Electrophilic substitution on aromatics addition-elimination

Electrophiles may add to aromatics, generating transient cationic intermediates. The driving force to regain aromaticity, however, is typically too strong for subsequent addition of a 

Qualitatively similar mechanisms may be written for a host of other electrophilic substitutions such as sulfonation, halogenation, Friedel-Crafts alkylation and acylation, and thallation. Weaker electrophiles such as molecular halogens and alkyl halides may need activation by a Lewis acid such as AICI3, as shown below, a point we will discuss in Section 3.1  

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