VKC Notes · Reaction mechanisms · Civil Services P-II · IFoS P-II · Geo-Scientist P-III
2.3 Reactive Intermediates: Carbocations, Carbanions, Radicals, Carbenes, Nitrenes, Benzyne
★★★ 2021–2025: 30 parts, 203 marks (Civil Services 12 parts / 100 m · IFoS 12 / 65 m · Geo-Scientist 6 / 38 m), plus about ten Geo-Scientist Prelims questions on stability orders and benzyne.
Syllabus lines. Civil Services: “Reactive intermediates: generation, geometry, stability and reactions of carbonium ions and carbanions, free radicals, carbenes, benzynes and nitrenes”. IFoS: the same, with “carbonium and carbenium ions”. Geo-Scientist: “Structure and reactivity of carbenes and nitrenes and their rearrangements (Reimer-Tiemann, Hofmann, Curtius, Lossen and Schmidt)”. The rearrangements themselves (Wolff, Curtius, Hofmann) are in 2.6.
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2.3.1 Four intermediates at a glance ★★★
| Carbocation R₃C⁺ | Carbanion R₃C⁻ | Radical R₃C• | Carbene R₂C: | |
|---|---|---|---|---|
| Electrons on C | 6, empty p | 8, lone pair | 7, one unpaired | 6, two non-bonding |
| Shape | planar, sp² | pyramidal, sp³ (planar if conjugated) | planar or shallow pyramid | bent (singlet) or wider (triplet) |
| Made by | ionisation, H⁺ + alkene | base on C–H, metal–C | homolysis (heat, hν) | diazo − N₂; α-elimination |
| Behaves as | electrophile | nucleophile, base | abstracts H, adds to C=C | adds to C=C, inserts into C–H |
Draw it in 60 s: three columns of five lines; write the strongest stabiliser at the top of each and “opposite order” under carbanions.
- Anything that spreads a positive charge or an odd electron (alkyl groups by hyperconjugation, conjugation with C=C or a ring, a lone pair next door) stabilises carbocations and radicals; the same alkyl groups destabilise carbanions.SN
- Prelims checks the orders directly: carbocations Geo Pre 2021 Q87 Geo Pre 2023 Q82 Geo Pre 2023 Q86 Geo Pre 2025 Q84; hyperconjugative structures = number of α-C–H bonds Geo Pre 2021 Q84; radicals and C–H bond strengths Geo Pre 2022 Q84 Geo Pre 2023 Q85 Geo Pre 2025 Q85.
2.3.2 Carbocations ★★★
- Generation: ionisation of R–X or ROH₂⁺ (SN1, E1), protonation of an alkene or C=O, loss of N₂ from a diazonium ion. Fate: captured by a nucleophile, loses H⁺ (alkene), or rearranges to a more stable cation (2.6).SN
- Stability: 3° > 2° > 1° (hyperconjugation and +I); allyl and benzyl by resonance; a lone pair next door (CH₃O–CH₂⁺) by +M; aromatic tropylium (6π) is so stable it is isolable as a salt.SN
- Cyclopropylmethyl cation: in the bisected geometry the empty p orbital overlaps the bent C–C bonds of the ring; it is more stable than benzyl, solvolyses abnormally fast and gives cyclopropylmethyl, cyclobutyl and homoallyl products CSE 2021 Q2(b) 15m.SN
- Bridgehead cations cannot become planar: a small cage is worst. Order acyclic 3° > bicyclo[2.2.2]octyl > norbornyl (bicyclo[2.2.1]) IFoS 2024 Q2(b)(ii) 5m.SN
- Non-classical ions (2-norbornyl) share σ electrons over three centres; that is why norbornyl reactions give exo products and scrambled labels (2.4).
2.3.3 Carbanions and enolates ★★
- Shape: pyramidal sp³ with a lone pair, inverting rapidly; planar when the charge is delocalised into C=O, C=N or a ring.
- s character: R₃C–CH₂⁻ < R₂C=CH⁻ < RC≡C⁻ (25, 33, 50 % s): the more s character, the closer the pair is held; terminal alkynes (pKa 25) are far more acidic than alkenes or alkanes IFoS 2022 Q3(e) 5m.SN
- Substituents: a –M group (NO₂, C=O, CN) at an ortho or para position takes the negative charge on to oxygen or nitrogen and stabilises it; NH₂ or OMe (+M) pushes more charge in and destabilises it IFoS 2022 Q1(e) 5m.SN
- 1,3-Diones: cyclohexane-1,3-dione (pKa ≈ 5) is a strong carbon acid because its enolate is delocalised over both C=O; a bridgehead C–H between two C=O in a small bicycle is not acidic: its enolate would need a bridgehead C=C (Bredt's rule) CSE 2021 Q2(b).
- Enolate: carbon or oxygen? The enolate has two nucleophilic ends. Soft CH₃I reacts at carbon (2-methylcyclohexanone); hard, oxophilic Me₃SiCl at oxygen (silyl enol ether, strong Si–O bond) CSE 2021 Q2(c)(ii) 5m.SN
- Racemisation through a planar anion: ethoxide removes the benzylic α-H of the chiral imine Ph(CH₃)CH–N=CPh₂ to give a planar 2-aza-allyl anion; reprotonation on either face or at the other carbon gives the racemate and the achiral isomer Ph(CH₃)C=N–CHPh₂ CSE 2022 Q4(a)(ii) 5m.
2.3.4 Free radicals ★★★
- Generation: homolysis of weak bonds (peroxides, AIBN, Br₂ or Cl₂ in light). Reactions: atom abstraction, addition to C=C, coupling, disproportionation. Chain: initiation, propagation, termination.SN
- Stability: 3° > 2° > 1° > CH₃•; benzyl and allyl by delocalisation. Bromine is much more selective than chlorine because H-abstraction by Br• is endothermic (late, product-like transition state).SN
- Stereochemistry: a planar radical is attacked from either face. Cyclopentane + excess Br₂ (heat): 1,1-, cis- and trans-1,2-, cis- and trans-1,3-dibromides; cis isomers are meso, trans are racemic: seven stereoisomers, five separable fractions CSE 2022 Q4(b)(i) 10m.
- Triarylmethyl radicals (Gomberg): persistent because the odd electron spreads into three rings, and the propeller shape shields the central carbon; they “dimerise” at a para carbon (quinoid dimer). Blocking the para positions (p-F) stops dimerisation; replacing one aryl by t-Bu reduces delocalisation and increases it: (p-FC₆H₄)₃C• < Ph₃C• < Ph₂(t-Bu)C• CSE 2023 Q1(b)(ii) 5m. Rings held flat by O bridges delocalise even better; ortho-OMe twists rings out of plane IFoS 2021 Q7(d) 5m.SN
- Radical additions: BrCCl₃ in light adds to styrene as •CCl₃ to the CH₂ end (benzylic radical), then Br transfer: PhCH(Br)CH₂CCl₃ (Kharasch) CSE 2025 Q2(b)(ii) 5m. NBS on 1,2-dihydronaphthalene brominates the allylic C-2 (radical also benzylic through C-4); NaOEt then eliminates HBr to naphthalene CSE 2022 Q2(b)(ii) 5m. Peroxide HBr addition: 2.4.
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