Polarity of Phenol

Phenol, also known as benzenol or carbolic acid, is an aromatic compound that consists of a benzene ring bonded to a hydroxyl group (-OH). The presence of the hydroxyl group makes phenol a polar molecule because oxygen is more electronegative than carbon and hydrogen.

Acidity of Phenol

Phenol is considered an acid because it can donate a proton (H+) and form a conjugate base. The acidity of phenol is attributed to resonance stabilization of its conjugate base, the phenoxide ion.

Resonance Stabilization of Phenoxide Ion

When phenol loses a proton, it forms the phenoxide ion (C6H5O-). The phenoxide ion is resonance stabilized, which means that the negative charge can be delocalized over the entire ring system through resonance structures.

Resonance Structures of Phenoxide Ion

The resonance structures of the phenoxide ion can be represented as follows:

1. In the first resonance structure, the lone pair on the oxygen atom forms a double bond with carbon, while the negative charge is located on the oxygen atom.

2. In the second resonance structure, the lone pair on the oxygen atom forms a double bond with one of the carbon atoms in the ring, while the negative charge is located on that carbon atom.

3. In the third resonance structure, the lone pair on the oxygen atom forms a double bond with another carbon atom in the ring, while the negative charge is located on that carbon atom.

The resonance structures demonstrate that the negative charge of the phenoxide ion is delocalized over the entire ring system, resulting in stability.

Comparison with Other Options

a) O-H: The O-H bond in phenol is the source of acidity, but it does not explain the resonance stabilization of the conjugate base.

b) Benzyl alcohol: While benzyl alcohol also has an -OH group, it lacks the aromatic ring required for resonance stabilization of the conjugate base.

c) Alkoxide ion: Alkoxide ions are formed when an alcohol loses a proton, but they do not have the same resonance stabilization as the phenoxide ion.

Conclusion

The correct answer is option 'D' (Phenoxide ion) because the resonance stabilization of the phenoxide ion is responsible for the acidity of phenol. The delocalization of the negative charge over the aromatic ring system increases the stability of the conjugate base, making phenol acidic.