Classification of alcohols

(a) According to the number of –OH groups

• Monohydric: one –OH: CH₃OH, C₂H₅OH.
• Dihydric: two –OH: ethylene glycol HOCH₂CH₂OH (1,2-ethanediol).
• Trihydric: three –OH: glycerol (glycerine) HOCH₂–CH(OH)–CH₂OH (1,2,3-propanetriol).
• Polyhydric: more than three –OH: sorbitol (six –OH groups).

(b) According to the type of carbinol carbon (the carbon carrying –OH), for monohydric alcohols

• Primary (1°): carbinol carbon is attached to one other carbon (and two H): R–CH₂OH, e.g. ethanol, 1-propanol.
• Secondary (2°): carbinol carbon is attached to two carbons (and one H): R₂CHOH, e.g. 2-propanol, 2-butanol.
• Tertiary (3°): carbinol carbon is attached to three carbons (no H): R₃C–OH, e.g. 2-methyl-2-propanol (tert-butyl alcohol). (Methanol, the exception, has the carbinol carbon attached to no carbon and is counted with the primary alcohols.)