Chapter 22 - Chapter 21-1 Electrophilic Aromatic...

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Br + FeBr 4 Br—Br + FeBr 3 b) Nitration H H E E Electrophilic Aromatic Substitution Although the aromatic ring systems are stable to simple electrophilic addition of E + A type reagents, the aromatic ring can be caused to react with powerful electrophilic reagents in a substitution reaction. The reaction is written as (using benzene as the example): arenium ion E + A + + HA N + O O H—O N O O H—O H Note that E + has "substituted" for a hydrogen atom, hence the name electrophilic aromatic substitution. Note also that A does not add to the arenium ion since this would result in the loss of aromaticity. Thus the lowest energy path involves loss of H+ (similar to the E1 path in carbocation reactions). Electrophilic Reagents for Electrophilic Aromatic Substitution a) Halogenation Lewis base Lewis Acid Cl—Cl + FeCl 3 Chapter 21-1 Chem 62 Electrophilic Aromatic Substitution + H 2 O + NO 2 + + H 2 SO 4 E Rxn Coord. H E + –A E H E H + H E + A δ + Energy profile of an electrophilic aromatic substitution reaction N + O O – H + Cl + FeCl 4 HSO 4
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R—X + AlCl 3 + R 2 C—CHR R + + H 2 O + R O H C(CH 3 ) 3 c) Alkylation R + AlCl 3 X (Friedel Crafts Alkylation) R 2 C=CHR + H + R—OH + H + Strong acids must be used in order to generate high concentrations of R+ (acids such as H 2 SO 4 , ArSO 3 H, AlCl 3 , H 3 PO 4 , etc) Carbocation rearrangements of R+ will be observed For example: (100%) (CH 3 ) 2 CHCH 2 Cl + AlCl 3 + Chapter 21-2 Chem 62 Electrophilic Aromatic Substitution H 3 C C H CH 3 CH 2 –Cl–AlCl 3 H 3 C C CH 3 CH 3 + H H R R + H R +
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R C O Cl R C O OCOR R C + AlCl 4 :O: d) Acetylation O S O O O S + HSO 4 OH O Chapter 21-3 Chem 62 Electrophilic Aromatic Substitution Friedel Crafts Acylation + AlCl 3 + AlCl 3 e) Sulfonation + H 2 SO 4 C O + R H H + O R + R O R O H H + O S + OH O O S OH O O S OH O acylium ion Acylation -Reduction AlCl 3 O NH 2 NH 2 , KOH ethylene glycol, >150 °C Zn(Hg), HCl Wolff-Kischner reduction Clemmensen reduction H 3 C HC C O H 3 C Cl CH 3 CH 3 CH 3 CH 3 CH 3 CH 3
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Chapter 21-4 Chem 62 Electrophilic Aromatic Substitution X NO 2 R COR SO 3 H Y H E Y Y Y H E E H H H Reaction of benzene with each of the electrophiles (a—e) provides the corresponding product SO 3 , H 2 SO 4 RCOCl, AlCl 3 RCl, AlCl 3 HNO 3 , H 2 SO 4 + E + A Electrophilic Aromatic Substitution Reactions of Substituted Aromatics Some substituted benzene derivatives react with E + A more rapidly than benzene; others react slower than benzene. The rate of reaction of a substituted benzene with E + A is governed by the stability of the transition state leading to the arenium ion.
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  • Spring '08
  • Forbes
  • pH, Chem, Nucleophilic substitution, Organic reaction, Substitution reaction, Nucleophilic Aromatic Substitution, Electrophilic aromatic substitution

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