aromatic - Substitution Reactions of Benzene and Other...

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Substitution Reactions of Benzene and Other Aromatic Compounds The remarkable stability of the unsaturated hydrocarbon benzene has been discussed in an earlier section . The chemical reactivity of benzene contrasts with that of the alkenes in that substitution reactions occur in preference to addition reactions, as illustrated in the following diagram (some comparable reactions of cyclohexene are shown in the green box). A demonstration of bromine substitution and addition reactions is helpful at this point, and a virtual demonstration may be initiated by clicking here . Many other substitution reactions of benzene have been observed, the five most useful are listed below (chlorination and bromination are the most common halogenation reactions). Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as Electrophilic Aromatic Substitution . The catalysts and co-reagents serve to generate the strong electrophilic species needed to effect the initial step of the substitution. The specific electrophile believed to function in each type of reaction is listed in the right hand column. Reaction Type Typical Equation Electrophile E (+) Halogenation: C 6 H 6 + Cl 2 & heat FeCl 3 catalyst —— > C 6 H 5 Cl + HCl Chlorobenzene Cl (+) or Br (+) Nitration: C 6 H 6 + HNO 3 & heat H 2 SO 4 catalyst —— > C 6 H 5 NO 2 + H 2 O Nitrobenzene NO 2 (+) Sulfonation: C 6 H 6 + H 2 SO 4 + SO 3 & heat —— > C 6 H 5 SO 3 H + H 2 O Benzenesulfonic acid SO 3 H (+) Alkylation: Friedel-Crafts C 6 H 6 + R-Cl & heat AlCl 3 catalyst —— > C 6 H 5 -R + HCl An Arene R (+) Acylation: Friedel-Crafts C 6 H 6 + RCOCl & heat AlCl 3 catalyst —— > C 6 H 5 COR + HCl An Aryl Ketone RCO (+) 1. A Mechanism for Electrophilic Substitution Reactions of Benzene
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A two-step mechanism has been proposed for these electrophilic substitution reactions. In the first, slow or rate-determining, step the electrophile forms a sigma-bond to the benzene ring, generating a positively charged benzenonium intermediate . In the second, fast step, a proton is removed from this intermediate, yielding a substituted benzene ring. The following four-part illustration shows this mechanism for the bromination reaction. Also, an animated diagram may be viewed. This mechanism for electrophilic aromatic substitution should be considered in context with other mechanisms involving carbocation intermediates. These include S N 1 and E1 reactions of alkyl halides , and Brønsted acid addition reactions of alkenes . To summarize, when carbocation intermediates are formed one can expect them to react further by one or more of the following modes: 1. The cation may bond to a nucleophile to give a substitution or addition product. 2.
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aromatic - Substitution Reactions of Benzene and Other...

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