Preparing Carboxylic Acids by Oxidation

Introduction

The most common laboratory route to a carboxylic acid is oxidation. A primary alcohol can be oxidised first to an aldehyde and then, if the conditions allow, on to the carboxylic acid. The same chemistry turns wine into vinegar and explains why aldehydes are so easily detected by mild oxidising agents. This page connects the alcohol oxidation you met earlier with carboxylic acid formation, and shows how the choice of conditions decides whether the reaction stops at the aldehyde or goes all the way.

Core explanation

Oxidation levels. Counting bonds to oxygen at the functional carbon helps track oxidation. In a primary alcohol RCH₂OH the carbon has one C–O bond; in an aldehyde RCHO it has two (a double bond counts as two); in a carboxylic acid RCOOH it has three. Each step removes two hydrogen atoms or adds one oxygen atom. The final level, four bonds to oxygen, is carbon dioxide.

The two stages. Using [O] to represent the oxidising agent:

RCH₂OH + [O] → RCHO + H₂O (primary alcohol to aldehyde)

RCHO + [O] → RCOOH (aldehyde to carboxylic acid)

Overall: RCH₂OH + 2[O] → RCOOH + H₂O.

For ethanol: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.

The common oxidant. In school and university laboratories the classic reagent is potassium dichromate(VI) acidified with dilute sulfuric acid. The orange Cr₂O₇²⁻ ion is reduced to green Cr³⁺, giving a visible sign that oxidation has occurred. Acidified potassium manganate(VII) also oxidises primary alcohols and aldehydes to acids, turning from purple to colourless. Chromium(VI) compounds are toxic and carcinogenic, so modern practice favours smaller scales, careful containment and greener oxidants where possible.

Stopping or continuing. Aldehydes usually have lower boiling points than both the parent alcohol and the acid because they cannot hydrogen bond with each other. If the aldehyde is distilled out as soon as it forms, and the alcohol is in excess, it escapes further oxidation. If instead the mixture is heated under reflux with excess oxidant, the aldehyde vapour condenses back into the flask and is oxidised to the carboxylic acid. The acid can then be separated by distillation afterwards.

Why aldehydes oxidise easily. In aqueous acid an aldehyde is in equilibrium with its hydrate, RCH(OH)₂. The hydrate still has a C–H bond on a carbon bearing an OH group, just like an alcohol, so it is oxidised in the same way. Ketones lack this C–H bond, which is why they resist mild oxidation.

What cannot be oxidised to an acid. Secondary alcohols stop at ketones, and tertiary alcohols are not oxidised under these conditions because the carbinol carbon has no hydrogen. Only primary alcohols and aldehydes (and methanal, which can go further to CO₂) give carboxylic acids of the same carbon skeleton by this route.

Other oxidation routes. Alkyl side chains on benzene rings are oxidised by hot alkaline manganate(VII) to a –COOH group directly attached to the ring, so methylbenzene gives benzoic acid. Industrially, ethanoic acid is made mainly by carbonylation of methanol rather than oxidation, but oxidation of ethanol by bacteria remains the basis of vinegar production.

Step-by-step reasoning

To plan an oxidation to a carboxylic acid:

1. Check the starting material is a primary alcohol or an aldehyde. 2. Count carbons: the acid keeps the same carbon skeleton. 3. Choose conditions: excess acidified oxidant, heated under reflux. 4. Write the equation using [O], balancing H and O with water. 5. Predict the colour change of the oxidant as evidence of reaction.

Visual explanation

Imagine a ladder of oxidation levels drawn beside a carbon atom: CH₃OH-type carbon at the bottom rung, the aldehyde on the next, the acid above that and CO₂ at the top. Each rung climbed corresponds to swapping C–H bonds for C–O bonds, and reflux keeps the molecule on the ladder until it reaches the acid rung.

Real-world analogy

Distilling off the aldehyde is like taking a cake out of the oven the moment it rises; reflux is like leaving the lid on and the oven running until it is fully baked. The same ingredients give different products depending on whether the intermediate is allowed to escape.

Real-world example

Vinegar is made when Acetobacter bacteria oxidise the ethanol in wine or cider, using oxygen from air, via ethanal to ethanoic acid. Wine left open to the air turns sour for the same reason, and table vinegar typically contains about 4–8% ethanoic acid.

Why?

Why does reflux favour the carboxylic acid? The aldehyde intermediate is the most volatile component. In an open distillation it escapes, but under reflux it is condensed and returned to the hot mixture with excess oxidant, so it stays in contact with the reagent long enough to be oxidised again.

Common misconception

"Any alcohol can be oxidised to a carboxylic acid." Only primary alcohols give acids with the same carbon skeleton. Secondary alcohols stop at ketones and tertiary alcohols resist oxidation altogether under these conditions.

Worked example

Question: Butan-1-ol is heated under reflux with excess acidified potassium dichromate(VI). Name the organic product, write the equation and state the colour change.

Reasoning: Butan-1-ol is primary; reflux and excess oxidant take it past butanal to the acid, keeping four carbons.

Answer: Butanoic acid; CH₃CH₂CH₂CH₂OH + 2[O] → CH₃CH₂CH₂COOH + H₂O; orange to green.

Quick check

1. Which organic compound is formed when propanal is warmed with acidified potassium dichromate(VI), and what colour change is observed? Answer: Propanoic acid forms, and the solution turns from orange to green.

Exam focus

Distinguish clearly between "distil immediately" (aldehyde) and "heat under reflux with excess oxidant" (acid). Use [O] in equations, balance with water, and quote the orange-to-green colour change. Be ready to explain why tertiary alcohols give no colour change.

Advanced insight

Chromium(VI) oxidation proceeds through a chromate ester, R–O–CrO₃H, which breaks down when a base removes the C–H proton next to the oxygen. Anhydrous oxidants such as pyridinium chlorochromate stop at the aldehyde because, without water, no hydrate forms, so there is no alcohol-like carbon for the second oxidation.

Summary

Primary alcohols are oxidised first to aldehydes and then to carboxylic acids, each step increasing the number of bonds from carbon to oxygen. Heating under reflux with excess acidified dichromate(VI) gives the acid; immediate distillation gives the aldehyde. Aldehydes oxidise easily through their hydrates, whereas ketones and tertiary alcohols resist mild oxidation.

Practice questions

1. Write an equation, using [O], for the oxidation of methanol to methanoic acid. Answer: CH₃OH + 2[O] → HCOOH + H₂O. 2. Explain why 2-methylpropan-2-ol gives no colour change with acidified dichromate(VI). Answer: It is a tertiary alcohol; the carbon bearing OH has no hydrogen atom to remove, so it is not oxidised and the dichromate stays orange. 3. Which alcohol would you oxidise to make 3-methylbutanoic acid? Answer: 3-methylbutan-1-ol, the primary alcohol with the same carbon skeleton. 4. Why is ethanal more volatile than both ethanol and ethanoic acid? Answer: Ethanal molecules cannot form hydrogen bonds with each other, whereas ethanol and ethanoic acid molecules can, so ethanal has weaker intermolecular forces.