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

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 K1,1 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 pK1,1=–0.68pKa– 0.77 obtains. Compared with pyridines of comparable pKa 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 pKa value. The pKa values at 25° of 2-chloropyridine, 3,5-dichloropyridine, and 3-bromopyridine are 0.75, 0.70, and 2.85 respectively.