Steric and electronic effects on the mechanism of nucleophilic substitution (SNAr) reactions of some phenyl 2,4,6-trinitrophenyl ethers with aniline and N-methylaniline in acetonitrile and in dimethyl sulfoxide
作者:Chukwuemeka Isanbor、Thomas A. Emokpae、Michael R. Crampton
DOI:10.1039/b207997f
日期:2002.12.6
Rate data are reported for the reactions of a series of 3′- and 4′-substituted phenyl 2,4,6-trinitrophenyl ethers, 4, with aniline in acetonitrile, leading to 2,4,6-trinitrodiphenylamine. The main reaction flux occurs through a base catalysed pathway involving formation of a zwitterionic intermediate, equilibrium constant K1, and rate-limiting proton transfer to base, rate constant kAn. The effects of ring substituents on values of rate and equilibrium constants are discussed. In contrast the corresponding reactions in DMSO involve both uncatalysed and base catalysed pathways. Reactions of 4 with N-methylaniline are extremely slow, but values of rate constants for reaction of 4-nitrophenyl 2,4,6-trinitrophenyl ether were measured using 1H NMR spectroscopy. The value of the parameter K1kAn is lowered by a factor of 105 for N-methylaniline relative to aniline in both acetonitrile and in DMSO. This reduction is attributed to increased steric hindrance both in formation of the intermediate and to proton transfer.
速率数据针对一系列3′-和4′-取代苯基2,4,6-三硝基苯基醚4与苯胺在丙酮腈中的反应进行了报告,该反应生成2,4,6-三硝基二苯胺。主要反应通路经由酸催化途径,涉及形成内盐中间体、平衡常数K1以及限制速率的质子转移至碱基,速率常数kAn。讨论了环取代基对速率和平衡常数值的影响。相比之下,在二甲基亚砜中对应的反应涉及未催化与酸催化两条途径。4与N-甲基苯胺的反应极其缓慢,但使用1H核磁共振光谱测量了4-硝基苯基2,4,6-三硝基苯基醚反应的速率常数值。在丙酮腈和二甲基亚砜中,对于N-甲基苯胺相较于苯胺,参数K1kAn的值降低了10的5次方倍。这种降低归因于在中间体形成与质子转移过程中的空间位阻增加。