Aromatic Hydrocarbons: Why Benzene Substitutes Instead of Adding

In the paper

Unit 3, Organic chemistry, carries 17 of the 50 chemistry marks; with eleven chapters sharing them, this one averages a little over one and a half (MEC publishes weights by unit, not by chapter). Two things from it show up far beyond this chapter: the named electrophiles, and the directing table, which decides the products in phenol, aniline and nitrobenzene as well.

MEC scope: Aromaticity and resonance; Preparation and chemical properties of benzene.

Three ways it is asked:

• Recall: the catalyst for Friedel–Crafts; the product of benzene with chlorine in sunlight; the number of π electrons in naphthalene.
• Understanding: why benzene substitutes rather than adds; why –Cl deactivates yet directs ortho and para; why cyclooctatetraene is not aromatic.
• Application: work out the resonance energy from heats of hydrogenation; predict the major product when a second group is introduced into nitrobenzene or toluene.