A primary alcohol can be oxidized to an aldehyde or straight past it to a carboxylic acid, and the aldehyde is very often the thing you actually want. Learning this section is learning which reagent stops where, and the reason turns out to be a single mechanistic detail: whether there is water in the flask.
What each class of alcohol can become
| Substrate | Mild oxidant | Strong / aqueous oxidant |
|---|---|---|
| Primary alcohol | Aldehyde | Carboxylic acid |
| Secondary alcohol | Ketone | Ketone |
| Tertiary alcohol | No reaction | No reaction |
The secondary column is the reassuring one: a ketone has no hydrogen left on the carbonyl carbon, so it cannot be oxidized further under these conditions and both reagents give the same thing. The choice of reagent only matters for primary alcohols.
Why water decides where a chromium oxidation stops
Chromium(VI) reagents oxidize an alcohol by way of a chromate ester, which then loses the carbinol C–H to give the carbonyl. For a primary alcohol that gives an aldehyde — and an aldehyde in water is in equilibrium with its hydrate, the gem-diol formed by water adding across the C=O.
That hydrate is, structurally, an alcohol again: it has an OH and a hydrogen on the same carbon. So the chromium reagent oxidizes it a second time, and the product is the carboxylic acid. Take the water away and the hydrate never forms, so the oxidation has nothing left to grip and stops at the aldehyde.
The reagents worth knowing
- PCC (pyridinium chlorochromate), in anhydrous dichloromethane. Cr(VI) with no water present, so a primary alcohol stops at the aldehyde and a secondary gives the ketone. The standard answer when a question wants an aldehyde and offers a chromium reagent.
- Jones reagent (CrO₃ in aqueous sulfuric acid, usually in acetone). Cr(VI) with water, so a primary alcohol goes all the way to the carboxylic acid. Secondary alcohols give ketones. Also written as CrO₃/H₂SO₄ or as Na₂Cr₂O₇/H₂SO₄, which behave the same way.
- Swern oxidation (oxalyl chloride and DMSO at −78 °C, then triethylamine). No metal at all, and stops cleanly at the aldehyde. Mild enough for sensitive substrates, which is why it is common in real synthesis.
- Dess–Martin periodinane (DMP), a hypervalent iodine reagent at room temperature. Also stops at the aldehyde, also metal-free, and easier to handle than the Swern because it needs no cryogenic step.
Group them by outcome rather than by chemistry and the list is short: PCC, Swern and DMP give aldehydes; Jones and other aqueous Cr(VI) give carboxylic acids; all of them give ketones from secondary alcohols and nothing from tertiary ones.
You need to convert the primary alcohol of a molecule that also contains a secondary alcohol elsewhere, and you want the aldehyde.
Reagent choice. PCC, Swern or DMP will all stop at the aldehyde. None of them distinguishes a primary alcohol from a secondary one, though, so the secondary alcohol will be oxidized to a ketone at the same time.
The real answer is that you cannot solve this by reagent choice alone. You protect the secondary alcohol first, oxidize, then deprotect. Recognizing that a selectivity problem needs a protecting group rather than a cleverer oxidant is the actual skill being tested.
Oxidation levels, again
Read the whole section against the ladder from the last one and it compresses. An alcohol sits at one rung, an aldehyde or ketone one rung up, a carboxylic acid one rung above that. A primary alcohol has two rungs available because it has two hydrogens on the carbinol carbon; a secondary alcohol has one; a tertiary alcohol has none and therefore cannot climb at all.
What carries forward
Every one of these reactions runs backwards in the next section, where hydride reagents take carbonyls back down to alcohols. Holding both directions at once is what makes synthesis planning possible — a primary alcohol and an aldehyde are one oxidation apart in either direction, so a route that needs one from the other is a single step, and knowing which reagent gets you there without overshooting is the content.