Iodoform reaction
Ethanol is oxidised and cleaved by alkaline iodine to give triiodomethane and methanoate.
- Reagents
- I2(aq), NaOH(aq)
- Conditions
- warm gently
- Reaction class
- iodoform reaction
- Equation
- CH3CH2OH + 4 I2 + 6 OH- -> HCO2- + CHI3 + 5 I- + 5 H2O
Overview
Ethanol is oxidised and cleaved by alkaline iodine to give triiodomethane and methanoate.
Transformation
Ethanol → Triiodomethane products
- Equation
- CH3CH2OH + 4 I2 + 6 OH- -> HCO2- + CHI3 + 5 I- + 5 H2O
- Reagents
- I2(aq), NaOH(aq)
- Environment
- warm gently
- Reaction class
- iodoform reaction
- Mechanism
- oxidation followed by haloform reaction
- Evidence level
- textbook core
Reference procedure
Microscale iodoform test
Ethanol gives a yellow triiodomethane precipitate
Materials and quantities
- Ethanol sample
- Iodine solution and sodium hydroxide; use the cited microscale quantities
- Approved negative comparison sample
Apparatus
- Small labelled test tubes
- Dropping pipettes
Procedure
- Treat the sample and comparison using the same reagent sequence and observation period from the source.
- Record cloudiness and yellow precipitate formation separately from disappearance of iodine colour.
Work-up and isolation
- Keep the test mixture for the laboratory’s disposal procedure; do not isolate the diagnostic solid.
Critical controls
- A positive result supports the required methyl-carbonyl structure or an oxidisable precursor. It does not uniquely identify ethanol.
- Ethanol is the primary-alcohol exception; suitable methyl secondary alcohols and methyl ketones also respond.
- Use eye protection and handle iodine and alkali as specified by the source.
Practical techniques
Controlled comparison of reaction times
Compare a visible change while keeping variables other than the one under study consistent.
Setup
- Use equal concentrations, volumes and mixing methods, and equilibrate samples at the same temperature.
- Define the visible endpoint before starting and record repeated measurements.
Operating checks
- Precipitation time includes mixing and detection thresholds; its inverse is only a relative-rate proxy under matched conditions.
- Do not compare different carbon skeletons when isolating the effect of halogen identity.
Scope and limitations
- Scope
- Ethanol gives the iodoform reaction because it is oxidised under the conditions to ethanal before haloform cleavage.
- Limitations
- This route is ethanol-specific and should not be read as a general reaction of all primary alcohols.
Related reactions
- Iodoform reaction: Methyl ketones → Triiodomethane products
Alkaline iodine converts methyl ketones into yellow triiodomethane and a carboxylate with one fewer carbon.
- Iodoform reaction: Ethanal → Triiodomethane products
Ethanal gives triiodomethane and methanoate in the alkaline iodine iodoform reaction.
- Iodoform reaction: Methyl secondary alcohols → Triiodomethane products
Methyl secondary alcohols are oxidised to methyl ketones, then cleaved to triiodomethane and a one-carbon-shorter carboxylate.
- Partial oxidation: Ethanol → Ethanal
Ethanol is oxidised to ethanal.
- Oxidation: Ethanol → Ethanoic acid
Ethanol is oxidised to ethanoic acid.
- Reduction: Ethanal → Ethanol
Ethanal is reduced to ethanol.
- Esterification: Ethanol → Ethyl acetate
Ethanoic acid and ethanol combine reversibly to form ethyl acetate and water.
- Acid hydrolysis: Ethyl acetate → Ethanol
Ethyl acetate and water react reversibly to form ethanoic acid and ethanol.
- Hydration: Ethene → Ethanol
Ethene gives ethanol through hydration.
- Dehydration: Ethanol → Ethene
Ethanol gives ethene through dehydration.
- Intermolecular dehydration: Ethanol → Diethyl ether
Protonation turns one ethanol OH group into a better leaving group. The oxygen of another ethanol molecule displaces water, and deprotonation gives diethyl ether; stronger dehydration conditions instead favour ethene.
- Hydrolysis: Bromoethane → Ethanol
Hydroxide attacks the saturated carbon bearing bromine while the C-Br bond breaks. This primary-substrate SN2 route replaces bromide with OH and preserves both carbon atoms; aqueous conditions favour this branch over elimination.
- Acid hydrolysis: Ethyl butyrate → Ethanol
Water attack and proton transfers permit the ethoxy fragment to leave as ethanol. Re-forming the carbonyl produces butanoic acid on the other branch.
References
- Pearson Edexcel Level 3 Advanced GCE in Chemistry specificationPearson Education Limited · Pearson qualifications · 2024
Supports the textbook-core organic reaction routes used for displayed route and mechanism content.
- A test to distinguish between ethanol and methanolRoyal Society of Chemistry and Nuffield Foundation · RSC Education
Microscale iodoform observation and the functional-group scope of positive results.
- Rates of hydrolysis: practical videos for 16–18 studentsRoyal Society of Chemistry · RSC Education
Water is the nucleophile in the silver-nitrate comparison, and silver-halide precipitation measures released halide indirectly.