The regioselective bromination of acetanilide to yield para-bromoacetanilide as the major product is a standard textbook reaction driven by the strongly activating and para-directing effect of the acetamido group, making it common knowledge in organic chemistry that requires no specific citation.
The claim is a fundamental textbook fact of electrophilic aromatic substitution in organic chemistry. The acetamido group on acetanilide is strongly ortho/para-directing, with steric hindrance and electronic factors making the para-isomer the major product. This is a foundational observation taught universally in introductory organic chemistry, categorizing it as common knowledge.