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Extra info for Aromatic Chemistry
Sample text
They include not only halogens and the hydroxyl and exert a nitro groups, where an electronegative atom is attached to the ring, but also groups such as carbonyl and nitrile in which an electron-deficient carbon atom is bonded to the ring. Alkyl groups behave in the opposite and releasing electron density to the ring. manner, exerting a The is the analogous redistribution of electrons in n-bonds. However, this resonance effect is transmitted throughout the whole of a conjugated system and creates alternate polarity at the carbon atoms along the system.
Aryl halides only undergo substitution with extreme difficulty unless activated by electron-withdrawing groups, the role of which is to stabilize the intermediate species and so lower the energy of activation of the first step. In this respect, they serve the same purpose as donor groups in electrophilic substitution reactions. Nitro, nitrile and carbonyl are typical activating groups. Activation is best achieved when the group is ortho or para to the halogen, since both inductive and mesomeric withdrawal of electrons operate.
Nitration occurs almost exclusively in the 1 - or a-position of naphthalene. Consideration of the contributing structures to the hybrid carbocation indicates why this is so. For a-attack, the canonical structures include 20,21 and 22. Whereas in 20 and 21 the stable aromatic sextet is preserved, in 22 the aromaticity is disrupted. However, for attack at the 2- or P-position, only one structure, 23, can Aromatic Substitution be drawn in which the aromatic sextet is preserved. It is therefore expected that the carbocation produced during a-attack is more stable than that formed from P-attack and hence the rate of reaction at the a-position is significantly faster.