CHEM2522 · Sustainable Chemical Manufacture
Radical Reactions & Photoredox Catalysis
Week 3 covers radical chemistry as a green C-C bond-forming toolkit: how radicals form and are stabilised, the initiation/propagation/termination chain, selective halogenation and allylic bromination with NBS, and the sustainability trade-off of Bu3SnH/AIBN dehalogenation (step-economical but toxic tin waste). It ends with photoredox catalysis, where visible-light photocatalysts such as Ir(ppy)3 do single-electron transfer to run tin-free, scalable radical reactions. Exam questions ask for full chain mechanisms, selectivity arguments and a 'greenness' judgement.
What this chapter covers
- 01Radicals: an unpaired electron, stabilised by resonance, inductive and steric effects (trityl, TEMPO); the carbocation-carbene-radical-carbanion continuum
- 02Initiation by homolysis: dibenzoyl peroxide → 2 PhCO2· → 2 Ph· + 2 CO2; AIBN → 2 (CH3)2C·CN + N2
- 03The chain: initiation → propagation → termination (radical + radical → non-radical)
- 04Anti-Markovnikov HBr addition (radical) vs Markovnikov (polar); halogenation selectivity (tertiary C-H easier than primary)
- 05Allylic bromination with NBS — keeps [Br2] low and forms a resonance-stabilised allylic radical
- 06Dehalogenation with Bu3SnH/AIBN: step-economical but generates stoichiometric toxic organotin waste
- 07Nucleophilic vs electrophilic radicals matched to electron-poor/electron-rich alkenes (basis of co-polymerisation)
- 08Photoredox catalysis: Ir(ppy)3 / Ru(bpy)3(2+) absorb visible light and do single-electron transfer; decarboxylative couplings; tin-free
Allylic bromination of cyclohexene with NBS: write the chain
- +1Initiation: the initiator generates a bromine atom (Br·). NBS maintains a low, steady supply of Br2, which is homolysed to two Br· radicals that start the chain.
- +1Propagation step 1 (H-abstraction): Br· abstracts an allylic hydrogen from cyclohexene, giving HBr and a resonance-stabilised allylic radical (the radical is delocalised over the two allylic carbons).
- +1Propagation step 2 (Br transfer): the allylic radical abstracts Br from Br2 (regenerated by NBS + HBr → succinimide + Br2), giving 3-bromocyclohexene and a new Br· that continues the chain.
- +1Roles/selectivity: NBS consumes the HBr made in step 1 and releases Br2 in small amounts, keeping [Br2] low so radical allylic substitution outcompetes ionic addition of Br2 across the double bond. The allylic C-H is abstracted selectively because the resulting allylic radical is resonance-stabilised (the weakest, most easily broken C-H bond).
Key terms
- Radical
- A species with an unpaired electron (·), drawn with single-barbed 'fishhook' arrows. Stabilised by resonance, electron-donating (inductive) groups and steric bulk; long-lived examples include the trityl radical and TEMPO.
- Initiation / propagation / termination
- The three phases of a radical chain: initiation makes the first radicals by homolysis, propagation steps consume and regenerate radicals (carrying the chain), and termination combines two radicals into a non-radical, ending it.
- AIBN
- Azobisisobutyronitrile, a common radical initiator that fragments on gentle heating to two carbon radicals plus N2; used sub-stoichiometrically (a few mol%).
- NBS (N-bromosuccinimide)
- A reagent that supplies bromine at low, steady concentration by consuming HBr and releasing Br2 slowly, enabling selective allylic (radical) bromination rather than ionic addition to the alkene.
- Allylic radical
- A carbon radical adjacent to a C=C double bond, stabilised by resonance delocalisation over the pi system; the reason the allylic C-H is abstracted selectively.
- Photoredox catalysis
- Catalysis by a visible-light-absorbing complex (e.g. Ir(ppy)3, Ru(bpy)3(2+)) whose excited state performs single-electron transfer, acting as both a stronger oxidant and a stronger reductant — enabling tin-free, scalable radical reactions such as decarboxylative couplings.
Radical Reactions & Photoredox Catalysis FAQ
Why use NBS instead of just adding Br2 for allylic bromination?
Because free Br2 at high concentration adds ionically across the C=C double bond (electrophilic addition), which is not what you want. NBS keeps the bromine concentration very low by mopping up the HBr produced during propagation and releasing Br2 only in small amounts, so the slow radical pathway — H-abstraction at the allylic position followed by Br transfer — wins. Low [Br2] is the whole trick.
What makes photoredox catalysis 'green' compared with Bu3SnH chemistry?
Tin hydride chemistry (Bu3SnH/AIBN) is step-economical and works, but it produces stoichiometric toxic organotin waste that is hard to remove from products. Photoredox catalysts such as Ir(ppy)3 are used at low loadings (often ~1 mol%), are driven by visible light rather than heat, and avoid tin altogether — so they hit the catalysis, energy-efficiency and safer-chemistry principles. They are also scalable, which is why continuous-flow photoredox reactors appear in industry.
How do I tell a radical mechanism from a polar one in a question?
Look for the signposts: a radical initiator (peroxide, AIBN) or light, single-barbed 'fishhook' arrows, and anti-Markovnikov regiochemistry for HBr addition. Polar reactions use electron-pair (double-barbed) arrows, often need acid/base or a polar solvent, and give Markovnikov products. Naming the class correctly is usually the first mark; the mechanism arrows must then match it.
Can Sia help me practise radical chain mechanisms?
Yes. Sia can set you fresh radical or photoredox problems, check that your initiation/propagation/termination steps balance electrons with fishhook arrows, and explain selectivity (why the allylic or more substituted C-H reacts). It teaches the method and checks your reasoning step by step; it does not do graded work for you, and University of Sydney academic-integrity rules apply.
Exam move
Learn the radical chain as a fixed skeleton — initiation (homolysis), propagation (two steps that regenerate a radical), termination (two radicals combine) — and be able to write it with single-barbed arrows for any substrate. Commit the stability order and the selectivity logic (resonance-stabilised radicals form fastest, so allylic/benzylic and more-substituted C-H bonds react) because that is where the selectivity marks live. For NBS, always state 'low [Br2] favours radical allylic substitution over ionic addition'. Keep the sustainability angle ready: Bu3SnH is step-economical but leaves toxic tin, while photoredox (Ir(ppy)3, ~1 mol%, visible light, tin-free) is the greener modern answer. Practise the Week 3 tutorial mechanisms and be ready to judge the 'greenness' of a radical process — that judgement is a recurring exam ask.
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