Question

Which one of the following options is best suited for effecting the transformation?

• A. $MnO_2$
• B. $DMSO, (COCl)_2, Et_3N$
• C. $Al(Oi-Pr)_3$
• D. $Ag_2O/NH_4OH$

Answer 1

The Correct Option is D

$\text{1. The Transformation}$

The reaction requires a selective oxidizing agent that targets the $\text{aldehyde}$ functional group ($\text{R-CHO}$) while leaving the $\text{secondary alcohol}$ functional group ($\text{R}_2\text{CHOH}$) untouched.

$$\text{R-CHO} + [\text{Oxidation}] \rightarrow \text{R-COOH}$$

$\text{2. Analysis of Reagents}$

$\text{Option 4: } \text{Ag}_2\text{O}/\text{NH}_4\text{OH}$ ($\text{Tollens' Reagent}$)

Tollens' reagent is a mild, selective oxidizing agent for aldehydes.

It is an ammoniacal solution of silver oxide, which contains the diamminesilver(I) complex, $[\text{Ag}(\text{NH}_3)_2]^{+}$.

Function: It selectively oxidizes aldehydes ($\text{CHO}$) to carboxylic acids ($\text{COOH}$). The reaction is typically performed in a basic medium, but it does not generally oxidize alcohols.

Selectivity: This reagent is perfectly suited for the required transformation as it will oxidize the aldehyde to the carboxylic acid without affecting the secondary alcohol group.

$\text{Option 1: } \text{MnO}_2$ ($\text{Manganese Dioxide}$)

Function: $\text{MnO}_2$ is a selective oxidizing agent for allylic and benzylic alcohols to the corresponding aldehydes or ketones.

Selectivity: It is not suitable for oxidizing an alkyl aldehyde to a carboxylic acid, nor is it the correct choice for selectively avoiding a secondary alkyl alcohol while oxidizing an aldehyde. It might oxidize the secondary alcohol depending on conditions, or it might not react as desired.

$\text{Option 2: } \text{DMSO}, (\text{COCl})_2, \text{Et}_3\text{N}$ ($\text{Swern Oxidation}$)

Function: This is a classic method for the oxidation of primary alcohols ($\text{RCH}_2\text{OH}$) to aldehydes ($\text{RCHO}$) and secondary alcohols ($\text{R}_2\text{CHOH}$) to ketones ($\text{R}_2\text{CO}$).

Selectivity: It is designed to oxidize alcohols, not aldehydes. It would likely leave the aldehyde untouched and oxidize the secondary alcohol to a ketone ($\text{C}=\text{O}$), leading to an undesired product.

$\text{Option 3: } \text{Al}(\text{O}i-\text{Pr})_3$ ($\text{Aluminium Isopropoxide}$)

Function: This reagent is used in the Meerwein-Ponndorf-Verley (MPV) reduction (reduction of aldehydes/ketones to alcohols) and the Oppenauer oxidation (oxidation of alcohols to aldehydes/ketones) in the presence of a ketone/aldehyde.

Selectivity: It is primarily associated with alcohol/ketone interconversion, not the oxidation of an aldehyde to a carboxylic acid. It would lead to an incorrect transformation.

$\text{Conclusion}$

The transformation requires an oxidant that is mild enough to be selective for the aldehyde over the secondary alcohol. Tollens' reagent ($\text{Ag}_2\text{O}/\text{NH}_4\text{OH}$) is the specific reagent used for this type of selective oxidation in organic chemistry.

$$\text{The best suited option is } \mathbf{\text{Ag}_2\text{O}/\text{NH}_4\text{OH}}$$