Classify alcohols as primary, secondary or tertiary and describe their characteristic reactions including oxidation, dehydration and combustion

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:

Reflux versus distillation in oxidation

A practical detail WACE rewards is controlling how far a primary alcohol oxidises. To stop at the aldehyde, the aldehyde is distilled off as it forms, because its boiling point is lower than the alcohol's and it escapes the mixture before it can be oxidised further. To go all the way to the carboxylic acid, the mixture is heated under reflux (vapours condense and return to the flask) with excess oxidising agent, so the aldehyde stays in contact with the dichromate and is oxidised completely. The same reagent therefore gives different products depending on the apparatus, which is a classic short-answer point.

Physical properties from the hydroxyl group

The $\text{-OH}$ group also explains the physical behaviour of alcohols. Because $\text{-OH}$ allows hydrogen bonding between molecules, alcohols have much higher boiling points than alkanes of similar molar mass and the smaller alcohols (methanol, ethanol, propanol) are fully miscible with water. As the carbon chain lengthens, the non-polar hydrocarbon part dominates and water solubility falls, so longer-chain alcohols behave more like hydrocarbons. This trend connects directly to the intermolecular-forces reasoning used throughout Unit 4.

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.