Describe the oxidation of primary and secondary alcohols to carbonyl compounds and carboxylic acids using oxidising agents

What this dot point is asking

The oxidation of alcohols is one of the most examined organic reactions because the product reveals the class of the starting alcohol. Oxidation removes hydrogen from the carbon bearing the -OH group, converting C-OH into a C=O.

The oxidising agents

The usual reagent is acidified potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$ in dilute sulfuric acid), which is orange and turns green as the chromium is reduced from $+6$ (orange dichromate) to $+3$ (green chromium(III)). Acidified potassium permanganate ($\text{KMnO}_4$) can also be used; it is purple and turns colourless. The colour change is the visual evidence of oxidation.

Primary alcohols

A primary alcohol oxidises in two stages:

For example, ethanol oxidises to ethanal (an aldehyde), then to ethanoic acid (a carboxylic acid). To stop at the aldehyde you distil it off as it forms (aldehydes are volatile); to obtain the carboxylic acid you heat under reflux with excess oxidant so the reaction goes to completion.

Secondary alcohols

A secondary alcohol oxidises to a ketone, which resists further oxidation because there is no hydrogen on the carbonyl carbon to remove. For example, propan-2-ol oxidises to propanone.

Tertiary alcohols

A tertiary alcohol is not oxidised by acidified dichromate or permanganate. The carbon carrying -OH has no hydrogen, so there is nothing to remove, and the oxidant colour does not change. This lack of reaction is itself diagnostic.

Distinguishing aldehydes from ketones

Because aldehydes can be oxidised further (to carboxylic acids) but ketones cannot, you can distinguish them with a mild oxidant. An aldehyde will reduce a mild oxidising agent, whereas a ketone will not react, which provides a chemical test to tell the two carbonyl families apart.

Why the carbonyl carbon stops a ketone oxidising

It is worth understanding why oxidation halts at the ketone for secondary alcohols but continues for primary alcohols. Each oxidation step removes a hydrogen from the carbon that bears the oxygen. In an aldehyde ($\text{R-CHO}$) the carbonyl carbon still carries a hydrogen, so a mild oxidant can remove it and add an $\text{-OH}$, forming the carboxylic acid. In a ketone ($\text{R-CO-R}'$) the carbonyl carbon is bonded to two other carbons and has no hydrogen left to remove, so further oxidation would require breaking a strong $\text{C-C}$ bond, which acidified dichromate cannot do. This single structural idea, "is there still a hydrogen on the carbonyl carbon?", explains the whole pattern of which carbonyl compounds oxidise further.

Why this matters

Oxidation of alcohols is a key synthetic step linking alcohols to aldehydes, ketones and carboxylic acids, and the products identify the class of an unknown alcohol. It connects to the alcohols topic, to carboxylic acids and esters, and to the spectroscopic identification of carbonyl compounds in instrumental analysis.