Hydrocarbons - Chemical Properties of Benzene and Electrophilic Substitution Reactions

Benzene ($C_6H_6$) is an aromatic hydrocarbon characterized by a planar ring with delocalized $\pi$ electrons, making it highly stable due to resonance energy.

The primary chemical characteristic of Benzene is Electrophilic Substitution Reactions ($S_EAr$), where an electrophile ($E^+$) replaces a hydrogen atom on the ring without disrupting the aromaticity.

The mechanism of electrophilic substitution involves three steps: (1) Generation of the electrophile, (2) Formation of a carbocation intermediate known as the sigma complex or arenium ion, and (3) Loss of a proton ($H^+$) from the carbocation to restore aromaticity.

Nitration: Benzene reacts with a mixture of concentrated $HNO_3$ and concentrated $H_2SO_4$ (nitrating mixture) at $323-333 K$ to form Nitrobenzene. The electrophile is the nitronium ion ($NO_2^+$).

Halogenation: Benzene reacts with halogens ($Cl_2$, $Br_2$) in the presence of Lewis acids like anhydrous $FeCl_3$, $FeBr_3$, or $AlCl_3$ to form haloarenes. The Lewis acid helps generate the halonium ion (e.g., $Cl^+$).

Sulphonation: The process of heating benzene with fuming sulphuric acid ($H_2SO_4 + SO_3$) to produce Benzene sulphonic acid. The active electrophile is neutral $SO_3$.

Friedel-Crafts Alkylation: Benzene reacts with an alkyl halide ($R-X$) in the presence of anhydrous $AlCl_3$ to produce alkylbenzene. The electrophile is the carbocation ($R^+$).

Friedel-Crafts Acylation: Benzene reacts with an acyl halide ($R-COCl$) or acid anhydride in the presence of anhydrous $AlCl_3$ to produce an acyl benzene (ketone). The electrophile is the acylium ion ($R-C^+=O$).

Hückel's Rule: For a compound to be aromatic, it must be cyclic, planar, and possess $(4n + 2) \pi$ electrons, where $n$ is an integer ($0, 1, 2, ...$).