![]() ![]() Most elements other than metals and carbon have a significantly greater electronegativity than hydrogen. The first is the inductive effect of the substituent. The influence a substituent exerts on the reactivity of a benzene ring may be explained by the interaction of two effects: In the following diagram we see that electron donating substituents (blue dipoles) activate the benzene ring toward electrophilic attack, and electron withdrawing substituents (red dipoles) deactivate the ring (make it less reactive to electrophilic attack). This activation or deactivation of the benzene ring toward electrophilic substitution may be correlated with the electron donating or electron withdrawing influence of the substituents, as measured by molecular dipole moments. In contrast, a nitro substituent decreases the ring's reactivity by roughly a million. For example, a hydroxy or methoxy substituent increases the rate of electrophilic substitution about ten thousand fold, as illustrated by the case of anisole in the virtual demonstration (above). Experiments have shown that substituents on a benzene ring can influence reactivity in a profound manner. The first is the relative reactivity of the compound compared with benzene itself. When substituted benzene compounds undergo electrophilic substitution reactions of the kind discussed above, two related features must be considered: Substitution Reactions of Benzene Derivatives In principle it could react by either mode 1 or 2, but the energetic advantage of reforming an aromatic ring leads to exclusive reaction by mode 2 ( ie. The carbocation intermediate in electrophilic aromatic substitution (the benzenonium ion) is stabilized by charge delocalization (resonance) so it is not subject to rearrangement. The second step of alkene addition reactions proceeds by the first mode, and any of these three reactions may exhibit molecular rearrangement if an initial unstable carbocation is formed. S N1 and E1 reactions are respective examples of the first two modes of reaction. The cation may rearrange to a more stable carbocation, and then react by mode #1 or #2. 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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