A kinetic study on SNAr reactions of 1-(4-nitrophenoxy)-2,4-dinitrobenzene (1a) with various anionic nucleophiles in 80 mol% water – 20 mol% DMSO at 25.0 °C is reported. The Brønsted-type plot for the reaction of 1a with a series of substituted phenoxides and HOO− results in an excellent linear correlation with βnuc = 1.17. However, OH− exhibits dramatic negative deviation from the Brønsted-type plot, while N3−, C6H5S−, and butane-2,3-dione monoximate (Ox−) deviate positively from linearity. HOO− is 680-fold more reactive than OH− but does not exhibit the α-effect. In contrast, Ox− is 166-fold more reactive than isobasic 4-Cl−C6H4O− and exhibits the α-effect. Differential solvation effects have been suggested to be responsible for the α-effect in this study, i.e., Ox− exhibits the α-effect, since it is 5.7 kcal/mol less strongly solvated than 4-Cl−C6H4O− in the reaction medium, while HOO− does not show the α-effect due to a strong requirement for partial desolvation before nucleophilic attack. The highly enhanced reactivity of polarizable N3− and C6H5S− and extremely decreased reactivity of nonpolarizable OH− are in accord with the hard–soft acid and base principle.
一项关于1-(4-硝基苯氧基)-2,4-二硝基苯(1a)与80%水-20%二甲基亚砜中各种阴离子亲核试剂在25.0°C下进行的SNAr反应的动力学研究已经报道。对1a与一系列取代苯氧基和HOO-反应的Brønsted类型图表显示出与βnuc = 1.17的优秀线性相关性。然而,OH-显示出与Brønsted类型图表戏剧性的负偏差,而N3-、C6H5S-和丁二酮单肟(Ox-)则呈现出正偏离直线。HOO-比OH-反应性高出680倍,但不表现出α效应。相比之下,Ox-比同等碱性的4-Cl-C6H4O-高出166倍,且表现出α效应。本研究中已经提出差异溶剂化效应可能是导致α效应的原因,即Ox-表现出α效应,因为它在反应介质中的溶剂化程度比4-Cl-C6H4O-低5.7 kcal/mol,而HOO-由于在亲核攻击之前需要部分脱溶剂化,所以不显示α效应。极化性N3-和C6H5S-的反应性显著增强,而不极化性OH-的反应性极大降低,符合硬-软酸碱原理。