Quantitative aspects of Lewis acidity. Part XV. Basicity of substituted pyridines and quinolines towards metal halides in diethyl ether

Abstract

BF₃, GaCl₃, SnCl₄, ZnCl₂, and PhSnCl₃ form 1 : 1 adducts with substituted pyridines in diethyl ether solution. 1 : 1 Adducts are also usually formed with quinolines, but 2-substituted quinolines sometimes load to a 2 acid : 1 base species as the only observable adduct. Values of K_1,1 for 1 : 1 adduct formation with pyridines follow the sequence BF₃ SnCl₄ > GaCl₃ > ZnCl₂ > PhSnCl₃, and with quinolines the sequence BF₃ > GaCl₃ > ZnCl₂. For ZnCl₂ and unhindered quinolines the correlation equation pK_1,1=–0.68pK_a– 0.77 obtains. Compared with pyridines of comparable pK_a values in water, quinolines are relatively weaker bases towards metal halides in diethyl ether. Both quinolines and pyridines are more basic towards metal halides in diethyl ether than are anilines of the same pK_a value. The pK_a values at 25° of 2-chloropyridine, 3,5-dichloropyridine, and 3-bromopyridine are 0.75, 0.70, and 2.85 respectively.