Chapter 36 — Oxidation and Reduction Reactions: A Unified View

"The oxidation state of carbon is the language of metabolism, the language of synthesis, the language of organic transformation. Once you can read it, every reaction in the textbook becomes legible." — paraphrase from a synthesis textbook

"Half of organic chemistry is making C-C bonds. The other half is changing oxidation states — adding or removing H, O, N from carbon to interconvert functional groups. Master both halves, and you have the toolkit of synthesis."

This chapter takes a unified view of the oxidation and reduction reactions scattered across Parts IV-VI. The unifying concept: oxidation state of carbon. Every functional group has a specific oxidation level; transformations between functional groups are classified as oxidations (raising the level) or reductions (lowering the level). Once you can count oxidation levels, you can: - Predict which reagents are needed for any transformation. - Choose reagents for selectivity (chemoselectivity, regioselectivity). - Plan multi-step syntheses involving redox steps.

By the end of this chapter you should be able to: - Calculate the oxidation state of any carbon. - Classify any transformation as oxidation, reduction, or neutral. - Choose appropriate reagents for any oxidation or reduction with appropriate selectivity. - Recognize biological redox transformations (NAD/NADH, FAD/FADH₂, cytochromes). - Design multi-step syntheses involving multiple redox steps.

36.1 Counting oxidation states of carbon

The oxidation state of a carbon atom in an organic molecule is calculated by: - Bonds to H: each subtracts 1 (H is more electropositive than C, so the electrons go to C). - Bonds to O, N, X (halogen), or other electronegative atoms: each adds 1 (these are more electronegative, so electrons go to them). - Bonds to C: count as 0 (the bonding electrons are split equally).

Examples

For methane ($CH_4$): C has 4 bonds to H. Oxidation state = -4.

For methanol ($CH_3OH$): C has 3 bonds to H + 1 bond to O. Oxidation state = -3 + 1 = -2.

For formaldehyde ($H_2C=O$): C has 2 bonds to H + 2 bonds to O (the C=O is two bonds). Oxidation state = -2 + 2 = 0.

For formic acid ($HCOOH$): C has 1 bond to H + 3 bonds to O (one to OH, double bond to =O). Oxidation state = -1 + 3 = +2.

For CO₂ ($O=C=O$): C has 0 bonds to H + 4 bonds to O. Oxidation state = +4.

The oxidation state ladder for a single carbon

For a single carbon, the oxidation state can range from -4 (most reduced, methane) to +4 (most oxidized, CO₂). Each step is an oxidation or reduction.

Oxidation raises the level (e.g., alcohol → aldehyde → COOH). Reduction lowers the level (e.g., COOH → aldehyde → alcohol).

For larger molecules

For a molecule with multiple carbons, calculate each carbon's oxidation state separately. The "molecular oxidation state" is rarely useful; what matters is the oxidation state at the carbon undergoing transformation.

Worked Problem 36.1: Calculate the oxidation state of each carbon in 2-butanol ($CH_3-CH(OH)-CH_2-CH_3$).

Solution: - C1 (CH₃, terminal methyl): 3 H + 1 C = -3 + 0 = -3. - C2 (CH(OH)): 1 H + 2 C + 1 O = -1 + 0 + 1 = 0. - C3 (CH₂): 2 H + 2 C = -2 + 0 = -2. - C4 (CH₃, terminal methyl): 3 H + 1 C = -3 + 0 = -3.

Note that C2 (the alcohol-bearing carbon) is at oxidation level 0 — same as an aldehyde! This is because the alcohol's OH is balanced by an extra H. Oxidation of 2-butanol to butan-2-one (a ketone) raises C2 to +1 (oxidation state changes by +2 because we lose 2 H's and gain... wait, let me recount).

Wait, let me redo. 2-Butanol's C2 has: 1 H, 1 OH (= 1 bond to O), and 2 bonds to C. Oxidation state = -1 + 1 + 0 = 0. After oxidation to butan-2-one ($CH_3-CO-CH_2-CH_3$), C2 has: 0 H, 2 bonds to O (the C=O), and 2 bonds to C. Oxidation state = 0 + 2 + 0 = +2. So oxidation state changed from 0 to +2 — a 2-electron oxidation. (Each electron transferred per bond change: 1 oxidation = -1 reductant + +1 oxidant.)

36.2 Oxidation reagents for alcohols and aldehydes

The most common oxidation in organic synthesis is alcohol → carbonyl. Choosing the right reagent matters: some give aldehyde (stop at +0), some give COOH (go to +2).

Reagents that stop at the aldehyde (or ketone)

For primary alcohols → aldehydes (not COOH):

• PCC (pyridinium chlorochromate): $\text{Cr(VI)Cl}^-$ + pyridinium. Mild; doesn't over-oxidize. Standard for many syntheses but uses toxic Cr(VI).
• DMP (Dess-Martin periodinane): a hypervalent iodine reagent. Modern alternative to PCC; non-toxic; works at room temperature in DCM. Standard for clean aldehyde formation.
• Swern oxidation (DMSO + oxalyl chloride + base): a 4-step protocol; converts primary alcohol to aldehyde via an intermediate alkoxysulfonium. Mild, no metal toxicity. Standard for many syntheses.
• TEMPO + co-oxidant (NaOCl/NaBr or PhI(OAc)₂): radical oxidation. Used in industrial settings for greener alternatives to chromium oxidation.

For secondary alcohols → ketones: PCC, DMP, Swern all work.

Reagents that go all the way to COOH

For primary alcohols → carboxylic acids:

• Jones reagent (CrO₃ in dilute H₂SO₄ + acetone): $\text{Cr(VI)}$ + acid. Aggressive; goes to COOH. Used in industrial settings (toxic Cr).
• KMnO₄ (hot): aggressive; oxidizes primary alcohols to COOH. Also oxidizes alkenes and aromatic side chains.
• TEMPO + bleach (NaOCl): less hazardous than Jones; goes to COOH under acidic conditions.

Selective oxidation of allylic and benzylic alcohols

Allylic and benzylic alcohols (where C-OH is adjacent to a C=C or aromatic) are preferentially oxidized over saturated alcohols. Mild oxidants (MnO₂, DMP, IBX) selectively oxidize these.

Common Mistake 36.1: Using KMnO₄ for selective alcohol oxidation. KMnO₄ is too aggressive — it oxidizes everything (alkenes, aromatic side chains, primary alcohols all the way to COOH). For selective primary alcohol → aldehyde, use PCC, DMP, or Swern.

36.3 Oxidation of alkenes and alkynes (Ch 16-17 review)

Alkenes can be oxidized to: - syn-Diol (cis-1,2-diol): $OsO_4$ + NMO (or $OsO_4$ alone, with NMO recycling) → diol via [3+2] cycloaddition + hydrolysis. - anti-Diol (trans-1,2-diol): mCPBA → epoxide; then aqueous acid → trans-diol via SN2-like ring opening. - Epoxide: mCPBA (peracid) at room temperature. Stereospecific. - Cleavage to two carbonyls: $O_3$ (ozone) + reductive workup ($Me_2S$ or Zn/HOAc) → 2 aldehydes; oxidative workup ($H_2O_2$) → 2 carboxylic acids.

Alkynes: - Cis-alkene: Lindlar Pd + H₂ (Section 36.5). - Trans-alkene: Na/NH₃(l) (dissolving metal reduction).

36.4 Reduction of carbonyls (Ch 25 review)

The standard reductants:

For chiral reduction: - CBS (Corey-Bakshi-Shibata) reagent: chiral oxazaborolidine; gives enantioselective reduction. - Noyori catalyst (Ru/BINAP): asymmetric hydrogenation of ketones.

36.5 Catalytic hydrogenation

The standard reductant for C=C and other multiple bonds is $H_2$ + a metal catalyst.

Hydrogenation is stereospecific cis addition: H atoms add to the same face of the C=C. Catalyst surfaces hold the alkene; H atoms transfer to both adjacent carbons.

36.6 Dissolving metal reductions

For specific transformations where conventional reductants don't work:

Birch reduction (Na in liq. NH₃ + EtOH)

Reduces benzene → 1,4-cyclohexadiene (a non-conjugated diene). The mechanism involves single-electron reduction by Na to form a radical anion, then protonation. Used to make 1,4-cyclohexadiene starting materials for Diels-Alder.

For substituted benzenes, the ortho/para or meta orientation depends on the substituent: - Electron-donating groups (OMe): ipso/para protonation. - Electron-withdrawing groups (COOH): meta protonation.

Trans-alkene from alkyne

$Na$ in liq. $NH_3$ reduces alkyne → trans alkene (anti addition; opposite of Lindlar's cis).

Aromatic nitro to amine

$Sn/HCl$ or $Fe/HCl$: classic dissolving-metal reduction of $Ar-NO_2$ to $Ar-NH_2$. Modern alternative: $H_2$/Pd/C.

Ketyl reduction

$Na/NH_3$ or $K/THF$ can reduce ketones to ketyl radicals, used in pinacol coupling and McMurry reaction.

36.7 Selectivity in oxidation and reduction

A key challenge in synthesis: when a substrate has multiple potentially-reactive groups, choose conditions that target only one.

Chemoselectivity

When two functional groups are reduced/oxidized at different rates with different reagents.

Example: a substrate has both an aldehyde and an ester. Use NaBH₄ → reduces only the aldehyde (the ester is untouched). Use LiAlH₄ → reduces both.

Regioselectivity

When the same functional group can be at multiple positions, choose conditions favoring one.

Example: a substrate has two alcohols (primary and secondary). Use bulky oxidant → oxidizes primary preferentially.

Stereoselectivity

When the reaction creates a new stereocenter, choose conditions favoring one enantiomer/diastereomer.

Example: a chiral hydride (CBS, Noyori) reduces a ketone enantioselectively.

Mechanism Map 36.1: Choosing oxidation reagents.

• Primary alcohol → aldehyde (mild, selective): PCC, Swern, DMP.
• Primary alcohol → COOH (full): Jones, KMnO₄.
• Secondary alcohol → ketone: PCC, DMP, Swern.
• Allylic/benzylic alcohol → carbonyl (selective): MnO₂.
• Aldehyde → COOH: chromium reagents.
• C=C → diol (syn): OsO₄.
• C=C → diol (anti): mCPBA + acid.
• C=C → cleavage: O₃.

Mechanism Map 36.2: Choosing reduction reagents.

• Aldehyde/ketone → alcohol: NaBH₄ (mild, selective).
• Ester/amide/COOH → alcohol: LiAlH₄.
• Ester → aldehyde (partial): DIBAL-H, 1 equiv, -78 °C.
• C=C, C≡C → alkane: H₂/Pd/C.
• C≡C → cis alkene: Lindlar.
• C≡C → trans alkene: Na/NH₃.
• Nitro → amine: Sn/HCl or H₂/Pd.
• Asymmetric ketone → chiral alcohol: CBS reagent or Noyori.

36.8 Biological redox: NAD, FAD, cytochromes

In biology, redox cofactors carry electrons (often as hydrides) between metabolic substrates and the electron transport chain.

NAD⁺ / NADH (nicotinamide adenine dinucleotide)

• NAD⁺: the oxidized form. Accepts a hydride.
• NADH: the reduced form. Donates a hydride.
• Used in oxidations: lactate dehydrogenase (lactate → pyruvate); malate dehydrogenase; etc. — many dehydrogenase enzymes.
• Used in reductions: alcohol dehydrogenase (acetaldehyde → ethanol); etc.

The chemistry: NAD⁺'s nicotinamide ring (the C4 position) accepts a hydride, becoming NADH (a 1,4-dihydropyridine). Standard reaction: substrate-H ↔ NAD⁺ → substrate + NADH.

This is essentially the same chemistry as NaBH₄ (a hydride donor), but enzyme-catalyzed.

FAD / FADH₂ (flavin adenine dinucleotide)

• Different chemistry from NAD: FAD accepts 2 electrons + 2 protons, becoming FADH₂.
• Used in tighter electron transfer: succinate dehydrogenase; acyl-CoA dehydrogenase.
• Less common as a standard enzyme cofactor than NAD, but critical for the citric acid cycle's step 6.

Cytochrome P450 (CYP enzymes)

• Heme-containing enzymes that introduce oxygen into substrates (oxidation).
• Used by the body to detoxify drugs, metabolize hormones, and produce bile acids.
• Liver CYPs are particularly important pharmacologically; they cause many drug-drug interactions.

The chemistry: CYP enzymes use $O_2$ + NADPH to insert an oxygen atom into a C-H bond. Specifically: the Fe in the heme + $O_2$ generates a high-valent Fe=O (Compound I), which abstracts an H from the substrate, then transfers the oxygen.

Biological Connection 36.1: Drug oxidation by CYP3A4.

Many drugs are metabolized by CYP3A4 (the liver enzyme). The chemistry: CYP3A4 + drug + O₂ + NADPH → oxidized drug + H₂O + NADP⁺. The oxidized drug (often a hydroxylated metabolite) is more polar than the parent, easier to excrete.

Drug-drug interactions occur when one drug inhibits CYP3A4, slowing metabolism of another drug. E.g., grapefruit juice contains compounds that inhibit CYP3A4, raising blood levels of statins, antifungals, and many other drugs.

36.9 Industrial and green oxidations

Modern oxidations strive to avoid: - Cr(VI) reagents (PCC, Jones): toxic, environmentally problematic. - Stoichiometric oxidants: large amount of waste.

Green alternatives: - TEMPO + bleach: organic waste only. - Pd-catalyzed aerobic oxidation: O₂ as the terminal oxidant; Pd as catalyst. - Enzyme-catalyzed oxidation: e.g., glucose oxidase for glucose → gluconic acid. - Photocatalytic oxidation: light-driven oxidations (a 2010s research area).

Modern process chemistry favors catalytic oxidations over stoichiometric ones to reduce environmental impact.

36.10 Why this chapter matters

Oxidation and reduction are essential to: 1. Synthesis: most multi-step syntheses include redox steps to interconvert functional groups. 2. Metabolism: every step of energy metabolism is redox chemistry. 3. Drug metabolism: CYP enzymes oxidize drugs in the liver. 4. Industrial chemistry: many processes (e.g., petrochemical refining) involve oxidation. 5. Environmental chemistry: oxidative degradation of pollutants.

Master the unified view of redox chemistry, and you master half of synthesis and most of metabolism.

36.11 Summary

1. Oxidation state of carbon: bonds to H decrease, bonds to O/N/X increase.
2. Oxidation level ladder: methane (-4) → alcohol (-2) → aldehyde (0) → COOH (+2) → CO₂ (+4).
3. Alcohol → aldehyde (selective, stop short): PCC, DMP, Swern, TEMPO.
4. Alcohol → COOH (full): Jones, KMnO₄, TEMPO + acid.
5. C=C → diol (syn): OsO₄.
6. C=C → diol (anti): mCPBA + acid.
7. C=C → cleavage: O₃ (with reductive or oxidative workup).
8. Carbonyl → alcohol: NaBH₄ (selective for aldehydes/ketones); LiAlH₄ (universal).
9. Ester → aldehyde (partial): DIBAL-H at -78 °C.
10. C=C → alkane: H₂/Pd/C.
11. C≡C → cis alkene: Lindlar Pd + H₂.
12. C≡C → trans alkene: Na in liquid NH₃.
13. Aromatic nitro → amine: Sn/HCl, Fe/HCl, or H₂/Pd.
14. Birch reduction (benzene → 1,4-cyclohexadiene): Na/NH₃/EtOH.
15. Asymmetric reduction: CBS reagent (for ketones), Noyori (for ketones with H₂).
16. NAD/NADH is biology's hydride carrier; FAD/FADH₂ is biology's electron-pair carrier; CYP enzymes are biology's oxidases.
17. Modern green redox: TEMPO/bleach, Pd-aerobic, biocatalysis, photocatalysis.

Chapter 37 turns to organometallic chemistry — the use of metals to enable C-C bond formation and selective transformations.