Iodoform Test, Phthalein Test and Other Tests for Alcohols and Phenols
Chemistry · Alcohols, Phenols And Ethers · NEET
The iodoform test gives a yellow solid (CHI3) with any alcohol that has the CH3-CH(OH)- group, like ethanol or 1-phenylethanol. The phthalein test and the FeCl3 test are used for phenols: phenol + FeCl3 gives a violet colour, and phenol + phthalic anhydride gives phenolphthalein that turns pink with NaOH. Memory hook: "Iodoform needs a CH3 next to the OH; phenol loves colour."
Quick chooser: the iodoform test (yellow CHI3) flags alcohols with a CH3 on the OH carbon; FeCl3 (violet) and the phthalein dye test (pink with NaOH) flag phenols.
Your doubts, answered
Which alcohols give a positive iodoform test?
Only alcohols with the CH3-CH(OH)- group give it. The reagent (I2 + NaOH, i.e. NaOI) first oxidises this alcohol to a methyl ketone CH3-CO-, then replaces the three CH3 hydrogens with iodine and breaks the bond to give yellow iodoform CHI3. Examples that pass: ethanol (CH3CH2OH), all secondary alcohols like propan-2-ol and 1-phenylethanol. So look for a CH3 directly attached to the C-OH carbon.
Why does methanol NOT give the iodoform test but ethanol does?
Methanol is CH3OH. Its carbon bearing OH has no CH3 group next to it, and it cannot form a CH3-CO- unit. Ethanol is CH3CH2OH; on oxidation it becomes acetaldehyde CH3CHO, which has the needed CH3-CO- part, so it gives iodoform. Rule: the carbon holding the OH must also carry a CH3. Methanol fails, ethanol passes.
What is the difference between the iodoform test and the phthalein test?
They detect different things. The iodoform test detects CH3CH(OH)- alcohols (and methyl ketones and acetaldehyde) and gives a yellow precipitate CHI3 with a sharp smell. The phthalein (dye) test detects a phenolic -OH: phenol is heated with phthalic anhydride and conc. H2SO4 to make phenolphthalein, which turns pink/red when NaOH is added. One is for special alcohols, the other is for phenols.
How do you tell an alcohol apart from a phenol in the lab?
Add neutral ferric chloride (FeCl3). Phenols give a violet, blue or green colour; simple alcohols give no colour. Phenols also turn blue litmus red (weakly acidic) and dissolve in NaOH, while alcohols do not. The phthalein dye test is another phenol-only test. So colour with FeCl3 = phenol.
Which functional group is identified by the phthalein dye test?
The phenolic -OH group. Aldehydes, ketones, carboxylic acids and ordinary alcohols do NOT respond. This is a common one-line NTA question, and it appeared in NEET 2026 with the answer 'Phenolic'.
⚠️ The NEET trap ✗ Thinking every alcohol (including methanol) gives the iodoform test, or that the phthalein test detects alcohols. ✓ Iodoform needs the CH3-CH(OH)- unit, so methanol fails but ethanol, propan-2-ol and 1-phenylethanol pass. The phthalein dye test detects a phenolic -OH only, not alcohols. 🧠 CH3 must sit on the OH carbon for iodoform. Phthalein and FeCl3 belong to phenol.
Real NEET questions
NEET 2018
Compound A, C8H10O, reacts with NaOI (produced by reacting Y with NaOH) to yield a yellow precipitate with a characteristic smell. A and Y are respectively:
A · C6H5CH(OH)CH3 (1-phenylethanol) and I2 ✓
B · C6H5CH2CH2OH (2-phenylethanol) and I2
C · p-CH3C6H4CH2OH (4-methylbenzyl alcohol) and I2
D · p-Cresol (4-methylphenol) and I2
Solution: The yellow precipitate with a sharp smell is iodoform CHI3, so this is a positive iodoform test. NaOI is made from I2 + NaOH, so Y = I2. The test needs the CH3-CH(OH)- group. Among the C8H10O choices, only 1-phenylethanol C6H5CH(OH)CH3 has this unit. It is oxidised to the methyl ketone C6H5COCH3 and then gives CHI3. Answer: A.
NEET 2026 (1)
The functional group that can be identified through the phthalein dye test is:
A · Aldehyde
B · Phenolic ✓
C · Carboxylic acid
D · Alcohol
Solution: In the phthalein dye test a phenol is heated with phthalic anhydride and conc. H2SO4 to form phenolphthalein, which turns pink/red on adding NaOH. This confirms a phenolic -OH group. Aldehydes, carboxylic acids and ordinary alcohols do not respond. Answer: B (Phenolic).
NEET 2018
Identify the major products P, Q and R: C6H6 + CH3CH2CH2Cl --(anhyd. AlCl3)--> P ; P --(i) O2 (ii) H3O+/heat--> Q + R.
A · P: CH3CH(OH)CH3, Q: acetone, R: phenol
B · P: C6H5CH2CH2CH3, Q: benzaldehyde, R: benzoic acid
C · P: C6H5CH2CH2CH3, Q: benzaldehyde, R: CH3CH2OH
Solution: In Friedel-Crafts alkylation the 1-propyl cation rearranges by a 1,2-hydride shift to the more stable isopropyl cation, giving cumene (P). Air (O2) forms cumene hydroperoxide, which with H3O+/heat splits into phenol (Q) and acetone (R). This is the cumene process; the phenol produced would then answer FeCl3 and phthalein tests, the acetone gives iodoform. Answer: D.
Solved Alcohols, Phenols And Ethers NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Only secondary alcohols with a CH3 on the C-OH carbon (that is, CH3-CH(OH)-R) give it, because only these can form a methyl ketone. Propan-2-ol and 1-phenylethanol pass; cyclohexanol does not.
What colour does phenol give with neutral FeCl3?
Phenol gives a violet or purple colour with neutral ferric chloride. This is a quick way to confirm a phenolic -OH. Alcohols give no such colour.
Does acetone give the iodoform test?
Yes. Acetone (CH3-CO-CH3) is a methyl ketone, so it gives a positive iodoform test. Any CH3-CO- or CH3-CH(OH)- compound gives it.
Is the iodoform test the same as the haloform test?
Yes. The iodoform test is the iodine version of the general haloform test. With I2/NaOH it gives yellow iodoform CHI3; with Cl2 or Br2 it gives the corresponding haloform.
Why is the iodoform test important for NEET?
NEET often gives a compound and asks which one forms a yellow precipitate or gives the iodoform test. Knowing the CH3-CH(OH)- and CH3-CO- rule lets you pick the answer in seconds, as in the 2018 1-phenylethanol question.
