Alcohols are classified as primary, secondary and tertiary.

The idea

The class of an alcohol is decided only at the carbon carrying the hydroxyl group. Count the alkyl groups attached to that carbon: one gives a primary alcohol, two a secondary, three a tertiary. Methanol has none and is counted as primary. Nothing else matters, so the chain length, the total number of carbons and any branch further along are irrelevant. Butan-1-ol and 2-methylpropan-1-ol are both primary, despite the branch in the second; butan-2-ol is secondary; 2-methylpropan-2-ol is tertiary. The classification is worth getting right because it predicts everything that follows: primary alcohols can be oxidised twice, secondary once, tertiary not at all under the usual conditions.

What earns the marks

• Counting the alkyl groups attached to the carbon bearing the hydroxyl group.
• The correct classification of a named or drawn alcohol.
• A correctly drawn example of each class with the same molecular formula.
• Linking the class to the product obtained on oxidation.

Common mistakes and how to put them right

• Classifying from the locant in the name, so calling butan-2-ol secondary but 2-methylpropan-1-ol secondary as well.

• Counting hydrogen atoms on the carbon instead of alkyl groups.

• Calling methanol secondary or tertiary because it has no alkyl group, when it is primary.

• Letting a branch elsewhere in the chain change the classification.

Exam tips

• Circle the carbon holding the hydroxyl group first, then count only what is bonded to that carbon.
• Draw the structure out fully before classifying; names alone invite mistakes.
• Use the classification to predict the oxidation product, since the two are almost always tested together.