Question

The oxidation of benzene by ${{V}_{2}}{{O}_{5}}$ in the presence of air produces:
A. benzoic acid
B. benzaldehyde
C. benzoic anhydride
D. maleic anhydride

Answer 1

Hint: Think about the D.A.Dowden mechanism that shows the oxidation of benzene by vanadium oxide in the presence of air. Consider where the cleavage will take place and what kind of anhydride will be formed.

Complete step by step answer:
We know that during this process, the cleavage of the benzene ring will occur and the formation of a dioic acid will take place. This dioic acid will then undergo dehydration to form an anhydride by the same vanadium oxide molecules. Vanadium oxide will cleave any double bonds that are present in the ring (but not all of them) and oxidize the terminal ends to form carboxylic acid groups with the help of the oxygen present in the air.
When one molecule of benzene reacts with two molecules of vanadium pentoxide and one and a half mole of oxygen, we get one mole of maleic anhydride, one mole of glyoxylic or aldehydic acid, two moles of vanadium trioxide, and one mole of water. The general reaction of this process is:

Now, let us look at the mechanism to get a deeper understanding of how this reaction works. The vanadium pentoxide attaches itself to one of the double bonds and breaks the ring. Then the aldehydic groups at the terminal ends are oxidized to form carboxylic acid groups. Then vanadium pentoxide attacks the second double bond, this causes another breakage and aldehydic acid to be liberated. Dehydration takes place after this and the anhydride of maleic acid is formed. The mechanism is as follows:

So, the correct answer is “Option D”.

Note: Note that in the second to last step of the mechanism, oxygen has a positive charge despite forming just a single bond. Remember that although oxygen has formed only one bond, it has been formed due to oxygen donating one of its lone pairs and not due to receiving a lone pair. Hence, that atom has a positive charge.