Electrophilic Aromatic Substitution of Polycyclic Aromatics

• Polycyclic aromatics such as naphthalene, anthracene etc. react with similar reagents to those used for benzene
• Typically they are more reactive than benzene
• Reactions are usually less selective than those of benzene and mixtures of products are often obtained

For example, the nitration of naphthalene proceeds to give mainly 1-nitronaphthalene:

This selectivity can be rationalized by drawing the resonance structures for intermediates produced by attack of the electrophile at the 1- and 2-positions.  Attack at the 1- position gives an intermediate that is represented by 7 resonance contributors of which 4 leave the aromaticity of the other ring intact. In contrast attack at the 2-position gives an intermediate with 6 resonance contributors in which only 2 have the aromaticity of the other ring intact.