The cation may transfer a proton to a base, giving a double bond product.ģ. The cation may bond to a nucleophile to give a substitution or addition product.Ģ. To summarize, when carbocation intermediates are formed one can expect them to react further by one or more of the following modes:ġ. These include S N1 and E1 reactions of alkyl halides, and Brønsted acid addition reactions of alkenes. This mechanism for electrophilic aromatic substitution should be considered in context with other mechanisms involving carbocation intermediates. To see an animated model of this reaction using ball&stick models. These may be viewed repeatedly by continued clicking of the "Next Slide" button. There are four stages to this slide show. Also, an animated diagram may be viewed.īromination of Benzene - An Example of Electrophilic Aromatic Substitution The following four-part illustration shows this mechanism for the bromination reaction. In the second, fast step, a proton is removed from this intermediate, yielding a substituted benzene ring. In the first, slow or rate-determining, step the electrophile forms a sigma-bond to the benzene ring, generating a positively charged benzenonium intermediate. A Mechanism for Electrophilic Substitution Reactions of BenzeneĪ two-step mechanism has been proposed for these electrophilic substitution reactions. The specific electrophile believed to function in each type of reaction is listed in the right hand column.ġ. The catalysts and co-reagents serve to generate the strong electrophilic species needed to effect the initial step of the substitution. Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as Electrophilic Aromatic Substitution. 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). 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).Ī demonstration of bromine substitution and addition reactions is helpful at this point, and a virtual demonstration may be initiated by clicking here.
The remarkable stability of the unsaturated hydrocarbon benzene has been discussed in an earlier section. Substitution Reactions of Benzene and Other Aromatic Compounds
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