VKC Notes · Photochemistry · Civil Services P-II · IFoS P-II · Geo-Scientist P-III

12.1 Organic Photochemistry: Norrish Reactions and More

★★★ 2021–2025: 37 parts, 251 marks (Civil Services 16 parts / 130 m · IFoS 17 / 100 m · Geo-Scientist 4 / 21 m). Norrish type II alone is asked almost every year.

Syllabus lines. Civil Services and IFoS: “Photochemistry: photochemical reactions of simple organic compounds, excited and ground states, singlet and triplet states, Norrish type-I and type-II reactions”. Geo-Scientist: “Simple cases of Norrish type-I and type-II reactions”. Photochemical pericyclic reactions are in the pericyclic notes; photophysics (quantum yield laws) in physical photochemistry.

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CSE 2024 Q7(a) = asked there; tap to open the solved answerSN = line kept in the short notes

12.1.1 Excited states, Jablonski diagram, sensitisation ★★

Fig. 12.1.1 Jablonski diagram.

Draw it in 60 s: two lines S₀ and S₁ on the left, T₁ lower on the right; straight arrows up (hν) and down (fluorescence), a wavy one S₁→T₁ (ISC), and down from T₁ (phosphorescence) or to products.

Ketones absorb at 280–300 nm (n→π*). The n,π* state has one electron in the oxygen n orbital: the oxygen behaves like an alkoxy radical (abstracts H, adds to alkenes).SN
S₀→S₁ is spin-allowed; S₀→T₁ is spin-forbidden (ε ≈ 0). Reach T₁ by absorbing into S₁ and letting intersystem crossing do the rest; aryl ketones cross with Φ_ISC ≈ 1 CSE 2025 Q5(c) 10m.SN
Sensitisation: a donor with high triplet energy and Φ_ISC ≈ 1 (benzophenone, E_T ≈ 289 kJ mol⁻¹) transfers triplet energy to an acceptor of lower E_T; quenchers (dienes, O₂) do the reverse. Butadiene: direct light (S₁) gives cyclobutene and bicyclobutane; benzophenone-sensitised (T₁) gives [2+2] and [4+2] dimers CSE 2023 Q7(b)(ii) 5m IFoS 2021 Q7(e).SN

(12.1.1)

Φ > 1 means a chain: photobromination of cinnamic acid at 480 nm gives Φ = 17.3 CSE 2025 Q6(c)(ii) 5m.

12.1.2 Norrish type I: α-cleavage ★★★

Fig. 12.1.2 Norrish type I and type II side by side.

Draw it in 60 s: two columns of six lines: α-cleavage → radicals → −CO or ketene; γ-H → 1,4-biradical → enol + alkene or cyclobutanol.

Fig. 12.1.3 Norrish type I drawn with fishhooks: one electron of the C–C bond next to C=O goes to each carbon.

Draw it in 60 s: acetone, two half-headed arrows from the middle of the C–CH₃ bond, one to each carbon → CH₃C•=O + •CH₃. Ring: same on C1–C2 → open chain with a dot at each end → H to the acyl end: alkenal; H to the alkyl end: ketene → ester.

The C–C bond next to the excited C=O breaks homolytically; the bond to the more substituted α-carbon breaks (more stable alkyl radical).SN
The acyl radical loses CO when it is hot (vapour phase) or when that leaves a stabilised radical (benzylic). Acetone: vapour → ethane + CO; room temperature → biacetyl CSE 2023 Q5(e) 10m.SN
Cyclic ketones give an acyl–alkyl biradical: H transfer to the acyl end → ω-unsaturated aldehyde; H transfer to the alkyl end → ketene, trapped by ROH as an ester; loss of CO → a ring one carbon smaller or an alkene. 2-Methylcyclohexanone → hept-5-enal + heptanoic ester; cyclohexanone in ROH → alkyl hexanoate CSE 2022 Q5(b) 10m IFoS 2023 Q6(b) 10m CSE 2023 Q7(b)(i) 10m IFoS 2021 Q1(g) 5m.SN
Radical cage test: photolyse Ph₂CHCOCHPh₂ with PhCH₂COCH₂Ph: three products 1 : 2 : 1, including the crossed Ph₂CH–CH₂Ph, so free radicals escape the cage CSE 2025 Q6(c)(iv) 5m CSE 2021 Q6(c)(ii).
Strained or bicyclic ketones (thujone, 4-caranone) cleave towards the more substituted carbon and rearrange; cyclopropyl ketones open the three-membered ring IFoS 2025 Q6(a)(i) IFoS 2024 Q6(c)(i) 10m CSE 2021 Q6(c)(i) IFoS 2022 Q6(e)(i) IFoS 2022 Q6(e)(ii) 10m CSE 2024 Q6(b) 15m.

12.1.3 Norrish type II and Norrish–Yang cyclisation ★★★ asked every year

Fig. 12.1.4 Norrish type II drawn with fishhooks: the n,π* oxygen takes the γ-H through a six-membered TS; the 1,4-biradical cleaves (a) or closes (b).

Draw it in 60 s: six-ring: O•, C•, Cα, Cβ, Cγ, H with the O···H gap open → three fishhooks (O• to the gap, C–H to the gap, C–H to Cγ) → biradical: four fishhooks break Cα–Cβ into two π bonds → enol + alkene.

(12.1.2)

The n,π* oxygen abstracts a γ-hydrogen through a six-membered cyclic transition state: a 1,4-biradical.SN
The biradical breaks its Cα–Cβ bond: an enol (→ methyl ketone or aldehyde) + an alkene. Hexan-2-one → acetone + propene. Or it closes to a cyclobutanol (Yang); or it hands the H back Geo 2025 Q7(c)(ii) 5m Geo 2024 Q8(b).SN
Selectivity: 3° > 2° > 1° γ-H (weaker C–H, more stable radical); a rigid ring that holds the γ-H near oxygen speeds it CSE 2021 Q5(d) 10m CSE 2025 Q6(c)(iii) 5m Geo 2021 Q6(f) 6m IFoS 2024 Q5(e) 5m.SN
Type I competes: if α-cleavage gives a 3° or benzylic radical (t-Bu ketones), type I wins.

Solved PYQ CSE 2022 Q6(a)(ii) 10m

Optically active 4,5-dimethylheptan-2-one is irradiated. Products, and the chirality of recovered ketone.

Two γ-carbons: C-5 (tertiary H) and the C-4 methyl (primary H). The 3° γ-H goes first.
Major: acetone + 3-methylpent-2-ene (E and Z); minor: 3-methylpent-1-ene and a cyclobutanol.
When the biradical returns the H instead, C-5 is planar for a moment: recovered ketone is epimerised at C-5; C-4 keeps its configuration. This proves a discrete biradical.SN

Intramolecular proof: irradiate valerophenone with 5,5-dideuteriohexan-2-one together: no D moves into the other ketone's products CSE 2022 Q6(a)(i) 5m.
Aryl ketones react from T₁; a para donor (OMe, OH) makes the unreactive π,π* triplet lowest and slows type II; a para acceptor (CF₃, Cl) keeps n,π* lowest IFoS 2024 Q6(c)(ii) 5m.SN
α-Keto esters in alcohols: γ-H from the O–CH₂, then cleavage to a hydroxyketene, which the solvent alcohol traps: the ester group is exchanged CSE 2024 Q5(c) 10m. Isomesityl oxide → Yang cyclobutanol IFoS 2023 Q5(c) 5m; 2-benzoylnorbornane, a rigid ketone IFoS 2023 Q7(a) 5m; ring-opening of 8-methyl-1-decalone Geo 2023 Q7(c) 5m.

Also asked: IFoS 2022 Q6(d) RCOCH₂CH(R)CHPh₂ · IFoS 2025 Q6(a)(ii) an α-amino ketone.

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