An unsaturated hydrocarbon X on ozonolysis gives A. Compound A when warmed with ammonical silver nitrate forms a bright silver mirror along the sides of the test tube. The unsaturated hydrocarbon X is :

Solution

Chemistry ma’am · 3 steps

A silver mirror means an aldehyde, so look for a multiple-bond carbon that still carries an H.

1. Read the test. A silver mirror with ammoniacal silver nitrate (Tollens' reagent) is the test for an aldehyde group, -CHO, or for HCOOH. Ketones do not give it. So compound A must carry a C-H bond on a carbonyl carbon.
2. Where CHO comes from. Ozonolysis cuts the multiple bond and turns each of its carbons into a C=O carbon. Whatever was attached to that carbon stays attached. A carbon that carried an H becomes -CHO (or HCOOH). A carbon carrying only alkyl groups becomes a ketone carbon.

   Hydrocarbon	H on a multiple-bond carbon?	Ozonolysis gives	Silver mirror?
   (a) HC≡C-CH₂CH₃	Yes, the terminal ≡ CH	CH₃CH₂COCHO, or CH₃CH₂COOH + HCOOH after full cleavage	Yes
   (b) (CH₃)₂C=cyclopropylidene	No	cyclopropanone + (CH₃)₂CO	No
   (c) CH₃C≡CCH₃	No	CH₃COCOCH₃, or 2CH₃COOH	No
   (d) (CH₃)₂C=C(CH₃)₂	No	2(CH₃)₂CO	No

3. So. Only but-1-yne has an H on a carbon of the multiple bond. Its ozonolysis product keeps a -CHO end (or gives HCOOH), and that gives the silver mirror. X is (a).

The idea to keep. Before ozonolysis, check each carbon of the double or triple bond. One with an H gives an aldehyde (Tollens' positive). One with only alkyl groups gives a ketone (Tollens' negative).

Answer from the NTA final answer key. Solution by PrepWiser.