Electrophilic substitution reactions are the most characteristic reactions of aromatic hydrocarbons. These reactions involve the substitution of a hydrogen atom on the aromatic ring with an electrophile. This is due to the unique electronic structure of aromatic compounds, which contains a delocalized pi system of electrons.

The key features of electrophilic substitution reactions in aromatic hydrocarbons are as follows:

1. Electrophile attack: In electrophilic substitution reactions, an electrophile attacks the aromatic ring and forms a sigma bond with one of the carbon atoms in the ring. This leads to the formation of a resonance-stabilized carbocation intermediate.

2. Resonance stabilization: The resonance-stabilized carbocation intermediate formed during electrophilic substitution reactions is stabilized by the delocalization of the positive charge over the aromatic ring. This resonance stabilization is a unique feature of aromatic compounds and contributes to the high reactivity of these compounds towards electrophiles.

3. Rearrangement: In some cases, rearrangement of the carbocation intermediate may occur to form a more stable carbocation before the final substitution reaction takes place. This rearrangement can result in the formation of different substitution products.

4. Substituent effects: The presence of substituents on the aromatic ring can significantly affect the reactivity and regioselectivity of electrophilic substitution reactions. Substituents can either activate or deactivate the ring towards electrophilic attack, and can also influence the position at which the substitution occurs.

Examples of electrophilic substitution reactions in aromatic hydrocarbons include nitration, halogenation, sulfonation, Friedel-Crafts alkylation, and Friedel-Crafts acylation.

In summary, aromatic hydrocarbons undergo electrophilic substitution reactions due to their unique electronic structure and the ability to form resonance-stabilized carbocations. These reactions are characterized by the substitution of a hydrogen atom on the aromatic ring with an electrophile, leading to the formation of a new substituted product.