链型脂肪族饱和二元酸的结构与反应性的讨论

Structure and reactivity of aliphatic saturated hydrocarbon dicarboxylic acids

通过对脂肪族二元酸的空间构象和电子结构分析,结果表明:(1)乙二酸的两C O键处于反式为能量最优势构象,单键自由运动的最大与最小能差12.313 kcal/mol;丙二酸的羧基O—C O键平面与C—C—C键平面的二面角分别呈59.6°和69.5°为能量最优势构象,单键自由运动的最大与最小能差14.282 kcal/mol;丁二酸的羧基O—C O键平面与C-C-C键平面的二面角在0°为能量最优势构象,单键自由运动的最大与最小能差2.3 745 kcal/mol;(2)羧基的拉电子作用增加了O—H键极性,羧基氢有发生一级离解趋势,二元酸中一旦一个H+离解,形成氧负离子,该羧酸根对另一羧基的吸电子能力下降,二级离解小于一级离解;(3)羧基的拉电子作用使丙二酸的α-C负电荷和α-H的正电荷密度相对于丁二酸、戊二酸较大,随着碳链增长α-H的活性减小;(4)脂肪族二元酸(H2A)及其酸根(HA-,A2-)的能量随碳链n具有良好的线性关系,E(H2A)=-39.037 63814n-376.375 932 46,E(HA-)=0.000 457 52n2-39.021 080 65n-375.584 376 84,E(A2-)=-39.027 086 8n-374.880 594 49;(5)脂肪族二元酸的HOMO能随碳链n具有较好的线性关系,EH(H2A)=0.007 534n-0.474 696,EH(HA-)=0.001 752 3n-0.160 646 36,EH(A2-)=-0.0116 898 8n-0.010 166 55;阐明了脂肪族二元酸中2个羧基官能团的相互作用对二元酸性能的影响.

In this paper, the relationship between structure and reactivity is discussed via the stereo effect and electronic structure analysis of aliphatie diearboxylie acids, respectively. （1） A potential energy plot for rotation about the C--C bond is shown that the eclipsed conformation in oxalic acid is 12.313 kcal/ mol higher in energy than the more stable staggered conformation. In malonie acid the barrier to rotation about the C--C bond for R - COOH is 14.282 kcal/mol,the stable conformation is that the dihedral angle between the two O--C =0 planes with C--C--C plane is 59.6° and 69.5°, respectively. The stable conformation is that the dihedral angle berween the two O--C = 0 planes with C--C--C plane is 0° in succinic acid, and the barrier to rotation about the C--C bond for R--COOH is 2. 3745 kcal/mol. （2） Thecarbonyl group and oxygen atom of O--H group that have high electronegativity tend to withdraw electron density from the hydrogen and have a marked acid-strengthening effect. The acid-strengthening effect of a carboxylic acid is not unexpected. All carbonyl group have this effect because of the associated dipole which provides electrostatic stabilization of the negative charge of a carboxylate anion. K2 for a dicarboxylic acid is generally less than the dissociation constant of K1. （3） The higher negative charge of α- C and positive charge of α - H density in malonic acid compared to succinic, glutaric, adipic acid... , this effect weskening and α - H activity of dicarboxyhc acids with distance down a chain. （4） The relationship between energy and a chain n of dicarboxylic acids,carboxylate anion and dicarboxylate ion is linear, E（H2A） = -39.03763814n -376.375 932 46, E（HA- ） =0.（300 4575 2n^2 -39.021 080 65n - 375.584 376 84, E（A^2- ） = -39.027 086 8n -374.880 594 49. （5） The relationship between HO- MO and a chain n of dicarboxylic acids, carboxylate anion and dicarboxylate ion is hnear, EH （ H2 A） = 0.007 534n-0.474 696,EH（HA-） = 0.001 752 3n -0. 160 646 36,EH（A^2-） = -0.011 689 88n -0.010 166 55. Some results obtained may be useful as references for teaching.