Question

Consider a compound A with formula ${{C}_{3}}{{H}_{6}}O$ forms phenyl hydrazine and gives a negative Tollens test. Compound A on reduction gives propane. Then identify compound A.
(A) it is a primary alcohol
(B) it is a aldehyde
(C) it is a ketone
(D) it is a secondary alcohol

Answer 1

Hint: From the question we can see that it gives phenylhydrazone on reaction. Phenylhydrazone can be formed only from carbonyl compounds. Further it has given that the compound A gives negative tollens test. This shows that it cannot be an aldehyde.

Complete answer:
Compound A is said to form phenylhydrazone on reaction, so it must be either aldehyde or ketone. Later, they are saying that it tests negative for the Tollens test. The Tollens test is also known as the Silver mirror test. Tollen’s reagent contains silver ammonia complex in ammonia solution. Tollens reagent should be used freshly. Aldehyde reacts with Tollen’s reagent to give silver mirror or grey-black precipitate. When aldehyde reacts with the Tollen’s reagent to form corresponding acids and silver will be reduced from the +1-oxidation state to the silver metal. Since any of these changes are not found, it cannot be aldehyde.
Ketones are the ones which test negative for the Tollen’s test. Hence the compound ‘A’ with the formula${{C}_{3}}{{H}_{6}}O$ will be acetone ($C{{H}_{3}}COC{{H}_{3}}$).
Further compound A, Acetone on reduction gives propane.
$C{{H}_{3}}COC{{H}_{3}}\xrightarrow{[H]}C{{H}_{3}}C{{H}_{2}}C{{H}_{3}}$

Additional Information:
Inorder to distinguish aldehyde and ketones the following tests can be done
- Schiff’s test
- Fehling’s test
- Test with chromic acid
- Sodium nitroprusside test

Note: - Acetone and Phenylhydrazine undergo condensation reaction by the removal of water molecules to give acetone phenylhydrazone.
- Primary alcohol on oxidation gives aldehyde.
- Secondary alcohol on oxidation gives ketone.