JNTU B.Pharmacy II PHARMACEUTICAL ORGANIC CHEMISTRY-II Previous Question Paper (March-2022 - R17)

Benzene exhibits resonance due to the delocalization of its six pi-electrons across the entire planar ring system. The structure is a resonance hybrid of two Kekulé structures, where the carbon-carbon bonds are intermediate in length between single (1.54 Å) and double (1.34 Å) bonds, measuring approximately 1.39 Å. This delocalization provides significant thermodynamic stability, known as resonance energy (approximately 150 kJ/mol), which prevents benzene from undergoing typical addition reactions characteristic of alkenes.

The aromatic character of benzene is defined by Hückel's rule, which states that a planar, cyclic, conjugated system must possess (4n + 2) pi-electrons to be aromatic. Benzene satisfies this with n = 1, resulting in 6 pi-electrons. This electronic configuration creates a closed-shell system that is exceptionally stable, favoring electrophilic substitution reactions that preserve the aromatic sextet rather than addition reactions that would destroy it.

Friedel-Crafts alkylation is an electrophilic aromatic substitution reaction used to introduce an alkyl group onto a benzene ring. The reaction involves the treatment of benzene with an alkyl halide in the presence of a strong Lewis acid catalyst, such as anhydrous aluminum chloride (AlCl₃).

The reaction mechanism proceeds in three main steps:

1. Generation of the electrophile: The Lewis acid reacts with the alkyl halide to form a carbocation intermediate (R⁺).
2. Electrophilic attack: The carbocation acts as an electrophile, attacking the benzene ring to form a non-aromatic resonance-stabilized sigma complex (arenium ion).
3. Deprotonation: A base (often the AlCl₄^- ion) removes a proton from the sp³ hybridized carbon of the sigma complex, restoring the aromaticity of the ring and yielding the alkylbenzene product.

Limitations of this reaction include the potential for carbocation rearrangement, the requirement for non-deactivated rings, and the tendency for polyalkylation, as the newly introduced alkyl group activates the ring toward further substitution.