Alkanals can be distinguished from alkanones by the reaction with

Analysis

The question asks for a chemical reagent that can differentiate between alkanals (aldehydes) and alkanones (ketones). Both groups contain the carbonyl functional group ($C=O$), but they differ in their reactivity due to the position of this group.

Concept Explanation

• Alkanals (Aldehydes): These have the carbonyl group at the end of the carbon chain. Because of this position, they are easily oxidized to carboxylic acids.
• Alkanones (Ketones): These have the carbonyl group in the middle of the carbon chain. They are resistant to mild oxidation.

To distinguish them, we use mild oxidizing agents. When an alkanal is heated with a mild oxidizing agent, it gets oxidized, causing a visible color change. An alkanone will not react under the same conditions, so no color change occurs.

Step-by-Step Solution

1. Evaluate the options: • A. Sudan III stain: This is used in biology to test for the presence of fats/lipids, not for distinguishing carbonyl compounds. • B. Starch iodide paper: This is typically used to test for the presence of oxidizing agents (like chlorine or peroxides), not specifically to distinguish aldehydes from ketones. • C. Lithium tetrahydrido aluminate (III) ($LiAlH_4$): This is a strong reducing agent. It will reduce both alkanals and alkanones to their respective alcohols, so it cannot be used to distinguish them. • D. Fehling's solution: This is a mild oxidizing agent containing copper(II) ions. • When heated with an alkanal, the blue $Cu^{2+}$ ions are reduced to a reddish-brown precipitate of copper(I) oxide ($Cu_2O$). • When heated with an alkanone, there is no reaction, and the solution remains blue.

2. Conclusion: Because Fehling's solution reacts with alkanals but not with alkanones, it is the perfect reagent for distinguishing between the two.

Exam Tip

Remember the "Tollens' and Fehling's" rule: Aldehydes are "easy" to oxidize. They will react with mild oxidizing agents like Fehling's solution and Tollens' reagent (silver mirror test). Ketones are "stubborn" and will only react with very strong oxidizing agents under harsh conditions.