Questions: Oxidation of Alcohols to Aldehydes and Ketones

Question 1

A chemist needs to convert a primary alcohol to an aldehyde and must avoid any carboxylic acid product. Which reagent and conditions should they choose?

• A. Jones oxidation (CrO₃ in aqueous H₂SO₄) — a powerful, reliable oxidant
• B. PCC (pyridinium chlorochromate) in anhydrous CH₂Cl₂ — mild, non-aqueous conditions
• C. Acidic potassium permanganate — strong oxidant for difficult substrates
• D. Concentrated H₂SO₄ — dehydrates the alcohol to an alkene instead

Answer: (B)

PCC in anhydrous CH₂Cl₂ stops cleanly at the aldehyde because there is no water present to facilitate further oxidation of the aldehyde intermediate to a carboxylic acid. Jones oxidation is powerful but aqueous and acidic — the aldehyde is immediately over-oxidized to the carboxylic acid under those conditions. Acidic KMnO₄ similarly over-oxidizes. Concentrated H₂SO₄ is not an oxidant for this purpose. DMP and Swern oxidation are equally valid choices to PCC for this goal.

Question 2

Why does oxidation of a secondary alcohol stop at the ketone stage, while oxidation of a primary alcohol can proceed further to a carboxylic acid?

• A. Ketones are thermodynamically more stable products and resist further reaction
• B. The ketone's carbonyl carbon has no remaining C–H bond, so there is no hydrogen to remove in a subsequent oxidation step
• C. Secondary alcohols react more slowly with oxidizing agents than primary alcohols
• D. Ketones immediately form stable hydrates that protect them from further oxidation

Answer: (B)

Oxidation of carbon involves removing hydrogen from the C–OH unit. An aldehyde (product of primary alcohol oxidation) still has one C–H at the carbonyl carbon — this is the handle for a second oxidation to carboxylic acid. A ketone has NO hydrogen on the carbonyl carbon (both bonds go to carbon substituents), so there is structurally nothing left to remove. The reaction simply cannot proceed further. Thermodynamic stability and reaction rate are not the correct mechanistic explanation.

Question 3

A tertiary alcohol such as 2-methyl-2-propanol can be oxidized to a ketone if a sufficiently strong oxidizing agent is used.

• T. True
• F. False

Answer: False

Tertiary alcohols have zero hydrogen atoms on the carbon bonded to the –OH group. Oxidation of a C–OH unit requires removing that C–H hydrogen to form the C=O double bond. With no such hydrogen available, the reaction simply cannot occur. No amount of oxidant strength overcomes this structural limitation under standard conditions. Tertiary alcohols resist oxidation unless the molecule undergoes degradation via entirely different mechanisms.

Question 4

Under mild, anhydrous conditions such as PCC in dichloromethane, a primary alcohol is oxidized selectively to the aldehyde without further oxidation to the carboxylic acid.

• T. True
• F. False

Answer: True

The key to stopping at the aldehyde is removing water from the system. In aqueous acidic conditions (Jones oxidation), the aldehyde intermediate is in equilibrium with its hydrate (geminal diol), which is then oxidized to the carboxylic acid. PCC in anhydrous CH₂Cl₂ eliminates this pathway: without water, no hydrate forms, and the aldehyde is the terminal product. DMP and Swern oxidation work by the same logic — mild, non-aqueous conditions prevent over-oxidation.

Question 5

A student treats 2-methyl-2-propanol (a tertiary alcohol) with Jones oxidation conditions and observes no carbonyl product. Explain why.

Model answer: 2-Methyl-2-propanol is a tertiary alcohol — the carbon bearing the –OH group has three carbon substituents and zero hydrogens. Oxidation of an alcohol to a carbonyl compound proceeds by removing a hydrogen from that carbon along with the –OH hydrogen to form the C=O double bond. With no C–H bond available at the carbinol carbon, there is nothing to abstract, and the oxidation cannot occur under standard conditions regardless of oxidant strength.

This question tests whether students understand that the structural classification of an alcohol (primary/secondary/tertiary) is not just a naming convention but directly determines what chemistry is possible. The number of hydrogens on the carbinol carbon controls oxidation reactivity. Jones oxidation is a strong enough oxidant that it would readily react if there were a C–H bond present — its failure here confirms the structural argument, not an issue of reagent potency.