Q.There are two isomers with the formula C2H5NO2. How will you distinguish between them?

Step 1. C2H5NO2 has exactly two constitutional (functional) isomers on a 2-carbon skeleton: nitroethane, CH3CH2-NO2 (nitrogen bonded to carbon), and ethyl nitrite, CH3CH2-O-N=O (nitrogen bonded through oxygen, an ester of nitrous acid).

Step 2. Nitroethane is a primary nitroalkane with an alpha-hydrogen, so it exists in equilibrium with a small amount of its aci tautomer -- and the unit's own nitro/aci comparison table states the nitro form specifically DECOLOURISES FeCl3 solution, a distinctive test response tied directly to this tautomeric system.

Step 3. Ethyl nitrite has no C-NO2 bond and no nitro/aci tautomerism at all -- it is simply an ester-like N-O-C linkage -- so it gives NO such FeCl3 colour response.

Step 4. Instead, ethyl nitrite (like any alkyl nitrite) hydrolyses readily on treatment with dilute acid, releasing nitrous acid, which decomposes to give characteristic brown/reddish nitrous fumes -- a response nitroethane, being far more hydrolytically stable, does not show under the same mild conditions.

Step 5. So testing a sample with FeCl3 (nitroethane responds, ethyl nitrite does not) or with dilute acid (ethyl nitrite readily liberates brown nitrous fumes, nitroethane does not) cleanly distinguishes the pair.

Use the structural difference (C-bonded nitro vs O-bonded nitrite) to predict two independent, testable differences: the FeCl3/nitro-aci response (nitroalkane only) and ease of acid hydrolysis releasing nitrous fumes (nitrite ester, much more readily).

• Trying to distinguish the pair purely by smell or physical state -- both are simple small liquids with only subtle sensory differences; the FeCl3 colour test and the differential ease of acid hydrolysis are the chemically reliable distinguishing tests, directly grounded in each compound's own bonding.