Question

Which of the following does not reduce Tollens reagent?
A. \[C{H_3}CHO\]\[\]
B.\[{C_6}{H_5}NHOH\]
C.\[HCOOH\]
D.\[{C_6}{H_5}N{O_2}\]
E. None of these

Answer 1

Hint: Tollen’s reagent is the ammonical solution of silver nitrate used in aldehyde testing. It does not test with ketones. Its formula of tollen’s reagent is \[2Ag{\left( {N{H_3}} \right)_2}OH\].

Complete Step by Step Solution:
Tollen’s reagent requires a strong reducing agent to get reduced. In order to reduce tollens reagent, the electron donating tendency of a reducing agent should be higher because reduction is the gaining of electrons. From the given option, the first three are strong reducing agents as compared to option D, i.e., nitrobenzene. Nitrobenzene is an oxidising agent, so it cannot reduce tollen’s reagent.

We can easily analyse their oxidising power by their electron donating tendency. In option A, acetaldehyde has pie electrons in the carbonyl functional group which are readily available for donation. In option B, phenylhydroxylamine has a highly acidic proton in the hydroxyl functional group and similarly in option C, formic acid has an acidic proton in the carboxylic functional group. These acidic protons are used to reduce tollen’s reagent. In option D, nitrobenzene has a lone pair of electrons of nitrogen which is generally used for donation but in this compound, they are not localised. They are delocalised in the compound. Due to this, they are not available for reducing tollens reagent.

So, option D is correct.

Additional Information:Tollens reagent is used in the silver mirror test. This test is used to distinguish aldehydes from ketones. Tollen’s reagent on reaction with aldehydes gives a silver mirror-like compound which is clearly visible.

Note: Best reducing agents are those which can undergo oxidation means which can donate their electrons easily so that they can give electrons to other compounds to get reduced, as gaining electrons is reduction.