Alkaline hydrolysis and acidification

Hydroxide adds at the ester carbonyl and displaces the methoxy fragment, which becomes methanol. Alkaline conditions leave the salicylate groups deprotonated; subsequent acidification restores the neutral acid and phenolic OH.

Reagents
1. Excess NaOH(aq); 2. HCl(aq) work-up
Conditions
Heat under reflux with aqueous alkali, then cool and acidify
Reaction class
Alkaline hydrolysis and acidification
Equation
C8H8O3 + 2NaOH + 2HCl -> C7H6O3 + CH3OH + 2NaCl + H2O

Overview

Hydroxide adds at the ester carbonyl and displaces the methoxy fragment, which becomes methanol. Alkaline conditions leave the salicylate groups deprotonated; subsequent acidification restores the neutral acid and phenolic OH.

Transformation

Methyl salicylate → Salicylic acid

Equation
C8H8O3 + 2NaOH + 2HCl -> C7H6O3 + CH3OH + 2NaCl + H2O
Reactants
Methyl salicylate + 2 Sodium hydroxide + 2 Hydrogen chloride
Products
Salicylic acid + Methanol + 2 Sodium chloride + Water
Reagents
1. Excess NaOH(aq); 2. HCl(aq) work-up
Environment
Heat under reflux with aqueous alkali, then cool and acidify
Reaction class
Alkaline hydrolysis and acidification
Mechanism
base-promoted acyl substitution followed by protonation
Evidence level
source-backed molecular example

Reference procedure

Salicylic-acid recovery from methyl salicylate

Methyl salicylate → salicylate under alkaline hydrolysis → salicylic acid after acidification

Materials and quantities

  • Methyl salicylate
  • Aqueous sodium hydroxide
  • Hydrochloric acid for the prescribed work-up

Apparatus

  • Reflux apparatus
  • Cooling bath
  • Suction-filtration apparatus

Procedure

  1. Heat the ester with aqueous alkali under the source’s reflux conditions.
  2. Cool before controlled acidification to convert the carboxylate into the less soluble free acid.

Work-up and isolation

  1. Collect, wash and dry the precipitated solid; assess its melting range.
  2. Compare starting material and product chromatographically when a purity claim requires additional evidence.

Critical controls

  • The immediate alkaline product is a salt; acidification is part of obtaining the named neutral product.
  • Both methyl salicylate and salicylic acid contain a free phenolic group; compare their other analytical signals.
  • Residual water or mother liquor changes mass and melting behaviour.

Practical techniques

Reflux setup

Boil the reaction while returning condensed vapour to the same flask, so volatile material is not continuously lost.

Setup

  1. Charge the flask before fitting the condenser; do not fill the flask more than about half full.
  2. Run cooling water into the lower condenser inlet and out from the upper outlet.
  3. Keep the top of an ordinary reflux condenser open to the atmosphere unless the cited procedure specifies a pressure-rated closed system.
  4. Heat to a gentle, steady boil so the condensation line remains within the condenser and liquid returns continuously to the flask.

Operating checks

  • Use an electric bath or mantle for flammable organic liquids; do not use a naked flame.
  • Start condenser water before heating and confirm that hoses are secure.
  • If addition is exothermic, complete the controlled addition and cooling stage before sustained reflux.

Filtration under reduced pressure

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

Setup

  1. Clamp the flask and seat the paper with the specified solvent before collecting a cooled crystal suspension.
  2. 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.

Melting range and solid purity

Record the interval from first melting to complete melting to compare a dry solid with reference data.

Setup

  1. Load a small, compact sample and heat slowly near its expected melting temperature.
  2. Record the start and end temperatures for crude, purified and reference samples under comparable conditions.

Operating checks

  • Impurities often lower and broaden the range; residual solvent can do the same.
  • A matching range alone cannot uniquely identify a compound. Decomposition is not ordinary melting.

Thin-layer chromatography

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

Setup

  1. Draw a pencil baseline above the solvent and apply small, separate sample spots.
  2. Develop the plate, mark the solvent front promptly and visualise using the approved method.
  3. 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
Hydrolysis of the methyl ester; acid work-up recovers salicylic acid.
Limitations
The net equation includes phenolic deprotonation and its reversal on acidification. Base and acid are used in sequence, not mixed at the outset; the initial hydrolysis product is a salt rather than neutral salicylic acid.

Related reactions

  • O acetylation: Salicylic acid → Aspirin

    The phenolic oxygen of salicylic acid receives an acetyl group from ethanoic anhydride. Acyl substitution forms an ester while retaining the separate carboxyl group; proton transfer gives ethanoic acid as the co-product.

  • Acid hydrolysis: Aspirin → Salicylic acid

    Water attacks the acid-activated acetyl carbonyl. Acyl-oxygen cleavage and proton transfer restore the phenolic OH of salicylic acid, while the removed acetyl group becomes ethanoic acid.

References

  1. The preparation of 2-hydroxybenzoic acidRoyal Society of Chemistry · RSC Education · 2023

    Official indexed content and teacher notes checked 2026-09-16. Supports methyl salicylate hydrolysis with aqueous sodium hydroxide followed by hydrochloric-acid work-up to salicylic acid. Exact work-up and isolation are in the linked practical source.

  2. The preparation of 2-hydroxybenzoic acidRoyal Society of Chemistry · RSC Education

    Alkaline ester hydrolysis followed by acidification, reduced-pressure filtration and purity assessment.

  3. The melting point of aspirinRoyal Society of Chemistry · RSC Education

    Dry samples and observed melting ranges as evidence of purity rather than a stand-alone identity claim.

  4. 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.

  5. Pearson Edexcel International Advanced Level Chemistry Student Practical GuidePearson Education Limited · Pearson qualifications · 2018

    Supports the practical distinctions between reflux, distillation, liquid-liquid separation, washing and drying in organic preparations.

  6. Reflux and distillation – practical videosRoyal Society of Chemistry · RSC Education

    Explains the different purposes and apparatus arrangements for reflux and distillation, including organic preparation and work-up.

  7. Evaporation, filtration and crystallisation: teaching practical scienceRoyal Society of Chemistry · RSC Education

    Distinguishes filtration, crystallisation and post-16 reduced-pressure isolation.