From the compounds given below, the number of compounds which give a positive Fehling’s test is____.
Compounds: Benzaldehyde, Acetaldehyde, Acetone, Acetophenone, Methanal, 4-nitrobenzaldehyde, cyclohexane carbaldehyde.
Compounds: Benzaldehyde, Acetaldehyde, Acetone, Acetophenone, Methanal, 4-nitrobenzaldehyde, cyclohexane carbaldehyde.
Correct Answer: 3
Approach Solution - 1
To determine which compounds give a positive Fehling's test, we need to understand that Fehling's test is used to identify reducing sugars and aldehydes, except for aromatic aldehydes. It won't respond to ketones or aromatic aldehydes.
Analysis of given compounds:
- Benzaldehyde: An aromatic aldehyde, does not give a positive Fehling's test.
- Acetaldehyde (ethanal): An aliphatic aldehyde, gives a positive Fehling's test.
- Acetone: A ketone, does not give a positive Fehling's test.
- Acetophenone: A ketone with aromatic character, does not give a positive Fehling's test.
- Methanal (formaldehyde): An aldehyde, gives a positive Fehling's test.
- 4-nitrobenzaldehyde: An aromatic aldehyde, does not give a positive Fehling's test.
- Cyclohexane carbaldehyde: An aliphatic aldehyde, gives a positive Fehling's test.
Conclusion: The compounds that give a positive Fehling's test are Acetaldehyde, Methanal, and Cyclohexane carbaldehyde.
Computation: Total compounds = 3.
Approach Solution -2
Fehling’s test is positive for compounds with an aldehyde group (except for aromatic aldehydes) or alpha-hydroxy ketones.
The compounds that give a positive Fehling’s test are:
- Acetaldehyde (CH3CHO)
- Methanal (HCHO)
- Cyclohexane carbaldehyde
Other compounds, such as benzaldehyde, acetophenone, and 4-nitrobenzaldehyde, do not give a positive Fehling’s test because they are aromatic aldehydes or ketones.
Thus, the correct count is:
So, the correct answer is: 3
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