Alcohols: Reactions and Oxidation

This guide covers Topic 16, Hydroxy compounds — subtopic 16.1 Alcohols in full — from Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is AS Level content.

Before studying this

At IGCSE / O Level (subtopic 11.6), you learned two named routes to ethanol — fermentation of aqueous glucose, and catalytic addition of steam to ethene — plus combustion and two uses (solvent, fuel). That is a narrow, single-alcohol treatment with no mechanism and no reference to oxidation states.

AS Level treats alcohols as a full functional-group class: six production routes rather than two, a distinguishing test between primary, secondary and tertiary alcohols, and the oxidation-state logic that explains why each class behaves differently. You should already have worked through Hydrocarbons: Alkanes and Alkenes and Halogenoalkanes, since three of the six production routes here start from those two classes.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — AS Level, Topic 16

16.1 Alcohols — the reactions by which alcohols are produced: electrophilic addition of steam to an alkene (H₂O(g)/H₃PO₄ catalyst); reaction of an alkene with cold dilute acidified potassium manganate(VII) to form a diol; substitution of a halogenoalkane using NaOH(aq) and heat; reduction of an aldehyde or ketone using NaBH₄ or LiAlH₄; reduction of a carboxylic acid using LiAlH₄; hydrolysis of an ester using dilute acid or dilute alkali and heat. The reactions of alcohols: combustion; substitution to a halogenoalkane (by HX(g); by KCl and concentrated H₂SO₄ or concentrated H₃PO₄; by PCl₃ and heat; by PCl₅; or by SOCl₂); reaction with Na(s); oxidation with acidified K₂Cr₂O₇ or acidified KMnO₄ to a carbonyl compound by distillation, or to a carboxylic acid by refluxing, given that primary alcohols yield aldehydes (further oxidisable to carboxylic acids), secondary alcohols yield ketones, and tertiary alcohols cannot be oxidised this way; dehydration to an alkene using a heated catalyst such as Al₂O₃ or a concentrated acid; and ester formation by reaction with a carboxylic acid and concentrated H₂SO₄ as catalyst, exemplified by ethanol. Also: classifying alcohols as primary, secondary or tertiary, including examples with more than one alcohol group; stating a characteristic distinguishing reaction — mild oxidation with acidified K₂Cr₂O₇, colour change orange to green; deducing the presence of a CH₃CH(OH)– group from its reaction with alkaline I₂(aq) to form a yellow precipitate of tri-iodomethane and an ion, RCO₂⁻; and explaining the acidity of alcohols compared with water.

Six routes to an alcohol

Three of the six production routes connect directly back to classes you’ve already studied: electrophilic addition of steam to an alkene (with H₃PO₄ catalyst), the diol formed by reacting an alkene with cold dilute acidified KMnO₄, and substitution of a halogenoalkane with NaOH(aq) and heat. The other three belong to functional groups covered later in the AS course — reduction of an aldehyde or ketone (NaBH₄ or LiAlH₄), reduction of a carboxylic acid (LiAlH₄ only, a stronger reducing agent is needed), and hydrolysis of an ester — and are worth returning to once those topics are covered.

Classifying alcohols

Alcohols are classified as primary, secondary or tertiary according to how many carbon groups are attached to the carbon bearing the –OH group (one, two or three respectively) — this classification is what determines the oxidation behaviour below. Some molecules carry more than one –OH group: ethane-1,2-diol, HOCH₂CH₂OH (a diol, used as antifreeze and in polyester production), has an –OH group on each of its two carbons, and propane-1,2,3-triol (glycerol), HOCH₂CH(OH)CH₂OH, carries three. Each –OH group in such a polyhydric alcohol can be classified and reacted independently of the others.

Oxidation and the primary/secondary/tertiary distinction

This is the central idea of the whole topic: how far an alcohol can be oxidised depends on how many hydrogens are on the carbon bearing the –OH group, using acidified K₂Cr₂O₇ or acidified KMnO₄ as the oxidising agent.

• A primary alcohol (–CH₂OH) is oxidised by distillation to an aldehyde, and further oxidised by refluxing to a carboxylic acid. Distillation removes the aldehyde from the reaction mixture as it forms, stopping oxidation at that stage; refluxing keeps everything in contact, allowing the second oxidation to complete.
• A secondary alcohol (–CHOH–) is oxidised to a ketone, and no further oxidation is possible under these conditions — a ketone has no hydrogen left on the carbonyl carbon for the oxidising agent to remove.
• A tertiary alcohol (–C(OH)–, three carbon substituents) cannot be oxidised by acidified K₂Cr₂O₇ or KMnO₄ at all — oxidation would require breaking a C–C bond, which these reagents cannot do.

Worked example. Butan-2-ol, CH₃CH(OH)CH₂CH₃, is heated under reflux with excess acidified potassium dichromate(VI). Identify the organic product and explain why no further oxidation occurs.

The –OH is on C2, which is bonded to two other carbons (C1 and C3) — this is a secondary alcohol. Oxidation removes two hydrogens from C2 (one from the O–H, one from the C–H) to form a C=O bond, giving butanone, CH₃COCH₂CH₃, a ketone. Butanone has no hydrogen remaining on the carbonyl carbon, so acidified dichromate has nothing further to remove — the reaction stops at the ketone regardless of how long reflux continues.

The distinguishing test

Mild oxidation with acidified K₂Cr₂O₇ gives a direct visual test: the orange Cr₂O₇²⁻ ion is reduced to the green Cr³⁺ ion whenever oxidation occurs. Primary and secondary alcohols turn the solution from orange to green; a tertiary alcohol leaves it orange, because no oxidation happens.

The iodoform (tri-iodomethane) test

Any compound containing a CH₃CH(OH)– group reacts with alkaline I₂(aq) to give a yellow precipitate of tri-iodomethane (CHI₃) and an ion RCO₂⁻. This test identifies the CH₃CH(OH)– fragment specifically — ethanol gives a positive result (R = H), as does any alcohol with that exact grouping; alcohols without a methyl group directly on the carbinol carbon give a negative result.

The other reactions of alcohols

Beyond oxidation, alcohols undergo several further reactions:

• Combustion: like any compound containing C, H and O, alcohols burn completely in excess oxygen to give CO₂ and H₂O — for example, C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
• Substitution to a halogenoalkane: the –OH is replaced by a halogen using HX(g) (e.g. C₂H₅OH + HBr → C₂H₅Br + H₂O), a mixture of KCl or KBr with concentrated H₂SO₄ or concentrated H₃PO₄, PCl₃ with heat, PCl₅ at room temperature (giving steamy HCl fumes — the classic test for an –OH group), or SOCl₂.
• Reaction with sodium: alcohols react with sodium metal, releasing hydrogen gas and forming a sodium alkoxide — 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. This parallels water’s reaction with sodium but is less vigorous, because the electron-donating alkyl group makes the O–H bond less polar.
• Dehydration to an alkene: heating an alcohol with a catalyst such as Al₂O₃, or with a concentrated acid such as concentrated H₂SO₄, eliminates H₂O to form an alkene — C₂H₅OH → CH₂=CH₂ + H₂O.
• Ester formation: refluxing an alcohol with a carboxylic acid, using concentrated H₂SO₄ as catalyst, forms an ester and water — CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. This is an equilibrium, so yield is limited unless the water (or the ester) is removed as it forms.

Acidity of alcohols compared with water

Alcohols are weaker acids than water. The alkyl group attached to the oxygen is electron-donating by the inductive effect, pushing electron density onto the oxygen. This makes the O–H bond’s hydrogen less easily released as H⁺, and — once released — makes the resulting alkoxide ion (RO⁻) less stable than hydroxide (OH⁻), since the same electron-donating effect concentrates negative charge on an already electron-rich oxygen rather than dispersing it. Both effects push the equilibrium away from ionisation, so alcohols ionise less readily than water and are the weaker acid.

Common mistakes

• Forgetting the distillation-vs-reflux distinction. The same reagent (acidified K₂Cr₂O₇ or KMnO₄) gives a different product from a primary alcohol depending only on whether the mixture is distilled (aldehyde) or refluxed (carboxylic acid) — the reagent doesn’t change, the setup does.
• Trying to oxidise a tertiary alcohol. If a question gives a tertiary alcohol and acidified dichromate, the answer is “no reaction,” not a strained attempt to name a product.
• Applying the iodoform test to any alcohol. It is specific to the CH₃CH(OH)– grouping (and, in Topic 17, the analogous CH₃CO– grouping in carbonyl compounds) — not a general test for all alcohols.
• Getting the acidity comparison backwards. Alcohols are less acidic than water, not more — the electron-donating alkyl group destabilises the conjugate base, it doesn’t stabilise it.

Quick revision checklist

• Six production routes, and which three connect back to alkenes and halogenoalkanes
• Reactions: combustion, substitution to a halogenoalkane, reaction with Na(s), oxidation, dehydration, esterification
• Primary → aldehyde (distillation) → carboxylic acid (reflux); secondary → ketone (no further oxidation); tertiary → no reaction
• The K₂Cr₂O₇ colour-change test: orange to green for primary/secondary, stays orange for tertiary
• The iodoform test for a CH₃CH(OH)– group
• Why alcohols are weaker acids than water

Related resources

• Halogenoalkanes: Nucleophilic Substitution and Elimination — the alcohol’s own route back to a halogenoalkane
• Hydrocarbons: Alkanes and Alkenes — the alkene route to an alcohol
• Carbonyl Compounds: Aldehydes and Ketones — where oxidation of an alcohol leads
• Carboxylic Acids and Esters — full oxidation, and esterification from the acid’s side
• Organic Mechanisms: An Introduction — naming and functional-group vocabulary
• Phenol: Reactions and Acidity — the A Level hydroxy compound, and the acidity comparison this resource sets up
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.