Q.Why is benzene extra ordinarily stable though it contains three double bonds?

Benzene’s extraordinary stability comes from aromatic stabilisation — the delocalisation of $\pi$ electrons over the entire ring, which lowers its energy far below that of a hypothetical cyclohexatriene with three localised double bonds.

1. Structure: Benzene is a planar, cyclic molecule with six $p$ orbitals (one on each carbon) overlapping sideways to form a continuous $\pi$ system.
2. Hückel’s rule: It has $4n+2$ $\pi$ electrons with $n=1$ (i.e., 6 $\pi$ electrons), satisfying the condition for aromaticity.
3. Delocalisation energy: The actual $\pi$ electron energy is about 150 kJ/mol lower than that of the localised Kekulé structure. This extra stabilisation is called resonance energy. …

1. The puzzle: three double bonds, but not like alkenes

If you look at benzene’s structure — a six-carbon ring with alternating single and double bonds — you might expect it to behave like three ethene molecules stuck together. Ethene adds bromine instantly; benzene does not. Ethene is easily hydrogenated; benzene requires a catalyst and high pressure. Something is fundamentally different.

The key is that benzene’s double bonds are not localised between specific carbon pairs. The true structure is a resonance hybrid of two Kekulé structures, but even that description understates the reality.

2. The real picture: a delocalised π-system

Each carbon in benzene is $sp^2$ hybridised. That leaves one unhybridised $p_z$ orbital perpendicular to the ring plane. These six $p_z$ orbitals overlap sideways with their neighbours, forming a continuous ring of electron density above and below the plane.

The six π-electrons are not confined to any one C–C pair. They are delocalised over the entire ring. This delocalisation is the source of stability.

3. How do we know this? Experimental evidence

The stability is not just theoretical. Three classic experiments confirm it:

1. Hydrogenation enthalpy: Hydrogenating cyclohexene (one double bond) releases about 120 kJ/mol. If benzene had three isolated double bonds, you would expect $3 \times 120 = 360$ kJ/mol. The actual hydrogenation enthalpy of benzene to cyclohexane is only about 208 kJ/mol. The difference — roughly 152 kJ/mol — is the resonance energy.

2. Bond lengths: X-ray diffraction shows all six C–C bonds are identical at 139 pm. No alternating short (double) and long (single) bonds are seen.

3. Chemical inertness: Benzene undergoes substitution (electrophilic aromatic substitution) rather than addition. Addition would destroy the delocalised π-system; substitution preserves it. This preference is a direct consequence of the stabilisation.

4. The Hückel rule: why six π-electrons is special

Benzene’s stability is not just about delocalisation — it is about the number of π-electrons. The Hückel rule states that planar, monocyclic, fully conjugated systems are aromatic (extraordinarily stable) when they contain $4n+2$ π-electrons, where $n$ is a non-negative integer. …