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=-CH
2OAc) of compound 2(R
1=t-Bu and R
2=-Me)vields 60%-60% ipso-product 2b(X=NO
2,R
1=t-Bu and R
2=Me) for the nitration in a solution of 65% nitric acid and Ac
2O, and 84% ipso-roduct 3b(X=Br, R
1=-t-Bu and R
2=Me)for the bromination in a solution of 65% nitric acid,NaBr and EtOH. Under the same conditions.the esteric group(X=CH
2ONO
2) of the compound 4(R
1=t-Bu and R
2=Me)gives 30% 2
band 95% 3
brespectively. An interesting bromination of compound 3(R
1=t-Bu,R
2=Me and X=CH
2Br)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 NO
3- and then the esteric group(X=CH
2ONO
2)of intermediate 4 is substituted by Br
2resulting from the oxidation of Br
-by nitric acid. No matter what the alkyl groups,R1 and R
2, are, no ipso-product was found from the nitration of thecompounds,where X=-CH
2Cl and-CH
2Br in acetic anhydride,which means that the esteric groups,-CH
2OAc and-CH
2ONO
2,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=-CH
2OAc, of compounds 8(R
1=R
2=Me) gives only 5% ipso-product for the nitration.
