Complete oxidation

Oxidation first changes the benzylic CH2OH group into an aldehyde and then into carboxylate under the alkaline conditions. Acid work-up gives benzoic acid; the ring is retained while the side-chain carbon becomes more oxidised.

Reagents: Excess KMnO4 / OH-; then acid work-up
Conditions: Heat with aqueous oxidant, then acidify
Reaction class: Complete oxidation
Equation: C6H5CH2OH + 2[O] -> C6H5COOH + H2O

Overview

Transformation

Benzyl alcohol → Benzoic acid

Equation: C6H5CH2OH + 2[O] -> C6H5COOH + H2O
Reagents: Excess KMnO4 / OH-; then acid work-up
Environment: Heat with aqueous oxidant, then acidify
Reaction class: Complete oxidation
Mechanism: alcohol oxidation
Evidence level: source-backed molecular example

Reference procedure

Benzoic-acid purification and benzyl-alcohol separation

Crude benzoic acid containing residual benzyl alcohol → recrystallised benzoic acid

Materials and quantities

Crude benzoic-acid sample
Water as recrystallisation solvent
Benzoic-acid and benzyl-alcohol reference samples
Reference TLC solvents

Apparatus

Recrystallisation and hot-filtration apparatus
Suction-filtration apparatus
TLC plate and development chamber

Procedure

Dissolve the crude solid in a suitable minimum amount of hot solvent and remove insoluble contamination.
Allow crystallisation, then collect and dry the solid.

Work-up and isolation

Compare crude material, recovered crystals and reference samples on the same TLC plate.
Use the reference’s complementary visualisation methods when spots are poorly separated or visible.

Critical controls

The source uses a model impure benzoic-acid sample; oxidation reagents and their work-up remain route-specific.
A single apparent TLC spot may hide overlapping compounds.
Lower recovered mass can reflect removal of impurities as well as loss of product.

Practical techniques

Recrystallisation

Purify a solid using its greater solubility in hot solvent than in cold solvent.

Setup

Choose a compatible solvent and dissolve the crude solid in the minimum practical amount of hot solvent.
Remove insoluble matter while hot if required; cool the clear solution before collecting the crystals.

Operating checks

Excess solvent reduces recovery; some product remains in the mother liquor.
Wash with a small amount of cold solvent and dry before weighing. Wet crystals give misleading yields.

Filtration under reduced pressure

Collect an insoluble solid while drawing the liquid through a filter using reduced pressure.

Setup

Clamp the flask and seat the paper with the specified solvent before collecting a cooled crystal suspension.
Retain the filtrate until the desired solid has been recovered.

Operating checks

Use vacuum-rated glassware and the laboratory-approved vacuum setup.
Release the vacuum safely before shutting down the suction source. Filtration removes bulk liquid; further drying may be needed.

Thin-layer chromatography

Compare starting material, product and reference samples using migration on the same stationary phase.

Setup

Draw a pencil baseline above the solvent and apply small, separate sample spots.
Develop the plate, mark the solvent front promptly and visualise using the approved method.
Calculate Rf as spot distance divided by solvent-front distance from the same baseline.

Operating checks

Compare samples on the same TLC plate using the same solvent.
Protect eyes and skin during UV visualisation; avoid inhaling eluent vapour.

Scope and limitations

Scope: Primary benzylic alcohol; oxidation occurs on the side chain.
Limitations: Benzoate forms in alkaline solution; acid work-up gives benzoic acid. An aldehyde is an intermediate, not the endpoint under these conditions.

Related reactions

Side chain oxidation: Toluene → Benzoic acid
Toluene gives benzoic acid through side-chain oxidation.
Carbonyl reduction: Benzaldehyde → Benzyl alcohol
Hydride attacks the aldehyde carbon and the C=O pi electrons move to oxygen. Protonation of the alkoxide gives benzyl alcohol; neither a new carbon-carbon bond nor reduction of the aromatic ring is involved.
Aldehyde oxidation: Benzaldehyde → Benzoic acid
Water reversibly adds to the aldehyde to form a hydrate, which can then be oxidised. The aldehyde carbon becomes the acid carbonyl carbon; acid work-up converts the initially formed benzoate into benzoic acid.
Acyl chloride preparation: Benzoic acid → Benzoyl chloride
Reaction with phosphorus(V) chloride replaces the carboxyl OH group with chlorine, retaining the acyl carbon and aromatic ring. The new acyl chloride is more susceptible to nucleophilic substitution, while POCl3 and HCl account for the remaining atoms.
Esterification: Benzoic acid → Ethyl benzoate
Acid activation allows ethanol to add to benzoic acid; proton transfers and water loss produce ethyl benzoate. The aromatic ring stays intact throughout the acyl substitution.

References

Organic Chemistry: Oxidation of AlcoholsJohn McMurry · OpenStax Organic Chemistry · 2023
Supports functional-group chemistry used to curate the linked examples. Checked 2026-09-16; named examples are applications of textbook scope, without claimed experimental yields.
Benzoic-acid purification and TLC: Coursework conundrumRoyal Society of Chemistry · RSC Education
Purification and analysis of benzoic acid contaminated with benzyl alcohol and a model inorganic impurity.
Purifying by recrystallisationRoyal Society of Chemistry · RSC Education
Solvent selection, soluble and insoluble impurities, recovery losses and residual-solvent error.
Evaporation, filtration and crystallisation: teaching practical scienceRoyal Society of Chemistry · RSC Education
Distinguishes filtration, crystallisation and post-16 reduced-pressure isolation.
Using thin-layer chromatography to investigate the reaction: teacher notesRoyal Society of Chemistry · RSC Education
Compares crude and recrystallised aspirin with a reference sample; extra spots suggest impurities.