Organic Compounds Giving Positive Tollens Test

To determine which of the given compounds give a positive Tollens' test, we analyze the reactivity of each compound towards Tollens' reagent (ammoniacal silver nitrate solution, $[\text{Ag}(\text{NH}_3)_2]^+\text{OH}^-$):

1. Option (A): Propenal ($\text{CH}_2=\text{CH}-\text{CHO}$) Propenal is an aliphatic aldehyde. All aldehydes (aliphatic and aromatic) readily reduce Tollens' reagent to metallic silver, forming a silver mirror: $\text{CH}_2=\text{CH}-\text{CHO} + 2[\text{Ag}(\text{NH}_3)_2]^+ + 3\text{OH}^- \longrightarrow \text{CH}_2=\text{CH}-\text{COO}^- + 2\text{Ag}\downarrow + 4\text{NH}_3 + 2\text{H}_2\text{O}$ Therefore, (A) gives a positive Tollens' test.

2. Option (B): Benzaldehyde ($\text{C}_6\text{H}_5\text{CHO}$) Benzaldehyde is an aromatic aldehyde. Like aliphatic aldehydes, it reduces Tollens' reagent to metallic silver: $\text{C}_6\text{H}_5\text{CHO} + 2[\text{Ag}(\text{NH}_3)_2]^+ + 3\text{OH}^- \longrightarrow \text{C}_6\text{H}_5\text{COO}^- + 2\text{Ag}\downarrow + 4\text{NH}_3 + 2\text{H}_2\text{O}$ Therefore, (B) gives a positive Tollens' test.

3. Option (C): Benzoin ($\text{Ph}-\text{CH(OH)}-\text{C}(=\text{O})-\text{Ph}$) Benzoin is an $\alpha$-hydroxy ketone. Under the alkaline conditions of Tollens' reagent, $\alpha$-hydroxy ketones tautomerize through an enediol intermediate and undergo oxidation to 1,2-diketones (benzil), reducing $\text{Ag}^+$ ions to metallic silver: $\text{Ph}-\text{CH(OH)}-\text{C}(=\text{O})-\text{Ph} + 2[\text{Ag}(\text{NH}_3)_2]^+ + 2\text{OH}^- \longrightarrow \text{Ph}-\text{C}(=\text{O})-\text{C}(=\text{O})-\text{Ph} + 2\text{Ag}\downarrow + 4\text{NH}_3 + 2\text{H}_2\text{O}$ Therefore, (C) gives a positive Tollens' test.

4. Option (D): Chalcone ($\text{Ph}-\text{CH}=\text{CH}-\text{C}(=\text{O})-\text{Ph}$) Chalcone is an $\alpha,\beta$-unsaturated ketone. Simple ketones and $\alpha,\beta$-unsaturated ketones do not possess the easily oxidizable aldehyde functional group or an $\alpha$-hydroxy ketone moiety; hence, they do not reduce Tollens' reagent. Therefore, (D) does not give a positive Tollens' test.

Thus, the correct options are (A), (B), and (C).