Producing alcohols

· Alkenes → alcohols by electrophilic addition of steam: H₂O(g), H₃PO₄ catalyst.
· Alkenes → diols using cold dilute acidified KMnO₄.
· Halogenoalkanes → alcohols by nucleophilic substitution with NaOH(aq), heat.
· Aldehydes/ketones → alcohols by reduction using NaBH₄ or LiAlH₄.
· Carboxylic acids → primary alcohols by reduction using LiAlH₄.
· Esters → alcohol + carboxylic acid/salt by hydrolysis using dilute acid or dilute alkali, heat.

This diagram shows examples of molecules containing the alcohol functional group, –OH. It is useful for recognising alcohols from displayed or skeletal formulae. Source

Classification of alcohols

· Primary alcohol: carbon bonded to –OH is attached to one carbon atom; general form RCH₂OH.
· Secondary alcohol: carbon bonded to –OH is attached to two carbon atoms; general form R₂CHOH.
· Tertiary alcohol: carbon bonded to –OH is attached to three carbon atoms; general form R₃COH.
· Molecules may contain more than one alcohol group, e.g. diols.
· Classification is essential for predicting oxidation products.

These images help compare primary, secondary and tertiary alcohols by looking at the carbon attached to the –OH group. This is the key structural feature tested in oxidation and identification questions. Source

Reactions of alcohols

· Combustion: alcohol + O₂ → CO₂ + H₂O in complete combustion.
· Substitution to halogenoalkanes: replace –OH with halogen using HX(g), KCl + concentrated H₂SO₄/H₃PO₄, PCl₃ + heat, PCl₅, or SOCl₂.
· With Na(s): alcohol + sodium → sodium alkoxide + H₂(g).
· Dehydration: alcohol → alkene + H₂O using heated Al₂O₃ or concentrated acid.
· Esterification: alcohol + carboxylic acid ⇌ ester + water, using concentrated H₂SO₄ catalyst.

The diagram shows ester formation from an alcohol and a carboxylic acid. For CIE, remember the reagent condition: concentrated H₂SO₄ catalyst. Source

Oxidation of alcohols

· Oxidising agents: acidified K₂Cr₂O₇ or acidified KMnO₄.
· Acidified K₂Cr₂O₇ colour change: orange → green if oxidation occurs.
· Primary alcohol + distillation → aldehyde.
· Primary alcohol + reflux → carboxylic acid.
· Secondary alcohol → ketone.
· Tertiary alcohols: not oxidised under these conditions.
· Use oxidation as a distinguishing test for primary, secondary and tertiary alcohols.

Distillation vs reflux in primary alcohol oxidation

· Use distillation to collect the aldehyde as it forms, preventing further oxidation.
· Use reflux with excess oxidising agent to keep the aldehyde in the mixture so it oxidises further to a carboxylic acid.
· Exam phrase: primary alcohols give aldehydes by distillation and carboxylic acids by reflux.
· Secondary alcohols do not need this distinction because they stop at ketones.

Triiodomethane / iodoform test

· Tests for the group CH₃CH(OH)– in alcohols.
· Reagent: alkaline iodine, I₂(aq)/OH⁻(aq).
· Positive result: yellow precipitate of tri-iodomethane, CHI₃.
· Alcohol pattern tested: CH₃CH(OH)–R.
· Product also includes an ion: RCO₂⁻.

This page illustrates the iodoform test for alcohols containing CH₃CH(OH)–. The key observation is a yellow precipitate of CHI₃. Source

Acidity of alcohols

· Alcohols are very weak acids.
· Alcohols react with Na(s) to form alkoxides and H₂(g), showing they can donate H⁺ from the O–H bond.
· Alcohols are less acidic than water because the alkyl group donates electron density, making loss of H⁺ less favourable.
· Exam comparison: water is more acidic than alcohols.