Alcohols

Alcohols can be made in several ways (many revisit earlier topics):

• Hydration of an alkene: steam (H₂O(g)) + H₃PO₄ catalyst → alcohol (14.2).
• Cold dilute acidified KMnO₄ on an alkene → a diol.
• Substitution of a halogenoalkane: RX + NaOH(aq), heat → ROH (15.1).
• Reduction of an aldehyde or ketone: NaBH₄ or LiAlH₄ → alcohol (17.1).
• Reduction of a carboxylic acid: LiAlH₄ → primary alcohol (18.1).
• Hydrolysis of an ester: dilute acid or alkali, heat → alcohol (+ acid/salt) (18.2).

Reaction	Reagents / conditions	Product
Combustion	O₂	CO₂ + H₂O
With sodium	Na(s)	sodium alkoxide + H₂
Substitution to halogenoalkane	HX; or KCl + conc H₂SO₄; or conc H₃PO₄; or PCl₃ + heat; or PCl₅; or SOCl₂	halogenoalkane
Dehydration	Al₂O₃ (heated) or conc H₂SO₄/H₃PO₄	alkene + H₂O
Esterification	carboxylic acid + conc H₂SO₄ (catalyst)	ester + H₂O

With sodium: $2\text{CH}_3\text{CH}_2\text{OH} + 2\text{Na} \rightarrow 2\text{CH}_3\text{CH}_2\text{ONa} + \text{H}_2$ (effervescence — gentler than Na with water).

Useful test note: PCl₅ with an alcohol gives steamy HCl fumes (a test for an –OH group).

Heating an alcohol with acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄) — orange — oxidises it (the dichromate turns green, Cr³⁺):

• Primary alcohol → aldehyde (if you distil off as it forms) → carboxylic acid (if you reflux for full oxidation). Uses [O] from the oxidising agent: $\text{CH}_3\text{CH}_2\text{OH} + [\text{O}] \rightarrow \text{CH}_3\text{CHO} + \text{H}_2\text{O}$ $\text{CH}_3\text{CHO} + [\text{O}] \rightarrow \text{CH}_3\text{COOH}$
• Secondary alcohol → ketone (cannot be oxidised further easily): $\text{CH}_3\text{CH(OH)CH}_3 + [\text{O}] \rightarrow \text{CH}_3\text{COCH}_3 + \text{H}_2\text{O}$
• Tertiary alcohol → not oxidised (no H on the carbon bearing –OH). The dichromate stays orange.

Distinguishing test: add acidified K₂Cr₂O₇ and warm — if it turns orange → green, the alcohol is primary or secondary; if it stays orange, it is tertiary. (Distillation isolates the aldehyde; reflux gives the acid.)

Iodoform (tri-iodomethane) test. Warming with alkaline aqueous iodine (I₂/NaOH) gives a pale yellow precipitate of CHI₃ if the molecule contains a CH₃CH(OH)– group (or CH₃CO–). It produces the carboxylate ion RCO₂⁻ as well: $\text{CH}_3\text{CH(OH)R} + \text{I}_2/\text{OH}^- \rightarrow \text{CHI}_3 + \text{RCO}_2^- + \dots$ e.g. ethanol and propan-2-ol give a positive test; methanol and propan-1-ol do not.

Acidity of alcohols vs water. Alcohols are weakly acidic (they react with reactive metals like Na), but they are weaker acids than water. This is because the alkyl group is electron-donating (+I effect): it pushes electron density onto the alkoxide ion (RO⁻), intensifying its negative charge and making it less stable. A less stable conjugate base means the dissociation is less favourable → alcohols are less acidic than water.