An Unlabelled Propanol Bottle and a Failed Propanoic Acid Synthesis
A student finds a bottle labelled only “propanol” and needs to prepare propanoic acid by oxidation. The bottle’s contents are clear and miscible with water. The student runs three tests. (i) Lucas test: the mixture turns cloudy only after about 10 min at room temperature. (ii) Iodoform test: a yellow precipitate forms after warming with I2/NaOH. (iii) Oxidation with hot acidified K2Cr2O7: the distillate gives a yellow precipitate with 2,4-dinitrophenylhydrazine but no silver mirror with Tollens’ reagent. The student also has a separate bottle of propan-1-ol available. The planned route is: oxidize the unknown with excess acidified dichromate under reflux, then isolate the acid. The student asks what the unknown is, why the oxidation did not give an acid, and whether the route can be rescued by using harsher conditions or by switching to the propan-1-ol bottle.
1. Identify the unknown alcohol from the three tests and explain the chemistry of each observation. Include the iodoform reaction and the oxidation product.
Read the worked answer
The unknown is propan-2-ol, a secondary alcohol.
Lucas test: propan-2-ol is secondary. It is protonated and displaced by Cl− more slowly than a tertiary alcohol but faster than a primary alcohol, so cloudiness appears after several minutes at room temperature. Propan-1-ol would remain clear for much longer under the same conditions; a tertiary alcohol such as tert-butanol would turn cloudy almost immediately.
Iodoform test: propan-2-ol contains the CH3–CH(OH)– unit required for the haloform reaction. With I2/NaOH it is oxidized to propanone and gives yellow CHI3: CH3CH(OH)CH3 + 4I2 + 6NaOH → CHI3 + CH3COONa + 5NaI + 5H2O. Propan-1-ol does not give this precipitate.
Oxidation: hot acidified dichromate oxidizes propan-2-ol to propanone: 3CH3CH(OH)CH3 + K2Cr2O7 + 4H2SO4 → 3CH3COCH3 + Cr2(SO4)3 + K2SO4 + 7H2O. Propanone gives a yellow/orange 2,4-dinitrophenylhydrazone but does not reduce Tollens’ reagent, so the distillate is a ketone, not an aldehyde or acid.
2. Explain why oxidation of propan-2-ol stops at propanone and cannot give propanoic acid under these conditions. What would be required to make propanoic acid from the available materials?
Read the worked answer
A secondary alcohol has one hydrogen on the carbinol carbon and no hydrogen on an adjacent carbon that can be removed to form a C=O without breaking the carbon skeleton. Oxidation removes the carbinol H and forms propanone; the carbon skeleton C–C–C is retained. Further oxidation of propanone would require cleavage of a C–C bond. Hot alkaline KMnO4 or strong dichromate under drastic conditions can cleave propanone, but the products are mixtures such as acetic acid and CO2, not propanoic acid. There is no clean two-electron oxidation that converts CH3COCH3 into CH3CH2COOH without adding a carbon or rearranging the skeleton.
To make propanoic acid, use the propan-1-ol bottle. Propan-1-ol is a primary alcohol and is oxidized first to propanal and then to propanoic acid: CH3CH2CH2OH → CH3CH2CHO → CH3CH2COOH. A controlled oxidation with excess acidified K2Cr2O7 under reflux drives the reaction to the acid. If propanal is wanted, distill it as it forms or use a milder oxidant such as PCC, but for propanoic acid the acidified dichromate route is appropriate. Do not try to rescue the propan-2-ol route; switch starting materials.
3. Using the propan-1-ol bottle, propose a practical sequence that maximizes propanoic acid yield and avoids isolating the aldehyde intermediate. Include one chemical test to confirm the final acid.
Read the worked answer
Place propan-1-ol in a round-bottomed flask with excess acidified potassium dichromate (K2Cr2O7/H2SO4) and a few anti-bumping granules. Attach a reflux condenser and heat gently for long enough to oxidize the intermediate propanal to propanoic acid. Reflux is preferred over simple distillation because propanal has a low boiling point and would escape before further oxidation; keeping it in the reaction mixture lets it be oxidized to the acid.
After reaction, cool and isolate the acid. A common procedure is to distill or steam-distil the mixture, neutralize the acid with sodium carbonate or sodium hydroxide to form sodium propanoate, evaporate excess water, then acidify the concentrated salt with dilute sulfuric acid and isolate propanoic acid by distillation or extraction. Simpler for a school scale: extract the cooled mixture with a suitable organic solvent, dry, and distil.
Confirm the product is a carboxylic acid by adding a small portion to aqueous sodium hydrogen carbonate. Effervescence of CO2 shows an acidic proton stronger than that of an alcohol or phenol. The product should also give a broad O–H stretch and a strong C=O stretch in its IR spectrum if instrumentation is available, but the bicarbonate test is the quick bench confirmation.