Nitration is an example of aromatic electrophilic substitution and its rate depends upon the group already pre

NCERT Exemplar · Q42

Q. Nitration is an example of aromatic electrophilic substitution and its rate depends upon the group already present in the benzene ring. Out of benzene and phenol, which one is more easily nitrated and why?

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Concept understanding — Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution – The First Meeting

Imagine you have a benzene ring — that perfect, flat hexagon of six carbons with alternating double bonds. It's stable, almost stubbornly so. You want to attach something new to it, say a bromine atom or a nitro group. But benzene doesn't react like an alkene. It doesn't just add across a double bond. Instead, it does something more elegant: it kicks out a hydrogen and keeps its aromatic ring intact.

That's the heart of Electrophilic Aromatic Substitution (EAS).

The Intuition: Why "Substitution" and Not "Addition"?

Benzene's stability comes from its delocalised $\pi$ electrons — a cloud above and below the ring. This cloud is electron-rich, so it attracts electrophiles (electron-loving species). But if an electrophile simply added to a double bond, the ring would break its aromaticity, losing that huge stabilisation. That would be energetically costly.

So benzene does something smarter: it lets the electrophile attack, temporarily breaks aromaticity to form a high-energy intermediate (the arenium ion), and then loses a proton to restore the aromatic ring. The net result? A hydrogen is replaced by the electrophile. The ring is back to its stable, aromatic self.

Note

The key trade-off: temporary loss of aromaticity is acceptable because the final product regains it. Addition reactions would permanently destroy aromaticity — benzene avoids that.

The Precise Statement

Electrophilic Aromatic Substitution is a reaction in which an electrophile ( $E^+$ ) replaces a hydrogen atom on an aromatic ring, proceeding through a sigma complex (arenium ion) intermediate, and restoring aromaticity after deprotonation.

The general equation:

\ce{Ar-H + E+ -> Ar-E + H+}

where Ar represents an aromatic ring.

The Mechanism in Three Steps

Generation of the electrophile – Many EAS reactions need a catalyst to create a strong enough $E^+$ . For example, bromination uses $\ce{FeBr3}$ to polarise $\ce{Br2}$ into $\ce{Br^+}$ .
Attack by the aromatic ring – The $\pi$ electrons of benzene attack the electrophile, forming a sigma complex (also called the arenium ion or Wheland intermediate). This intermediate is non-aromatic — it has four $\pi$ electrons delocalised over five carbons, and one $sp^3$ carbon bearing the electrophile and a hydrogen.

\text{Benzene} + E^+ \longrightarrow \text{Sigma complex (non-aromatic)}

Deprotonation – A base (often the counterion of the catalyst, like $\ce{FeBr4-}$ ) removes the proton from the $sp^3$ carbon. The pair of electrons from the C–H bond flows back into the ring, restoring the aromatic sextet.

\text{Sigma complex} + \text{Base} \longrightarrow \text{Product} + \ce{H+}

Watch out

The sigma complex is not aromatic. It's a high-energy intermediate. Students often mistakenly think it's still aromatic — it isn't. That's why the step is fast and the complex is short-lived.

Why This Matters for Exams

EAS is the gateway to understanding how to put groups onto benzene rings. The rate-determining step is usually the formation of the sigma complex (step 2). The regiochemistry (where the electrophile goes) depends on whether the ring already has a substituent — that's the topic of activating/deactivating groups and ortho/para vs. meta directors.

But for now, remember this: …

Why this formula?

Electrophilic Aromatic Substitution: Why the Mechanism Holds

The Core Puzzle: Why Benzene Doesn't Just Add

Benzene ( $C_6H_6$ ) has three double bonds — so why doesn't it undergo addition reactions like alkenes?

The answer lies in aromatic stabilisation: benzene's delocalised $\pi$ -electron cloud (the "aromatic sextet") is about 150 kJ/mol more stable than a hypothetical cyclohexatriene with localised double bonds.

If benzene simply added an electrophile (like $Br_2$ ), it would lose this stabilisation — a huge energy penalty.

So, nature chooses a different path: substitution instead of addition, preserving the aromatic ring.

The Key Formula: The Reaction Profile

The rate-determining step in electrophilic aromatic substitution (EAS) is the formation of the arenium ion (σ-complex):

\text{Ar-H} + E^+ \xrightarrow{\text{slow}} \text{Ar-E}^+ \text{H} \quad (\text{σ-complex})

Then, fast deprotonation restores aromaticity:

\text{Ar-E}^+ \text{H} + B^- \xrightarrow{\text{fast}} \text{Ar-E} + B\text{H}

Why This Holds: The Energy Barrier Logic

First step (slow): The electrophile $E^+$ attacks the electron-rich ring. The σ-complex is non-aromatic — it has only 4 π-electrons delocalised over 5 carbons (the sixth carbon is $sp^3$ hybridised). This intermediate is higher in energy than the starting benzene.
Second step (fast): A base removes the proton, restoring the aromatic sextet. This step is strongly exothermic — the system regains ~150 kJ/mol of stabilisation.

The overall reaction is exothermic, but the activation energy is dominated by the destabilisation of the σ-complex.

The Rate Law: Why It's First Order in Both

From the mechanism:

\text{Rate} = k[\text{Arene}][E^+]

Reasoning:

The slow step involves one molecule of arene and one molecule of electrophile.
No other species appear before the rate-determining step.
Therefore, the rate law is bimolecular — first order in each reactant.

This is not derived from the overall stoichiometry — it comes directly from the molecularity of the slow step.

The Hammett Equation: Quantifying Substituent Effects

For substituted benzenes, the rate constant $k$ relative to benzene ( $k_0$ ) follows:

\log \frac{k}{k_0} = \sigma \rho

Why This Holds

$\sigma$ (sigma constant): Measures the electronic effect of a substituent (electron-donating or withdrawing) relative to hydrogen. It is derived from the ionisation constants of benzoic acids — a purely empirical scale.
$\rho$ (rho constant): Measures the sensitivity of the reaction to substituent effects. A positive $\rho$ means the reaction is favoured by electron-withdrawing groups (rare in EAS); a negative $\rho$ means electron-donating groups accelerate the reaction.

Why it works:

The σ-complex has a positive charge delocalised over the ring. Substituents that stabilise this positive charge (electron-donating groups like $-\text{OH}$ , $-\text{NH}_2$ ) lower the activation energy — hence $\sigma$ is negative for such groups. Electron-withdrawing groups ( $-\text{NO}_2$ , $-\text{CN}$ ) destabilise the σ-complex — $\sigma$ is positive.

The linear free-energy relationship holds because the transition state resembles the σ-complex in charge distribution.

The Directing Effect: Why Ortho/Para vs Meta

The position of substitution is governed by the stability of the σ-complex for each possible attack site. …

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Phenol is nitrated far more easily than benzene because the –OH group is a strong activating group that donates electron density into the ring via resonance, making the aromatic ring much more reactive toward electrophilic attack.

The core idea: what makes a ring “easy” to nitrate?

Nitration is an electrophilic aromatic substitution reaction. The attacking species is the nitronium ion ( $\text{NO}_2^+$ ), which is an electron-seeking (electrophilic) reagent. For the reaction to happen quickly, the benzene ring must be electron-rich — it must have a high electron density that can attract and stabilise the incoming $\text{NO}_2^+$ .

So the question reduces to: Which ring — benzene or phenol — has higher electron density in the aromatic π-system?

1. Benzene: the neutral baseline

Benzene has six π-electrons perfectly delocalised over six carbons. There is no substituent to push or pull electron density. Its electron density is uniform and moderate. The nitronium ion finds it “okay” to attack, but the reaction requires fairly harsh conditions — concentrated nitric and sulphuric acids, often with heat.

2. Phenol: the –OH group changes everything

In phenol, the –OH group is attached directly to the ring. Oxygen is highly electronegative, so you might guess it withdraws electrons. But that’s not the full story — and this is where the classic exam trap lies.

Watch out

A common mistake is to think that because oxygen is electronegative, the –OH group withdraws electrons by induction. In fact, the resonance effect dominates here, and it is strongly electron-donating.

The lone pairs on oxygen can delocalise into the π-system of the benzene ring. Draw the resonance structures:

One lone pair from oxygen forms a π-bond to the ipso carbon (the carbon attached to –OH).
This pushes negative charge into the ortho and para positions of the ring.
The result: the ring becomes significantly more electron-rich than benzene, especially at the ortho and para carbons.

\text{–OH} \xrightarrow{\text{resonance}} \text{increases electron density at ortho and para positions}

3. Why this makes nitration faster

The rate-determining step in nitration is the attack of $\text{NO}_2^+$ on the ring to form a σ-complex (arenium ion). This intermediate is positively charged and unstable. A ring that already has high electron density can better stabilise this positive charge — it “gives” electrons more readily. …

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Here are the common mistakes students make when answering this question, along with how to avoid each one.

1. Mistake: Saying "Phenol is more reactive because it has an –OH group" without explaining why

What students write: "Phenol is more reactive because of the –OH group."
Why it’s wrong: This is incomplete. The examiner expects the mechanism — how the –OH group affects the ring.

How to avoid: Always link the –OH group to electron donation and activation of the ring.

Correct logic: The –OH group donates electrons via resonance (lone pair on oxygen delocalises into the ring). This increases electron density at ortho and para positions, making the ring more nucleophilic and thus more reactive toward electrophilic substitution (nitration).

2. Mistake: Confusing activating vs deactivating groups

What students write: "Phenol is more reactive because –OH is a deactivating group."
Why it’s wrong: –OH is a strong activating group, not deactivating.

How to avoid: Memorise the shortlist:

Activating (electron-donating): –OH, –NH₂, –OCH₃, –CH₃ (alkyl groups)
Deactivating (electron-withdrawing): –NO₂, –CN, –COOH, –CHO, –SO₃H

Tip: For exams, remember: All ortho/para directors (except halogens) are activating.

3. Mistake: Forgetting to mention resonance vs inductive effect

What students write: "–OH group donates electrons by inductive effect."
Why it’s wrong: The inductive effect of –OH is actually electron-withdrawing (due to high electronegativity of oxygen). The net activation comes from resonance, which dominates.

How to avoid: Clearly state:

"The –OH group shows a +R effect (resonance donation) that outweighs its –I effect (inductive withdrawal). This makes the ring electron-rich."

4. Mistake: Not comparing with benzene explicitly

What students write: "Phenol is easily nitrated."
Why it’s wrong: The question asks for a comparison — you must state why phenol is more reactive than benzene.

How to avoid: Always write a direct comparison:

"Benzene has no activating group, so its electron density is uniform and lower. Phenol, due to the –OH group, has higher electron density on the ring, making it more reactive toward electrophiles like NO₂⁺."

5. Mistake: Writing the wrong reaction conditions

What students write: "Phenol undergoes nitration with conc. HNO₃ and conc. H₂SO₄ at 50–60°C."
Why it’s wrong: Phenol is so reactive that it undergoes nitration even with dilute HNO₃ at room temperature — no need for conc. acid or heat.

How to avoid: Know the conditions:

Benzene: Conc. HNO₃ + conc. H₂SO₄, 50–60°C
Phenol: Dilute HNO₃, room temperature (or even with nitrating mixture, but much milder) …

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Q.Nitration is an example of aromatic electrophilic substitution and its rate depends upon the group already present in the benzene ring. Out of benzene and phenol, which one is more easily nitrated and why?

Concept understanding — Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution – The First Meeting

The Intuition: Why "Substitution" and Not "Addition"?

The Precise Statement

The Mechanism in Three Steps

Why This Matters for Exams

Why this formula?

Electrophilic Aromatic Substitution: Why the Mechanism Holds

The Core Puzzle: Why Benzene Doesn't Just Add

The Key Formula: The Reaction Profile

Why This Holds: The Energy Barrier Logic

The Rate Law: Why It's First Order in Both

The Hammett Equation: Quantifying Substituent Effects

Why This Holds

The Directing Effect: Why Ortho/Para vs Meta

The core idea: what makes a ring “easy” to nitrate?

1. Benzene: the neutral baseline

2. Phenol: the –OH group changes everything

3. Why this makes nitration faster

Concept: Acidity of Phenol — Activating Effect of –OH Group

Method: Resonance Effect Analysis for Electrophilic Substitution

Steps

1. Mistake: Saying "Phenol is more reactive because it has an –OH group" without explaining why

2. Mistake: Confusing activating vs deactivating groups

3. Mistake: Forgetting to mention resonance vs inductive effect

4. Mistake: Not comparing with benzene explicitly

5. Mistake: Writing the wrong reaction conditions

Q.Nitration is an example of aromatic electrophilic substitution and its rate depends upon the group already present in the benzene ring. Out of benzene and phenol, which one is more easily nitrated and why?

The core idea: what makes a ring “easy” to nitrate?

1. Benzene: the neutral baseline

2. Phenol: the –OH group changes everything

3. Why this makes nitration faster

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