for the condensation reaction which agrees with rate relationships found. Gas‐liquid partition chromatography was used to follow the kinetics of the condensation reactions. A rate constant of 5.41 × 10−2M−1min−1 was found for the reaction of benzaldehyde with 2‐methyl‐quinoline using zinc chloride as a catalyst at 104.0°. Rate constants of 1.28 × 10−2 MT−1 min−1 and 1.05 × 10−2 M−1 min−1 were found for the reactions of p‐methylbenzaldehyde and p‐methoxybenzaldehyde with quinaldine to form 2‐(p‐methylstyryl)quinoline and 2‐(p‐methoxystyryl)‐quinoline, respectively at 92.4°. A linear relationship was found using the Hammett equation. An Arrhenius plot was constructed from rate constants determined at five different temperatures for the reaction of benzaldehyde and quinaldine to form 2‐styrylquinoline, using zinc chloride as a catalyst. The energy of activation, Ea, was found to be 22.2 kcal/mole for this reaction. The enthalpy of activation, ΔH‡, free energy of activation, ΔF‡, and entropy of activation, ΔS‡, were found to be 21.4 kcal/mole, 27.7 kcal/mole and ‐16.7 eu/mole, respectively, at 104.0°. The mechanism proposed in the formation of 2‐styrylquinoline involves the fast formation of a carbanion‐zinc chloride complex, which then attacks, in the rate determining step, the aldehyde utilized in the reaction. The lack of reaction of certain methylquinolines is attributed to the inadequacy of the carbanion formed and not to the difficulty involved in the initial formation of the carbanion.