关键词: 多取代苯, 自位硝化, 自位溴化, 六元环环外定位效应

Abstract: In order to study substituent effects on the substitution reactions of 10 multi-sub-stituted benzene, (这里有图片199510-1549-1.gif)the relative yields of the products from their nitration and bromination in different solventsare investigated.The esteric group(X=-CH₂OAc) of compound 2(R₁=t-Bu and R₂=-Me)vields 60%-60% ipso-product 2b(X=NO₂,R₁=t-Bu and R₂=Me) for the nitration in a solution of 65% nitric acid and Ac₂O, and 84% ipso-roduct 3b(X=Br, R₁=-t-Bu and R₂=Me)for the bromination in a solution of 65% nitric acid,NaBr and EtOH. Under the same conditions.the esteric group(X=CH₂ONO₂) of the compound 4(R₁=t-Bu and R₂=Me)gives 30% 2_band 95% 3_brespectively. An interesting bromination of compound 3(R₁=t-Bu,R₂=Me and X=CH₂Br)in a solution of 65% nitric acid and EtOH yields 95% ipso-product 3b. Firstiy, this bromination proceeds practically through a intermediate 4 which resulted from the replacement of Br^-by NO₃^- and then the esteric group(X=CH₂ONO₂)of intermediate 4 is substituted by Br₂resulting from the oxidation of Br^-by nitric acid. No matter what the alkyl groups,R1 and R₂, are, no ipso-product was found from the nitration of thecompounds,where X=-CH₂Cl and-CH₂Br in acetic anhydride,which means that the esteric groups,-CH₂OAc and-CH₂ONO₂,and only them are ipso-directors in the case of thepresented compounds, The esteric groups would seem to make it possible for the reactions toproceed through the six-membered cyclic transition states such as:  Steric effect of the alkyl groups helps the esteric group to yield ipso-product.the group,X=-CH₂OAc, of compounds 8(R₁=R₂=Me) gives only 5% ipso-product for the nitration.

Key words: Multi-substituted benzene, Ipso nitration, Ipso bromination, Six-membered cyclic transition state