Important Concepts
1. Substituents on the benzene ring can be divided into two classes: those that activate the ring by electron donation and those that deactivate it by electron withdrawal. The mechanisms of donation and withdrawal are based on induction or resonance. These effects may operate simultaneously to either reinforce or oppose each other. Amino and alkoxy substituents are strongly activating, alkyl and phenyl groups weakly so; nitro, trifluoromethyl, sulfonyl, oxo, nitrile, and cationic groups are strongly deactivating, whereas halogens are weakly so.
2. Activators direct electrophiles ortho and para; deactivators direct meta, although at a much lower rate. The exceptions are the halogens,which deactivate but direct ortho and para.
3. When there are several substituents, the strongest activator (or weakest deactivator) controls the regioselectivity of attack, the extent of control decreasing in the following order: NR2, OR > X2, R > meta directors
4. Strategies for the synthesis of highly substituted benzenes rely on the directing power of the substituents, the synthetic ability to change the sense of direction of these substituents by chemical manipulation, and the use of blocking and protecting groups.
5. Naphthalene undergoes preferred electrophilic substitution at C1 because of the relative stability of the intermediate carbocation.
6. Electron-donating substituents on one of the naphthalene rings direct electrophiles to the same ring, ortho and para. Electron-withdrawing substituents direct electrophiles away from that ring; substitution is mainly at C5 and C8.
7.The actual carcinogen derived from benzo[a]pyrene appears to be an oxacyclopropanediol in which C7 and C8 bear hydroxy groups and C9 and C10 are bridged by oxygen. This molecule alkylates one of the nitrogens of one of the DNA bases, thus causing mutations.
From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE
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