Lucas test turbidity timing for primary, secondary and tertiary alcohols, and why neutral FeCl3 works only for phenol
Ishita GoyalDropper Sample
15 Oct 2026 · 1 min read
I'm building my one-page template for alcohols, phenols and ethers (distinction tests on top, named reactions below) and a few cells are still empty.
Lucas test: I know the reagent is conc. HCl with anhydrous ZnCl2. I know the three alcohol classes give different results, but my notes say "3° immediate, 2° five minutes, 1° none" in one place and I have seen the opposite order in a friend's notes. I want to understand why so I never mix it up.
Phenol tests: neutral FeCl3 gives a coloured complex with phenol and nothing with ethanol, and bromine water gives a white precipitate. I can memorise but I would like to know why only phenol responds.
Named reactions: Kolbe's reaction, Reimer–Tiemann reaction, and Williamson synthesis. I have the products but not the exact reagents and conditions, and I'm not sure what goes wrong when you use a tertiary alkyl halide in Williamson.
I made a table with columns for reagent, observation, and reason. Anyone can fill in the gaps, or tell me if my "reason" column is wrong.
7 answers & replies
Sign in to join the conversation.
[+][–]Zoya Khan12thSample· 5d ago
Lucas reagent: conc. HCl + anhydrous ZnCl2. The alcohol is converted to an alkyl chloride, which is insoluble in the reagent and so makes the clear solution turn cloudy. The rate depends on how easily the carbocation forms, since the reaction goes via SN1.
- Tertiary alcohols: turbidity appears immediately
- Secondary alcohols: turbidity in about 5 minutes
- Primary alcohols: no turbidity at room temperature (only on heating)
The test is useful for alcohols that dissolve in the reagent, up to about six carbons.
[+][–]Ishita GoyalDropperSample· 4d agoAuthor
"cloudy because the alkyl chloride is insoluble" is the reason I was missing. Updating the reason column now, thanks all!
[+][–]Harsh Gupta12thSample· 5d ago
I'd guess primary is fastest, it's the least crowded, nothing blocks the attack. Tertiary is bulky and should be slowest. So the opposite of what your notes say, ishita 😅
[+][–]Neel Chatterjee12thSample· 5d ago
Harsh, this is SN1, not SN2, so crowding at the carbon doesn't matter. What matters is the stability of the carbocation intermediate: . The tertiary carbocation is stabilised by three alkyl groups (inductive effect and hyperconjugation), so it forms fastest and the reaction goes immediately. The primary carbocation is so unstable that the reaction barely goes at room temperature. Your SN2 intuition would be correct for the Williamson ether synthesis, which is the next question.
[+][–]Harsh Gupta12thSample· 5d ago
okay SN1 vs SN2 mix-up, as usual. thanks neel, I'll go sit in the corner
[+][–]Mehak AroraDropperSample· 5d ago
Phenol tests. Neutral gives a violet (blue-violet) colour with phenol because the phenolic OH forms a coloured iron complex; simple alcohols don't give it. Bromine water: the –OH group strongly activates the benzene ring toward electrophilic substitution, so phenol reacts with bromine water without any catalyst and gives a white precipitate of 2,4,6-tribromophenol, and the bromine water is decolourised.
Another tip: phenol is acidic enough to dissolve in NaOH, but it's weaker than carbonic acid so it doesn't react with NaHCO3, while a carboxylic acid does.
[+][–]Vivek Mishra12thSample· 5d ago
Named reactions with conditions:
- Kolbe's reaction: sodium phenoxide + CO2 at about 400 K and 4–7 atm, then acidify, gives salicylic acid.
- Reimer–Tiemann: phenol + chloroform + aqueous NaOH at about 340 K, then hydrolysis, gives salicylaldehyde (o-hydroxybenzaldehyde). The electrophile is dichlorocarbene, .
- Williamson synthesis: sodium alkoxide + alkyl halide, SN2, gives ether. Works well with primary alkyl halides. With a tertiary alkyl halide the alkoxide acts as a base and you get an alkene by elimination, not an ether.
