Fig. illustrates the EAS mechanism using the bromination of fluorobenzene as an example. The mechanism is relatively simple and involves the addition of an electrophile to the aromatic ring to form a σ-complex (also called a Wheland intermediate or an arenium ion), which usually determines the regioselectivity of EAS reactions with Br and Cl. Hence, free energy calculations of different protonated regioisomers, corresponding to different σcomplexes and thereby different reaction pathways, can be used to predict the regioselectivity.