Heterocyclic Chemistry

1. To determine if a compound is aromatic, it must satisfy the following criteria:
   1. It must be cyclic.
   2. It must be planar.
   3. It must have (4n+2) π electrons (Hückel's Rule).
2. Analyzing each compound:
   1. Benzene (6 π electrons): Aromatic, satisfies Hückel's Rule with n=1.
   2. Pyridine (6 π electrons): Aromatic, follows Hückel's Rule with n=1.
   3. Furan (6 π electrons): Aromatic, has lone pair contributing to π system.
   4. Pyrrole (6 π electrons): Aromatic, lone pair contributes to π system.
   5. Naphthalene (10 π electrons): Aromatic, follows Hückel's Rule with n=2.
   6. Pyrimidine (6 π electrons): Aromatic, follows Hückel's Rule with n=1.
   7. Others fail due to non-planarity, wrong electron count, or non-conjugation.
3. Thus, the number of aromatic compounds is 5.

Compound I (the methyl group attached to a benzene ring with two hydrogen atoms attached to the other carbon atoms) has low acidity due to the electron-donating effects of the methyl group, which stabilize the negative charge on the conjugate base.

Compound II is more acidic because the electron-withdrawing substituent (the –NO₂ group) helps stabilize the negative charge on the conjugate base by delocalizing it.

Compound III has a conjugate base that is stabilized by the aromatic ring, but it is still less acidic than II.

Compound IV is less acidic because the lack of any electron-withdrawing group makes it more difficult to stabilize the negative charge on the conjugate base.

Thus, the order of acidity follows making (B) the correct answer.