Mechanism of Electrophilic Aromatic Substitution Step 1:Attack on the electrophile forms the sigma complex. H 6 H H E+ H H H sigma complex(arenium ion) Copyright 2010 Pearson Prentice Hall,Inc. Step 2:Loss of a proton gives the substitution product. H H H H H base: base-H H H H H Copyright @2010 Pearson Prentice Hall,Inc. Chapter 17 3Chapter 17 3 Mechanism of Electrophilic Aromatic Substitution