Question

Which of the following is not a criterion for aromaticity? (1) An aromatic compound is cyclic and planar (2) Each atom in aromatic ring has p-orbital. These p-orbitals must be parallel so that a continuous overlap is possible around the ring (3) The cyclic p-molecular orbital (electron cloud) formed by overlap of p-orbitals must contain \((4 n+2) \mathrm{p}\) -electrons (4) It should decolourise \(\mathrm{Br} / \mathrm{CCl}_{4}\) and alkaline \(\mathrm{KMnO}_{4}\) solution

Short answer

Option 4 is not a criterion for aromaticity.

Step-by-step solution

Understand Aromaticity Criteria

To determine which option is not a criterion for aromaticity, recall the rules for a compound to be considered aromatic. Aromatic compounds must be cyclic, planar, have conjugated systems (p-orbitals on each atom in the ring) and follow Hückel's rule (\(4n+2\) π-electrons).

Evaluate Option 1

Option 1 states: 'An aromatic compound is cyclic and planar.' This is a valid criterion because aromatic compounds must form a cyclic structure and be planar to allow for effective overlap of p-orbitals.

Evaluate Option 2

Option 2 states: 'Each atom in aromatic ring has p-orbital. These p-orbitals must be parallel so that a continuous overlap is possible around the ring.' This is also a valid criterion, as continuous overlap of p-orbitals ensures delocalization of electrons.

Evaluate Option 3

Option 3 states: 'The cyclic p-molecular orbital (electron cloud) formed by overlap of p-orbitals must contain \(4 n+2\) \(π\)-electrons.' This follows Hückel's rule, another essential criterion for aromaticity.

Evaluate Option 4

Option 4 states: 'It should decolourise \(Br/ CCl_{4}\) and alkaline \(KMnO_{4}\) solution.' This is incorrect because decolourisation of these solutions is a characteristic of unsaturation, typically indicating the presence of double bonds, but it is not a criterion for aromaticity.

Identify the Incorrect Criterion

Aromaticity is defined by the structure and electronic configuration, not by chemical reactivity with bromine or potassium permanganate. Therefore, Option 4 does not describe a criterion for aromaticity.

Key concepts

Hückel's Rule

Hückel's rule is a fundamental guideline for determining if a compound is aromatic. It states that a molecule must have a specific number of π (pi) electrons. This number is given by the formula \(4n+2\), where \ is a non-negative integer (0, 1, 2, 3, etc.). This means that the molecule could have 2, 6, 10, 14, and so on, π-electrons to be considered aromatic.
For example, benzene has 6 π-electrons. Substituting this into Hückel's rule formula, where \ = 1\, we get \(4 \cdot 1 + 2 = 6\), confirming its aromaticity.
A non-aromatic compound will not follow this rule, making it crucial for identifying aromatic compounds.

p-Orbital Overlap

The continuous overlap of p-orbitals across the aromatic ring is essential for delocalization of electrons. Each atom in the aromatic ring must have a p-orbital that is parallel to the others. This allows these p-orbitals to overlap side-by-side, forming a continuous π-electron cloud above and below the plane of the ring.
This overlap leads to the stabilization of the molecule, due to the delocalization of electrons. Delocalized electrons can move freely over the entire ring, lowering the overall energy of the molecule. This is a key requirement for aromaticity.
Without p-orbital overlap, the electrons cannot delocalize, and the compound would not be aromatic.

Cyclic and Planar Structure

For a molecule to be aromatic, it must have a cyclic and planar structure. This means that all the atoms in the molecule form a closed ring that lies flat in a single plane.
This geometric arrangement is necessary so that the p-orbitals on the atoms can align properly and overlap. If the molecule was not planar, these p-orbitals would not be parallel and continuous overlap would be impossible.
Benzene is a classic example of a molecule that is both cyclic and planar. Its flat, hexagonal structure allows for the necessary p-orbital overlap to create a stable, aromatic system.

Unsaturation Tests

While conducting unsaturation tests, such as decolourising bromine in carbon tetrachloride (\(Br/ CCl_{4}\)) or potassium permanganate (\(KMnO_{4}\)), can indicate the presence of double or triple bonds, it is not a criterion for aromaticity. Aromaticity depends on the molecule's structure and electron configuration, not on chemical reactivity.
Unsaturated compounds tend to react with these solutions, indicating double or triple bonds, but this does not mean they are aromatic.
Option 4 in the exercise is incorrect because aromatic compounds are not defined by their ability to decolourise these solutions.