苯氧乙酸类化合物的酸离解常数测定及其量子化学计算

Determination and Quantum Chemical Calculation of the Acid Dissociation Constants of Some Phenoxy Acetic Acids

用电位滴定法测定了 7种苯氧乙酸类化合物的酸离解常数 ,其酸性均强于乙酸和苯甲酸 ,且其酸性变化规律与经典的有机化学理论完全相符 ,氧原子的诱导效应和对苯环的共轭效应使苯氧乙酸类化合物容易离解出质子 ,而氧原子对苯环的供电子作用使得含有 2个或 2个以上—OCH2 COOH基团的苯氧乙酸类化合物的酸性比苯氧乙酸弱。用半经验量子化学CNDO/2方法 ,分别计算了 7种苯氧乙酸化合物分子中各个羧基的O—H键和C—O单键的键电荷密度 ,计算了键电荷密度比 :f =ρO—H/ρC—O。结果表明 ,苯氧乙酸的羧基f值大小及其它 6种苯氧乙酸类化合物最易发生离解的羧基 f值大小顺序 ,分别与它们的pKa 或 pKa1值的大小顺序相对应 ,故 f值可以作为苯氧乙酸类化合物酸性强弱的判据。

The acid dissociation constants of seven phenoxy acetic acids were determined by potentiometric titration. It has been shown that phenoxy acetic acids exhibit stronger acidity than both acetic acid and benzoic acid due to the induction and conjugation of the oxygen atom on —OCH 2COOH that links to the benzene ring. Acidity of phenoxy acetic acids containing two or more —OCH 2COOH groups is lower than that of phenoxy acetic acid by reason of electron donation of oxygen atom to the benzene ring. Quantum chemical calculation of the phenoxy acetic acids was carried out using CNDO/2 method. The bond charge density(ρ) of O—H and C—O bond in carboxyl group of seven phenoxy acetic acids was calculated, and the value f= ρ O—H/ρ C—O, was figured out. The f value of the carboxyl group in phenoxy acetic acid as well as the f values of the most likely dissociated carboxyl group in other six phenoxyl acetic acid derivatives are changed in the corresponding change order of their pK a or pK a1 values. It is concluded that the f value can be used as a judgement criterion of relative acidity for phenoxy acetic acids.