Oxidizing an aldehyde — Carbonyl Chemistry

An aldehyde is the one carbonyl in this chapter that is still going somewhere. Leave it in a flask with almost any oxidant — leave it in a flask with air — and it climbs one more rung to the carboxylic acid. A ketone, sitting at the same oxidation level, does nothing at all.

The difference is one hydrogen

Oxidation at a carbon means replacing a C–H bond with a bond to oxygen. An aldehyde's carbonyl carbon still carries a hydrogen, so there is something to take. A ketone's carries two carbons instead, so oxidizing it would mean breaking a C–C bond — which is why ordinary oxidants leave ketones untouched.

It is the same hydrogen count the next chapter will use to decide which alcohols can be oxidized and how many times, applied one rung higher up. Primary alcohols have two hydrogens on the carbon and go twice; secondary have one and go once; tertiary have none and do not go at all.

What is actually oxidized is the hydrate

A carbonyl carbon has no hydroxyl for an oxidant to grip. The hydrate does. The section before this one showed water adding across the C=O to give a gem-diol, and that gem-diol has an OH and a hydrogen on the same carbon — which is to say it is an alcohol, and it is oxidized exactly as a primary alcohol is.

This is the mechanism behind a reagent rule the next chapter will state without explaining. Jones reagent, Cr(VI) in aqueous acid, carries a primary alcohol past the aldehyde to the carboxylic acid because the water in the flask keeps regenerating the hydrate. PCC in anhydrous dichloromethane stops at the aldehyde because there is no water, therefore no hydrate, therefore nothing with an O–H left to attack.

Tollens' reagent and the silver mirror

The step everyone skips. An oxidant needs an O–H and a C–H on the same carbon, and a C=O offers neither — so what is actually attacked is the hydrate. That single box is the whole difference between a reagent that stops at the aldehyde and one that does not.The same argument explains a sugar. A cyclic hemiacetal holds only a trace of the open-chain aldehyde, and yet glucose gives a full silver mirror, because the equilibrium delivers that trace continuously and the oxidant consumes it as fast as it appears. A species can be present in traces and still control the product, provided it is the only form that can react.

Tollens' reagent is the diamminesilver(I) ion, Ag(NH₃)₂⁺, in basic solution. The aldehyde is oxidized to the carboxylate, and the silver(I) is reduced to metallic silver, which plates out on clean glass as a silver mirror. That visible deposit is the whole point.

Tollens' is a very mild oxidant, and the things it does not do are what make it useful. It leaves alcohols alone and it leaves alkenes alone. A reagent that attacks one functional group and ignores its neighbors is a test rather than a synthesis — nobody makes a carboxylic acid this way, because silver is expensive.

Fehling's and Benedict's solutions

Both use Cu(II) held in solution by a chelating ligand under basic conditions, and both signal a positive result by dropping a brick-red precipitate of Cu₂O out of a deep blue solution.

Reagent	Cu(II) complexed by	Notes
Fehling's	tartrate	Reliable for aliphatic aldehydes; unreliable with aromatic ones
Benedict's	citrate	More robust; the version used clinically

Why glucose tests positive and sucrose does not

Glucose is drawn as a ring, and a ring has no aldehyde in it. It tests positive anyway, because the ring is a hemiacetal and hemiacetals open. The equilibrium holds only a trace of the open-chain aldehyde at any moment, but it delivers that trace continuously, and the oxidant consumes it as fast as it appears. Sugars that behave this way are called reducing sugars — named for what they do to the metal, not for what happens to them.

A ketose tests positive too, which is worth knowing because it looks like it should not. Fructose has no aldehyde at any point in the equilibrium, and Benedict's finds it anyway: these reagents are basic, and base isomerizes a ketose to the corresponding aldose through an enediol. So the property that actually decides a positive test is a free anomeric carbon, not an aldehyde group — which is how the carbohydrate section defines a reducing sugar, and it is the more general statement.

Sucrose is negative, because its glycosidic bond ties up both anomeric carbons; neither ring can open. Lactose and maltose each leave one anomeric carbon free, so both are positive. The carbohydrate section develops that bookkeeping properly; here it is the hydrate argument again, with a hemiacetal in place of the gem-diol.

The consequence on the shelf

Aldehydes oxidize in air on standing, by a radical chain that needs no reagent at all. An old bottle of benzaldehyde grows crystals of benzoic acid around the cap, and that contamination is a routine cause of unexplained failures. Aldehydes are therefore stored under inert gas, and are often made immediately before use.

A ketone can be oxidized, but only by breaking a C–C bond, and both routes this course covers do exactly that. The Baeyer–Villiger inserts an oxygen into one of those bonds to give an ester. Hot, concentrated KMnO₄ cleaves straight through. Neither is something that happens by accident in a bottle.

What carries forward

Aldehyde to carboxylic acid is a one-step move in any synthesis, and the reagent question is only ever how much else in the molecule you are willing to lose. Run it in reverse and the same fact becomes a warning: any route that makes an aldehyde and leaves it sitting has a problem, because air will finish the job for you.