Question

For the substituent in this experiment that was found to be least activating, explain why bromination took place at the position on the ring indicated by the experimental results.

Experiment 40 (relative reactivities of several aromatic compounds) out of the Microscale Approach to Organic Laboratory Techniques.

Answer 1

The -NHCOCH3 group is a less powerful ortho,para directing group than -NH2 due to the presence of the electron   withdrawing carbonyl group,which renders the aromatic ringless electron-rich. Electrophilic substitution by bromine is stilldirected electronically to theortho and para positions on the ring.the acetamido group,-NHCOCH3, however, effectively blocks the ortho positions by steric hindrance. For thesereasons, only para substitution is usually observed.