Chapter 36 — Key Takeaways

What you should leave Chapter 36 with

  1. Oxidation state of carbon is the unifying concept of redox chemistry. Calculate it as: -1 per bond to H, +1 per bond to O/N/X (halogen), 0 per bond to C.

  2. Oxidation level ladder for one carbon: - $CH_4$: -4 (most reduced). - Alcohol ($RCH_2OH$): -2. - Aldehyde ($RCHO$): 0. - Carboxylic acid ($RCOOH$): +2. - $CO_2$: +4 (most oxidized). - Each step up = oxidation; each step down = reduction.

  3. A ketone and an aldehyde are at the same oxidation level (0). An ester, amide, COOH, acid halide are all at +2 (the same as COOH).

  4. Alcohol oxidation reagents (chemoselective): - PCC: 1° → aldehyde; 2° → ketone. Mild, Cr(VI). - DMP: 1° → aldehyde; 2° → ketone. Modern, mild, Cr-free. - Swern: 1° → aldehyde; 2° → ketone. Mild, Cr-free, requires DMSO + oxalyl chloride + base. - Jones: 1° → COOH (full); 2° → ketone. - KMnO₄ (hot): 1° → COOH; aggressive, also oxidizes alkenes and aromatic side chains.

  5. Carbonyl reduction reagents (chemoselective): - NaBH₄: aldehydes/ketones only; mild. - LiAlH₄: everything reducible (aldehydes, ketones, esters, amides, COOH, nitriles, epoxides). - DIBAL-H (1 equiv, -78 °C): partial reduction of esters/nitriles → aldehyde. - L-Selectride: bulky; stereoselective for axial attack. - CBS reagent: chiral oxazaborolidine; enantioselective ketone reduction.

  6. Alkene oxidations (Ch 16 review): - OsO₄: syn-1,2-diol. - mCPBA + acid: anti-1,2-diol via epoxide. - mCPBA: epoxide. - O₃: cleavage to two carbonyls.

  7. Alkyne reductions (Ch 17 review): - Lindlar Pd + H₂: cis-alkene. - Na/NH₃(l): trans-alkene. - Pd/C + H₂: alkane.

  8. Catalytic hydrogenation (H₂/Pd or Pt): - Reduces alkenes, alkynes, nitros. - Doesn't reduce esters, amides under normal conditions. - Pd/C is mild; PtO₂ more vigorous; Raney Ni reduces most things.

  9. Asymmetric reductions: - CBS reagent: enantioselective ketone → chiral alcohol. - Noyori (Ru/BINAP): asymmetric hydrogenation of ketones with H₂. - Knowles (Rh/DiPAMP): asymmetric hydrogenation of α,β-unsaturated acids → L-DOPA.

  10. Asymmetric oxidations:

    • Sharpless AE (asymmetric epoxidation): allylic alcohol + TBHP + Ti(OiPr)₄ + chiral DET → chiral epoxide.
    • Sharpless AD (asymmetric dihydroxylation): alkene + OsO₄ + NMO + chiral cinchona ligand → chiral cis-diol.
    • Jacobsen epoxidation: Mn-salen catalyst for unfunctionalized alkenes.

  11. Dissolving metal reductions:

    • Birch (Na/NH₃/EtOH): benzene → 1,4-cyclohexadiene.
    • Aromatic NO₂ → NH₂: Sn/HCl, Fe/HCl, or H₂/Pd.
    • Alkyne → trans-alkene: Na/NH₃.

  12. Biological redox uses cofactors:

    • NAD⁺/NADH: hydride carrier; oxidative metabolism.
    • NADP⁺/NADPH: hydride carrier; biosynthesis (fatty acids, cholesterol).
    • FAD/FADH₂: electron-pair carrier; succinate dehydrogenase, β-oxidation.
    • Cytochrome P450 (CYP): oxygen activation for hydroxylation.

  13. CYP3A4 is the major drug-metabolizing CYP. Drug-drug interactions via CYP3A4 inhibition (e.g., grapefruit juice) are a major clinical concern.

  14. NAD⁺ chemistry is hydride transfer, the same mechanism as NaBH₄. Hundreds of dehydrogenase enzymes use NAD⁺.

  15. Modern green oxidations:

    • TEMPO + bleach: organic radical catalysis.
    • Pd-catalyzed aerobic oxidation: O₂ as terminal oxidant.
    • Photocatalytic oxidation: light-driven.
    • Biocatalysis: enzyme-catalyzed for stereo- and chemoselectivity.

  16. Selectivity rules:

    • Chemoselectivity: choose the right reagent (NaBH₄ vs LiAlH₄).
    • Regioselectivity: position-selective (e.g., Birch protonation; Sharpless ligand control).
    • Stereoselectivity: face-selective (chiral catalysts).

  17. Sharpless's Nobel Prizes: 2001 (asymmetric oxidations) and 2022 (click chemistry). Two-time Nobelist; rare.

  18. Multistep synthesis design with redox: count oxidation levels of starting material and target. Each step changes one or more levels. Plan reagents accordingly.

  19. For asymmetric synthesis: choose chiral catalysts (CBS for reductions; Sharpless ligands for oxidations; Noyori or Knowles for hydrogenations).

  20. Mastery of Chapter 36 is essential for synthesis (most multi-step routes have redox steps), pharmacology (drug metabolism), and biochemistry (energy metabolism).

Cross-references

  • Chapter 16 — Alkene oxidation reactions.
  • Chapter 17 — Alkyne reductions.
  • Chapter 25 — Carbonyl reduction (NaBH₄, LiAlH₄).
  • Chapter 26 — Carboxylic acid reduction.
  • Chapter 32-34 — Biological redox (NAD/FAD in glycolysis, β-oxidation).
  • Chapter 35 — Drug metabolism by CYP enzymes.
  • Chapter 37 — Organometallic chemistry; asymmetric catalysis.
  • Chapter 38 — Total synthesis; many redox steps.
  • Appendix C — Oxidation reagent reference.
  • Appendix F — Named reactions: Sharpless, Jacobsen, Birch, etc.

Study tip

For each redox transformation, ask: 1. What's the oxidation level change? Up (oxidation) or down (reduction)? 2. Which functional group is the target? Alcohol, carbonyl, alkene, nitro, etc. 3. What other groups are present? What reagent has the right chemoselectivity? 4. Is stereochemistry a concern? Choose chiral catalyst if so.

If you can answer these for any redox step, you've internalized Chapter 36.