Part 1: Properties, classification and oxidation

Use hydrogen bonding to explain properties and the carbon bearing OH to predict oxidation products.

Hydrogen bonding changes volatility and solubility

Alcohols contain an OH group bonded to a saturated carbon. Their molecules hydrogen bond to each other, so they generally boil higher and are less volatile than comparable hydrocarbons. London forces also increase with size; compare similar molecules when isolating the OH effect.

Small alcohols mix well with water through hydrogen bonding. As the hydrocarbon section grows, water solubility generally decreases. The general formula CnH2n+1OH applies to saturated acyclic monohydric alcohols, not every compound with an OH group.

Oxygen is more electronegative than carbon and hydrogen, so the C–O and O–H bonds are polar. The Oδ− and Hδ+ of the OH group, together with oxygen’s lone pairs, enable hydrogen bonding.

Count carbon neighbours of the C–OH carbon

A primary alcohol has one carbon group attached to the carbon bearing OH, a secondary two and a tertiary three. Do not classify by the total carbon count or OH position alone. Methanol is a special zero-carbon-neighbour case that follows the primary oxidation pattern.

Propan-1-ol, CH₃CH₂CH₂OH, is primary. Propan-2-ol, CH₃CH(OH)CH₃, is secondary. 2-Methylpropan-2-ol, (CH₃)₃COH, is tertiary. The last has no H attached to its C–OH carbon.

Primary, secondary and tertiary behaviour

Use acidified potassium or sodium dichromate(VI), with dilute sulfuric acid, and suitable heating. Reduction of orange dichromate to green Cr³⁺ accompanies oxidation of a suitable alcohol. Write [O] for one oxygen equivalent in the organic equation, rather than introducing an unbalanced dichromate formula.

A primary alcohol oxidises to an aldehyde, then a carboxylic acid. A secondary alcohol oxidises to a ketone, which resists further oxidation under these conditions. Tertiary alcohols resist oxidation by this reagent under ordinary laboratory conditions because the C–OH carbon lacks a hydrogen; oxidation would require breaking the carbon framework.

Conditions decide how far oxidation goes

An aldehyde is more easily protected from further oxidation by distilling it out as it forms. Reflux instead returns volatile material to the reacting flask and allows prolonged reaction with excess oxidant. The same primary alcohol can therefore give different isolated products.

H032/01 June 2025 Q25(a) highlighted mistakes with full oxidation and carbonyl connectivity. Keep the same carbon skeleton and remove/add the correct atoms: an internal carbonyl carbon cannot retain an extra H as well as two carbon neighbours and a double-bonded O.

A branched alcohol is not automatically tertiary

In (CH₃)₂CHCH₂OH, find the carbon actually bonded to oxygen: it is CH₂OH. That carbon has only one carbon neighbour, so 2-methylpropan-1-ol is primary even though the molecule has a branch. In (CH₃)₃COH, the OH-bearing carbon has three carbon neighbours and no hydrogen, so 2-methylpropan-2-ol is tertiary.

This local classification predicts the usual oxidation pattern. Primary alcohols have two H atoms on the C–OH carbon before oxidation, secondary alcohols have one, and tertiary alcohols have none. Count neighbours rather than using the total number of carbons or the apparent length of the name.

Track the same carbon from alcohol to carbonyl

For 2-methylpropan-1-ol, (CH₃)₂CHCH₂OH, the terminal CH₂OH carbon becomes the aldehyde carbon in (CH₃)₂CHCHO, 2-methylpropanal. With full oxidation it becomes the acid carbon in (CH₃)₂CHCOOH, 2-methylpropanoic acid. The branch remains on the adjacent carbon; oxidation does not move it.

For pentan-2-ol, CH₃CH(OH)CH₂CH₂CH₃, the C–OH carbon loses its H while O–H also loses H, and a C=O bond forms. The product is CH₃COCH₂CH₂CH₃, pentan-2-one. The carbonyl carbon already has two single bonds to carbon and a double bond to oxygen: adding an H there would exceed valency four.

In the [O] equations, one oxygen equivalent removes two H atoms as water in alcohol-to-carbonyl oxidation. Aldehyde-to-acid oxidation then adds a further oxygen equivalent. Balance the complete organic equation rather than treating [O] as a condition written without consequence.

What the orange-to-green change can and cannot identify

Under the specified oxidation conditions, an orange-to-green change supports reduction of dichromate by an oxidisable substance. Both primary and secondary alcohols can give it, so colour alone does not distinguish them. Determine the product or combine the observation with structural information.

In a comparison restricted to known alcohols, no change under an effective heated test is consistent with a tertiary alcohol. Without that restriction, a negative result does not uniquely prove the tertiary family. Check reagent condition, heating and whether the sample contacted the reagent before drawing the inference.

When comparing boiling and solubility, remember there are two competing structural features: OH allows hydrogen bonding, while the hydrocarbon region becomes increasingly significant with chain length. Do not explain poor water solubility of a larger alcohol by claiming its O–H bond has stopped being polar.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources