Department of Chemistry, University of Sheffield, Sheffield, S3 7HF United Kingdom.
J Am Chem Soc. 2013 Aug 14;135(32):12091-100. doi: 10.1021/ja405799q. Epub 2013 Aug 5.
The association constants for formation a 1:1 complex between 4-phenyl azophenol and tri-n-butylphosphine oxide were measured in mixtures of n-octane and n-decanol, n-octane and n-hexanoic acid, and n-octane and 2-ethylhexyl acetamide. The experiments provide insight into the competition between solvent self-association and solvent-solute interactions in these systems. The solvation properties of the three polar solvents are quite different from one another and from polar solvents that do not self-associate. Carboxylic acids form dimers in concentrated solution (>1 mM in alkanes). Carboxylic acid dimers have exposed H-bond acceptor sites that solvate H-bond donor solutes with a similar binding affinity to carboxylic acid monomers. The carboxylic acid H-bond donor site is inaccessible in the dimer and is not available to solvate H-bond acceptor solutes. The result is that solvation of H-bond acceptor solutes is in competition with solvent dimerization, whereas solvation of H-bond donor solutes is not. Secondary amides form linear polymers in concentrated solution (>10 mM in alkanes). The solvation properties of the secondary amide aggregates are similar to those of carboxylic acid dimers. Solvation of H-bond acceptor solutes must compete with solvent self-association, because the amide H-bond donor site is not accessible in the middle of a polymeric aggregate. However, the amide aggregates have exposed H-bond acceptor sites, which solvate H-bond donor solutes with similar binding affinity to amide monomers. Alcohols form cyclic tetramers at concentrations of 100 mM in alkanes, and these cyclic aggregates are in equilibrium with linear polymeric aggregates at concentrations above 1 M. The alcohol aggregates have exposed H-bond acceptor sites that solvate H-bond donor solutes with similar binding affinity to alcohol monomers. Although the alcohol H-bond donor sites are involved in H-bond interactions with other alcohols in the aggregates, these sites are sufficiently exposed to form a second bifurcated H-bond with H-bond acceptor solutes, and these interactions have a similar binding affinity to alcohol monomers. The result is that self-association of alcohols does not compete with solvation of solutes, and alcohols are significantly more polar solvents than expected based on the properties of alcohol monomers.
在正辛烷和正癸醇、正辛烷和正己酸以及正辛烷和 2-乙基己基乙酰胺的混合物中，测量了 4-苯基偶氮苯酚与三正丁基氧化膦形成 1:1 配合物的缔合常数。这些实验深入了解了在这些体系中溶剂自缔合与溶剂-溶质相互作用之间的竞争。三种极性溶剂的溶剂化性质彼此之间以及与不发生自缔合的极性溶剂有很大的不同。羧酸在浓溶液（烷烃中大于 1mM）中形成二聚体。羧酸二聚体具有暴露的 H-键接受体位点，可与 H-键供体溶质以类似于羧酸单体的结合亲和力进行溶剂化。二聚体中的羧酸 H-键供体位点不可用，无法用于溶剂化 H-键接受体溶质。结果是，H-键接受体溶质的溶剂化与溶剂二聚化竞争，而 H-键供体溶质的溶剂化则不竞争。仲酰胺在浓溶液（烷烃中大于 10mM）中形成线性聚合物。仲酰胺聚合体的溶剂化性质与羧酸二聚体相似。H-键接受体溶质的溶剂化必须与溶剂自缔合竞争，因为酰胺 H-键供体位点在聚合物聚合体的中间不可用。然而，酰胺聚合体具有暴露的 H-键接受体位点，可与酰胺单体具有相似结合亲和力的 H-键供体溶质进行溶剂化。醇在烷烃中的浓度为 100mM 时形成环状四聚体，这些环状聚集体在浓度高于 1M 时与线性聚合物聚集体处于平衡状态。醇聚合体具有暴露的 H-键接受体位点，可与醇单体具有相似结合亲和力的 H-键供体溶质进行溶剂化。尽管醇 H-键供体位点参与聚合体中与其他醇的 H-键相互作用，但这些位点足够暴露，可以与 H-键接受体溶质形成第二个分叉 H-键，这些相互作用与醇单体具有相似的结合亲和力。结果是，醇的自缔合不与溶质的溶剂化竞争，并且醇是比基于醇单体性质预期的极性更大的溶剂。
