Questions: Oxidation Reactions in Organic Chemistry

Question 1

A student needs to convert a primary alcohol to an aldehyde for a synthesis. They choose Jones reagent (CrO₃/H₂SO₄), reasoning it is the most powerful and reliable chromium oxidant. What product do they most likely obtain?

• A. The desired aldehyde — Jones reagent is selective for the first oxidation
• B. A carboxylic acid — Jones reagent over-oxidizes primary alcohols past the aldehyde stage
• C. A ketone — Jones reagent rearranges primary alcohols under acidic conditions
• D. No reaction — Jones reagent only oxidizes secondary alcohols

Answer: (B)

Jones reagent operates in aqueous acidic conditions, which allows the aldehyde product to hydrate into a geminal diol. That diol has a new C–H bond that gets oxidized, pushing the reaction all the way to a carboxylic acid. The student should have used PCC or Swern oxidation — not because they are 'weaker,' but because they work in anhydrous conditions that prevent aldehyde hydration and therefore prevent over-oxidation.

Question 2

Why does PCC stop the oxidation of a primary alcohol at the aldehyde stage rather than continuing to the carboxylic acid?

• A. PCC contains less chromium and therefore runs out of oxidizing capacity after the first step
• B. PCC cannot react with aldehydes or ketones at all
• C. PCC is used in anhydrous CH₂Cl₂, preventing the aldehyde from forming a hydrate intermediate that would expose a new C–H for oxidation
• D. The pyridine in PCC acts as a proton scavenger that deactivates the chromium after one oxidation

Answer: (C)

The key is conditions, not intrinsic strength. In anhydrous CH₂Cl₂, the aldehyde product cannot form a geminal diol hydrate, so there is no new C–H bond for further oxidation. Jones reagent, by contrast, operates in aqueous conditions where hydrate formation is possible, enabling the second oxidation to carboxylic acid. This illustrates the broader principle: selectivity is controlled by reaction conditions, not reagent 'strength' alone.

Question 3

Tertiary alcohols resist oxidation under normal conditions because they lack a hydrogen atom on the carbon bearing the hydroxyl group.

• T. True
• F. False

Answer: True

Oxidation of an alcohol to a carbonyl requires removal of the C–H bond on the carbon that bears the OH. Primary alcohols have two such hydrogens, secondary alcohols have one, and tertiary alcohols have none — the carbon is fully substituted. With no C–H to remove, the oxidation cannot proceed by the standard chromate ester mechanism. Strong oxidants like hot concentrated KMnO₄ can cleave the C–C bond instead, but this is a different reaction (oxidative cleavage), not simple alcohol oxidation.

Question 4

PCC is a weaker oxidant than Jones reagent, which is why it stops at the aldehyde stage when oxidizing primary alcohols.

• T. True
• F. False

Answer: False

This is the central misconception. PCC stops at the aldehyde not because of inherent weakness but because of reaction conditions. PCC is used in anhydrous dichloromethane, which prevents the aldehyde from hydrating into a geminal diol intermediate. Without that hydrate, the second oxidation cannot occur. Jones reagent in aqueous acid allows hydration, enabling over-oxidation. The principle: the same transformation at a different oxidation state becomes accessible because the conditions change what intermediates are available.

Question 5

In terms of oxidation-state bookkeeping, explain the progression from a primary alcohol → aldehyde → carboxylic acid. What changes at each step, and why does this progression stop at the carboxylic acid stage?

Model answer: At each step, one C–H bond is broken and one new C–O bond forms, increasing the carbon's oxidation state by approximately +1. A primary alcohol carbon (one C–O bond, two C–H bonds) steps up to an aldehyde (two C–O bonds via C=O, one C–H). The aldehyde then steps up to a carboxylic acid (two C–O bonds plus an OH, no C–H on the carbonyl carbon). The progression stops at the carboxylic acid because the carbonyl carbon no longer has any C–H bonds to remove — the ladder has no more rungs. Further oxidation would require C–C bond cleavage.

This oxidation-state framework is the essential tool for retrosynthetic analysis: working backward from a target, you ask what oxidation state the relevant carbon was in the starting material and which reagent achieves that specific transition. PCC for +1 step (alcohol → aldehyde), Jones for +2 steps (alcohol → carboxylic acid). Recognizing that each oxidation step consumes one C–H bond is what makes tertiary alcohol resistance immediately intuitive — no C–H, no oxidation.