Question

Which aldehyde does not give Fehling test?

• A. Methanal
• B. Propanal
• C. Ethanal
• D. Butanal
• E. Phenylmethanal

Hint:

Aromatic aldehydes like phenylmethanal do not give Fehling's test, whereas aliphatic aldehydes do.

Answer 1

Answer: (E)

Fehling's test is used to distinguish between aldehydes and ketones. Aldehydes generally react with Fehling's solution (a mixture of copper sulfate, sodium potassium tartrate, and sodium hydroxide), reducing the blue copper(II) ion to red copper(I) oxide, while ketones do not undergo this reaction. Step 1: Fehling's test principle.
Fehling's test involves the reduction of Cu²⁺ ions to Cu₂O (copper(I) oxide) in the presence of aldehydes. Most aldehydes, except aromatic aldehydes, give a positive result, while ketones and aromatic aldehydes generally do not.
Step 2: Explanation of options.
- Methanal (formaldehyde) is an aldehyde and gives a positive Fehling's test.
- Propanal and Butanal are also aldehydes and give a positive test.
- Ethanal (acetaldehyde) also reacts positively with Fehling's solution.
- Phenylmethanal (benzaldehyde) is an aromatic aldehyde, and aromatic aldehydes generally do not react with Fehling's solution.

Step 3: Conclusion.
Thus, the correct answer is (E) Phenylmethanal, as it does not give a positive Fehling's test.
Final Answer:} (E) Phenylmethanal