Benzene Reactions: Substitution vs. Addition Revision Notes | A Level AQA Chemistry

1. The Central Question: Why Does Benzene Refuse to Add?

Alkenes react readily with bromine water via electrophilic addition — the pi bond breaks, bromine adds across it, and the product is a saturated dibromoalkane. So when you first encounter benzene, which appears in the Kekule model to have three double bonds (though this is only a simplified representation), you might expect it to react the same way, only faster.

It does not. Benzene does not decolourise bromine water under normal conditions. Instead, when it does react with electrophiles, it does so by substitution — replacing a hydrogen atom rather than adding across a double bond. The reason for this lies entirely in the nature of benzene's bonding, and understanding it is the key to this whole topic.

2. Benzene's Bonding: The Source of Its Stability

Benzene ($\text{C}_6\text{H}_6$) is a planar, regular hexagonal molecule. Each carbon is $\text{sp}^2$ hybridised, forming three sigma bonds — two to adjacent carbons and one to a hydrogen. This leaves one unhybridised p orbital on each carbon, perpendicular to the ring plane.

These six p orbitals overlap sideways, above and below the ring, to form a continuous, cyclic pi system. Crucially, the six pi electrons are not localised between specific pairs of carbon atoms — they are delocalised across all six carbons simultaneously. This is what makes benzene fundamentally different from cyclohexatriene (a hypothetical molecule with alternating single and double bonds).

The enthalpy of hydrogenation of cyclohexene (one double bond) is approximately $-120$ kJ mol$^{-1}$. A hypothetical cyclohexatriene with three isolated double bonds would therefore be expected to have an enthalpy of hydrogenation of $3 \times (-120) = -360$ kJ mol$^{-1}$. The measured value for benzene is only approximately $-208$ kJ mol$^{-1}$. Benzene releases 152 kJ mol$^{-1}$ less energy than expected — meaning it is 152 kJ mol$^{-1}$ more stable than the hypothetical structure.

3. Why Addition Would Destroy the Stability

Here is the core of the argument. If benzene underwent electrophilic addition — as an alkene does — the product would be a cyclohexadiene derivative. The delocalised pi system would be broken: two of the six pi electrons would be used to form new sigma bonds with the adding atoms, leaving a non-aromatic cyclohexadiene ring that lacks the full delocalised pi system of benzene.

That means the product would not benefit from the 152 kJ mol$^{-1}$ of delocalisation energy. The reaction would be energetically costly — you would be converting a highly stable aromatic system into a less stable, non-aromatic one. The thermodynamics strongly disfavour this pathway.

Substitution, by contrast, replaces one hydrogen with another group while leaving the pi system intact. The aromatic ring is restored at the end of the mechanism, and the full delocalisation energy is retained in the product. This is why substitution is thermodynamically preferred.

4. The Mechanism: How Substitution Preserves Aromaticity

Electrophilic aromatic substitution proceeds in two stages. In the first stage, an electrophile ($\text{E}^+$) attacks the pi system of the benzene ring, forming a bond with one carbon. This disrupts the delocalised system temporarily, producing a positively charged, non-aromatic intermediate called the Wheland intermediate (or arenium ion). At this point, the ring has lost its aromaticity — it is a cyclohexadienyl cation.

In the second stage, a base (typically the conjugate base of the catalyst, such as $\text{AlCl}_4^-$ in halogenation or $\text{HSO}_4^-$ in nitration) removes the hydrogen atom from the carbon that bonded to the electrophile. This restores the six-electron delocalised pi system — aromaticity is regained, and the product is the substituted benzene ring.

The driving force for step 2 is precisely the restoration of aromaticity. The Wheland intermediate is high in energy because it lacks the delocalisation energy of the aromatic ring. Losing a proton to regain aromaticity releases approximately 152 kJ mol$^{-1}$ of stabilisation energy — this is a powerful thermodynamic driving force that makes proton loss, rather than nucleophilic attack on the carbocation, the preferred pathway.

Worked example: Tracing the Substitution vs Addition Decision

Explain, with reference to energy changes, why benzene undergoes electrophilic substitution rather than electrophilic addition when treated with an electrophile E+.

1. Benzene has a delocalised pi system of 6 electrons across all 6 carbons. Delocalisation energy $\approx 152$ kJ mol$^{-1}$. — This is the starting point for any explanation. Benzene is not a triene — it is an aromatic system with exceptional thermodynamic stability.
2. Addition: $\text{E}^+$ and a nucleophile add across one C=C. Product is a cyclohexadiene derivative. The delocalised pi system is broken. — The addition product is non-aromatic. It does not benefit from the 152 kJ mol⁻¹ delocalisation energy. The reaction converts a stable aromatic compound into a less stable non-aromatic one — thermodynamically unfavourable.
3. Substitution: $\text{E}^+$ attacks the ring forming a Wheland intermediate, then $\text{H}^+$ is lost. Product is $\text{C}_6\text{H}_5\text{E}$ — still aromatic. — The product retains the full delocalised pi system. The 152 kJ mol⁻¹ stabilisation energy is preserved. Thermodynamically, this pathway is strongly favoured.
4. Addition product energy: higher (no aromaticity). Substitution product energy: lower (aromaticity retained). Energy difference $\approx 152$ kJ mol$^{-1}$. — The substitution product is approximately 152 kJ mol⁻¹ more stable than the addition product. This large energy difference makes substitution overwhelmingly preferred.
5. Benzene undergoes electrophilic substitution because the product retains the aromatic delocalised pi system, whereas addition would destroy it, resulting in a significantly less stable product. — This is the mark-winning statement. Always link the preference for substitution explicitly to the preservation of the delocalised pi system and the associated stabilisation energy.

Answer: Benzene undergoes electrophilic substitution rather than addition because substitution preserves the delocalised pi system and its associated stabilisation energy (approximately 152 kJ mol⁻¹), whereas addition would destroy aromaticity, producing a thermodynamically less stable non-aromatic product.

5. Comparing Benzene and Alkenes: The Key Differences

It is worth making the contrast with alkenes explicit, because examiners frequently ask you to compare the two. An alkene has a localised pi bond between two specific carbon atoms. When an electrophile attacks, the pi bond breaks completely and both electrons are used to form a new sigma bond. There is no aromatic system to preserve, so addition is the energetically straightforward outcome.

Notice the practical consequence: benzene does not decolourise bromine water, whereas cyclohexene does. This is a classic observation used in AQA questions to distinguish aromatic from non-aromatic compounds. The reason benzene fails to react is not that it lacks pi electrons — it has six of them — but that reacting via addition would cost it the 152 kJ mol$^{-1}$ of delocalisation energy, making the reaction thermodynamically unfavourable under normal conditions.

6. Exam Technique and Common Mistakes

AQA questions on this topic almost always use the command word 'explain'. That means you must give reasons, not just observations. Stating that 'benzene undergoes substitution' earns no marks on its own — you must explain why, linking your answer to the delocalised pi system and the thermodynamic cost of destroying aromaticity.

A high-scoring answer will typically include three elements: a reference to the delocalised pi system and its associated stability, a statement that addition would destroy this system, and a conclusion that substitution is preferred because it restores aromaticity in the product. Quantifying the stabilisation energy (approximately 152 kJ mol$^{-1}$) where relevant adds precision and signals genuine understanding to the examiner.

The central lesson of this topic is that benzene's behaviour is governed by thermodynamics. Its delocalised pi system is not just a structural feature — it is a reservoir of approximately 152 kJ mol$^{-1}$ of stability that the molecule 'defends' in every reaction it undergoes. Substitution is the preferred pathway precisely because it is the only one that allows benzene to keep that stability intact.