When we synthesize benzene derivatives with two or more substituents the effect of directing groups must be taken into account. Often the order of reactions can change the products produced.
From benzene synthesize:
Two reactions an acylation and a bromination. In this case the two substituents are meta to each other. This means that the effects of a meta directing groups must be utilized. Of the two reactions the acylation puts a meta director on the benzene ring. The means the acylation need to come first.
If the reaction is reversed an ortho/para directing bromine is added first. The end products are different.
Prof. Steven Farmer (Sonoma State University)