A Mechanism for Electrophilic Substitution Reactions of Benzene

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.

Preliminary step: Formation of the strongly electrophilic bromine cation

Step 1: The electrophile forms a sigma-bond to the benzene ring, generating a positively charged benzenonium intermediate

Step 2: A proton is removed from this intermediate, yielding a substituted benzene ring

This mechanism for electrophilic aromatic substitution should be considered in context with other mechanisms involving carbocation intermediates. These include S_N1 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. The cation may transfer a proton to a base, giving a double bond product.
3. The cation may rearrange to a more stable carbocation, and then react by mode #1 or #2.

S_N1 and E1 reactions are respective examples of the first two modes of reaction. 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. The carbocation intermediate in electrophilic aromatic substitution (the benzenonium ion) is stabilized by charge delocalization (resonance) so it is not subject to rearrangement. 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. proton loss).

مواضيع ذات صلة

مركبات الآزو متجانسة الحلقة Homocyclic azo compounds

تصنيف مركبات الآزو Classification of azo compound

أصباغ الآزو Azo Dye

تفاعلات الازدواج Reaction Coupling

أملاح الدايازونيوم الاروماتية      Aromtic Diazonium Salt    

أهمية الكواشف العضوية reagents Importance of organic

الثايوزبينات Thiazepines

الدايازبينات Diazepines

الاوكسازبينات Oxazepines

المركبات الحلقية غير المتجانسة Heterocyclic Compounds

نبذة عن مظاهر كيمياء الكلورامين-T الحيوية واهميته

بعض مظاهر كيمياء الكلورامين-T