2.4 Addition to C=C and C≡C: Markovnikov, Stereochemistry, Dienes
★★★ 2021–2025: 35 parts, 196 marks (Civil Services 12 parts / 85 m · IFoS 12 / 60 m · Geo-Scientist 11 / 51 m), plus about fifteen Geo-Scientist Prelims questions. Short 4–10-mark parts, nearly every paper.
Syllabus lines. Civil Services and IFoS: “Electrophilic addition to C=C and C≡C; nucleophilic addition to C=O, C≡N, conjugated olefins and carbonyls”. Geo-Scientist: “Addition to C-C multiple bonds: mechanism of addition involving electrophiles, nucleophiles and free radicals …, addition to conjugated systems …, orientation and reactivity (on simple cis- and trans- alkenes)”. Addition to C=O is in 7.1, reagent details in 11.1, polymerisation in 9.1.
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CSE 2024 Q7(a) = asked there; tap to open the solved answerSN = line kept in the short notes
2.4.1 Electrophilic addition and Markovnikov's rule ★★★
- The π bond attacks the electrophile in the slow step; the electrophile goes to the carbon that leaves the more stable carbocation (Markovnikov). Then the nucleophile adds Geo Pre 2021 Q110 Geo Pre 2025 Q105.SN
- Reactivity follows the stability of that cation: electron-donating groups speed it up, electron-withdrawing groups slow it. CCl₃CH=CH₂ < Cl₂CHCH=CH₂ < ClCH₂CH=CH₂ < CH₃CH=CH₂ Geo 2021 Q6(a) 4m; (CH₃)₂C=CH₂ fastest with H₂SO₄ Geo Pre 2021 Q109 Geo Pre 2025 Q81. Styrene adds Br₂ faster than propene: benzylic cation Geo 2025 Q6(b) 4m.SN
- Rearrangement: 3,3-dimethylbut-1-ene + HCl gives 3-chloro-2,2-dimethylbutane and, after a methyl shift, 2-chloro-2,3-dimethylbutane CSE 2021 Q4(b)(i) 5m.SN
- Electron-poor alkenes add “anti-Markovnikov”: CH₂=CH–COOH + HCl gives ClCH₂CH₂COOH, because H⁺ goes to the carbonyl O and Cl⁻ to the β-carbon (1,4-addition); an α-cation next to C=O is too unstable Geo 2025 Q6(c) 4m.SN
- Allenes: H⁺ adds to the central carbon; the cation turns into an allyl (and, with phenylallene, benzylic) cation before Cl⁻ adds. Tetramethylethylene + Cl₂ gives mostly the allylic chloride CH₂=C(CH₃)C(CH₃)₂Cl: the crowded 3° cation loses H⁺ faster than it takes Cl⁻ IFoS 2023 Q4(a) 5m Geo Pre 2025 Q106.
Solved PYQ CSE 2025 Q2(b)(i) 10m
3,3-Dimethylbut-1-ene with dil. H₂SO₄ does not give 3,3-dimethylbutan-2-ol. Major product and mechanism; conditions that would give 3,3-dimethylbutan-2-ol.
- H⁺ adds to CH₂: 2° cation at C-2 next to the 3° C(CH₃)₃.
- 1,2-Methyl shift gives the 3° cation; water adds, H⁺ is lost: 2,3-dimethylbutan-2-ol.
- Use oxymercuration–demercuration (Hg(OAc)₂, H₂O–THF; then NaBH₄): Markovnikov OH, no free cation, no shift: 3,3-dimethylbutan-2-ol.SN
2.4.2 Halogen addition: the bromonium ion and anti addition ★★★
- Br₂ is polarised by the π bond; a bridged bromonium ion forms; Br⁻ opens it from the back: anti addition, stereospecific. cis-But-2-ene → racemic 2,3-dibromobutane; trans → meso CSE 2024 Q3(a)(ii) 5m Geo 2021 Q8(d) 8m Geo Pre 2024 Q108.SN
- In water or methanol the solvent competes with Br⁻ and opens the bromonium at the more substituted carbon (more δ+): bromohydrins and bromo ethers, Br and OH/OMe trans Geo Pre 2021 Q108. More alkyl groups on the C=C, faster bromination CSE 2023 Q4(a)(ii) 5m.SN
- Iodolactonisation: I₂/NaHCO₃ on an unsaturated acid; the carboxylate opens the iodonium from the back to give a γ-lactone with I and O trans Geo 2024 Q7(d)(i) 5m.
- Styrene-type alkenes form open benzylic cations, so stereospecificity is partly lost.
Stereospecific additions in Prelims Geo Pre 2025 Q98 Geo Pre 2026 Q105.
2.4.3 Hydration: acid, oxymercuration, hydroboration ★★★
| Method | Where OH goes | Stereochemistry | Rearrangement? |
|---|---|---|---|
| H₃O⁺ (dil. H₂SO₄) | Markovnikov | none (free cation) | yes |
| Hg(OAc)₂, H₂O; NaBH₄ | Markovnikov | anti (Hg and OH) | no |
| BH₃·THF; H₂O₂, OH⁻ | anti-Markovnikov | syn (H and OH) | no |
| Alkyne + H₂O, HgSO₄, H₂SO₄ | Markovnikov → methyl ketone | enol → keto | – |
- Oxymercuration goes through a bridged mercurinium ion; water attacks the more substituted carbon. Norbornene gives exo-norbornan-2-ol (exo face open) IFoS 2025 Q1(e) 5m. Other nucleophiles: an alcohol in the same molecule closes a ring (4-methylpent-4-en-1-ol → 2,2-dimethyltetrahydrofuran) Geo 2024 Q6(e) 4m; CH₃CN gives an amide (hex-1-ene → N-(hexan-2-yl)acetamide) Geo 2023 Q6(e) 4m; ROH gives ethers Geo Pre 2022 Q103 Geo Pre 2022 Q104 Geo Pre 2023 Q108 Geo Pre 2024 Q103.SN
- Hydroboration (detail in 11.1): allylbenzene → PhCH₂CH(OH)CH₃ by oxymercuration, PhCH₂CH₂CH₂OH by hydroboration IFoS 2021 Q4(d)(i) 5m; a tetrasubstituted alkene: B and H add syn from the less hindered face CSE 2023 Q4(a)(iii) 10m.
- Phenylacetylene + H₂O (HgSO₄, H₂SO₄) → acetophenone: ¹H NMR δ 2.6 (s, 3H) and 7.4–8.0 (5H) IFoS 2023 Q7(h) 5m.SN
2.4.4 Epoxides and diols ★★
- Peracid gives a syn epoxide (geometry kept); opening by H₂O, OH⁻ or an amine is SN2 from the back. Net result: anti-dihydroxylation. trans-But-2-ene + PhCO₃H, then OH⁻ → meso-butane-2,3-diol CSE 2022 Q4(a)(i) 10m.SN
- Cyclohexene: mCPBA then H₃O⁺ → trans-cyclohexane-1,2-diol; OsO₄ → cis-diol IFoS 2025 Q6(c)(ii) 5m. cis-But-2-ene + cold KMnO₄ (syn) → meso diol Geo 2021 Q8(d).SN
- Cyclopentene oxide + Me₂NH → trans-2-(dimethylamino)cyclopentanol, racemic IFoS 2022 Q5(g) 5m. LiAlH₄ opens an epoxide at the less hindered carbon, so the OH ends up on the more substituted one IFoS 2024 Q6(a)(ii) 5m.
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