primary, secondary, and tertiary alcohols

Not all alcohols behave the same way, and the difference often comes down to one thing: how crowded the carbon bearing the -OH is. We sort alcohols into primary (1°), secondary (2°), and tertiary (3°) by counting how many other carbons are attached to that carbinol carbon — the carbon that holds the -OH.

Count the carbon neighbours of the C-OH carbon. If it is bonded to just one other carbon (the rest of its bonds being to hydrogens or the OH), the alcohol is primary, as in ethanol, CH3-CH2-OH. If it is bonded to two other carbons it is secondary, like isopropanol, (CH3)2CH-OH. If it is bonded to three other carbons it is tertiary, like tert-butanol, (CH3)3C-OH. Methanol, CH3-OH, sits in a class of its own (no carbon neighbours) and usually behaves like a primary alcohol.

This label is not bookkeeping — it predicts reactivity. Oxidation cares a lot: primary alcohols can be oxidised to aldehydes or carboxylic acids, secondary to ketones, while tertiary alcohols resist oxidation entirely because the carbinol carbon has no hydrogen to remove. Substitution and dehydration mechanisms also split along these lines — tertiary substrates favour SN1/E1 pathways through stable carbocations, primary ones favour SN2/E2. So the first thing a chemist asks about an alcohol is: 1°, 2°, or 3°?