Oxidation and Reduction Reactions
High-Yield Summary
- Aldehyde (-CHO, still has a carbonyl-C hydrogen) + oxidant stronger than PCC (KMnO₄, CrO₃, Ag₂O/Tollens', H₂O₂) → carboxylic acid (-COOH).
- Ketones resist this oxidation entirely — no carbonyl-carbon hydrogen for the oxidant to remove — regardless of oxidant strength.
- Tollens' reagent (Ag(NH₃)₂⁺, from Ag₂O): positive test = silver mirror deposits with an aldehyde; negative test (no mirror) = ketone.
- Hydride reduction: aldehyde → primary alcohol (-CH₂OH); ketone → secondary alcohol (-CHOH-). Hydride (H⁻) adds to the electrophilic carbonyl carbon.
- LiAlH₄ (strong): reduces aldehydes, ketones, esters, and carboxylic acids. NaBH₄ (mild): reduces only aldehydes/ketones, leaves esters/acids untouched — key for selective reduction.
LiAlH₄ vs. NaBH₄
| Reagent | Reduces |
|---|---|
| LiAlH₄ (strong) | Aldehydes, ketones, esters, carboxylic acids — all → alcohols |
| NaBH₄ (mild) | Aldehydes, ketones only — esters/acids untouched |
Key Terms
- Tollens' reagent / Tollens' test
- Diamminesilver(I) complex (Ag(NH₃)₂⁺, from Ag₂O); oxidizes aldehydes to carboxylic acids while depositing a visible silver mirror — positive with aldehydes, negative with ketones.
- Hydride-reducing agent
- Delivers H⁻ to the electrophilic carbonyl carbon, converting C=O into C-OH (an alcohol); LiAlH₄ and NaBH₄ are the two common examples.
Common MCAT Trap
- A negative Tollens' test doesn't mean 'no carbonyl' — it specifically distinguishes a ketone (negative) from an aldehyde (positive); both still have a carbonyl group.
- NaBH₄'s mildness is a selectivity feature, not a weakness — use it deliberately when a molecule has an ester/acid you want to leave untouched while reducing a ketone/aldehyde elsewhere.
- Aldehyde reduction gives a primary alcohol, ketone reduction gives a secondary alcohol — matches the number of R groups already on the carbonyl carbon plus the one new H from the hydride.
Quick Recall
Why can aldehydes be oxidized to carboxylic acids but ketones can't?
What does a positive Tollens' test look like, and what does it indicate?
Which reagent would you choose to reduce a ketone while leaving a co-existing ester unreacted, and why?