How are alcohols classified, and what reactions do they undergo?
Classify alcohols as primary, secondary or tertiary and describe their characteristic reactions including oxidation, dehydration and combustion
A focused answer to the WACE Year 12 Chemistry dot point on alcohols, their classification as primary, secondary and tertiary, and their key reactions including oxidation to carbonyl compounds, dehydration to alkenes, and combustion, with a worked example and common exam mistakes.
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What this dot point is asking
An alcohol contains the hydroxyl functional group, -OH, attached to a carbon atom. The -OH group makes alcohols polar and able to hydrogen bond, and it is the reactive site for their characteristic reactions.
Classifying alcohols
Alcohols are classified by the number of carbon atoms bonded to the carbon that carries the -OH group:
- Primary (1 degree): the -OH carbon is attached to one other carbon (for example ethanol).
- Secondary (2 degrees): the -OH carbon is attached to two other carbons (for example propan-2-ol).
- Tertiary (3 degrees): the -OH carbon is attached to three other carbons (for example 2-methylpropan-2-ol).
Oxidation
Oxidation uses an oxidising agent such as acidified potassium dichromate (orange, turning green) or permanganate (purple, turning colourless).
- Primary: ethanol oxidises first to ethanal (an aldehyde), then with further oxidation to ethanoic acid (a carboxylic acid).
- Secondary: propan-2-ol oxidises to propanone (a ketone), which does not oxidise further easily.
- Tertiary: no reaction; the carbon bearing -OH has no hydrogen to lose.
Dehydration
Heating an alcohol with a suitable acid catalyst (such as concentrated sulfuric acid) removes a water molecule to form an alkene. This is an elimination reaction; for example ethanol dehydrates to ethene:
Combustion
Like all organic compounds, alcohols burn. Complete combustion in plentiful oxygen gives carbon dioxide and water and releases a large amount of energy, which is why ethanol is used as a fuel:
Why this matters
Alcohols sit at the centre of organic synthesis: they can be made from alkenes (hydration) or haloalkanes (substitution), and converted into aldehydes, ketones, carboxylic acids, esters and alkenes. Their reactions are essential building blocks for the multi-step pathways examined in Unit 4.