Properties of alcohols

The essentials

Alcohols have higher boiling points than hydrocarbons, ethers and haloalkanes of similar molar mass because of intermolecular hydrogen bonding. Lower alcohols are soluble in water because they hydrogen bond with water; solubility falls as the hydrocarbon part grows. Boiling point rises with the number of carbon atoms and falls with branching.

Key points

• O–H cleavage: alcohols react with active metals (Na, K) to give alkoxides and H₂, showing they are weak acids. Acidity order: 1° > 2° > 3°, since alkyl groups push electron density onto oxygen. Alcohols are weaker acids than water.
• Esterification: alcohol + carboxylic acid (conc. H₂SO₄) ⇌ ester + water; acid chlorides and anhydrides also give esters.
• C–O cleavage: with HX gives alkyl halides. Lucas test (conc. HCl + anhydrous ZnCl₂): tertiary alcohols give turbidity at once; primary alcohols give no turbidity at room temperature; secondary alcohols are in between.
• Dehydration: heating with conc. H₂SO₄ or H₃PO₄ gives alkenes; ethanol at 443 K gives ethene. Ease: 3° > 2° > 1°.
• Oxidation: primary alcohols give aldehydes (with PCC or CrO₃ in anhydrous medium) and then carboxylic acids (with acidified KMnO₄); secondary alcohols give ketones; tertiary alcohols resist oxidation.
• Heated copper at 573 K: primary alcohols give aldehydes, secondary give ketones, tertiary alcohols undergo dehydration to alkenes.

Exam tips

• Lucas test speed follows carbocation stability, so it distinguishes 1°, 2° and 3° alcohols.

Common mistakes

• Thinking alcohols are stronger acids than water; sodium alkoxides are strong bases that react with water.