19.3 Oxidation of Aldehydes and Ketones

Oxidation of Aldehydes and Ketones

Aldehydes and ketones react very differently when exposed to oxidizing agents. Aldehydes are easily oxidized to carboxylic acids, while ketones strongly resist oxidation under normal conditions. This difference comes down to what's bonded to the carbonyl carbon: aldehydes have a hydrogen there, and ketones don't. That single structural difference controls most of the reactivity you need to know for this topic.

Aldehyde Oxidation to Carboxylic Acids

Aldehydes oxidize readily because the carbonyl carbon bears a C–H bond that can be broken relatively easily. The overall transformation converts the $-CHO$ group into a $-COOH$ group:

$RCHO + [O] \rightarrow RCOOH$

where $R$ is an alkyl group (or hydrogen for formaldehyde) and $[O]$ represents the oxidizing agent.

The mechanism generally proceeds through these steps:

1. The aldehyde reacts with water (or the oxidizing agent) to form a hydrate intermediate, where the carbonyl carbon now has two $-OH$ groups attached.
2. The oxidizing agent removes electrons from this intermediate, breaking the C–H bond at the carbonyl carbon.
3. A proton transfer and loss of water yield the carboxylic acid product.

The electron-withdrawing nature of the carbonyl group makes the carbonyl carbon electrophilic, which helps the initial nucleophilic addition step proceed smoothly.

Ketone Resistance to Oxidation

Ketones resist oxidation under conditions that easily oxidize aldehydes. Two factors explain this:

• No C–H bond on the carbonyl carbon. Ketones have two carbon groups (alkyl or aryl) flanking the carbonyl. Since there's no hydrogen to remove, the hydrate-based oxidation pathway that works for aldehydes simply isn't available.
• Steric hindrance. The two substituents on the carbonyl carbon physically block oxidizing agents from approaching.

To oxidize a ketone, you'd have to break a carbon–carbon bond, which requires significantly more energy than breaking the C–H bond in an aldehyde. This only happens under harsh conditions. For example, hot, concentrated $KMnO_4$ under acidic conditions can cleave the C–C bond adjacent to the carbonyl, producing two carboxylic acid fragments. This is called oxidative cleavage, and it's not a typical laboratory goal since it destroys the carbon skeleton.

Oxidizing Agents for Aldehydes

Different oxidizing agents vary in strength and selectivity. Here are the ones you should know:

• Chromic acid ($H_2CrO_4$): Made by dissolving $CrO_3$ in aqueous $H_2SO_4$. This is a strong oxidizing agent that reliably converts aldehydes to carboxylic acids but can also oxidize other sensitive functional groups (like primary alcohols).
• Potassium permanganate ($KMnO_4$): Another strong oxidant. Under acidic conditions it oxidizes aldehydes to carboxylic acids. It's powerful enough to oxidize many other functional groups too, so it's not very selective.
• Pyridinium chlorochromate (PCC): This is the tricky one. PCC is a mild chromium-based oxidant typically used to oxidize primary alcohols to aldehydes without further oxidation to carboxylic acids. It works in anhydrous conditions (dichloromethane solvent), which prevents over-oxidation. Don't confuse its role: PCC stops at the aldehyde stage when starting from an alcohol.
• Tollens' reagent (silver mirror test): A solution of $AgNO_3$ and $NH_3$ that provides $Ag^+$ ions as the mild oxidizing agent. Aldehydes reduce $Ag^+$ to metallic silver, which deposits as a shiny mirror on the glass. The aldehyde is oxidized to a carboxylate salt. Ketones give no reaction, making this a useful qualitative test to distinguish aldehydes from ketones.
• Fehling's solution: Made from $CuSO_4$ mixed with sodium potassium tartrate (Rochelle salt) in $NaOH$. The $Cu^{2+}$ ions oxidize aldehydes to carboxylate salts, and the copper is reduced to $Cu_2O$, which appears as a brick-red precipitate. Like Tollens' test, ketones give no reaction, so this also distinguishes aldehydes from ketones.

Oxidation and Reduction in Carbonyl Chemistry

It helps to think about oxidation state when comparing carbonyl compounds. Carbon in an aldehyde is at a lower oxidation state than carbon in a carboxylic acid, which is why converting an aldehyde to a carboxylic acid is an oxidation (increase in oxidation state). The carbonyl carbon in a ketone is already at a higher oxidation state than in an aldehyde, but further oxidation would require C–C bond cleavage rather than simple C–H bond breaking.

Going in the other direction, reduction of aldehydes and ketones produces alcohols. Hydride reagents like $NaBH_4$ or $LiAlH_4$ deliver $H^-$ to the electrophilic carbonyl carbon through nucleophilic addition. Aldehydes reduce to primary alcohols; ketones reduce to secondary alcohols. This is the reverse relationship: oxidation moves you up the oxidation-state ladder (alcohol → aldehyde → carboxylic acid), and reduction moves you back down.