Second-order rate constants have been measured for reactions of Y-substituted phenyl diphenylphosphinates (1a–h) with EtO−K+ in anhydrous ethanol. A linear Brønsted-type plot is obtained with βLg = −0.54, a typical βLg value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with σo and σ− constants are linear but exhibit many scattered points, while the corresponding Yukawa–Tsuno plot results in excellent linear correlation with r = 0.41. The r value of 0.41 indicates that the leaving group departs at the rate-determining step (RDS) whether the reactions proceed through either a concerted or a stepwise mechanism. However, a stepwise mechanism in which departure of the leaving group occurs at the RDS is excluded since the incoming EtO− ion is much more basic and a poorer leaving group than the leaving aryloxide. The ΔH‡ values determined in the current reactions are strongly dependent on the nature of the substituent Y, while the ΔS‡ values remain constant on changing the substituent Y in the leaving group, i.e., from Y = H to Y = 4-NO2 and Y = 3,4-(NO2)2. These ΔH‡ and ΔS‡ trends also support a concerted mechanism.
测量了 Y 取代苯基
二苯基膦酸盐 (1a-h) 与 EtO-K+ 在无
水乙醇中反应的二阶速率常数。得到的布氏线性图为 βLg = -0.54,这是通过协同机理进行反应的典型 βLg 值。与 σo 和 σ- 常数相关的 Hammett 图是线性的,但表现出许多分散的点,而相应的 Yukawa-Tsuno 图则具有极好的线性相关性,r = 0.41。0.41 的 r 值表明,无论反应是通过协同机理还是逐步机理进行,离去基团都会在速率决定步骤(RDS)上离去。然而,由于进入的 EtO- 离子比离开的芳基氧化物碱性更强,离开基团的能力更弱,因此排除了离开基团在 RDS 发生时离开的分步机理。在当前反应中测定的 ΔH‡ 值与取代基 Y 的性质密切相关,而 ΔS‡ 值则在离去基团中的取代基 Y 发生变化时保持不变,即从 Y = H 到 Y = 4-
NO2 和 Y = 3,4-( )2 时保持不变。这些 ΔH‡ 和 ΔS‡ 趋势也支持一种协同机制。