ChemistryISC Class 12 · Haloalkanes and Haloarenes

Chloroform and Iodoform: ISC Class 12 Polyhalogen Compounds

Updated 15 October 2026 · Fact-checked

Chloroform (CHCl₃) is a sweet-smelling liquid that air and light slowly oxidise to poisonous phosgene, so it is kept in dark, full bottles with a little ethanol. Iodoform (CHI₃) is a yellow solid formed when a compound with a CH₃CO– or CH₃CH(OH)– group is warmed with iodine and alkali: the iodoform test.

Understand Chloroform and Iodoform

Chloroform, trichloromethane (CHCl₃). A colourless, dense, sweet-smelling liquid, not miscible with water. It is made by chlorination of methane, $C H_{4} + 3 C l_{2} \to C H C l_{3} + 3 H C l$ (in steps, with light), and in the laboratory by warming ethanol or propanone with bleaching powder (a haloform reaction).

Reactions of chloroform.
• Oxidation in air and light gives the highly poisonous gas phosgene (carbonyl chloride): $2 C H C l_{3} + O_{2} \to 2 C O C l_{2} + 2 H C l$ . So chloroform is stored in dark coloured bottles, completely filled (no air space), and about 1% ethanol is added, which converts any phosgene into harmless diethyl carbonate (ethyl carbonate): $C O C l_{2} + 2 C_{2} H_{5} O H \to (C_{2} H_{5} O)_{2} C O + 2 H C l$ .
• Carbylamine (isocyanide) reaction: warmed with a primary amine and alcoholic KOH, chloroform gives an isocyanide with an extremely offensive smell: $C H C l_{3} + 3 K O H + C_{2} H_{5} N H_{2} \to C_{2} H_{5} N C + 3 K C l + 3 H_{2} O$ . Secondary and tertiary amines do not react, so this is a test for primary amines (and for chloroform).
• Nitration: with concentrated nitric acid it gives chloropicrin (trichloronitromethane), used as an insecticide: $C H C l_{3} + H N O_{3} \to C C l_{3} N O_{2} + H_{2} O$ .
• Hydrolysis with aqueous alkali gives a formate, because the three Cl atoms on one carbon are replaced and the unstable triol loses water: $C H C l_{3} + 4 N a O H \to H C O O N a + 3 N a C l + 2 H_{2} O$ .
• Reduction with zinc and HCl gives dichloromethane: $C H C l_{3} + 2 [H] \to C H_{2} C l_{2} + H C l$ .
• Heating with silver powder gives ethyne: $2 C H C l_{3} + 6 A g \to C_{2} H_{2} + 6 A g C l$ .
Uses: solvent for fats, alkaloids and iodine; major use today is making the refrigerant R-22 (CHClF₂). It was once a surgical anaesthetic but has been replaced by safer substances. Effects: breathing its vapour depresses the central nervous system (dizziness, fatigue, headache); long exposure can damage the liver and kidneys.

Iodoform, triiodomethane (CHI₃). A yellow crystalline solid with a distinctive smell. Preparation: warming ethanol or propanone with iodine and sodium hydroxide (or sodium carbonate): $C H_{3} C O C H_{3} + 3 I_{2} + 4 N a O H \to C H I_{3} + C H_{3} C O O N a + 3 N a I + 3 H_{2} O$ and $C_{2} H_{5} O H + 4 I_{2} + 6 N a O H \to C H I_{3} + H C O O N a + 5 N a I + 5 H_{2} O$ . Use: it was used as an antiseptic; its action is due to the iodine it liberates, not to iodoform itself. Because of its objectionable smell it has been replaced by other iodine formulations. With silver powder it gives ethyne: $2 C H I_{3} + 6 A g \to C_{2} H_{2} + 6 A g I$ .

The iodoform test. A compound that contains a CH₃CO– group (methyl ketones, ethanal) or a CH₃CH(OH)– group (ethanol and secondary alcohols of the type CH₃CH(OH)R, which iodine/alkali first oxidises to CH₃CO–) gives a yellow precipitate of iodoform when warmed with I₂ and NaOH. Methanol, propan-1-ol, propanal and pentan-3-one do not.

Other polyhalogen compounds (uses and effects). Dichloromethane: paint remover, aerosol propellant, process solvent; it harms the central nervous system and can impair hearing and vision. Tetrachloromethane (CCl₄): used to make refrigerants and aerosol propellants and as a solvent; it can cause liver damage and, in the upper atmosphere, depletes ozone. Freons (chlorofluorocarbons such as CCl₂F₂): unreactive, non-toxic refrigerants and propellants, but in the stratosphere UV light releases chlorine radicals that destroy ozone. DDT (p,p′-dichlorodiphenyltrichloroethane): an insecticide that is very persistent, accumulates in fatty tissue and builds up along food chains.

Chloroform and Iodoform: formulas and key facts

Phosgene formation: $2 C H C l_{3} + O_{2} light 2 C O C l_{2} + 2 H C l$; Why chloroform is kept dark, full and with ethanol.
Removal of phosgene by ethanol: $C O C l_{2} + 2 C_{2} H_{5} O H \to (C_{2} H_{5} O)_{2} C O + 2 H C l$; Gives harmless diethyl carbonate.
Carbylamine reaction: $R N H_{2} + C H C l_{3} + 3 K O H heat R N C + 3 K C l + 3 H_{2} O$; Offensive smell; primary amines only.
Iodoform from propanone: $C H_{3} C O C H_{3} + 3 I_{2} + 4 N a O H \to C H I_{3} + C H_{3} C O O N a + 3 N a I + 3 H_{2} O$; Yellow precipitate: positive iodoform test.
Iodoform from ethanol: $C_{2} H_{5} O H + 4 I_{2} + 6 N a O H \to C H I_{3} + H C O O N a + 5 N a I + 5 H_{2} O$; Ethanol is first oxidised to ethanal.
Alkaline hydrolysis of chloroform: $C H C l_{3} + 4 N a O H \to H C O O N a + 3 N a C l + 2 H_{2} O$; Gives sodium formate.

How to solve Chloroform and Iodoform questions

Questions on these compounds are recall of reactions and uses, a few conversions, and the iodoform test applied to unknowns. Organise answers around the functional change.

1Identify which reaction is asked: oxidation (phosgene), substitution by amine (carbylamine), nitration (chloropicrin), hydrolysis (formate), reduction (CH₂Cl₂) or coupling with Ag (ethyne).
2Write the balanced equation with reagent and condition, and name the organic product.
3For the iodoform test, look for CH₃CO– or CH₃CH(OH)– in each compound: methyl ketones, ethanal, ethanol and CH₃CH(OH)R alcohols give a yellow precipitate.
4For storage questions, give all three precautions (dark bottle, filled to the top, about 1% ethanol) and the phosgene equation.
5For environmental questions, give one use and one harmful effect for each compound named.
6In quantitative questions on iodoform, use 1 mol of substrate → 1 mol of CHI₃ (M = 394 g mol⁻¹).

Chloroform and Iodoform in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Iodoform yield from propanone

Suppose a pharmacy laboratory prepares iodoform for an old-style antiseptic dusting powder by warming 5.8 g of propanone (M = 58 g mol⁻¹) with excess iodine and sodium hydroxide. After filtering and drying, 30.0 g of yellow crystals is obtained. (CHI₃: M = 394 g mol⁻¹)

Write the equation, calculate the theoretical yield and the percentage yield, and say why the powder acts as an antiseptic.

Try it first, then show the solutionSolution

$C H_{3} C O C H_{3} + 3 I_{2} + 4 N a O H \to C H I_{3} + C H_{3} C O O N a + 3 N a I + 3 H_{2} O$ : 1 mol propanone gives 1 mol iodoform.
Moles of propanone $= \frac{5.8}{58} = 0.10$ mol, so the theoretical mass of CHI₃ $= 0.10 \times 394 = 39.4$ g.
Percentage yield $= \frac{30.0}{39.4} \times 100 = 76%$ .
In contact with wounds iodoform slowly liberates iodine, which kills microbes.

Answer: Theoretical yield 39.4 g; percentage yield about 76%. The antiseptic action comes from the iodine it liberates.

Memory hook: One methyl ketone, one iodoform: count moles, then grams.

Tricks to remember Chloroform and Iodoform

Short memory aids. Say each one aloud once, then use it on a question the same day.

Dark, Full, Alcohol
Store chloroform like a secret: in the Dark, Full to the top, with a drop of Alcohol (ethanol).
Use it when: Storage questions and the phosgene fill-in.
Methyl next to the carbonyl
Iodoform test needs CH₃ next to C=O (or next to CH–OH, which becomes C=O). No methyl there, no yellow precipitate.
Use it when: Deciding which compounds give the test.
One carbon, three halogens, three reactions with ammonia-like partners
CHCl₃ + RNH₂ (+KOH) → RNC (stink); CHCl₃ + HNO₃ → CCl₃NO₂ (tears); CHCl₃ + NaOH → HCOONa (formate).
Use it when: Recalling chloroform reactions as a set.

Chloroform and Iodoform: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: What forms in an old bottle · 1 mark · MCQBoard level

Chloroform kept in a clear bottle exposed to air and sunlight slowly forms a poisonous gas. The gas is:
(a) chlorine
(b) phosgene, COCl₂
(c) carbon monoxide
(d) methyl isocyanide

Try it first, then show the solutionSolution

Air and light oxidise chloroform: $2 C H C l_{3} + O_{2} \to 2 C O C l_{2} + 2 H C l$ .
COCl₂ is carbonyl chloride, phosgene.

Answer: (b) phosgene.

Example 2: Hydrolysis of chloroform · 2 marks · Short answerBoard level

What is formed when chloroform is boiled with aqueous sodium hydroxide? Write the balanced equation.

Try it first, then show the solutionSolution

All three Cl atoms are replaced by OH; the unstable CH(OH)₃ loses water to give formic acid, which forms its sodium salt (1 mark).
$C H C l_{3} + 4 N a O H \to H C O O N a + 3 N a C l + 2 H_{2} O$ (1 mark).

Answer: Sodium formate (sodium methanoate): $C H C l_{3} + 4 N a O H \to H C O O N a + 3 N a C l + 2 H_{2} O$ .

Example 3: Applying the iodoform test · 3 marks · Short answerHarder board level

Write the equation for the formation of iodoform from propanone. Which of the following give a positive iodoform test: methanol, ethanol, propan-1-ol, propan-2-ol? Give a reason.

Try it first, then show the solutionSolution

$C H_{3} C O C H_{3} + 3 I_{2} + 4 N a O H \to C H I_{3} + C H_{3} C O O N a + 3 N a I + 3 H_{2} O$ ; yellow precipitate (1 mark).
Ethanol (CH₃CH₂OH, oxidised to CH₃CHO) and propan-2-ol (CH₃CH(OH)CH₃, oxidised to CH₃COCH₃) give the test (1 mark).
Methanol and propan-1-ol cannot give a CH₃CO– group on oxidation, so they do not (1 mark).

Answer: Positive: ethanol and propan-2-ol (both contain CH₃CH(OH)–). Negative: methanol and propan-1-ol.

Example 4: Three conversions · 3 marks · Short answerHarder board level

How will you bring about the following? Give equations. (a) Chloroform to dichloromethane. (b) Methylamine to methyl isocyanide. (c) Removal of phosgene from stored chloroform.

Try it first, then show the solutionSolution

(a) Reduce with zinc and HCl: $C H C l_{3} + 2 [H] \to C H_{2} C l_{2} + H C l$ (1 mark).
(b) Warm with chloroform and alcoholic KOH (carbylamine reaction): $C H_{3} N H_{2} + C H C l_{3} + 3 K O H \to C H_{3} N C + 3 K C l + 3 H_{2} O$ (1 mark).
(c) Add about 1% ethanol: $C O C l_{2} + 2 C_{2} H_{5} O H \to (C_{2} H_{5} O)_{2} C O + 2 H C l$ (1 mark).

Answer: (a) Zn/HCl. (b) CHCl₃ + alcoholic KOH. (c) Ethanol converts phosgene to diethyl carbonate.

Example 5: Identifying C₃ compounds with the iodoform test · 5 marks · Long answerHOTS / competency

Compound A (C₃H₆O) gives a yellow precipitate when warmed with I₂ and NaOH, but does not reduce Tollens' reagent. Compound B (C₃H₈O) also gives a yellow precipitate, and on mild oxidation gives A. Compound C, an alcohol isomeric with B, gives no yellow precipitate.
(a) Identify A, B and C. [3]
(b) Write the equation for the iodoform reaction of A. [1]
(c) Name a reagent that converts A into chloroform. [1]

Try it first, then show the solutionSolution

A has a CH₃CO– group and is not an aldehyde: propanone, CH₃COCH₃ (1 mark).
B is oxidised to propanone and contains CH₃CH(OH)–: propan-2-ol, CH₃CH(OH)CH₃ (1 mark).
C is the isomeric alcohol without a CH₃CH(OH)– group: propan-1-ol, CH₃CH₂CH₂OH (1 mark).
(b) $C H_{3} C O C H_{3} + 3 I_{2} + 4 N a O H \to C H I_{3} + C H_{3} C O O N a + 3 N a I + 3 H_{2} O$ (1 mark).
(c) Bleaching powder (calcium hypochlorite) on warming (1 mark).

Answer: (a) A propanone, B propan-2-ol, C propan-1-ol. (b) As shown. (c) Bleaching powder.

Where students lose marks in Chloroform and Iodoform

The slips examiners see most often, and what to write instead.

Mistake: Wrong product names in the chloroform storage fill-in
ISC examiners reported 'diethyl ether' instead of phosgene and 'ethene' instead of ethyl carbonate (2024).
Do this: Learn the pair: ethanol prevents the build-up of phosgene (COCl₂) by converting it into diethyl (ethyl) carbonate.
Usually costs: 1 mark.
Mistake: Wrong metal or missing heat in iodoform to ethyne
Examiners noted sodium used instead of silver, or heating omitted, in 2024.
Wrong working
$C H I_{3} + N a \to C_{2} H_{2}$
Right working
$2 C H I_{3} + 6 A g heat C_{2} H_{2} + 6 A g I$
Do this: Silver powder and heat; balance with 6Ag and 6AgI.
Usually costs: 1 mark.
Mistake: Applying the iodoform test to any carbonyl compound
Students mark propanal or pentan-3-one as positive.
Do this: Only CH₃CO– (or CH₃CH(OH)–) gives the test.
Usually costs: ½ mark per compound.
Mistake: Using a secondary amine in the carbylamine test
Students assume all amines give the smell.
Do this: Only primary amines (aliphatic or aromatic) give isocyanides.
Usually costs: 1 mark.

Exam tips

For each polyhalogen compound in the syllabus (CH₂Cl₂, CHCl₃, CCl₄, CHI₃, freons, DDT), learn one use and one environmental or health effect.
Write reagents in full: 'alcoholic KOH' for carbylamine, 'I₂ and NaOH' for the iodoform test.
In identification problems, combine the iodoform test with a second clue (Tollens' or oxidation) to decide between aldehydes, ketones and alcohols.
Balance the haloform equations carefully: ketone + 3I₂ + 4NaOH; ethanol + 4I₂ + 6NaOH.
Name products by their common and IUPAC names where both are used (chloroform/trichloromethane, iodoform/triiodomethane).

Practice questions on Chloroform and Iodoform

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Iodoform is used as an antiseptic because:
(a) it is a yellow solid
(b) it liberates iodine
(c) it is soluble in water
(d) it oxidises to phosgene
Hint
The active substance is an element.
Answer
(b).
Q2 · 2 marks · Short answerBoard level
Give two uses of dichloromethane and one harmful effect of exposure to it.
Hint
Paints, aerosols, solvents.
Answer
Uses: paint remover; propellant in aerosols; process solvent (e.g. in drug manufacture), metal cleaning. Harm: it affects the central nervous system; it can impair hearing and vision (and cause dizziness, nausea).
Q3 · 2 marks · Short answerHarder board level
Why is a small amount of ethanol added to chloroform kept for use? Write the equations involved.
Hint
Two equations: forming and removing phosgene.
Answer
Air and light oxidise chloroform to poisonous phosgene: $2 C H C l_{3} + O_{2} \to 2 C O C l_{2} + 2 H C l$ . Ethanol converts any phosgene formed into harmless diethyl carbonate: $C O C l_{2} + 2 C_{2} H_{5} O H \to (C_{2} H_{5} O)_{2} C O + 2 H C l$ .
Q4 · 3 marks · Short answerHarder board level
Which of the following give a positive iodoform test? Give a reason for each: ethanal, propanal, butan-2-one, pentan-3-one, ethanol, methanol.
Hint
Look for CH₃CO– or CH₃CH(OH)–.
Answer
Positive: ethanal (CH₃CHO), butan-2-one (CH₃COC₂H₅) and ethanol (CH₃CH₂OH, oxidised to CH₃CHO). Negative: propanal (CH₃CH₂CHO, no CH₃ on the carbonyl carbon), pentan-3-one (C₂H₅COC₂H₅) and methanol (gives HCHO).
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Tetrachloromethane has zero dipole moment.
Reason (R): The four C–Cl bond dipoles point to the corners of a regular tetrahedron and cancel one another.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Symmetry.
Answer
(a).
Q6 · 3 marks · Short answerHarder board level
Describe the carbylamine test for ethylamine with an equation. Why does diethylamine not respond to it?
Hint
The NH₂ group provides both H atoms lost.
Answer
Warm ethylamine with chloroform and alcoholic KOH: an extremely offensive smell of ethyl isocyanide is produced. $C_{2} H_{5} N H_{2} + C H C l_{3} + 3 K O H \to C_{2} H_{5} N C + 3 K C l + 3 H_{2} O$ . The reaction needs two H atoms on nitrogen (–NH₂); diethylamine has only one, so it cannot form an isocyanide.
Q7 · 3 marks · Short answerHOTS / competency
Chloroform and tetrachloromethane are both colourless, dense liquids. Each is boiled with aqueous KOH and the solution is then warmed with Tollens' reagent. Predict and explain the observations, with equations for the hydrolysis.
Hint
What does each give on hydrolysis? Which product can reduce Tollens' reagent?
Answer
Chloroform: $C H C l_{3} + 4 K O H \to H C O O K + 3 K C l + 2 H_{2} O$ ; formate ion has an H on the carbonyl carbon and reduces Tollens' reagent, giving a silver mirror. CCl₄: $C C l_{4} + 6 K O H \to K_{2} C O_{3} + 4 K C l + 3 H_{2} O$ ; carbonate does not reduce Tollens' reagent, so no mirror.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a perfumer suspects that an aromatic liquid in a sample is acetophenone (C₆H₅COCH₃) rather than benzaldehyde. She warms a little of it with iodine and sodium hydroxide, and a yellow precipitate with a distinctive smell appears.
(i) Name the yellow precipitate and give its formula. [1]
(ii) Write the balanced equation for the reaction. [1]
(iii) Would benzaldehyde have given the same result? Explain. [1]
Hint
A methyl ketone gives the test.
Answer
(i) Iodoform, CHI₃. (ii) $C_{6} H_{5} C O C H_{3} + 3 I_{2} + 4 N a O H \to C H I_{3} + C_{6} H_{5} C O O N a + 3 N a I + 3 H_{2} O$ . (iii) No. Benzaldehyde (C₆H₅CHO) has no CH₃CO– group, so it does not give iodoform.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Formula and name of phosgene?Show answer
COCl₂, carbonyl chloride.
2.What is chloropicrin and how is it made?Show answer
CCl₃NO₂ (trichloronitromethane), from chloroform and concentrated HNO₃.
3.Colour of iodoform?Show answer
Yellow.
4.Which group gives the iodoform test?Show answer
CH₃CO– or CH₃CH(OH)–.
5.Product of chloroform with aqueous NaOH?Show answer
Sodium formate.
6.Main current use of chloroform?Show answer
Making the refrigerant R-22 (CHClF₂); also as a solvent.

Chloroform and Iodoform: frequently asked questions

Why is chloroform stored in dark coloured bottles?

Light and air oxidise it to poisonous phosgene; dark bottles cut out light, filling them to the top excludes air, and added ethanol destroys any phosgene.

What is the iodoform test?

Warming a compound with iodine and sodium hydroxide; a yellow precipitate of iodoform shows a CH₃CO– or CH₃CH(OH)– group.

What is the carbylamine reaction?

Heating a primary amine with chloroform and alcoholic KOH gives an isocyanide with a very unpleasant smell; it is a test for primary amines.

Why is iodoform an antiseptic?

It liberates free iodine, which kills microbes.

How do freons damage the ozone layer?

In the stratosphere, UV light breaks C–Cl bonds to give chlorine radicals, which react with ozone in a chain reaction and deplete it.