The correct order of the pK_a of the following compounds is

Concept:

Acid Strength and Aromatacity:

• A substance that gives H⁺ ion in the solution and forms salts by combining with some metals is called acid. According to Arrhenius, acid is a substance that releases an H+ ion in the solution and a base is a substance that accepts an H+ ion from the solution.
• The pKa value indicates the strength of an acid in the solution. The lower the value of pKa, the more the acid's strength will be.
• Aromatic compounds follow Huckel's rule, according to which a cyclic, planar, and conjugated species having (4n+2)pi electrons (n = 0,1,3...) is aromatic.
• Aromatic compounds are very stable.
• Whereas, the anti-aromatic compounds follow the 4npi electrons rule. According to this a cyclic, planar, and conjugated species having (4n+2)pi electrons (n = 0,1,3...) is aromatic.
• Anti-aromatic compounds are very unstable.

Explanation:

• The conjugate base formed by compound A after deprotonation is an anti-aromatic compound with 4npi electrons for n=1, which is unstable. Thus, compound A has lower acid strength and a high value of pKa.
• The conjugate base formed by compound C after deprotonation is an anti-aromatic compound with 8npi electrons for n=2, which is unstable. Thus, compound B also has a lower acid strength and a high pKa value.
• The conjugate base formed by compound B after deprotonation is an aromatic compound with 6npi electrons for n=1, which is very stable. Thus, compound B has the highest acid strength and lowest value of pKa.

Conclusion:

• Thus, the correct order of the pKa of the following compounds is A > C > B.