Question

Which of the following oxidizing agents is mild enough to selectively oxidize an aldehyde to a carboxylic acid without cleaving other sensitive carbon-carbon single bonds?

• A. Tollens' reagent
• B. Concentrated $\text{H}_2\text{SO}_4$ at high temperatures
• C. Ozone followed by $\text{Zn / H}_2\text{O}$
• D. $\text{LiAlH}_4$ in dry ether

Hint:

Mild oxidants like Tollens' or Fehling's reagents can oxidize aldehydes, but they are not strong enough to oxidize ketones.

Answer 1

The Correct Option is A

Concept: Aldehydes ($\text{R-CHO}$) are easily oxidized because they possess a reactive hydrogen atom directly bonded to the carbonyl carbon. This structural feature allows them to undergo oxidation under mild conditions. In contrast, ketones ($\text{R-CO-R}'$) lack this specific carbonyl hydrogen atom, meaning their oxidation requires the cleavage of strong carbon-carbon single bonds under harsh conditions. Consequently, mild laboratory oxidizing agents can selectively oxidize aldehydes while leaving ketones and other functional groups untouched.

Step 1: Analyzing the chemical behavior of Tollens' reagent as a selective oxidant.
Tollens' reagent is an aqueous alkaline solution containing the diamminesilver(I) coordination complex, formulated as $[\text{Ag}(\text{NH}_3)_2]^+$. The silver ions ($\text{Ag}^+$) in this complex serve as a gentle, mild electron acceptor. When mixed with an aldehyde, Tollens' reagent selectively targets and oxidizes the carbonyl carbon, transforming the aldehyde group into a carboxylate anion ($\text{R-COO}^-$): \[ \text{R-CHO} + 2[\text{Ag}(\text{NH}_3)_2]^+ + 3\text{OH}^- \rightarrow \text{R-COO}^- + 2\text{Ag(s)} \downarrow + 4\text{NH}_3 + 2\text{H}_2\text{O} \]

Step 2: Ruling out alternative aggressive or reducing reagents.
We evaluate the alternative choices to confirm selectivity:
• Hot concentrated $\text{H}_2\text{SO}_4$ acts as a powerful, non-selective oxidizing and dehydrating agent that causes widespread decomposition.
• Ozone ($\text{O}_3$) followed by a zinc workup is used to cleave carbon-carbon double bonds in alkenes rather than selectively oxidizing terminal carbonyls.
• $\text{LiAlH}_4$ is a strong reducing agent that performs the opposite transformation, converting carbonyls back into primary alcohols. Thus, Tollens' reagent is the ideal mild oxidant for this selective transformation.