Question

The most suitable reagent for the conversion of $R-C{{H}_{2}}-OH$ to $RCHO$ is
(A) $Cr{{O}_{3}}$
(B) $PCC$
(C)$KMn{{O}_{4}}$
(D)${{K}_{2}}C{{r}_{2}}{{O}_{7}}$

Answer 1

In the process of conversion of $R-C{{H}_{2}}-OH$ to $RCHO$ is an oxidation reaction, the reagent must be an oxidizing agent. In order to correctly understand the question well, we should have an idea of the preparation of aldehyde and carboxylic acid from alcohol.

Complete Solution :
$R-C{{H}_{2}}-OH+[O]\xrightarrow{REAGENT}RCHO\xrightarrow{REAGENT}RCOOH$ ………..equation 1
The primary alcohol reacts with oxygen in the presence of a certain oxidizing agent to give aldehyde its yield, sometimes the aldehyde yield is further oxidized to carboxylic acid in the presence of an oxidizing agent.
The regent given in all the options is an oxidizing agent. It is a bit hard to choose between them. Let’s look into each option in depth.
- Option A is $Cr{{O}_{3}}$, this reagent is prepared in-situ from $Cr{{O}_{3}}$ with dilute sulphuric acid. This is also a good oxidizing agent but as shown in equation 1, it further oxidizes aldehyde to the carboxylic acid. According to the question the reaction should stop with the aldehyde. Therefore, option A is the wrong answer.
- Option B is PCC. The full form of ”PCC” is pyridinium chlorochromate. This is also an oxidizing agent. PCC in dichloromethane solvent is the best combination as an oxidizing agent. The specialty of this is it selectively oxidizes the –OH to –COOH and it won’t form carboxylic acid. This exactly meets the condition of this question. Therefore, Option B is the correct answer.
- Option C is $KMn{{O}_{4}}$. Potassium permanganate is also an oxidizing agent but it acts as an oxidizing agent only in acidic medium as it itself is alkaline in nature. Alcohol acts as weak acids in basic medium here the basic medium is $KMn{{O}_{4}}$, thus it reacts and forms aldehyde but it also readily prone to form carboxylic acid as aldehyde is acidic in nature. Therefore, option C is the wrong answer.
- Option D is ${{K}_{2}}C{{r}_{2}}{{O}_{7}}$. Potassium dichromate is an oxidizing agent but this also readily forms carboxylic acid. Therefore option D is the wrong answer.
So, the correct answer is “Option B”.

Note: There are three important points to observe:
-All four reagents are good oxidizing agents.
-PCC does not oxidize aldehyde to the carboxylic acid.
-$KMn{{O}_{4}}$ acts as an oxidizing agent only in an acidic medium as it, itself is alkaline in nature.