Haloform

8 MCQs9-step worked example
Source: NCERT BiomoleculesPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

The haloform reaction is one of the cleanest "yes/no" tests in organic chemistry — and the trap that costs marks is treating it as if every ketone qualifies.

What the reaction is. When a compound containing a methyl carbonyl group (CH₃–CO–) or a structure oxidisable to one is treated with excess NaOH/I₂ (or NaOH/Cl₂, NaOH/Br₂), the three α-hydrogens on the methyl group are successively replaced by halogen atoms. The resulting trihalomethyl group is then cleaved by base, yielding a haloform (CHI₃, CHCl₃, or CHBr₃) and the sodium salt of a carboxylic acid with one fewer carbon.

The iodoform variant uses I₂/NaOH. A yellow precipitate of iodoform (CHI₃, mp 119 °C) is the positive test result, as noted in NCERT Class 12 Chemistry Chapter 12, Part 2, page 18.

Which substrates give a positive iodoform test — the complete list:

  • Methyl ketones: CH₃–CO–R (acetone, acetophenone, etc.)
  • Ethanal (acetaldehyde): CH₃–CHO
  • Ethanol: CH₃–CH₂–OH (oxidised in situ to ethanal)
  • Secondary alcohols of the form CH₃–CH(OH)–R (oxidised in situ to methyl ketones)

The high-frequency trap: aspirants mark non-methyl ketones (e.g., diethyl ketone, benzophenone) or primary alcohols longer than ethanol as positive. They are not. The structural requirement is strict — a CH₃CO– unit must be present or generatable by mild oxidation. Pentan-3-one (CH₃CH₂COCH₂CH₃) has no methyl group adjacent to the carbonyl and does NOT give the test.

NEET context. This topic appears as a direct recall or single-step application question, typically asking "which of the following gives a positive iodoform test?" with four structures. Speed matters: if you can spot the CH₃CO– motif in seconds, the mark is free. If you guess, the –1 negative marking penalty hits.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

Which one of the following compounds gives a positive iodoform test?

MCQ 2Easy RecallPractice

The iodoform test uses I₂/NaOH. What is the characteristic observation for a positive test?

MCQ 3Easy RecallPractice

Which of the following alcohols gives a positive iodoform test?

MCQ 4Direct ApplicationPractice

Among the following, how many compounds give a positive iodoform test?

MCQ 5Direct ApplicationPractice

Propan-2-ol gives a positive iodoform test, but propan-1-ol does not. The correct reason is:

MCQ 6Direct ApplicationPractice

Compound X gives a positive iodoform test and on treatment with I₂/NaOH produces sodium benzoate as one of the products. Compound X is:

MCQ 7Concept TrapPractice

A student claims: "All ketones give a positive iodoform test." This claim is:

MCQ 8CalculationPractice

An unknown liquid Y (molecular formula C₄H₈O) gives a positive iodoform test and does not reduce Tollens' reagent. The structure of Y is:

Worked Example

  1. 1

    Given

    Five organic compounds to evaluate for the iodoform test (I₂/NaOH).

  2. 2

    Required

    Identify which compounds give a positive iodoform test (yellow precipitate of CHI₃).

  3. 3

    Concept

    The haloform reaction requires a CH₃–CO– group, either already present or generated by in-situ oxidation of the substrate. Methyl ketones, ethanal, ethanol, and secondary alcohols of the form CH₃–CH(OH)–R qualify.

  4. 4

    Formula / Rule

    Structural test: does the compound have or generate a CH₃CO– unit under I₂/NaOH conditions?

  5. 5

    Substitution (compound-by-compound check)

    | Compound | Structure | Oxidation product (if alcohol) | CH₃CO– present? | Result | |---|---|---|---|---| | Ethanol | CH₃CH₂OH | → CH₃CHO (ethanal) | Yes | ✓ | | Propan-1-ol | CH₃CH₂CH₂OH | → CH₃CH₂CHO (propanal) | No (–CH₂CH₃ on C=O) | ✗ | | Propan-2-ol | CH₃CH(OH)CH₃ | → CH₃COCH₃ (acetone) | Yes | ✓ | | Pentan-3-one | CH₃CH₂COCH₂CH₃ | (already a ketone) | No (–CH₂CH₃ on both sides) | ✗ | | Ethanal | CH₃CHO | (already an aldehyde) | Yes | ✓ |

  6. 6

    Calculation

    Count of positive results = 3 (ethanol, propan-2-ol, ethanal).

  7. 7

    Final answer

    Ethanol, propan-2-ol, and ethanal give a positive iodoform test. Propan-1-ol and pentan-3-one do not.

  8. 8

    Common trap

    The most common error is including pentan-3-one because "it's a ketone" — but having a carbonyl is not enough. The CH₃ must be directly bonded to the C=O carbon. Pentan-3-one has –CH₂CH₃ groups on both sides. A second common error is excluding ethanol because "it's an alcohol, not a carbonyl compound" — but ethanol is oxidised in situ to ethanal.

  9. 9

    Similar NEET-style question

    "Among the following — acetophenone, benzophenone, butan-2-one, pentan-3-one, and methanol — how many give a positive iodoform test?" (Answer: 2 — acetophenone and butan-2-one are methyl ketones; the others lack the CH₃CO– unit.) ---

Before solving, remember these

Key Fact

Haloform reaction

Methyl ketones (or 2° alcohols oxidisable to methyl ketones, ethanol) + X₂/NaOH → CHX₃ (haloform) + RCOO⁻. Iodoform test: distinguishes methyl ketones; yellow precipitate of CHI₃.

-- NCERT, p. 18

Formulas

pKa of carboxylic acid

Stronger acid than phenol due to more effective resonance over two equivalent oxygens. EWG substituents (Cl, NO2) increase acidity.

SymbolQuantitySI Unit
pKa-log Ka-

Valid when

  • Aqueous solution
  • α-substituent effects strongest

pKa of phenol vs aliphatic alcohol

Phenols ~10⁶× more acidic than aliphatic alcohols due to resonance stabilisation of phenoxide ion. Electron-withdrawing substituents lower pKa further.

SymbolQuantitySI Unit
pKa-log Ka-

Valid when

  • Aqueous solution
  • Substituent effects shift values

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Oxidation product depends on reagent strength

Category: Organic Reaction Conditions

1° alcohol: PCC/PDC → aldehyde (stops). KMnO4/K2Cr2O7 in acidic → carboxylic acid (continues). 2° alcohol: any oxidiser → ketone. 3° alcohol: not oxidised by ordinary reagents.

When it triggers

Question gives 1° alcohol oxidation with specified reagent.

How to avoid

PCC, PDC, Swern, DMP: mild → stop at aldehyde. KMnO4, K2Cr2O7, CrO3, jones: strong → carboxylic acid. Reagent choice matches desired product.

Past Year Questions

20 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2024Revised key

Given below are two statements: Statement I : Aniline does not undergo Friedel-Crafts alkylation reaction. Statement II : Aniline cannot be prepared through Gabriel synthesis . In the light of the above statements, choose the correct answer from the options given below:

1Both statement I and Statement II are true
2Both Statement I and Statement II are false
3Statement I is correct but Statement II is false
4Statement I is incorrect but Statement II is true
NTA Answer: Option 1(revised_final)
NEET 2024Revised key

Fehling’s solution ‘A’ is

1aqueous copper sulphate
2alkaline copper sulphate
3alkaline solution of sodium potassium tartrate (Rochelle’s salt)
4aqueous sodium citrate
NTA Answer: Option 1(revised_final)
NEET 2022

Given below are two statements: Statement I: In Lucas test, primary, secondary and tertiary alcohols are distinguished on the basis of their reactivity with conc. HCl + ZnCl , known as Lucas Reagent. 2 Statement II: Primary alcohols are most reactive and immediately produce turbidity at room temperature on reaction with Lucas Reagent. In the light of the above statements, choose the most appropriate answer from the options given below:

1Statement I is incorrect but Statement II is correct
2Both Statement I and Statement II are correct
3Both Statement I and Statement II are incorrect
4Statement I is correct but Statement II is incorrect
NTA Answer: Option 4(final)
NEET 2021

Match List-I with List-II List-I List-II CO, HCI (a) (i) Hell-Volhard-Zelinsky Anhyd. AlCl/ Cu3Cl reaction O R — C — CH + (b) (ii) Gattermann-Koch 3 NaOX reaction R — CH — OH (c) (iii) Haloform reaction 2 +R′ COOH Conc. HSO 2 4 (d) R — CH COOH (iv) Esterification 2 ( II) X /Red P 2 (ii) HO 2 Choose the correct answer from the options given below.

1(a) - (ii), (b) - (iii), (c) - (iv), (d) - (i)
2(a) - (iv), (b) - (i), (c) - (ii), (d) - (iii)
3(a) - (iii), (b) - (ii), (c) - (i), (d) - (iv)
4(a) - (i), (b) - (iv), (c) - (iii), (d) - (ii)
NTA Answer: Option 1(final)

How NEET usually asks this

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