001_ES-2

001_ES-2 - Electrophilic Aromatic Substitution...

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1 Nucleophile- Lewis base. Shorthand: Ar-H E + + H E + H + Electrophile- Lewis acid: may be or neutral. Product: neutral if electrophile is ; if electrophile is neutral. Shorthand: Ar-E H 2 SO 4 Ar-H + HNO 3 Ar-NO 2 + H 2 O Electrophilic Aromatic Substitution Electrophilic substitution is the typical reaction type for aromatic rings. Generalized electrophilic aromatic substitution: 1. Nitration —
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2 + H 2 SO 4 NO 2 fast + HSO 3 - H NO 2 (3) Addition of a basic group, eg HSO 4 - , to the σ complex would result in formation of a non-aromatic compound, whereas expulsion of H + results in an aromatic product. This intermediate carbocation is sometimes called a σ complex. σ bond NO 2 H NO 2 H NO 2 H slow (2) O=N=O + a nitronium ion, cf. NO 2 + BF 4 - (1) HONO 2 + 2 H 2 SO 4 H 3 O HSO 4 NO 2 + 2 + Mechanism Since the nitro group can often be reduced to the amine group (tin or iron and HCl are frequently used to effect this reduction), ArNO 2 > Ar-NH 2 , nitration is often used to ultimately make an aryl amine.
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3 SO 3 ArSO 3 H + H 2 O Ar-H + H 2 SO 4 strong acid Ar-SO 3 H + H 2 O + H 3 O + (4) Ar-SO 3 - + H 2 SO 4 SO 3 - + HSO 4 - H - O 3 S (3) σ complex H - O 3 S + (2) H 3 O + + HSO 4 - + SO 3 2 H 2 SO 4 (1) S O O O 2. Sulfonation — Mechanism
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4 Note the following — + Sulfonic acids ( eg , ArSO 3 H), derivatives of sulfuric acid, are strong acids and are highly ionized in water. + Each step in the sulfonation mechanism is an equilibrium; therefore, the entire reaction is an equilibrium. Thus, Ar-H can be sulfonated using fuming sulfuric acid, H 2 SO 4 C SO 3 , and Ar-SO 3 H can be desulfonated (to Ar-H) by boiling it in a dilute solution of sulfuric acid. + Sulfonation of an aromatic ring can provide a route to a phenol , Ar-OH. If a sulfonic acid is fused with solid KOH, the -SO 3 H group is replaced by -OH. [Owing to the vigorous reaction conditions, there are limitations with regard to which substituents may be present on the ring.] + Sulfonation of an aromatic ring provides a highly polar site capable of hydrogen bonding; this gives rise to water solubility. Dyes are sometimes made
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001_ES-2 - Electrophilic Aromatic Substitution...

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