Lewis acid-base titrations employing megacycle-frequency oscillators : Titration involving stannic chloride in acetonitrile and benzene solution

Abstract

By using a megacycle-frequency oscillator to follow the reaction, the Lewis acid, stannic chloride, can be titrated with nitrogen bases in acetonitrile as solvent and with oxygen bases in benzene as solvent with an error of 0.5-4%; reverse-order titrations were equally successful. The characteristic maxima and minima in the titration curves indicate that in acetonitrile stannic chloride probably forms AB, A3B4 and A4B3 adducts with piperidine, and AB and A4B3 adducts with pyridine; no adduct was indicated for diphenylamine. In benzene solution, stannic chloride forms (a) AB2 adducts with MeOH, EtOH, n-PrOH, iso-PrOH, n-BuOH, sec.-BuOH and iso-BuOH, (b) AB and AB2 adducts with acetone and tetrahydrofuran, and (c) an AB adduct with dioxane; the stoichiometry for a group of ethers is less decisive. The presence of the 1:1 tetrahydrofuran-stannic chloride adduct in benzene supports the belief that pentacoordinate tin exists in certain adducts with oxygen bases. The megacycle-frequency oscillator was also applied to the estimation of the relative base strength of Lewis bases toward a given Lewis acid by assuming that the instrument response increase, as an ether or alcohol was added to stannic chloride in benzene, is due to the formation of the new coordinate bond. Agreement of the data obtained with the limited existing data on relative base strengths of ethers is good in those cases where comparable steric factors are involved.