Question

Which of the following reagents cannot be used to distinguish between pentanal and 2-pentanone?

• A. Tollen’s reagent
• B. Fehling’s solution
• C. Br\(_2\) in CCl\(_4\)
• D. I\(_2\) in NaOH

Hint:

Br\(_2\) in CCl\(_4\) is a test for double or triple bonds, not for differentiating aldehydes and ketones.

Answer 1

The Correct Option is C

Step 1: Pentanal is an aldehyde, while 2-pentanone is a ketone.
Step 2: Tollen’s reagent gives a silver mirror with aldehydes but not with ketones, so it can distinguish them.
Step 3: Fehling’s solution is reduced by aliphatic aldehydes like pentanal but not by ketones, hence it can distinguish them.
Step 4: Iodoform test is given by methyl ketones. 2-pentanone (\(\text{CH}_3\text{CO}-\)) gives a positive test, while pentanal does not.
Step 5: Br\(_2\) in CCl\(_4\) is used to test unsaturation. Since both pentanal and 2-pentanone are saturated compounds, neither reacts, so this reagent cannot distinguish between them.