What does the Fehling’s test detect?

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Question restatement: What does the Fehling’s test detect?

Option a. all of above: This is not correct because Fehling’s test does not indicate all possible substances; it is specific to reducing sugars, not proteins or fats.

Option b. Presence of polypeptide chain: Fehling’s test does not detect polypeptides or proteins. Protein components are not oxidized in this reaction, so this option is inaccurate.

Option c. Presence of fatty acids: Fehling’s test is not used for fatty acids or lipid oxidation. Lipids do not produce the red copper(I) oxide result characteristic of Fehling’s test.

Option d. Presence of reducing sugars: This is correct. Fehling’s test identifies reducing sugars (e.g., glucose, galactose) because they can reduce cupric ions (Cu2+) to red cuprous oxide (Cu2O) under alkaline conditions, producing the characteristic brick-red precipitate.

Explanation of why the correct option works: Reducing sugars possess free aldehyde or ketone groups that can act as reducing agents. In the Fehling’s reaction, these groups reduce the copper(II) ions in the solution to copper(I) oxide, yielding the visible red precipitate. This assay is historically used to distinguish reducing sugars from non-reducing sugars and to test for the presence of aldehyde or enediol functional groups that can be oxidized.

Why the other options are incorrect: The test is not designed to detect polypeptide chains or proteins, so option b mischaracterizes the assay’s scope. It is also not a test for fatty acids or general lipids, so option c is irrelevant to Fehling’s chemistry. Finally, option a would imply a broad detection that includes substances Fehling’s cannot detect, which is inaccurate since the reaction is specific to reducing sugars and does not sense the other classes listed.

What does the Fehling’s test detect?

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