Um Ik-Hwan, Han Jeong-Yoon, Buncel Erwin
Division of Nano Sciences and Department of Chemistry, Ewha Womans University, Seoul, Korea.
Chemistry. 2009;15(4):1011-7. doi: 10.1002/chem.200801534.
Second-order rate constants have been measured spectrophotometrically for the reactions of O-p-nitrophenyl thionobenzoate (1, PNPTB) with HO(-), butan-2,3-dione monoximate (Ox(-), alpha-nucleophile), and p-chlorophenoxide (p-ClPhO(-), normal nucleophile) in DMSO/H(2)O of varying mixtures at (25.0+/-0.1) degrees C. Reactivity of these nucleophiles significantly increases with increasing DMSO content. HO(-) is less reactive than p-ClPhO(-) toward 1 up to 70 mol % DMSO although HO(-) is over six pK(a) units more basic in these media. Ox(-) is more reactive than p-ClPhO(-) in all media studied, indicating that the alpha-effect is in effect. The magnitude of the alpha-effect (i.e., k(Ox(-) )/k(p) (-ClPhO(-) )) increases with the DMSO content up to 50 mol % DMSO and decreases beyond that point. However, the dependency of the alpha-effect profile on the solvent for reactions of 1 contrasts to that reported previously for the corresponding reactions of p-nitrophenyl benzoate (2, PNPB); reactions of 1 result in much smaller alpha-effects than those of 2. Breakdown of the alpha-effect into ground-state (GS) and transition-state (TS) effects shows that the GS effect is not responsible for the alpha-effect across the solvent mixtures. The role of the solvent has been discussed on the basis of the bell-shaped alpha-effect profiles found in the current study as well as in our previous studies, that is, a GS effect in the H(2)O-rich region through H-bonding interactions and a TS effect in the DMSO-rich media through mutual polarizability interactions.
在（25.0±0.1）℃下，采用分光光度法测定了硫代苯甲酸对硝基苯酯（1，PNPTB）与HO⁻、丁二酮一肟（Ox⁻，α-亲核试剂）和对氯苯氧负离子（p-ClPhO⁻，正常亲核试剂）在不同混合比例的DMSO/H₂O中的二级反应速率常数。随着DMSO含量的增加，这些亲核试剂的反应活性显著提高。在高达70 mol% DMSO的体系中，HO⁻对1的反应活性低于p-ClPhO⁻，尽管在这些介质中HO⁻的碱性比p-ClPhO⁻高六个以上的pKₐ单位。在所有研究的介质中，Ox⁻比p-ClPhO⁻更具反应活性，这表明α-效应起作用。α-效应的大小（即k(Ox⁻)/k(p-ClPhO⁻)）在DMSO含量高达50 mol%时随DMSO含量增加而增大，超过该点则减小。然而，1反应的α-效应曲线对溶剂的依赖性与先前报道的苯甲酸对硝基苯酯（2，PNPB）相应反应的情况不同；1的反应产生的α-效应比2的小得多。将α-效应分解为基态（GS）和过渡态（TS）效应表明，在整个溶剂混合物中，GS效应并非α-效应的原因。基于本研究以及我们先前研究中发现的钟形α-效应曲线，讨论了溶剂的作用，即在富水区域通过氢键相互作用产生GS效应，在富DMSO介质中通过相互极化率相互作用产生TS效应。
