Benzaldehyde gives Tollens' test but not Fehling's, and which compounds give the iodoform test?
Nisha Verma12th Sample
16 Oct 2026 · 1 min read
Revision month, chemistry, distinction tests. Every board and NEET paper has at least one "which reagent distinguishes X from Y" question, so I need this airtight.
The thing I got wrong in a mock: "Benzaldehyde and propanal can be distinguished by..." I said Tollens' reagent, and apparently the right answer was Fehling's. I thought both tests detect aldehydes so either should work. What is the real difference between the two?
Second, the iodoform test. I know the reagent is with NaOH and the product is yellow . I also know ethanal and propanone are positive. But I'm not sure about acetophenone, pentan-2-one, pentan-3-one, ethanol, and propan-2-ol. What's the rule? Any pattern I can apply to an unfamiliar structure?
Third: how do I distinguish a carboxylic acid from a phenol, or from an aldehyde? There are always three or four near-identical options and I panic.
I'd like a list of compounds with ✓ or ✗ for Tollens, Fehling, and iodoform. I'll make the master table if you give me data.
7 answers & replies
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[+][–]Ishita GoyalDropperSample· 4d ago
Here's the table:
Compound Tollens Fehling Iodoform Methanal ✓ ✓ ✗ Ethanal ✓ ✓ ✓ Propanal ✓ ✓ ✗ Benzaldehyde ✓ ✗ ✗ Propanone ✗ ✗ ✓ Pentan-2-one ✗ ✗ ✓ Pentan-3-one ✗ ✗ ✗ Acetophenone ✗ ✗ ✓ Benzophenone ✗ ✗ ✗ Benzaldehyde vs propanal is exactly why Fehling's was the answer: propanal reduces it, benzaldehyde doesn't.
[+][–]Nisha Verma12thSample· 3d agoAuthor
Master table done. Pasted in, thanks Ishita. And Tushar, the strength explanation is what I'll write as the reason.
[+][–]Rohan Sharma11thSample· 4d ago
isn't fehling basically the same test as tollens, just with copper instead of silver? benzaldehyde is an aldehyde so it should give both. also tollens is for aldehydes and fehling for ketones, i think? 😅
[+][–]Tushar PawarDropperSample· 3d ago
Rohan, partly. Both detect the aldehyde group because it's easily oxidised, but they differ in strength. Tollens' reagent (ammoniacal silver nitrate) is the milder-but-effective oxidant that works with both aliphatic and aromatic aldehydes: silver mirror. Fehling's solution is in alkaline tartrate, and it oxidises only aliphatic aldehydes: red-brown precipitate. Aromatic aldehydes like benzaldehyde don't respond. Ketones give neither (they can't be oxidised easily). So neither of them is for ketones.
[+][–]Rohan Sharma11thSample· 3d ago
oh so tollens = both aldehydes, fehling = aliphatic only. and ketones neither. okay, that makes sense now
[+][–]Aman YadavDropperSample· 3d ago
Iodoform rule. Positive if the compound has (methyl ketone or ethanal) or (ethanol, propan-2-ol, and other secondary alcohols with methyl on the carbinol carbon). Methanol: no. Acetic acid: no. Pentan-3-one: no, both sides are ethyl. Yellow ppt, antiseptic smell.
Another test to know: 2,4-DNP reagent gives an orange-yellow precipitate with both aldehydes and ketones. That separates carbonyls from alcohols and acids.
[+][–]Riya Singh12thSample· 3d ago
Acid tests. Sodium bicarbonate: carboxylic acids give brisk effervescence of CO2, phenols don't (phenol is weaker than carbonic acid). So benzoic acid vs phenol: use NaHCO3. Neutral gives violet with phenol, not with benzoic acid (benzoic acid gives a buff-coloured precipitate with it, but NEET won't ask that).
Special case: formic acid, which has an aldehyde-like H, reduces Tollens' and Fehling's reagents, unlike other carboxylic acids.
