Organic Chemistry · Alcohols and Phenols
Oxidation of Alcohols
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In 30 seconds
In organic chemistry, "oxidation" means a change in the number of C–H and C–O bonds at the carbon of interest: a carbon is oxidized when it gains C–O bonds or loses C–H bonds. An alcohol carbon already holds one C–O bond; oxidation converts it to a carbonyl (a second C–O bond) or, for primary alcohols, all the way to a carboxylic acid (a C–O double bond plus an O–H). The carbon skeleton is never broken by these oxidations, and the product is dictated by two factors: whether the alcohol is primary, secondary, or tertiary, and how strong and how wet the oxidizing agent is.
The reagents form a predictable ladder. Mild, anhydrous oxidants such as PCC Pyridinium chlorochromate, an anhydrous Cr(VI) reagent Full entry → (pyridinium chlorochromate) stop at the Aldehyde RCHO; the two-hydrogen oxidation product of a 1° alcohol Full entry → for 1° alcohols. Strong aqueous oxidants such as CrO3/H2SO4 (Jones reagent CrO3 + H2SO4 in aqueous acetone Full entry →) or KMnO4 push 1° alcohols to carboxylic acids and convert 2° alcohols to ketones. Tertiary alcohols have no hydrogen on the –OH carbon, so they resist oxidation under normal conditions — a fact that is both a synthetic blessing and a classic exam question.
Why this matters
Alcohol oxidation is the industrial and laboratory route to aldehydes, ketones, and carboxylic acids — three of the most important carbonyl families. Formaldehyde (from methanol oxidation) is a building block for resins and disinfectants; acetaldehyde and acetic acid come from ethanol oxidation; cyclohexanone (from cyclohexanol) is the feedstock for nylon-6,6. In medicine, the same chemistry runs in the liver: Alcohol dehydrogenase Enzyme that oxidizes alcohols with NAD⁺ Full entry → oxidizes ethanol to acetaldehyde using NAD⁺, and aldehyde dehydrogenase converts it to acetate. That two-step pathway explains both the effects of drinking and the toxicity of methanol, which is oxidized first to formaldehyde and then to formic acid. Knowing which functional group an oxidant delivers — and why a 3° alcohol survives — lets you read a synthesis plan, predict a metabolic fate, and spot the trap in a "product prediction" problem.
The college version
Core Concepts
What "oxidation" means for an alcohol carbon
Count bonds, not electrons. At the –OH carbon:
- 1° alcohol (RCH2OH): one C–O bond, three C–H bonds. Oxidation to aldehyde removes two C–H bonds and adds a second C–O bond (a C=O). Further oxidation to the carboxylic acid adds an O–H.
- 2° alcohol (R2CHOH): one C–O bond, one C–H bond. Oxidation to a ketone removes the C–H and forms the C=O. Ketones are resistant to further oxidation under normal conditions.
- 3° alcohol (R3COH): one C–O bond, zero C–H bonds on that carbon. There is nothing to remove, so no oxidation occurs with typical reagents.
Chromium(VI) reagents: PCC vs Jones
Chromium(VI) oxidants are the workhorses. The mechanism is an esterification–elimination: the alcohol's oxygen attacks the electrophilic Cr(VI), forming a Chromate ester R–O–Cr(VI) intermediate in chromium oxidations Full entry →; then a base removes the α-hydrogen while the Cr–O bond breaks, giving the carbonyl and a reduced chromium species. The oxidation state change is Cr(VI) → Cr(III), a three-electron reduction per chromium.
- PCC (pyridinium chlorochromate, C5H5NH+CrO3Cl-) in dry CH2Cl2: anhydrous, so the aldehyde cannot hydrate to the gem-diol that would be further oxidized. 1° alcohol → aldehyde; 2° alcohol → ketone.
- Jones reagent (CrO3 + H2SO4 in aqueous acetone): water is present, so 1° alcohols go all the way to carboxylic acids; 2° alcohols give ketones. Mild enough that C=C bonds usually survive.
- Other Cr(VI) forms: K2Cr2O7/H2SO4 behaves like Jones; CrO3 in pyridine is a milder variant.
Other oxidants worth knowing
- KMnO4 (aqueous, often basic or with acid): strong; 1° → carboxylic acid, 2° → ketone. Harsher than Jones; it can also cleave alkenes, so it is not the gentle choice when a double bond is present.
- NaOCl (household bleach) with a catalytic amount of TEMPO A nitroxyl radical catalyst used with NaOCl Full entry →: a mild, cheap, scalable route from 1° alcohols to aldehydes or, in buffered aqueous conditions, to carboxylic acids; 2° alcohols to ketones. Industrially important because it avoids toxic chromium waste.
- Ag2O/NH3 (Tollens' reagent) and Cu2+ (Benedict's/Fehling's): selectively oxidize aldehydes to carboxylic acids; a 1° alcohol is first converted to the aldehyde by the reagent itself, but the practical classic use is distinguishing aldehydes from ketones.
Biological oxidation: NAD⁺ and the liver
Enzymes use NAD⁺ (nicotinamide adenine dinucleotide) as the oxidant. Alcohol dehydrogenase removes a hydride (H-) from the –OH carbon and transfers it to NAD⁺, giving NADH and a carbonyl:
CH3CH2OH + NAD+ ⟶ CH3CHO + NADH + H+
The reaction is formally the same loss of C–H / gain of C=O as the chromium reagents, but it runs at body temperature in water. Because methanol is oxidized to formaldehyde and then formic acid (both toxic), methanol poisoning is treated with ethanol: ethanol competes for alcohol dehydrogenase, slowing the production of the toxic metabolites.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| PCC vs Jones reagent | Both Cr(VI), both oxidize alcohols | PCC is anhydrous → stops at aldehyde. Jones is aqueous → carboxylic acid from 1° alcohols |
| Aldehyde vs ketone product | 1° vs 2° alcohol oxidation | 1° gives RCHO (then RCOOH with aqueous oxidants); 2° gives R₂C=O only |
| "Tertiary alcohols don't oxidize" | "Tertiary alcohols are inert" | They resist oxidation at the –OH carbon, but they still react as bases, form alkoxides, dehydrate, and can be cleaved by very harsh oxidants |
| Oxidation vs combustion | Controlled C–H/C–O changes vs burning | Laboratory oxidation preserves the skeleton; combustion destroys it to CO₂ and H₂O |
| NAD⁺ vs NADH | Oxidized vs reduced form | NAD⁺ accepts a hydride (oxidizes the alcohol); NADH is the electron-rich product that later donates electrons |
| Acetaldehyde from ethanol | "Alcohols make acids directly" | Ethanol → acetaldehyde (2 e⁻) → acetic acid (2 more e⁻): two separate enzyme/reagent steps |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of the –OH carbon as having two "slots" that can hold hydrogen atoms. Oxidation is like taking hydrogen atoms out of those slots and putting in a double bond to oxygen instead. A secondary alcohol has one hydrogen to lose, so it can only become a ketone. A primary alcohol has two hydrogens to lose, so it can go one step (aldehyde) or two steps (carboxylic acid). A tertiary alcohol has no hydrogen in its slots at all — there is nothing to remove, so it just sits there while the oxidant fumes.
Worked example
Example 1: Choose the reagent to make hexanal, not hexanoic acid
Problem. Convert 1-hexanol to hexanal without over-oxidation. Show the reaction.
Step 1 — recognize the target. Hexanal is the aldehyde; 1-hexanol is primary. Aldehydes are oxidized faster than alcohols in aqueous solution, so water must be excluded.
Step 2 — reagent choice. PCC in dry dichloromethane. (Jones or KMnO₄ would give hexanoic acid.)
Step 3 — write the balanced transformation:
CH3(CH2)4CH2OH PCC, CH2Cl2⟶ CH3(CH2)4CHO
Answer. Hexanal forms in high yield because the anhydrous conditions prevent hydration of the aldehyde.
Example 2: Stoichiometry of a Jones oxidation with dimensional analysis
Problem. How many grams of CrO3 are required to oxidize 0.150 mol of cyclohexanol to cyclohexanone, assuming each CrO3 accepts 3 electrons and each alcohol donates 2?
Step 1 — electron bookkeeping. One 2° alcohol → ketone is a 2-electron oxidation. Chromium(VI) → chromium(III) is a 3-electron reduction.
Step 2 — ratio. To balance electrons, 3 alcohols donate 3 × 2 = 6 e⁻ and 2 chromiums accept 2 × 3 = 6 e⁻. Mole ratio: 3 mol alcohol per 2 mol CrO3.
2 mol CrO33 mol alcohol
Step 3 — dimensional analysis. Molar mass of CrO3 = 52.00 + 3 × 16.00 = 100.00 g/mol.
0.150 mol alcohol × 23 = 0.100 mol CrO3
0.100 mol CrO3 × 100.00 gmol = 10.0 g CrO3
Answer. 10.0 g of CrO3 provides the electrons needed for 0.150 mol of cyclohexanol. (In practice you use excess oxidant to drive the reaction to completion.)
Example 3: Predict the product of 2-methyl-2-propanol + KMnO₄
Problem. Treat 2-methyl-2-propanol (tert-butyl alcohol) with hot aqueous KMnO4. What forms?
Step 1 — classify. The –OH carbon (central carbon of the tert-butyl group) bears three methyl groups and no hydrogen.
Step 2 — apply the rule. Oxidation requires removal of a C–H bond at the –OH carbon; there is none.
Answer. No oxidation occurs under normal conditions; the alcohol is recovered unchanged (this is why tert-butyl alcohol is used as a solvent in oxidations). Note the contrast: 2-methyl-2-propanol is the classic "unoxidizable" alcohol, whereas 2-propanol cleanly gives acetone.
Key takeaways
- Oxidation = loss of C–H bonds and/or gain of C–O bonds at the –OH carbon; the carbon skeleton is preserved.
- 1° + PCC (dry) → aldehyde; 1° + Jones/KMnO₄ (aqueous) → carboxylic acid.
- 2° + any oxidant → ketone (ketones resist further oxidation).
- 3° alcohols do not oxidize (no H on the –OH carbon).
- Jones reagent = CrO3/H2SO4 in aqueous acetone; mild enough to leave C=C intact.
- Chromium goes Cr(VI) → Cr(III) (3 e⁻ per Cr); the alcohol loses 2 e⁻ (1°→aldehyde or 2°→ketone) or 4 e⁻ (1°→acid).
- Biological oxidant: NAD⁺ → NADH; alcohol dehydrogenase converts ethanol → acetaldehyde.
- Aldehydes are more easily oxidized than alcohols — that is why anhydrous PCC is required to stop at the aldehyde.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What structural feature of a tertiary alcohol makes it resistant to oxidation?
Show answer
The –OH carbon has no hydrogen (three carbon groups attached, no C–H bonds), so there is no C–H to remove in forming a carbonyl.
Which reagent converts 1-butanol to butanal, and why does it not continue to butanoic acid?
Show answer
PCC in dry CH₂Cl₂. It is anhydrous, so the aldehyde cannot hydrate; hydration (gem-diol formation) is what allows further oxidation to the acid with aqueous reagents.
Jones reagent is used on 3-methyl-2-butanol. What is the major product?
Show answer
3-Methyl-2-butanone (a ketone). Jones oxidizes the 2° alcohol; the carbon skeleton (3-methylbutane with C=O at C2) is preserved.
How many grams of CrO3 (molar mass 100.00 g/mol) are needed to oxidize 0.300 mol of 2-butanol to 2-butanone (2 e⁻ per alcohol, 3 e⁻ per Cr)?
Show answer
0.300 mol alcohol × (2/3) = 0.200 mol CrO₃ × 100.00 g/mol = 20.0 g.
In the liver, ethanol is oxidized in two steps. Name the intermediates and the final product.
Show answer
Ethanol → acetaldehyde (alcohol dehydrogenase, NAD⁺) → acetic acid/acetate (aldehyde dehydrogenase, NAD⁺). The acid is then activated to acetyl-CoA for metabolism.
Why would a chemist choose NaOCl/TEMPO over Jones reagent for a large-scale oxidation?
Show answer
NaOCl/TEMPO avoids toxic chromium waste, is inexpensive, and runs safely on large scale in water; it gives the same aldehyde/ketone products with milder conditions.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Oxidation (organic)
- Gain of C–O bonds / loss of C–H bonds at carbon
- PCC
- Pyridinium chlorochromate, an anhydrous Cr(VI) reagent
- Jones reagent
- CrO3 + H2SO4 in aqueous acetone
- Chromate ester
- R–O–Cr(VI) intermediate in chromium oxidations
- NAD⁺ / NADH
- Nicotinamide adenine dinucleotide redox pair
- Aldehyde
- RCHO; the two-hydrogen oxidation product of a 1° alcohol
- TEMPO
- A nitroxyl radical catalyst used with NaOCl
- Alcohol dehydrogenase
- Enzyme that oxidizes alcohols with NAD⁺
- NAD⁺/NADH
- Nicotinamide adenine dinucleotide, oxidized/reduced forms
Sources & references
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