Which of the following is not a criterion for aromaticity?

It should decolourise \(Br_2/CCl_4\) and alkaline \(KMnO_4\) solution

The correct answer is option 4. It should decolourise \(Br_2/CCl_4\) and alkaline \(KMnO_4\) solution.

To identify the statement that is not a criterion for aromaticity, we need to understand the key principles and rules that define aromaticity. Let's discuss these criteria in detail:

Criteria for Aromaticity

Cyclic and Planar Structure:

Cyclic: The compound must be cyclic, meaning the atoms are arranged in a ring.

Planar: The compound must be planar, which means all the atoms in the ring lie in a single plane. This planarity allows the p-orbitals to overlap effectively.

Continuous Overlap of p-Orbitals:

p-Orbitals: Each atom in the ring must have a p-orbital that can participate in the delocalization of π-electrons.

Parallel p-Orbitals: These p-orbitals must be parallel to each other, enabling continuous overlap around the ring. This continuous overlap forms a conjugated π-system, which is essential for aromaticity.

Hückel's Rule:

(4n + 2) π-Electrons: The cyclic system must contain a specific number of π-electrons, given by the formula (4n + 2), where \( n \) is a non-negative integer (0, 1, 2, ...). This rule, known as Hückel's rule, ensures the system has a certain level of stability due to the delocalized π-electron cloud.

Examples: Benzene (C₆H₆) has 6 π-electrons (n=1). Naphthalene (C₁₀H₈) has 10 π-electrons (n=2).

Chemical Reactivity

Aromatic compounds are distinct from alkenes and alkynes in their chemical reactivity. Aromatic compounds generally do not participate in reactions that disrupt their π-electron system because this would destroy their aromaticity. This distinction is crucial for understanding why certain tests are not applicable to aromatic compounds.

Common Misconceptions

Decolorization of \(Br_2/CCl_4\) and Reaction with Alkaline \(KMnO_4\):

Bromine Test: Alkenes and alkynes react with bromine in carbon tetrachloride (\(Br_2/CCl_4\)), leading to the decolorization of bromine. This reaction is an addition reaction where bromine adds across the double or triple bonds.

Potassium Permanganate Test: Alkenes and alkynes react with alkaline potassium permanganate (\(KMnO_4\)), which results in oxidation. This reaction also indicates the presence of carbon-carbon double or triple bonds.

Aromatic Compounds: Aromatic compounds do not typically decolorize bromine in \(CCl_4\) or react with \(KMnO_4\) because these reactions would disrupt the aromatic system. Aromatic compounds undergo substitution reactions (e.g., electrophilic aromatic substitution) rather than addition reactions.

Detailed Explanation of the options

1. An aromatic compound is cyclic and planar: The option is true. Aromatic compounds must be cyclic to allow for a continuous π-electron system and must be planar to enable effective overlap of p-orbitals.

2. Each atom in the aromatic ring has a p-orbital. These p-orbitals must be parallel so that a continuous overlap is possible around the ring: The option is true. For aromaticity, each atom in the ring must have a p-orbital that overlaps with its neighbors, forming a delocalized π-system. These p-orbitals need to be parallel to ensure effective overlap and continuous conjugation.

3. The cyclic p-molecular orbital (electron cloud) formed by overlap of p-orbitals must contain (4n + 2) π-electrons:The option is trure. This is Hückel's rule, which is a fundamental criterion for aromaticity. The (4n + 2) π-electrons rule ensures the system has the required electron count for aromatic stabilization.

4. It should decolorize \(Br_2/CCl_4\) and alkaline \(KMnO_4\) solution: The option is false. This statement is incorrect because aromatic compounds do not typically react with \(Br_2/CCl_4\) or \(KMnO_4\) in the same way that alkenes or alkynes do. Aromatic compounds are resistant to these tests because such reactions would disrupt their aromatic π-electron system.

The incorrect statement that is not a criterion for aromaticity is: (4) It should decolorize \(Br_2/CCl_4\) and alkaline \(KMnO_4\) solution