Carboxylic Acid Synthesis
High-Yield Summary
- Carboxylic acids are made by oxidizing primary alcohols or aldehydes — both require strong oxidizing agents (mild conditions stop short).
- Strong oxidants: Na₂Cr₂O₇, K₂Cr₂O₇, CrO₃, KMnO₄ — e.g. ethanol + KMnO₄ → acetic acid.
- Only primary alcohols go all the way to a carboxylic acid. Secondary alcohols stop at the ketone. Tertiary alcohols can't be oxidized at all — no H on the hydroxyl-bearing carbon to remove.
- Aldehydes already have the carbonyl in place, so the same strong oxidants take them directly to the carboxylic acid with no extra steps.
- PCC (mild, anhydrous) stops a primary alcohol at the aldehyde stage — it does not continue to the carboxylic acid. Strong oxidants (especially aqueous acidic, e.g. Jones oxidation) hydrate and re-oxidize that aldehyde intermediate to finish the job.
Alcohol Class → Oxidation Endpoint
| Alcohol | Furthest oxidation state reached |
|---|---|
| Primary alcohol | Carboxylic acid (via aldehyde intermediate, with a strong oxidant) |
| Secondary alcohol | Ketone — stops here, can't be oxidized further |
| Tertiary alcohol | No reaction — no H on the hydroxyl-bearing carbon to remove |
Key Terms
- PCC (pyridinium chlorochromate)
- Mild, anhydrous oxidant; stops a primary alcohol at the aldehyde stage, does not continue on to the carboxylic acid.
- Jones oxidation
- Oxidation with a strong Cr(VI) reagent in aqueous acid; hydrates the aldehyde intermediate and re-oxidizes it, carrying a primary alcohol all the way to the carboxylic acid.
Common MCAT Trap
- The oxidant's strength (and whether it's anhydrous vs. aqueous), not just the alcohol's identity, determines whether the reaction stops at the aldehyde or continues to the carboxylic acid — PCC vs. Na₂Cr₂O₇/H₂SO₄ is a classic exam distinction.
- Tertiary alcohols aren't just "hard to oxidize" — they categorically cannot be oxidized under these conditions, because oxidation requires removing an H from the hydroxyl-bearing carbon and tertiary alcohols have none.
Quick Recall
What are the two classes of starting material for carboxylic acid synthesis, and what do they have in common?
Why can't a tertiary alcohol be oxidized to a carboxylic acid (or anything else)?
Why does PCC stop at the aldehyde while KMnO₄ or Jones oxidation continues to the carboxylic acid?