Alcohol oxidation is an important organic reaction. Primary alcohols (R-CH2-OH) can be oxidized either to aldehydes (R-CHO) or to carboxylic acids (R-CO2H), while the oxidation of secondary alcohols (R1R2CH-OH) normally terminates at the ketone (R1R2C=O) stage. Tertiary alcohols (R1R2R3C-OH) are resistant to oxidation.[1]

The direct oxidation of primary alcohols to carboxylic acids normally proceeds via the corresponding aldehyde, which is transformed via an aldehyde hydrate (R-CH(OH)2) by reaction with water before it can be further oxidized to the carboxylic acid.

Often it is possible to interrupt the oxidation of a primary alcohol at the aldehyde level by performing the reaction in absence of water, so that no aldehyde hydrate can be formed.

Primary alcohols

Primary alcohols can be oxidised to either aldehydes or carboxylic acids depending on the reaction conditions. In the case of the formation of carboxylic acids, the alcohol is first oxidised to an aldehyde which is then oxidised further to the acid.

Partial oxidation to aldehydes

You get an aldehyde if you use an excess of the alcohol, and distil off the aldehyde as soon as it forms.

The excess of the alcohol means that there isn't enough oxidising agent present to carry out the second stage. Removing the aldehyde as soon as it is formed means that it doesn't hang around waiting to be oxidised anyway!

If you used ethanol as a typical primary alcohol, you would produce the aldehyde ethanal, CH3CHO.

The full equation for this reaction is fairly complicated, and you need to understand about electron-half-equations in order to work it out.

In organic chemistry, simplified versions are often used which concentrate on what is happening to the organic substances. To do that, oxygen from an oxidising agent is represented as [O]. That would produce the much simpler equation:

It also helps in remembering what happens. You can draw simple structures to show the relationship between the primary alcohol and the aldehyde formed.

Important!  This is not intended to suggest any sort of mechanism for the reaction - it is just a way of helping you to remember what happens.

If you are in the UK A level system (or its equivalent), it is highly likely that your examiners will accept equations involving [O]. To be sure, consult your syllabus, past papers and mark schemes. If you are studying a UK-based syllabus and haven't got any of these things, follow this link to find out how to get them.

Full oxidation to carboxylic acids

1. Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid with the structural formula CH3CH2OH, often abbreviated as C2H5OH or C2H6O. Wikipedia

2. Formula: C2H6ODensity: 789.00 kg/m³Boiling point: 78.37 °CMelting point: -114 °CIUPAC ID: ethanolMolar mass: 46.06844 g/mol

All alcohols can be completely oxidized to carbon dioxide and water by oxygen in the air; that is, all

alcohols are combustible. Like hydrocarbons, combustion is an important reaction of alcohols, but more

controlled oxidation is even more important, because it can convert alcohols into other compounds that

are useful to society. The ease with which an alcohol can be oxidized and the extent of the oxidation

depends on whether the alcohol is primary, secondary, or tertiary.

For primary alcohols, controlled, stepwise oxidation first yields compounds called aldehydes; if more of

the oxidizing agent is available, then aldehydes can be further oxidized to carboxylic acids. SchematicallyPrimary alcohol → aldehyde → carboxylic acid

Oxidation of an organic compound can usually be recognized because either an oxygen atom is added to

a molecule or two hydrogen atoms are lost from a molecule. For example, stepwise oxidation of ethanol

first produces the aldehyde ethanal (commonly called acetaldehyde); further oxidation produces the

carboxylic acid, ethanoic acid (commonly called acetic acid). An aldehyde has the functional group –CHO,

where the carbon atom is double-bonded to an oxygen atom. A carboxylic acid has the functional group –

COOH, in which the carbon atom is double-bonded to an oxygen atom and single-bonded to an oxygen

atom in an OH group. The structures below show the differences between ethanol, ethanal, and ethanoic

acid.