JNTU B.Pharmacy II PHARMACEUTICAL ORGANIC CHEMISTRY-II Previous Question Paper (October/November-2020 - R17)

Hückel's rule states that a planar, cyclic, fully conjugated system is aromatic if it contains (4n + 2) π-electrons, where n is a non-negative integer (n = 0, 1, 2, ...). This rule provides a criterion for predicting the stability and chemical behavior of cyclic unsaturated compounds.

Merits:

• It successfully predicts the aromaticity of benzene (n=1, 6π electrons) and heterocyclic compounds like pyridine, furan, and pyrrole.
• It explains the unusual stability and characteristic electrophilic substitution reactions of aromatic systems compared to alkenes.
• It allows for the prediction of aromatic character in ions, such as the cyclopropenyl cation (n=0, 2π electrons) and the cyclopentadienyl anion (n=1, 6π electrons).

Demerits:

• The rule is strictly applicable only to monocyclic systems; it becomes less reliable for complex polycyclic aromatic hydrocarbons.
• It does not account for the influence of steric hindrance or non-planarity, which can disrupt conjugation and negate aromaticity even if the electron count satisfies the rule.
• It fails to predict the properties of anti-aromatic compounds (4n π-electrons) which are often highly unstable.

Friedel-Crafts acylation involves the reaction of benzene with an acyl halide or acid anhydride in the presence of a Lewis acid catalyst, such as anhydrous aluminum chloride (AlCl₃), to produce an aromatic ketone.

Mechanism:

1. Generation of the electrophile: The Lewis acid reacts with the acyl halide to form an acylium ion (R-C≡O⁺).
2. Electrophilic attack: The acylium ion attacks the benzene ring to form a resonance-stabilized sigma complex (arenium ion).
3. Deprotonation: A base removes a proton from the sigma complex to restore the aromaticity of the ring, yielding the ketone. The catalyst is regenerated.

Limitations:

• Deactivation: The product is a ketone, which is less reactive than benzene; therefore, the reaction does not proceed to polyacylation.
• Catalyst consumption: The ketone product forms a stable complex with the AlCl₃ catalyst, necessitating the use of more than one equivalent of the Lewis acid.
• Substrate restriction: The reaction fails if the benzene ring contains strongly electron-withdrawing groups (e.g., -NO₂, -CN, -SO₃H) or amino groups that coordinate with the catalyst.
• Rearrangement: Unlike alkylation, acylation does not involve carbocation rearrangements, but it is limited to the formation of ketones only.