Eleven symmetrically 2,6-disubstituted benzoic acids (with the following substituents: OCH3, OC2H5, OC3H7, OCH(CH3)2, OC4H9, CH3, F, Cl, Br, I, and NO2) have been synthesized and their dissociation constants measured potentiometrically in methanol, ethanol, propan-1-ol, propan-2-ol, butan-2-ol, acetone, dimethyl sulfoxide, dimethylformamide, acetonitrile, pyridine, and 1,2-dichloroethane. The experimental data obtained have been analyzed from the point of view of solvent effects on acidity of the individual derivatives. Different behaviour found with benzoic acid and the disubstituted derivatives in protic solvents is due to changes in solvation. The different character of solvation of benzoic acid and the disubstituted derivatives depends on the type of substitution, being manifested only in 2,6-disubstituted benzoic acids. The graphical analysis has shown a distinct trend in the increase of magnitude of deviation of the point of benzoic acid in the series: propan-2-ol, butan-2-ol, propan-1-ol, ethanol, methanol. This order correlates with the steric demands of carbon chain of the alcohols used. The abnormal behaviour of benzoic acid in the dissociation in these alcohols as compared with that of its 2,6-disubstituted derivatives is due to the different extent of solvation of the reaction centre caused by steric hindrance. Against the expectation, benzoic acid appears to be a weaker acid in protic solvents, whereas its alkoxy derivatives are stronger acids. The solvation also minimizes the inductive effect of alkoxy groups in the symmetrically 2,6-disubstituted derivatives. In aprotic solvents the acidity of 2,6-dialkoxybenzoic acids is also increased, in this case as a result of sterically forced deviation of the reaction centre and/or the substituents out of the plane of benzene ring.
 
                                    已合成了十一个对称的2,6-二取代
苯甲酸(具有以下取代基:OCH
3、OC
2H
5、OC
3H
7、OCH(CH
3)
2、OC
4H
9、CH
3、F、Cl、Br、I和NO
2),并在
甲醇、
乙醇、1-
丙醇、2-
丙醇、
2-丁醇、
丙酮、
二甲基亚砜、二甲基甲酰胺、
乙腈、
吡啶和
1,2-二氯乙烷中测量了它们的电位滴定解离常数。从溶剂对各个衍
生物酸度的影响角度分析了所得到的实验数据。在质子溶剂中,
苯甲酸和二取代衍
生物的不同行为是由于溶剂化的改变。
苯甲酸和二取代衍
生物的不同溶剂化特性取决于取代类型,仅在2,6-二取代
苯甲酸中表现出来。图形分析显示出
苯甲酸点的偏离程度在以下系列中呈现出明显的趋势:2-
丙醇、
2-丁醇、1-
丙醇、
乙醇、
甲醇。这个顺序与所使用
醇类的碳链的空间位阻有关。与其2,6-二取代衍
生物相比,在这些醇中
苯甲酸的解离行为异常,这是由于空间位阻引起的反应中心溶剂化程度不同。与预期相反,
苯甲酸在质子溶剂中似乎是较弱的酸,而其烷氧基衍
生物是较强的酸。溶剂化还使对称的2,6-二烷氧基
苯甲酸中烷氧基群的感应效应最小化。在无质子溶剂中,2,6-二烷氧基
苯甲酸的酸度也增加了,这是由于反应中心和/或取代基被迫离开苯环平面的空间位阻造成的。