Question
Distinguish between acetaldehyde () and acetone () using two simple chemical tests.
Solution — Step by Step
Add Tollens’ reagent (ammoniacal silver nitrate) to both. Acetaldehyde, being an aldehyde, reduces Tollens’ reagent to metallic silver, forming a shiny silver mirror on the test tube wall. Acetone (a ketone) does not react.
Reaction with aldehyde:
Both acetaldehyde and acetone give a positive iodoform test (yellow precipitate of CHI) because both have the group. So this test does NOT distinguish them.
Use Fehling’s solution instead. Fehling’s gives a brick-red precipitate of CuO with aldehydes; ketones do not react.
Final answer: (i) Tollens’ test: silver mirror with acetaldehyde, no reaction with acetone. (ii) Fehling’s test: red ppt with acetaldehyde, no reaction with acetone.
Why This Works
The C-H bond on the carbonyl carbon makes aldehydes easy to oxidise to carboxylic acids. Tollens’ (Ag) and Fehling’s (Cu) are mild oxidising agents that abstract this hydrogen, getting reduced themselves to metallic silver and CuO respectively. Ketones lack this α-H on the carbonyl carbon, so they don’t react.
The iodoform test is for methyl ketones AND ethanol/acetaldehyde — anything with a or group. Since both acetaldehyde and acetone contain , both give iodoform.
Alternative Method
Schiff’s reagent: pink colour with aldehydes (acetaldehyde gives pink), no colour with ketones (acetone stays colourless). Another quick visual test.
Memorise which tests separate aldehydes from ketones (Tollens’, Fehling’s, Schiff’s, Benedict’s) and which group them together (iodoform — only for methyl ketones and acetaldehyde). Confusion here costs marks in NEET every year.
Common Mistake
Students often suggest the iodoform test to distinguish acetaldehyde from acetone. Both give a yellow CHI precipitate, so the test fails to distinguish them. The correct distinguishers are Tollens’ and Fehling’s — tests that exploit aldehydic C-H specifically.