Question

Which among the following undergoes self-oxidation and self-reduction in the same reaction?
[A] ${{C}_{7}}{{H}_{8}}O$
[B] $C{{H}_{2}}O$
[C] ${{C}_{3}}{{H}_{7}}O$
[D] ${{C}_{2}}{{H}_{4}}O$

Answer 1

Hint: Here, the correct answer is the compound which does not have an alpha hydrogen. It undergoes Cannizzaro reaction in which it self-oxidises and self-reduces to form an alcohol and an aldehyde.

Complete step by step answer:
 A reaction where the same compound undergoes self-oxidation as well as self-reduction is the Cannizzaro reaction.
As we can understand, the Cannizzaro reaction is a disproportionation reaction but it is not just that. The Cannizzaro reaction is a base induced disproportionation reaction. In this reaction, we can convert two molecules of an aldehyde into a primary alcohol as well as an aldehyde. But it is important to remember that the aldehyde should not be enolizable. Generally, aldehydes with no alpha hydrogen atoms are non-enolizable.
In the given options, let us check which among the given options do not contain alpha hydrogen atoms and that will be our correct answer but before that we should know what an alpha hydrogen is.
The alpha carbon is the first carbon that is attached to the functional group. The hydrogen atoms attached to the alpha carbon atoms is known as alpha hydrogen. Let us check the presence of alpha hydrogen atoms in the given compounds.

Firstly, we have ${{C}_{7}}{{H}_{8}}O$. If we separate the aldehyde functional group which is –CHO, we can write it as ${{C}_{6}}{{H}_{7}}CHO$. As we can see it will have 2 alpha hydrogen atoms (there will be a $C{{H}_{2}}$ group attached to the functional group first and thus it is the alpha carbon). So this does not undergo Cannizzaro reaction.

Next we have $C{{H}_{2}}O$ and we can write it as $HCHO$ . As we can see it has only one carbon atom and that is the functional group therefore it has no alpha hydrogen. So it can undergo Cannizzaro reaction. It is formaldehyde and it undergoes base induced self-oxidation and self-reduction with potassium hydroxide. We can write the reaction as-
     \[2HCHO+KOH\to C{{H}_{3}}OH+HCOOK\]

Here, formaldehyde undergoes self-oxidation and self-reduction therefore this is the correct answer.
In the next two options, we have ${{C}_{3}}{{H}_{7}}O$ and ${{C}_{2}}{{H}_{4}}O$ and they will not undergo Cannizzaro reaction. We can see from the above discussion that HCHO undergoes self-oxidation and self-reduction in the same reaction. So, the correct answer is “Option B”.

Note: There are certain ketones which can undergo Cannizzaro type of reaction. It can undergo such a reaction by the transfer of one of its two carbon groups and not the hydride thus, making it act like an aldehyde. Like the Cannizzaro reaction, we have a cross-Cannizzaro where instead of using two molecules of the same aldehyde, we use formaldehyde along with any other aldehyde in presence of a base and we obtain sodium formate and an alcohol as product.