Relative Basicity of the Methylbenzenes
Relative Basicity of the Methylbenzenes
复制标题
甲苯的相对碱度
DOI：
10.1021/ja01149a033
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发表时间：
1951
影响因子：
15
通讯作者：
A. P. Lien
中科院分区：
文献类型：
--
作者：
D. A. Mccaulay;A. P. Lien
测定了所有甲苯的相对碱度。所采用的技术包括两个间歇萃取实验，其中甲苯混合物用氟化氢和控制量的三氟化硼处理，和ofvapor压力测量的各种甲苯与氟化氢和三氟化硼的混合物。结果表明，在从甲苯到六亚甲基的过程中，碱性程度随着每个连续甲基的加入而增加。碱性也受甲基取代基位置的影响，1，3-取向对碱性贡献最大。在氟化氢存在下甲苯分子与三氟化硼相互作用形成的络合物中，每摩尔芳烃含有一摩尔三氟化硼，可能由Ar-H ~+和BF_3 ~+两个离子组成。提出了阳离子是芳香取代反应中中间体结构的共振杂化，并用这一假设解释了随着甲基数目的增加和1，3-位向的增加，配合物的稳定性增加的现象。人们已经认识到芳烃是弱碱，最近已经发展了几种测量这一性质的方法。Andrews和Keefer 2使用银离子和芳香核之间的络合物形成反应作为芳香基强度的量度。Fairbrother 8将碘在各种烃类溶液中的偶极矩变化与每种烃类溶剂的基本性质联系起来。Benesi和Hildebrand 4对碘在芳烃中的溶液进行了光谱研究，以将碘-芳烃复合物的强度与芳烃的碱性关联起来。Brown和Brady 6发现，氯化氢在低温下在芳族溶剂中的溶解度可作为芳烃基本性质的量度。这些方法已被用来比较苯的相对碱度与其甲基取代同系物的碱度，包括三甲苯。一般来说，已发现苯环的碱性随甲基亚甲基的数目增加而增加，
The relative basicities of all the methylbenzenes were measured. The techniques employed consisted of both batch extrac-tion experiments, in which methylbenzene mixtures were treated with hydrogen fluoride and controlled amounts of boron trifluoride, and ofvapor pressure measurements of mixtures of various methylbenzenes with hydrogen fluoride and boron trifluoride. The results show that the degree of basicity increases with the addition of each successive methyl group in going from toluene to hexamethylbenzene. Basicity is also influenced by position of methyl substituents; the 1, 3-orientation con-tributes most to basic character. The complex formed by the interaction of a methylbenzene molecule with boron trifluoride in the presence ofhydrogen fluoride contains one mole of boron trifluoride per mole ofaromatic and probably consists of the two ions Ar-H+ and BF «~. It is suggested that the cation is a resonance hybrid of structures commonly assigned to inter-mediates in aromatic substitution reactions, and this postulate is used to explain the increasing stability of the complex with increasing number and with 1, 3-orientation of methyl groups.It has been recognized that aromatic hydrocarbons are weak bases, 1 and several methods have been developed recently for measuring this property. Andrews and Keefer2used the reaction of complex formation between silver ion and an aromatic nucleus as a measure of aromatic base strength. Fairbrother8 related the change in dipole moments of iodine in solution in various hydrocarbons with the basic character of each hydrocarbon solvent. Benesi and Hildebrand4 employed a spectrophoto-metric study of solutions of iodine in aromatic hydrocarbons to correlate the strength of iodine-aromatic complexes with the basicity of the aro-matic hydrocarbons. Brown and Brady6 found that the solubility of hydrogen chloride in aromatic solvents at low temperatures serves as a measure of the basic property of the aromatic hydrocarbons. These methods have been used to compare the relative basicity of benzene with the basicities of its methyl-substituted homologs up to and including the trimethylbenzenes. In general, it has been found that the basic character ofthe benzene ring increases with the number of methyl sub-