Oxidation and Reduction Reactions
High-Yield Summary
- LEO the lion says GER: Lose Electrons = Oxidation, Gain Electrons = Reduction. Oxidation increases positive charge; reduction increases negative charge.
- The reducing agent donates electrons (is itself oxidized); the oxidizing agent accepts electrons (is itself reduced).
- Structural shortcut: oxidation = more bonds to O/electronegative atoms + fewer H's; reduction = fewer bonds to electronegative atoms + more H's.
- Alcohol oxidation depends on class: primary → aldehyde (mild) or carboxylic acid (strong); secondary → ketone only (no further oxidation possible); tertiary resists oxidation entirely.
- Reduction reverses oxidation: aldehydes/ketones → alcohols (LiAlH₄ or NaBH₄); carboxylic acids/esters → alcohols (LiAlH₄ only); amides → primary amines (LiAlH₄, removes the oxygen).
Alcohol Oxidation by Class
| Alcohol | Product / reagent |
|---|---|
| Primary (mild) | → Aldehyde — PCC or CrO₃/pyridine |
| Primary (strong) | → Carboxylic acid — H₂CrO₄, KMnO₄, or H₂O₂ |
| Secondary | → Ketone — PCC or CrO₃/pyridine (no further oxidation possible) |
| Tertiary | Resists oxidation — no C–H bond on the carbinol carbon |
Alkene Ozonolysis Workup
| Workup | Product |
|---|---|
| Reducing (Zn, dimethyl sulfide) | Aldehydes and ketones (mild — no further oxidation) |
| Oxidizing (H₂O₂, or KMnO₄/heat/H₃O⁺) | Any aldehyde fragment → carboxylic acid; ketones unaffected |
Key Terms
- Hydroxylation
- Alkene → vicinal diol via OsO₄ or basic KMnO₄ (syn addition); diols cleave further to aldehydes/ketones via NaIO₄, Pb(OAc)₄, or HIO₄.
- Epoxidation
- Alkene → epoxide via a peracid (e.g. mCPBA); epoxides are highly reactive due to ring strain.
- Baeyer-Villiger oxidation
- mCPBA oxidizes a ketone to an ester by inserting an oxygen atom adjacent to the carbonyl carbon.
- Alkyne cleavage
- O₃/H₂O or KMnO₄/heat/H₃O⁺ cleave a triple bond; internal alkynes give two carboxylic acids, terminal alkynes give one carboxylic acid + CO₂.
Common MCAT Trap
- Secondary alcohols stop at ketones — there's no hydrogen on the carbonyl carbon of a ketone, so unlike aldehydes there's no 'strong vs. mild' distinction to worry about.
- NaBH₄ is milder than LiAlH₄ — NaBH₄ only reliably reduces aldehydes/ketones, while LiAlH₄ is strong enough for carboxylic acids, esters, and amides too.
- Amide reduction gives an amine, not an alcohol — LiAlH₄ removes the carbonyl oxygen entirely on amides, unlike its reduction of acids/esters.
Quick Recall
What does 'LEO the lion says GER' stand for?
Why can't a tertiary alcohol be oxidized under normal conditions?
What product forms from ozonolysis of an alkene with a reducing workup (Zn or DMS) vs. an oxidizing workup (H₂O₂)?
What reagent reduces an ester, and what does it produce?