From the compounds given below, number of compounds which give positive Fehling's test is _______
Benzaldehyde, Acetaldehyde, Acetone, Acetophenone, Methanal, 4-nitrobenzaldehyde, cyclohexane carbaldehyde.

We need to count the number of compounds that give a positive Fehling's test from: Benzaldehyde, Acetaldehyde, Acetone, Acetophenone, Methanal, 4-nitrobenzaldehyde, Cyclohexane carbaldehyde.

Recall what Fehling's test detects.

Fehling's test uses Fehling's reagent (an alkaline solution of cupric tartrate, deep blue in colour) to detect aliphatic aldehydes. Aldehydes reduce $$Cu^{2+}$$ (blue) to $$Cu_2O$$ (red-brown precipitate). Important: aromatic aldehydes do NOT give a positive Fehling's test because the benzene ring reduces the reactivity of the aldehyde group. Ketones also do not give a positive test.

Analyse each compound.

- Benzaldehyde ($$C_6H_5CHO$$): Aromatic aldehyde. Does NOT give positive Fehling's test.

- Acetaldehyde ($$CH_3CHO$$): Aliphatic aldehyde. Positive.

- Acetone ($$CH_3COCH_3$$): Ketone. Does NOT give positive test.

- Acetophenone ($$C_6H_5COCH_3$$): Aromatic ketone. Does NOT give positive test.

- Methanal ($$HCHO$$): Aliphatic aldehyde (simplest one). Positive.

- 4-Nitrobenzaldehyde ($$O_2N-C_6H_4-CHO$$): Aromatic aldehyde. Does NOT give positive Fehling's test.

- Cyclohexane carbaldehyde ($$C_6H_{11}CHO$$): Aliphatic aldehyde (cyclohexane ring is not aromatic). Positive.

Count.

Positive Fehling's test: Acetaldehyde, Methanal, Cyclohexane carbaldehyde = 3.

The answer is 3.