The Friedel-Crafts reaction creates new alkyl- or acyl-aromatic bonds, with or without cation rearrangement. Designing reaction sequences, especially those involving diazonium intermediates, so as to access a wide variety of substituted benzenes provides a good introduction to the challenges of synthetic organic chemistry. Aromatic rings with strong electron withdrawal can undergo nucleophilic aromatic substitution, which plays an important role in biochemistry. The special properties of phenyl-substituted alkanes, especially benzylic reactivity and steric hindrance, played an important role in the development of organic chemistry a century ago.