Ch 17 - Reactions of Aromatic Compounds

Ch 17 - Reactions of Aromatic Compounds - Reactions of...

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17 Reactions of Aromatic Compounds W ith an understanding of the properties that make a compound aromatic, we now consider the reactions of aromatic compounds. A large part of this chapter is devoted to electrphilic aRomatic substitutiOn, the most impor- tant mechanism involved in the reactions of aromatic compounds± Many reactions of benzene and its derivatives are expLained by minor variations of electrophiLic aromat² ic substitution. We wilL study severaL of these reactions and then consider how sub³ Stituents on the ring influence its reactivity towaRd eLectrophilic aromatic substitution and the regiochemistry seen in the products´ We wilL aLso study other reactions of aro- matic compoundsµ including nucLeophiLic aromatic substitution, addition reactions, reactionS of Side chainS¶ aNd SpeciaL reactions of phenolS. ·ike an aLkene¶ benzene has clouds of pi eLectrons above and below its sigma bond framework± Although benzene's pi e¸ectrons are in a stabLe aromatic system, they aRe availabLe to attack a strong electrophile to give a carbocation¹ This resonanceºstabi³ lized carbocation is caLled a sigma complex because the electrophile is joined to the benzene ring by a new sigma bond. + E H H H sigma complex The sigma complex (also caLLed an aRenium iOn) is not aRomatic because the »P 3 ºhybriD carbon atom interrupts the rIng of p orbitaLs± This Loss of aromaticity con¼ tributes to the highly endotHeRmic nature of this Frst step. The sigma compLex regains aromaticity either by a reversal of the Frst step ½returning to the reactants) or by ¸oss of the proton on the tetrahedral caRbon atom, Leading to the substitution product± The overa¾L reaction is the substitutiOn of an electrophile (E + ) for a pRoton (H + ) on the aromatic ring: electrophilic aromatic substitution. This class of reactions includes EPM of anisole 17-1 ElectrophiLic Aromatic SubstitutIon substituted E 749
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