Classification and nomenclature

The essentials

Alcohols and phenols are classified by the number of –OH groups: monohydric (one), dihydric (two, e.g. ethane-1,2-diol or ethylene glycol) and trihydric (three, e.g. propane-1,2,3-triol or glycerol). Monohydric alcohols with –OH on an sp³ carbon are primary, secondary or tertiary depending on how many carbon atoms are attached to the carbinol carbon. Allylic and benzylic alcohols are special cases. Ethers are simple (symmetrical, both groups the same) or mixed (unsymmetrical).

Key points

• IUPAC: replace the final "e" of the alkane by "ol": CH₃CH(OH)CH₃ is propan-2-ol; (CH₃)₃COH is 2-methylpropan-2-ol.
• Phenols: C₆H₅OH is phenol; methylphenols are cresols. Benzene-1,2-diol is catechol, benzene-1,3-diol is resorcinol and benzene-1,4-diol is hydroquinone (quinol).
• Ethers are named as alkoxyalkanes, the larger group being the parent: CH₃OC₂H₅ is methoxyethane; C₆H₅OCH₃ is methoxybenzene (anisole).
• Structure: in alcohols the oxygen is sp³ hybridised and the C–O–H angle is slightly less than tetrahedral (108.9° in methanol) because of lone pair repulsion. In ethers the C–O–C angle is slightly more than tetrahedral (111.7° in dimethyl ether) because of repulsion between bulky groups.
• In phenol the C–O bond (136 pm) is shorter than in methanol (142 pm) because of partial double-bond character from resonance and the sp² carbon.

Exam tips

• Count the carbon atoms attached to the carbon carrying –OH to classify an alcohol.

Common mistakes

• Calling C₆H₅CH₂OH a phenol; it is benzyl alcohol, a primary benzylic alcohol.