Oxidations & Reductions
How chemists move a molecule up and down the oxidation ladder — adding or removing C–O / C–H bonds under control. Two layers on every reagent: a hand-held beginner path (plain “what it does”, arrow-by-arrow mechanism, graded worked examples, trap boxes) and a research-grade advanced/reference path (stereoelectronics, transition-state and selectivity models, oxidation-state bookkeeping, chemoselectivity & FG-tolerance tables, and the asymmetric variants: Sharpless AE/AD mnemonics, the CBS transition state, and the Noyori/Knowles cycles). Oxidations: Swern, Dess–Martin/IBX, Cr(VI) (Jones/PCC/PDC/Collins), Corey–Kim, Oppenauer, ozonolysis, epoxidation + Sharpless AE, dihydroxylation + Sharpless AD, Baeyer–Villiger, Riley, Étard. Reductions: the hydride ladder (LiAlH₄/NaBH₄/DIBAL/L-Selectride/Luche), catalytic hydrogenation (Lindlar, Rosenmund), dissolving-metal (Birch, Bouveault–Blanc), Wolff–Kishner, Clemmensen, MPV, CBS, Noyori/Knowles, Stryker — closing with an oxidation ladder and a “which reductant do I use?” decision table.
The 26 sections in Part 4
- 1Oxidation-state bookkeeping for carbon (do this first) Free below
- 2The Swern oxidation (activated DMSO)
- 3Dess–Martin periodinane (DMP) & IBX (hypervalent iodine)
- 4Chromium(VI) oxidations: Jones, PCC, PDC, Collins
- 5The Corey–Kim oxidation
- 6The Oppenauer oxidation
- 7Ozonolysis (the Criegee mechanism)
- 8Epoxidation: peracids (mCPBA / Prilezhaev) and Sharpless AE
- 9Dihydroxylation: OsO₄ (Upjohn) & Sharpless AD
- 10The Baeyer–Villiger oxidation (migratory aptitude)
- 11The Riley oxidation (SeO₂, allylic / α-carbonyl)
- 12The Étard reaction (chromyl chloride)
- 13The master ‘which reductant?’ map
- 14The hydride ladder: LiAlH₄, NaBH₄, DIBAL, L-Selectride, Luche
- 15Catalytic hydrogenation (incl. Lindlar & Rosenmund)
- 16Dissolving-metal reductions: Birch & Bouveault–Blanc
- 17The Wolff–Kishner reduction
- 18The Clemmensen reduction
- 19The Meerwein–Ponndorf–Verley (MPV) reduction
- 20The CBS (Corey–Bakshi–Shibata) reduction
- 21Noyori / Knowles asymmetric hydrogenation
- 22Stryker’s reagent ([(Ph₃P)CuH]₆)
- 23The oxidation ladder — which oxidant, how far?
- 24‘Which reductant do I use?’ — master selector
- 25The three big ‘which-way’ dichotomies
- 26Asymmetric methods at a glance (Sharpless / CBS / Noyori–Knowles)
Oxidation-state bookkeeping for carbon (do this first)
Free extractSection E.0 of Part 4, reproduced in full from the book — figures and all. No sign-in, no paywall on this section.
A 60-second rule that tells you whether any reaction is an oxidation, a reduction, or neither.
Worked count — methane series. CH₄: four C–H = 4×(−1) = −4. CH₃OH: three C–H (−3) + one C–O (+1) = −2. HCHO: two C–H (−2) + two C=O bonds (+2) = 0. HCOOH: one C–H (−1) + three C–O bonds (+3) = +2. CO₂: four C–O bonds = +4. Every step is +2, i.e. loss of two electrons — the definition of a two-electron oxidation.
⚠ Common mistakes & exam traps
- A tertiary alcohol (R₃C–OH) has no C–H on the carbinol carbon, so ordinary oxidants cannot make a carbonyl there — they leave it alone (or, forcingly, cleave C–C).
- Going 1° alcohol → aldehyde is easy to over-shoot to the carboxylic acid, because the aldehyde hydrate (gem-diol) still has a C–H to lose. Controlling this over-oxidation is the entire reason “mild” reagents (Swern, DMP, PCC) exist.
- Not every O-adding reaction changes carbon’s oxidation state: e.g. simple hydration of an alkene (H–OH across C=C) is redox-neutral at the molecule level (one C goes up, the other down).
Two-electron vs one-electron oxidants, the ‘which oxidant’ logic, and how Part E is organised.
Most named oxidations here are formally two-electron processes, but they reach the carbon by very different chemistry: (i) activated-DMSO oxidations (Swern, Corey–Kim, Pfitzner–Moffatt) route through an alkoxysulfonium ylide; (ii) hypervalent-iodine oxidations (DMP, IBX) via ligand exchange at I(V); (iii) Cr(VI) oxidations (Jones, PCC, PDC, Collins) via a chromate ester with a β-hydride elimination-like collapse; (iv) hydride-transfer oxidations (Oppenauer) via a six-membered MPV-type TS; (v) peroxide/peracid chemistry (epoxidation, Baeyer–Villiger, dihydroxylation via metal-oxo); and (vi) allylic/benzylic C–H oxidations (SeO₂ Riley, CrO₂Cl₂ Étard). Sorting a reagent into its family instantly predicts its chemoselectivity and by-products.
| Oxidant family | Named examples | Delivers | Signature selectivity |
|---|---|---|---|
| Activated DMSO | Swern, Corey–Kim, Moffatt | alcohol → aldehyde/ketone | mild; stops at aldehyde; cold; no over-oxidation |
| Hypervalent I | Dess–Martin (DMP), IBX | alcohol → aldehyde/ketone | RT, near-neutral, FG-tolerant; IBX can give enones/α,β-unsat. |
| Cr(VI) | Jones, PCC, PDC, Collins | 1°→acid (Jones) or aldehyde (PCC) | PCC/PDC stop at aldehyde (anhydrous); Jones over-oxidises 1° to acid |
| Hydride transfer | Oppenauer (ox), MPV (red) | 2° alcohol ↔ ketone | chemoselective for alcohols; leaves C=C, needs sacrificial ketone |
| Peracid / peroxide | mCPBA, Baeyer–Villiger, Sharpless AE | C=C→epoxide, C=O→ester | electrophilic O-transfer; syn; migratory aptitude (BV) |
| Metal-oxo (Os/Mn) | OsO₄ dihydroxylation, Sharpless AD | C=C → syn-diol | syn addition; asymmetric with cinchona ligands |
| Ozone | Ozonolysis (Criegee) | C=C → two C=O | oxidative cleavage; reductive vs oxidative work-up decides product |
| Allylic/benzylic C–H | Riley (SeO₂), Étard (CrO₂Cl₂) | allylic→allylic alcohol/enal/enone; ArCH₃→ArCHO | positional selectivity at sp³ C–H next to π |
General framing; primary citations live in each reaction chapter.
- Oxidation-state bookkeeping for carbon — standard treatment in Clayden, Organic Chemistry, and Carey & Sundberg, Advanced Organic Chemistry Part B.
- Cross-check terminology with IUPAC recommendations on oxidation numbers.
Read the rest of Part 4
The remaining 25 sections of this part — The Swern oxidation (activated DMSO), Dess–Martin periodinane (DMP) & IBX (hypervalent iodine), Chromium(VI) oxidations: Jones, PCC, PDC, Collins, The Corey–Kim oxidation — and all nine parts of Named Reactions are part of ChemVidya Full Access, along with the other books, 55 Study Notes and 6,000+ practice questions.
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