Q.Write the equations involved in the following reactions:
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🔒 Start your 14-day free trial to unlock the full solution →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 π 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.
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:
Ar−H+EX+Ar−E+HX+
where Ar represents an aromatic ring.
The Mechanism in Three Steps
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Generation of the electrophile – Many EAS reactions need a catalyst to create a strong enough E+. For example, bromination uses FeBrX3 to polarise BrX2 into BrX+.
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Attack by the aromatic ring – The π 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 π electrons delocalised over five carbons, and one sp3 carbon bearing the electrophile and a hydrogen.
Benzene+E+⟶Sigma complex (non-aromatic)
- Deprotonation – A base (often the counterion of the catalyst, like FeBrX4X−) removes the proton from the sp3 carbon. The pair of electrons from the C–H bond flows back into the ring, restoring the aromatic sextet.
Sigma complex+Base⟶Product+HX+
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 (C6H6) has three double bonds — so why doesn't it undergo addition reactions like alkenes?
The answer lies in aromatic stabilisation: benzene's delocalised π-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 Br2), 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):
Ar-H+E+slowAr-E+H(σ-complex)
Then, fast deprotonation restores aromaticity:
Ar-E+H+B−fastAr-E+BH
Why This Holds: The Energy Barrier Logic
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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 sp3 hybridised). This intermediate is higher in energy than the starting benzene.
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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:
Rate=k[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 (k0) follows:
logk0k=σρ
Why This Holds
- σ (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.
- ρ (rho constant): Measures the sensitivity of the reaction to substituent effects. A positive ρ means the reaction is favoured by electron-withdrawing groups (rare in EAS); a negative ρ 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 −OH, −NH2) lower the activation energy — hence σ is negative for such groups. Electron-withdrawing groups (−NO2, −CN) destabilise the σ-complex — σ 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. …
(i) Reimer-Tiemann reaction — This is an electrophilic aromatic substitution where phenol is treated with chloroform in the presence of a strong base (NaOH) to introduce an aldehyde group at the ortho position. The key intermediate is a dichlorocarbene (:CCl₂), which attacks the electron-rich aromatic ring.
The overall equation is:
CX6HX5OH+CHClX3+3NaOH70X∘CCX6HX4(OH)CHO+3NaCl+2HX2O …
Both reactions are electrophilic aromatic substitution (EAS) on phenol, where a strong base generates a phenoxide ion that activates the ring, and a special electrophile attacks the ortho position. In the Reimer–Tiemann reaction, the electrophile is dichlorocarbene (:CCl₂), giving salicylaldehyde. In Kolbe’s reaction, the electrophile is carbon dioxide (CO₂), giving salicylic acid.
The Core Idea: Why Phenol is Special
Phenol (C6H5OH) is unusually reactive toward electrophilic substitution because the –OH group donates electrons into the ring by resonance. But in both reactions below, we first treat phenol with a strong base (NaOH or KOH). This deprotonates the –OH to give the phenoxide ion (C6H5O−), which is an even stronger activating group. The negative charge on oxygen pushes electrons into the ring so powerfully that the ortho and para positions become highly nucleophilic — ready to attack even weak or unusual electrophiles.
(i) Reimer–Tiemann Reaction
What happens: Phenol is treated with chloroform (CHCl₃) in the presence of aqueous NaOH (or KOH) at about 60–70 °C. After work‑up with acid, the product is salicylaldehyde (2‑hydroxybenzaldehyde).
Step‑by‑step reasoning
- Generation of the electrophile The base (NaOH) deprotonates chloroform to form the trichloromethyl carbanion (:CCl3−), which then loses a chloride ion to give dichlorocarbene (:CCl2). This is a highly reactive, electron‑deficient species — a powerful electrophile.
CHCl3+OH−⟶:CCl3−+H2O
:CCl3−⟶:CCl2+Cl−
- Attack on the phenoxide ion The phenoxide ion (formed from phenol + NaOH) attacks the electrophilic carbene at the ortho position (the para position is sterically hindered by the –OH group in the transition state). This gives an intermediate dichloromethyl‑substituted phenol.
C6H5O−+:CCl2⟶ortho-(Cl2CH)C6H4O−
- Hydrolysis to aldehyde The dichloromethyl group (–CHCl2) is hydrolysed by the aqueous base. Two successive nucleophilic substitutions (by OH⁻) replace both chlorines with –OH groups, which then tautomerise to give the aldehyde.
Ar–CHCl2+2OH−⟶Ar–CH(OH)2+2Cl−
Ar–CH(OH)2tautomerisationAr–CHO+H2O
- Acidification Finally, dilute acid is added to protonate the phenoxide back to phenol, yielding salicylaldehyde.
A common mistake is to think the carbene attacks the –OH group directly. It does not — the attack is on the ring at the ortho carbon. Also, the product is always the ortho isomer; para‑hydroxybenzaldehyde is not formed in significant amounts under these conditions.
Overall equation:
CX6HX5OH+CHClX3+3NaOH60−70°C2-HOCX6HX4CHO+3NaCl+2HX2O
(ii) Kolbe’s Reaction (Kolbe–Schmitt Reaction)
What happens: Sodium phenoxide is heated with carbon dioxide under pressure (about 125 °C, 4–7 atm), then acidified to give salicylic acid (2‑hydroxybenzoic acid).
Step‑by‑step reasoning
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Activation of the ring
Phenol is first converted to sodium phenoxide (C6H5ONa) using NaOH. The phenoxide ion is a much stronger nucleophile than phenol itself.
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Attack by CO₂
Carbon dioxide is a weak electrophile (the carbon is partially positive due to the two electronegative oxygens). The ortho carbon of the phenoxide ion attacks the carbon of CO₂, forming a carboxylate group at the ortho position. …
Here is the clear solution method for the given reactions, following a concept-first approach.
Method: Mechanism-Based Analysis of Named Reactions
This method focuses on understanding the key reactive intermediate (the electrophile) and the substrate (the aromatic ring) to write the correct chemical equation.
Steps:
- Identify the Substrate: Determine the starting aromatic compound (usually phenol, C6H5OH).
- Identify the Reagents & Conditions: Note the specific chemicals and environment (e.g., CHCl3/NaOH, CO2/NaOH).
- Identify the Electrophile: Determine the highly reactive species generated in situ (e.g., dichlorocarbene :CCl2, carbon dioxide CO2).
- Determine the Position of Attack: Recall the directing effect of the substituent already on the ring (e.g., -OH is ortho/para directing).
- Write the Final Product: Combine the aromatic ring with the new functional group at the correct position, accounting for any subsequent hydrolysis or acidification steps.
(i) Reimer-Tiemann Reaction
Concept: This is an electrophilic aromatic substitution where the electrophile is dichlorocarbene (:CCl2) , a very reactive species. The substrate is phenol.
Step-by-Step Equation:
- Generation of the Electrophile: Chloroform (CHCl3) reacts with a strong base (NaOH) to form dichlorocarbene.
CHCl3+NaOH⟶:CCl2+NaCl+H2O
- Attack on the Phenoxide Ion: Phenol first reacts with NaOH to form the more reactive phenoxide ion. The dichlorocarbene attacks the ortho position of the phenoxide ion.
C6H5OH+NaOH⟶C6H5O−Na++H2O
C6H5O−Na++:CCl2⟶Intermediate (o-hydroxybenzal chloride)
- Hydrolysis to Final Product: The intermediate is hydrolyzed during the work-up (acidification) to yield the final product.
IntermediateH3O+o-hydroxybenzaldehyde (Salicylaldehyde)
Final Balanced Equation:
C6H5OH+CHCl3+3NaOHΔC6H4(OH)CHO+3NaCl+2H2O
- Key Result: The product is Salicylaldehyde (2-hydroxybenzaldehyde). A small amount of the para isomer is also formed.
(ii) Kolbe's Reaction …
Here are the common mistakes students make with the Reimer-Tiemann and Kolbe’s reactions, along with precise corrections.
1. Confusing the Reagent (Reimer-Tiemann)
The Mistake: Students often write the wrong halogenated reagent or forget the base. They might write:
"Phenol + CHClX3 → Salicylaldehyde"
Why it’s wrong: The reaction requires a strong base (NaOH or KOH) to generate the reactive intermediate (:CClX2, dichlorocarbene). Without the base, no reaction occurs.
How to Avoid: Memorize the reagent pair: CHClX3 + NaOH (aq). Think of it as a carbene generation step first.
Correct Equation:
CX6HX5OH+CHClX3+3NaOHΔCX6HX4(OH)CHO+3NaCl+2HX2O
2. Forgetting the Ortho-Selectivity (Both Reactions)
The Mistake: Students write the product as para-substituted or as a mixture without specifying the major product.
Why it’s wrong: Both reactions are ortho-selective due to the intramolecular nature of the mechanism (the attacking group is held near the ortho position by the phenoxide ion).
How to Avoid: Always draw the –OH group and place the new group (−CHO or −COOH) ortho to it. The para product is a minor byproduct (often ignored in board exams unless specified).
Key Point: The major product is Salicylaldehyde (ortho-hydroxybenzaldehyde) and Salicylic acid (ortho-hydroxybenzoic acid).
3. Writing the Wrong Product for Kolbe’s Reaction
The Mistake: Students write the product as para-hydroxybenzoic acid or forget that COX2 adds directly to the ring.
Why it’s wrong: Kolbe’s reaction is an electrophilic substitution where COX2 (after activation by base) acts as the electrophile. The product is salicylic acid, not benzoic acid.
How to Avoid: Remember the order of addition: Phenoxide ion → attack by COX2 → ortho-substituted intermediate → acidification gives salicylic acid.
Correct Equation:
CX6HX5ONa+COX2125°C,4−7atmCX6HX4(OH)COONaHX+CX6HX4(OH)COOH
4. Mixing Up the Conditions (Temperature/Pressure)
The Mistake: Students write the same conditions for both reactions (e.g., "reflux" for both).
Why it’s wrong: Reimer-Tiemann requires aqueous base and heat (60–80°C). Kolbe’s requires high pressure (4–7 atm) and temperature (125°C) to force COX2 into the reaction.
How to Avoid: Make a quick table:
| Reaction | Reagent | Conditions |
| :--- | :--- | :--- | …
Showing the 12 most recent of 30 on this concept.
- CBSE 2026Set ANNUAL1 markQ.Complete the following reaction: aniline (benzene ring with an −NH2 substituent) +Br2(aq)→ ?
›Reveal solutionSolution
The −NH2 group is a powerful activating, ortho/para-directing group, so aniline reacts instantly with bromine water at all three activated ring positions to give 2,4,6-tribromoaniline as a white precipitate.
The lone pair on the amino nitrogen delocalises into the aromatic ring by resonance, strongly raising electron density especially at the ortho (2,6) and para (4) positions. This makes those three positions so reactive toward electrophiles that no Lewis-acid catalyst is required (unlike ordinary benzene bromination, which needs FeBr3), and substitution does not stop after one bromination — it proceeds at all three activated sites simultaneously: …
- CBSE 2026Set SEM31 markMCQQ.The reagent which can be used for the following transformation is: phenol (C6H5OH) -> salicylaldehyde (2-hydroxybenzaldehyde, OH and CHO on adjacent ring carbons)(a) i) CHCl3, NaOH, 60-80 C ii) dil. HCl(b) i) CO2, NaOH, 120-140 C ii) dil. HCl(c) i) CCl4, NaOH, 60-80 C ii) dil. HCl(d) i) HCHO, NaOH ii) dil. HCl
›Reveal solutionSolution
Phenol + CHCl3 + NaOH (60-80 C) then acidification gives 2-hydroxybenzaldehyde (salicylaldehyde) by the Reimer-Tiemann reaction. Correct option (a).
In the Reimer-Tiemann reaction, chloroform (CHCl3) with aqueous NaOH generates dichlorocarbene (:CCl2), the electrophile. It attacks the phenoxide ring, chiefly at the ortho position; subsequent hydrolysis on acidification (dil. HCl) converts the -CHCl2 group into -CHO, introducing an aldehyde group ortho to -OH.
Product: salicylaldehyde (2-hydroxybenzaldehyde).
- CO2/NaOH (option b) is the Kolbe reaction, giving salicylic acid (-COOH), not the aldehyde. …
- CBSE 2025Set ANNUAL1 markQ.Aniline does not undergo Friedel-Crafts reaction. Give reason.
›Reveal solutionSolution
The catalyst itself reacts with aniline's basic amino group, deactivating the ring before any substitution can occur.
Friedel–Crafts reactions (alkylation/acylation) require the Lewis acid catalyst AlCl3. Aniline's −NH2 group is strongly basic (it has a lone pair on nitrogen), so it readily reacts with AlCl3 to form a salt/complex (C6H5N+H2−AlCl3−).
…
- CBSE 2025Set ANNUAL1 markQ.Fill in the blank: When phenol is reacted with concentrated nitric acid, the product formed is ________.
›Reveal solutionSolution
Phenol reacts with concentrated nitric acid to give 2,4,6-trinitrophenol (picric acid) via nitration at all three activated ortho/para positions.
The -OH group of phenol is a strong activating, ortho/para-directing group. With concentrated HNO3 (a strong nitrating agent), phenol undergoes exhaustive electrophilic nitration at both ortho pos …
- CBSE 2025Set ANNUAL1 markQ.What happens when aniline is treated with bromine water?
›Reveal solutionSolution
The -NH2 group strongly activates the benzene ring, so aniline reacts with bromine water even without a catalyst, substituting at all three positions ortho/para to -NH2 at once.
Aniline's -NH2 group is a powerful electron-donating, ring-activating group (o,p-director). It makes the ring so reactive that bromine water reacts directly, without needing a Lewis-acid catalyst, substituting simultaneously at both ortho positions and the para position:
C6H5NH2+3Br2(aq)→2,4,6-tribromoaniline↓(white ppt)+3HBr
…
- CBSE 2025Set ANNUAL1 markMCQQ.Reaction of bromine water with phenol gives:(a) 2, 4, 6-Tribromophenol(b) o-Bromophenol and p-Bromophenol(c) o-Bromophenol(d) p-Bromophenol
›Reveal solutionSolution
Phenol's -OH group strongly activates the ring at all three of the ortho/ortho/para positions, so with excess aqueous bromine (bromine water) all three positions get substituted at once, giving 2,4,6-tribromophenol as a white precipitate — no catalyst needed.
The -OH group donates electron density into the ring by resonance, making the ortho and para positions highly electron-rich. Bromine water (dilute aqueous Br2) is reactive enough on its own (unlike with benzene, which needs a Lewis-acid catalyst like FeBr3) to brominate all three activated positions (2, …
- CBSE 2025Set ANNUAL1 markQ.Identify the structure of the missing component in the given reaction sequence : Toluene --(conc. HNO3 + conc. H2SO4)--> ? --(Fe/HCl)--> 4-aminotoluene
›Reveal solutionSolution
Nitration of toluene (methyl = o,p-director) followed by reduction of the nitro group gives the target amine — the missing intermediate is the nitro compound before reduction.
Toluene, treated with a nitrating mixture (conc. HNO3 + conc. H2SO4), undergoes electrophilic aromatic substitution. The methyl group is an ortho/para-directing, ring-activating substituent, so nitration occurs mainly at the para (and ortho) position, giving predominantly 4-nitrotoluene (p-nitrotoluene) as the major product. This nitro compound, on reduction with F …
- CBSE 2025Set ANNUAL1 markMCQQ.In the chlorination of benzene, the reactive species is(a) Cl+(b) Cl-(c) Cl2(d) Cl2-
›Reveal solutionSolution
Chlorination of benzene proceeds via electrophilic attack by Cl+.
In the presence of a Lewis acid catalyst such as anhydrous FeCl3 or AlCl3, Cl2 is polarised and heterolysed to generate an electrophilic chlorine species, Cl+ (as part of a complex with the catalyst, e.g. [FeCl4]- Cl+). This Cl+ then attacks the electron-rich benzene ring …
- CBSE 2024Set 56/3/11 markMCQQ.For the following question, two statements are given – one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer from the codes (A), (B), (C) and (D) as given below. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true. Assertion (A) : Aniline does not undergo Friedel-Crafts reaction. Reason (R) : Friedel-Crafts reaction is an electrophilic substitution reaction.
›Reveal solutionSolution
Aniline fails in Friedel-Crafts alkylation/acylation because the amino group forms a complex with the Lewis acid catalyst (AlCl₃), making the ring strongly deactivated. The Reason is true but does not explain this specific failure — it only states a general fact about the reaction type.
Concept first: Electrophilic Aromatic Substitution (EAS) and why aniline is special
Friedel-Crafts reactions are classic EAS reactions. In EAS, an electrophile attacks the electron-rich benzene ring. The more electron-rich the ring, the faster the reaction. Activating groups (like –NH₂, –OH, –OCH₃) donate electrons to the ring, making it more reactive toward electrophiles. So at first glance, aniline (C₆H₅NH₂) should be highly reactive in Friedel-Crafts reactions — the –NH₂ group is a strong activator.
But real chemistry is not that simple. The catalyst in Friedel-Crafts reactions is a Lewis acid, typically anhydrous AlCl₃. AlCl₃ is a strong electron-pair acceptor. The lone pair on the nitrogen of aniline is basic — it readily coordinates to AlCl₃, forming a salt-like complex. This complex changes everything.
Let’s walk through the reasoning step by step.
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What the Assertion says: Aniline does not undergo Friedel-Crafts reaction. This is a well-known experimental fact. If you try to alkylate or acylate aniline using AlCl₃ and an alkyl halide or acyl halide, you get either no reaction or a messy tar. The desired product is not formed.
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Why the Assertion is true: When aniline is mixed with AlCl₃, the nitrogen’s lone pair donates to the aluminium, forming C₆H₅NH₂·AlCl₃. This complex has a positive charge on nitrogen (or at least a strongly polarised N–Al bond). The –NH₂ group is no longer an electron-donating group — it becomes a strong electron-withdrawing group (–NH₂⁺AlCl₃⁻). This deactivates the ring so severely that even a powerful electrophile like the acylium ion cannot attack it. The ring becomes less reactive than nitrobenzene. So the reaction simply does not proceed.
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What the Reason says: Friedel-Crafts reaction is an electrophilic substitution reaction. This is a true statement — it is the textbook definition. Both alkylation and acylation proceed via an electrophilic attack on the aromatic ring. …
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- CBSE 2024Set A11 markMCQQ.Anisole on treatment with CH3Cl in the presence of anhydrous AlCl3 gives :(a) Toluene(b) O – chloroanisole(c) Ortho and para-methylanisoles(d) p – chloroanisole
›Reveal solutionSolution
Friedel–Crafts alkylation of anisole gives o- and p-methylanisole — option (c).
Anisole (C6H5OCH3) reacting with CH3Cl in the presence of anhydrous AlCl3 is a Friedel–Crafts alkylation: a methyl group is introduced onto the ring. The methoxy group −OCH3 is activating and ortho/para-directing, so the new methyl enters mainly at the ortho and para positions, giving a mixture of …
- CBSE 2024Set ANNUAL1 markQ.Write directive influence of -OCH3 group present in anisole for electrophilic substitution reaction.
›Reveal solutionSolution
The -OCH3 group in anisole donates electron density into the ring by resonance, activating the ring and directing incoming electrophiles preferentially to the ortho and para positions.
The oxygen of -OCH3 has lone pairs that can be delocalised into the benzene ring by resonance (+M/+R effect), increasing electron density specifically at the ortho and para positions relative to the -OCH3 group.
Although oxygen's electronegativity also exerts a small electron-withdrawing inductive (-I) effect, the stronger resonance donation dominates overall, making the ring more reactive than benzene itself (activating) towards electrophiles. …
- CBSE 2024Set ANNUAL1 markQ.Write chemical name of white precipitate obtained on the reaction of phenol with bromine water.
›Reveal solutionSolution
Phenol is highly reactive towards electrophilic bromination because the -OH group strongly activates the ring; even with dilute bromine water (no catalyst needed) it substitutes at all three available ortho/para positions at once.
The -OH group of phenol is a powerful activating, ortho/para-directing group (via resonance donation of its oxygen lone pair into the ring), making the ring far more reactive than benzene towards electrophiles. …
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