VKC Notes · Substitution reactions · Civil Services P-II · IFoS P-II · Geo-Scientist Prelims
2.5 Aromatic Substitution: Electrophilic, Nucleophilic and Heterocycles
★★★ 2021–2025: 27 parts, 180 marks (Civil Services 9 parts / 70 m · IFoS 18 / 110 m), plus 13 Geo-Scientist Prelims questions. IFoS sets heterocycles almost every year; Prelims tests directing effects and SNAr.
Syllabus lines. Civil Services: “electrophilic and nucleophilic reactions of aromatic compounds including heterocyclic compounds: pyrrole, furan, thiophene and indole”. IFoS: “electrophilic and nucleophilic reactions of aromatic compounds including simple heterocyclic compounds: pyrrole, thiophene, indole”. Aliphatic SN1/SN2 sit in the same syllabus line and get their own notes; benzyne is in 2.3 Reactive intermediates.
Two-page short notes for revisionAll VKC notes
Download PDF PLAN
PDF edition, print-ready. It opens with the VKC plan, together with every locked answer, note and test on the site.
CSE 2024 Q7(a) = asked there; tap to open the solved answerSN = line kept in the short notes
2.5.1 Electrophilic substitution: mechanism and energy profile ★★★
Draw it in 60 s: benzene → bracket with three cyclohexadienyl cations (H and E on the top carbon, + at 2, 4 and 6 o’clock) → substituted benzene; write “slow” on the first arrow.
- Make the electrophile: NO₂⁺ from HNO₃ + 2H₂SO₄; Cl⁺ equivalent from Cl₂ + FeCl₃ (Cl–Cl···FeCl₃); R⁺ or RC≡O⁺ from RCl or RCOCl + AlCl₃; SO₃ from fuming H₂SO₄.
- Slow step: the π system attacks E⁺; one carbon becomes sp³ and aromaticity is lost in the σ-complex (arenium, Wheland intermediate), its + spread over the two ortho and the para carbons.SN
- Fast step: a base removes H⁺ from the sp³ carbon and the sextet returns. Because C–H breaks after the slow step, nitration and chlorination show no primary kinetic isotope effect (kH/kD ≈ 1); sulphonation, which is reversible, does show one.SN
Draw it in 60 s: two humps, the first higher; a well between them labelled σ-complex; products lower than reactants. Mark both transition states with the bond that is changing.
This figure is the full answer to CSE 2024 Q4(b)(ii) 5m: draw TS1 as C···Cl forming with Cl···Cl···FeCl₃ breaking, TS2 as C···H breaking to FeCl₄⁻.
2.5.2 Directing effects and reactivity ★★★
| Group on the ring | Effect | Rate vs benzene | Directs |
|---|---|---|---|
| –NH₂, –NR₂, –OH, –O⁻ | +M ≫ −I | strongly activating | ortho/para |
| –OR, –NHCOR | +M > −I | activating | ortho/para |
| –R, –Ar | +I, hyperconjugation | weakly activating | ortho/para |
| –F, –Cl, –Br, –I | −I > +M | deactivating | ortho/para |
| –CHO, –COR, –CO₂H, –CN, –SO₃H, –CF₃, –NO₂, –NR₃⁺ | −M, −I | deactivating | meta |
- A donor helps only when the + sits on the carbon carrying it: that happens for ortho and para attack (an extra oxonium or iminium contributor). An acceptor puts + next to a δ+ atom for ortho/para attack, so meta is “least bad”.SN
- Halogens: −I deactivates the ring, but the lone pair still stabilises the ortho/para σ-complex: deactivating yet ortho/para directing.SN
- Two groups: the stronger activator wins; the new group avoids the crowded position between two groups. Bulky groups lower ortho attack Geo Pre 2022 Q112 Geo Pre 2024 Q114 Geo Pre 2023 Q113.SN
- ipso attack: an electrophile can add at a carbon that already carries a group and displace it. p-Bromoanisole with HNO₃ gives 4-bromo-2-nitroanisole and some 4-nitroanisole (Br⁺ leaves); p-chloroanisole gives only 4-chloro-2-nitroanisole, since Cl is not displaced IFoS 2023 Q3(b)(i) 3m IFoS 2023 Q3(b)(ii) 2m.
- Friedel–Crafts limits: alkyl cations rearrange (n-PrCl → cumene); fails on rings with –NO₂, –NH₂ (AlCl₃ complexes N) and on vinyl or aryl halides; alkylation over-alkylates, acylation does not Geo Pre 2023 Q112 Geo Pre 2023 Q118 Geo Pre 2022 Q113 Geo Pre 2024 Q113.SN
2.5.3 Synthesis: get the order of steps right ★★
- Make an o/p director work for a meta product, then remove it: use –NH₂ (as –NHAc) to place the new group, then delete it by diazotisation and H₃PO₂.
- p-Nitrotoluene → (Sn/HCl) p-toluidine → Ac₂O → HNO₃ (ortho to NHAc) → hydrolysis → NaNO₂/HCl, 0–5 °C → H₃PO₂ → m-nitrotoluene CSE 2022 Q4(b)(ii) 5m.
- p-Toluidine → acetylate → Br₂/AcOH (ortho to NHAc) → hydrolyse → diazotise → H₃PO₂ → m-bromotoluene IFoS 2021 Q2(c)(i) 5m.SN
- Groups that cannot be put on by SEAr come through the diazonium salt: ArN₂⁺ + CuCN → ArCN (Sandmeyer). Benzene → HNO₃/H₂SO₄ → Sn/HCl → NaNO₂/HCl (0–5 °C) → CuCN → benzonitrile CSE 2023 Q4(c)(ii) 10m.SN
- Von Richter reaction: p-bromonitrobenzene + KCN in hot aqueous ethanol gives m-bromobenzoic acid. CN⁻ adds ortho to NO₂, the nitro O closes on the nitrile, an N–N bond forms and N₂ is lost; the CO₂H ends up next to where NO₂ was (cine substitution), meta to Br CSE 2022 Q3(b)(ii) 5m.
- Haworth: anisole + succinic anhydride/SnCl₄ (para) → Clemmensen (Zn–Hg, HCl) → polyphosphoric acid closes the ring: 7-methoxy-1-tetralone IFoS 2021 Q3(d)(i) 5m.
- L-Dopa from L-tyrosine: protect (N-acetyl, ethyl ester) → dilute HNO₃ ortho to OH → H₂/Pd (NO₂ → NH₂) → NaNO₂/H₂SO₄, then warm water (NH₂ → OH) → acid hydrolysis. No bond to the stereocentre is touched, so the L-configuration is kept CSE 2022 Q2(c)(ii) 10m.
- Prelims: halogenate first, then nitrate, then SNAr with ethoxide Geo Pre 2022 Q115; nitration goes ortho to Cl and meta to CF₃, then MeO⁻ displaces the now-activated Cl Geo Pre 2024 Q110.
2.5.4 Pyrrole, furan, thiophene and indole ★★★ IFoS favourite
- The heteroatom lone pair is part of the 6π sextet, so these rings are electron-rich: pyrrole > furan > thiophene > benzene toward electrophiles. N donates best (2p–2p overlap); S (3p) overlaps poorly and thiophene is the most aromatic, so it reacts least IFoS 2025 Q1(c) 5m.SN
- Substitution goes to C-2 (three contributors for the cation, one an iminium/oxonium). Mild reagents only: strong acids polymerise pyrrole and furan.SN
Draw it in 60 s: two rows of pentagons with NH at the bottom: C-2 attack gives three contributors (last one N+=C), C-3 attack only two.
- Furan + Ac₂O, BF₃ → 2-acetylfuran. BF₃ activates the anhydride; the rate-determining step is attack of C-2 on the acyl carbon to form the σ-complex; loss of H⁺ restores the ring CSE 2024 Q4(a)(i) 10m.
- Vilsmeier–Haack: CH₃CON(CH₃)₂ + POCl₃ → chloroiminium ion [CH₃C(Cl)=N(CH₃)₂]⁺; pyrrole attacks it at C-2; aqueous Na₂CO₃ hydrolyses the iminium: 2-acetylpyrrole (DMF gives the 2-carbaldehyde) IFoS 2024 Q3(a) 10m.SN
- Pyrrole + CHCl₃/KOH: :CCl₂ forms; C-2 attack and hydrolysis give pyrrole-2-carbaldehyde (Reimer–Tiemann type); addition across C2=C3 then ring expansion gives 3-chloropyridine (Ciamician–Dennstedt) IFoS 2025 Q4(d) 10m.
- Metalation: n-BuLi removes the C-2 H of thiophene as readily as that of furan: the large, polarisable S stabilises the adjacent carbanion and Li coordinates to S; 3d-orbital arguments are not needed IFoS 2025 Q8(c) 5m.
Draw it in 60 s: indole twice: E on C-3 with + on C-2, then the N+=C-2 form; below it, E on C-2 with + on C-3 only.
- Indole: C-3, not C-2. C-3 attack puts + on C-2 next to N (iminium) with the benzene sextet intact; C-2 attack can use N only through an o-quinoid form CSE 2023 Q2(c)(ii) 5m CSE 2024 Q3(c)(ii) 10m.SN
- Do not nitrate indole with HNO₃/H₂SO₄ (protonation at C-3, oxidation, polymer); use benzoyl nitrate (PhCOCl + AgNO₃) or acetyl nitrate at low temperature: 3-nitroindole derivatives IFoS 2021 Q5(f) 5m.
Part of VKC Basic
Read all 11 sections of this topic, its short notes and the PDF with VKC Basic
VKC Basic (₹999 for 6 months) opens every solved answer, the notes, the handbook and the daily answer archive. Every Mains test series includes it.
Solved PYQs, short notes and books: vkcscience.comTelegram: t.me/upsc_chemistry_vkc