Aldehyde and Ketone Oxidation and Reduction Reactions

Aldehydes and ketones don't just form from oxidizing alcohols — they can themselves be pushed further in either direction, oxidized to carboxylic acids or reduced to alcohols. This article covers which oxidizing agents carry an aldehyde on to a carboxylic acid, why ketones can't follow the same path, how hydride-delivering reagents reduce a carbonyl back to an alcohol, and why the choice between two common reducing agents — LiAlH₄ and NaBH₄ — matters for selectivity.

Key Takeaways

• Aldehydes can be oxidized to carboxylic acids (-CHO → -COOH) using oxidizing agents stronger than PCC: KMnO₄, CrO₃, Ag₂O (Tollens' reagent), or H₂O₂.

• Tollens' reagent (the diamminesilver(I) complex from Ag₂O) gives a positive silver-mirror test with aldehydes and a negative test with ketones.

• Ketones resist this oxidation because their carbonyl carbon has no attached hydrogen for the oxidant to remove.

• Hydride-reducing agents deliver H⁻ to the electrophilic carbonyl carbon, reducing aldehydes to primary alcohols and ketones to secondary alcohols.

• LiAlH₄ is a strong reducing agent that also reduces esters and carboxylic acids; NaBH₄ is milder and selectively reduces only aldehydes and ketones.

Oxidizing Aldehydes to Carboxylic Acids

An aldehyde's carbonyl carbon still has a hydrogen attached to it, and that hydrogen is what makes further oxidation possible. Treating an aldehyde with an oxidizing agent stronger than pyridinium chlorochromate (PCC) converts the -CHO group into a -COOH group — a carboxylic acid — increasing the oxidation state of the carbonyl carbon.

Reagents commonly used for this conversion include:

• Potassium permanganate (KMnO₄)

• Chromic acid (CrO₃)

• Silver oxide (Ag₂O) — most often encountered in its ammoniacal form as Tollens' reagent, the diamminesilver(I) complex (Ag(NH₃)₂⁺). Tollens' reagent selectively oxidizes aldehydes while depositing metallic silver as a visible "silver mirror" on the reaction vessel — the classic qualitative Tollens' test used to identify aldehydes.

• Hydrogen peroxide (H₂O₂)

Each of these reagents is strong enough to complete the aldehyde-to-carboxylic-acid conversion, and this reaction is a common synthetic route to carboxylic acids.

Why Ketones Resist Further Oxidation

Ketones don't undergo this same reaction. A ketone's carbonyl carbon is bonded to two alkyl or aryl groups instead of a hydrogen, so there's no carbonyl-carbon hydrogen for an oxidizing agent to remove. Without that hydrogen available, ketones are resistant to oxidation by KMnO₄, CrO₃, Ag₂O, or H₂O₂ under normal conditions — which is also why a negative Tollens' test (no silver mirror) is used to distinguish a ketone from an aldehyde.

Reducing Aldehydes and Ketones to Alcohols

Both aldehydes and ketones can be pushed in the opposite direction — reduced to alcohols. This transformation is carried out with hydride-reducing reagents, which deliver a hydride ion (H⁻) to the electrophilic carbonyl carbon, adding a hydrogen where the double bond to oxygen used to be.

• Reducing an aldehyde produces a primary alcohol (-CH₂OH), since the carbonyl carbon already had one hydrogen and one R group before reduction added a second hydrogen.

• Reducing a ketone produces a secondary alcohol (-CHOH-), since the carbonyl carbon's two R groups remain attached after the hydride adds a single hydrogen.

LiAlH₄ vs. NaBH₄: Comparing Reducing Agent Strength

The two most common hydride-reducing agents differ in strength and selectivity:

• Lithium aluminum hydride (LiAlH₄) is a strong reducing agent. It reduces aldehydes and ketones, and is powerful enough to also reduce esters and carboxylic acids to alcohols.

• Sodium borohydride (NaBH₄) is a milder reducing agent. It selectively reduces aldehydes and ketones but does not reduce esters or carboxylic acids.

This difference matters whenever a molecule contains more than one reducible group: NaBH₄ can target a ketone or aldehyde while leaving an ester or carboxylic acid elsewhere in the molecule untouched, while LiAlH₄ will reduce all of them.

Common MCAT Mistakes

• Assuming ketones oxidize the same way aldehydes do. An aldehyde's carbonyl-carbon hydrogen is what a strong oxidant removes to form a carboxylic acid; a ketone has no such hydrogen, so KMnO₄, CrO₃, Ag₂O, and H₂O₂ all leave it untouched.

• Mixing up a positive vs. negative Tollens' test. A silver mirror forming means an aldehyde is present (it got oxidized); no silver mirror means a ketone (or no aldehyde present) — the test result identifies which carbonyl class you have.

• Forgetting which alcohol a reduction produces. Reducing an aldehyde (one carbonyl-carbon hydrogen plus one R group) gives a primary alcohol; reducing a ketone (two R groups) gives a secondary alcohol — the substitution pattern carries straight through.

• Treating LiAlH₄ and NaBH₄ as interchangeable. Both reduce aldehydes and ketones, but only LiAlH₄ is strong enough to also reduce esters and carboxylic acids — NaBH₄ is the milder, more selective choice when other reducible groups need to be left alone.

MCAT-Style Concept Check

Question: A molecule contains both a ketone and an ester. Which reagent reduces the ketone to a secondary alcohol while leaving the ester untouched?

• A) LiAlH₄, because it is the stronger of the two hydride reagents

• B) NaBH₄, because it selectively reduces aldehydes and ketones but not esters

• C) Tollens' reagent, because it selectively targets ketones over esters

• D) CrO₃, because it is a mild oxidizing agent

Answer: B

Explanation: NaBH₄ is a milder hydride-reducing agent that reduces aldehydes and ketones but does not reduce esters or carboxylic acids, so it converts the ketone to a secondary alcohol without touching the ester. LiAlH₄ would reduce both groups since it's strong enough to also reduce esters. Tollens' reagent and CrO₃ are oxidizing agents, not reducing agents, and neither reduces a ketone.

FAQ

Can ketones be oxidized to carboxylic acids the way aldehydes can?

No. A ketone's carbonyl carbon is bonded to two alkyl or aryl groups instead of a hydrogen, so there's no carbonyl-carbon hydrogen for an oxidizing agent like KMnO₄, CrO₃, Ag₂O, or H₂O₂ to remove. Ketones resist this oxidation under normal conditions.

What is Tollens' reagent, and what does a positive test look like?

Tollens' reagent is the diamminesilver(I) complex (Ag(NH₃)₂⁺), an ammoniacal form of silver oxide. It selectively oxidizes aldehydes to carboxylic acids while depositing metallic silver as a visible "silver mirror" on the reaction vessel — a positive result identifies an aldehyde; a negative result (no mirror) points to a ketone.

What's the difference between the alcohol you get from reducing an aldehyde vs. a ketone?

Reducing an aldehyde adds a hydride to a carbonyl carbon that already has one hydrogen and one R group, producing a primary alcohol. Reducing a ketone adds a hydride to a carbonyl carbon with two R groups, producing a secondary alcohol.

When would you choose NaBH₄ over LiAlH₄?

Choose NaBH₄ when a molecule has an ester or carboxylic acid elsewhere that needs to stay intact — NaBH₄ selectively reduces only aldehydes and ketones. LiAlH₄ is the stronger choice when esters or carboxylic acids also need to be reduced, since it reduces all of them.