Fehling's test principle

Fehling's solution is reduced by aliphatic aldehydes to give a red precipitate of cuprous oxide, $\text{Cu}_2\text{O}$.

• Aliphatic aldehydes: give positive test
• Aromatic aldehydes: generally do not give positive test
• Ketones: do not give positive test

Check each compound one by one

(i) Benzaldehyde

Structure: aromatic aldehyde

Therefore, it does not give Fehling's test.

(ii) Acetaldehyde

Structure: aliphatic aldehyde, $\text{CH}_3\text{CHO}$

Therefore, it gives positive Fehling's test.

(iii) Acetone

Structure: ketone, $\text{CH}_3\text{COCH}_3$

Therefore, it does not give Fehling's test.

(iv) Acetophenone

Structure: ketone, $\text{C}_6\text{H}_5\text{COCH}_3$

Therefore, it does not give Fehling's test.

(v) Methanal

Structure: aliphatic aldehyde, $\text{HCHO}$

Therefore, it gives positive Fehling's test.

(vi) 4-Nitrobenzaldehyde

Structure: aromatic aldehyde

Therefore, it does not give Fehling's test.

(vii) Cyclohexane carbaldehyde

Here the $-\text{CHO}$ group is attached to an alicyclic ring, not directly to an aromatic ring. It behaves like an aliphatic aldehyde.

Therefore, it gives positive Fehling's test.

Count positive cases

Positive Fehling's test is given by:

• Acetaldehyde
• Methanal
• Cyclohexane carbaldehyde

So, total number of compounds = $3$.

Comparison with stored correct answer

Stored correct answer = $3$

Derived answer = $3$

Hence, the derived answer agrees with the stored correct answer.