Nitration of Bromobenzene and Methyl Benzoate

Electrophilic aromatic substitutions (EAS) are excellent models for study of reaction mechanisms and selectivity. As such, they are extensively used in undergraduate organic chemistry laboratories. Typical sophomore organic chemistry student expects EAS of monosubstitued benzene to produce either a mixture of ortho/para isomers or a meta isomer depending on the nature of the substituent. However, most EAS result in a product mixture that is composed of all three regioisomers and, while either ortho/para or meta isomers predominate, they are not exclusive products. Furthermore, ortho/para ratio is frequently different from what would be expected by consideration of steric effects. In this exercise students carried out nitrations of bromobenzene and methyl benzoate on a microscale. The resulting product mixture was analyzed by means of GC-MS. The observed regioslectivities were explained by consideration of electronic effects of the substituents. This valence bond explanation was suitable for sophomore organic chemistry students including chemistry non-majors.