Assertion: Phenols give o- and p-nitrophenol on nitration with dilute nitric acid. Reason: -OH group present i

NCERT Exemplar · Q74

Q. Assertion: Phenols give o- and p-nitrophenol on nitration with dilute nitric acid. Reason: -OH group present in phenol is o-, p-directing.

(i) Assertion and reason both are correct statements and reason is the correct explanation for assertion.

(ii) Assertion and reason both are correct statements but reason is not the correct explanation for assertion.

(iii) Assertion is correct statement but reason is wrong statement.

(iv) Assertion is wrong statement but reason is correct statement.

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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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The -OH group is strongly activating and ortho/para-directing, and with dilute HNO3, phenol undergoes nitration to give a mixture of o- and p-nitrophenol -- so both the assertion and reason are correct, and the reason correctly explains the assertion. The answer is (i).

The key here is understanding Electrophilic Aromatic Substitution (EAS) -- how substituents already on the ring control where the next group goes. Phenol's -OH group has lone pairs that donate electron density into the ring through resonance, making the ortho and para positions more electron-rich than the meta position. When an electrophile like the nitronium ion (NO2+) attacks, it preferentially hits those activated positions.

The assertion says phenol gives o- and p-nitrophenol on nitration with dilute nitric acid -- true. (With concentrated nitric acid, phenol is heavily oxidised and tars form; dilute acid is mild enough for controlled nitration, giving a mixture of ortho and para isomers.)

The reason states that the -OH group is o-, p-directing -- also true, and this directly explains why nitration happens at those positions. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

KEAM 2026Set eng-2026-04174 marksMCQ
$Br_2$
›Reveal solutionSolution

Using $Br_2$ in the non-polar solvent $CS_2$ at low temperature (273 K) suppresses polybromination and gives p-bromophenol as the major monobrominated product.

Phenol is strongly activated, so bromine water (polar, ionising) gives 2,4,6-tribromophenol.

To stop at monobromination, a low-polarity solvent and low temperature are used, which lowers the electrophilicity/availability of $Br^+$ . …
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KEAM 2026Set eng-2026-04224 marksMCQ
$Q. When phenol is treated with excess of bromine water, it gives (A) o-bromophenol (B) o- and p-bromophenol (C) 1,3,5-tribromophenol (D) 2,4-dibromophenol (E) 2,4,6-tribromophenol$
›Reveal solutionSolution

Phenol with excess bromine water undergoes electrophilic substitution at the ortho and para positions, giving a white precipitate of 2,4,6-tribromophenol. …

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KEAM 2026Set pha-2026-0418F4 marksMCQ
$Br_2$
›Reveal solutionSolution

Aniline $\rightarrow$ acetanilide (acetic anhydride/pyridine) $\rightarrow$ bromination gives mainly p-bromoacetanilide.

Aniline is acetylated to acetanilide $C_6H_5NHCOCH_3$ . The acetamido group is an activating ortho/para director, but the bulky $-NHCOCH_3$ hinders the ortho positions, so electrophilic bromination with $Br_2$ / $CH_3COOH$ occurs predominantly at the para po …
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KEAM 2026Set pha-2026-0420F4 marksMCQ
$Q. Which of the following reaction yieldstarry oxidation products? (A) Sulphonation of aniline (B) Nitration of aniline (C) Firedel-Crafts alkylation aniline (D) Firedel-Crafts alkylation of aniline (E) Bromination of aniline$
›Reveal solutionSolution

Aniline is readily oxidised; direct nitration with HNO $_3$ /H $_2$ SO $_4$ oxidises it to dark tarry products, so the amino group is protected (acetylated) first.

Aniline is very easily oxidised because the ring is electron-rich. When it is subjected to direct nitration with the strongly oxidising nitrating mixture ( $\mathrm{HNO_3/H_2SO_4}$ ), a large part of it is oxidised to dark, tarry products rather than cleanly nitrated. This is exactly why, in practice, aniline is first acetylated ( …
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KEAM 2025Set eng-2025-04264 marksMCQ
$H_2SO_4$
›Reveal solutionSolution

Sulphonation of phenol gives phenol-2,4-disulphonic acid; subsequent nitration replaces the $–SO_3H$ groups to yield picric acid (2,4,6-trinitrophenol).

Treating phenol with concentrated $H_2SO_4$ introduces sulphonic acid groups, giving phenol-2,4-disulphonic acid (X). On treatment with concentrated $HNO_3$ , the readily displaceable sulphonic groups are replaced by nitro groups and the ring is further nitrated, producing picric acid (2,4,6-trinitrophenol, Y). …
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KEAM 2025Set eng-2025-04284 marksMCQ
$C_6H_5NH_2 \xrightarrow[Pyridine]{(CH_3CO)_2O} A \xrightarrow[CH_3COOH]{Br_2} B$
›Reveal solutionSolution

Acetylation moderates aniline; the acetamido group is an o/p-director and bromination gives mainly the para product.

$\mathrm{C_6H_5NH_2}$ + $(\mathrm{CH_3CO})_2\mathrm{O}$ /pyridine $\to$ acetanilide (A), $\mathrm{C_6H_5NHCOCH_3}$ . The acetamido group is a strong ortho/para director; steric factors make the para product dominant. With $\mathrm{Br_2/CH_3COOH}$ the final product B is **p-bromoacetan …
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KEAM 2025Set eng-2025-04294 marksMCQ
$+ Br_2 \xrightarrow{FeBr_3}$
›Reveal solutionSolution

$\text{-}OH$ activates and directs ortho/para. With the para position occupied by $\text{-}CH_3$ , electrophilic bromination goes ortho to the $\text{-}OH$ , yielding 2-bromo-4-methylphenol.

p-Cresol is 4-methylphenol, with $\text{-}OH$ and $\text{-}CH_3$ para to each other. Both substituents are ortho/para directors, but the $\text{-}OH$ group is a much stronger activator and controls the orientation. Its para position is already occupied by the methyl group, so electrophilic aromatic bromination (with $Br_2/FeBr_3$ ) occurs at the position ortho to the $\text{-}OH$ . The major product is **2-bromo-4-methylphen …
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KEAM 2025Set pha-2025-0424A4 marksMCQ
$AlCl_3$
›Reveal solutionSolution

$AlCl_3$ generates the acylium electrophile $CH_3CO^+$ , which substitutes a ring hydrogen (para to Cl). This is electrophilic aromatic substitution.

$CH_3COCl + AlCl_3 \rightarrow CH_3CO^+ + AlCl_4^-$ . The acylium ion attacks the electron-rich benzene ring of chlorobenzene at the para position (Cl is $o,p$ -directing), for …
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KEAM 2024Set eng-2024-06084 marksMCQ
$C_6H_6O$
›Reveal solutionSolution

X = phenol, Y = benzene, Z = acetophenone: phenol is reduced by Zn dust to benzene, which undergoes Friedel–Crafts acylation to give acetophenone ( $C_8H_8O$ ).

Identify X: $C_6H_6O$ is phenol. Heating phenol with zinc dust reduces it (removes the –OH):
$C_6H_5OH + Zn \longrightarrow C_6H_6 + ZnO$
so Y = benzene.

Benzene then undergoes Friedel–Crafts acylation with acetyl chloride and anhydrous $AlCl_3$ : …
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Q.Assertion: Phenols give o- and p-nitrophenol on nitration with dilute nitric acid.
Reason: -OH group present in phenol is o-, p-directing.

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

Method: Evaluate Assertion-Reason Statements in Organic Chemistry

Common Mistakes in This Electrophilic Aromatic Substitution Question

Mistake 1: Assuming the Reason is Incorrect

Mistake 2: Thinking the Assertion is Wrong

Mistake 3: Choosing Option (B) Instead of (A)

Q.Assertion: Phenols give o- and p-nitrophenol on nitration with dilute nitric acid. Reason: -OH group present in phenol is o-, p-directing.

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Q.Assertion: Phenols give o- and p-nitrophenol on nitration with dilute nitric acid.
Reason: -OH group present in phenol is o-, p-directing.

Unlock everything free for 14 days