Which of the following facts explains as to why p-nitrophenol is more acidic than phenol? 1\. \(-\) I effect of nitro group 2\. Greater resonance effect of p-nitrophenoxy group 3\. Gteric effect of bulky nitro group Select the correct answer using the codes given below: (a) 2 and 3 (b) 1 and 3 (c) 1 and 2 (d) 2 alone

The electron-withdrawing inductive effect is a crucial factor when evaluating the acidity of phenols. This effect occurs when an atom or a group of atoms attached to a phenol ring draws electron density away through the sigma bonds. This can have a substantial impact on the phenol's properties, particularly its acidity.

In the case of p-nitrophenol, the nitro group, which is known for being a strong electron-withdrawing group, exerts an inductive effect that pulls electron density away from the oxygen atom of the phenoxide ion. This withdrawal of electron density stabilizes the negative charge that forms on the oxygen when the phenol loses a proton, effectively increasing the phenol's tendency to release the proton, which is a measure of its acidity.

Impact of Inductive Effect on Acidity

The more powerful the electron-withdrawing effect, the more stable the remaining negatively charged phenoxide ion, and consequently, the greater the acidity of the phenol. Thus, compounds like p-nitrophenol, with strong electron-withdrawing groups, are observed to be more acidic than phenols without such substituents.

The resonance effect in phenols significantly influences their acidic behavior as well. Resonance is the delocalization of electrons across adjacent atoms within a molecule, allowing for the distribution of electronic charge more evenly.

Phenol molecules can exhibit resonance as the lone pair of electrons on the oxygen can participate in the conjugated system of the aromatic ring. This participation results in different resonating structures, where the negative charge of the deprotonated phenol, the phenoxide ion, is spread out across the molecule, reducing the electron density on any one atom.

Enhanced Resonance with Substituents

The presence of certain substituents, such as the nitro group in p-nitrophenol, can increase this resonance effect. They not only withdraw electrons via the inductive effect but also contribute to resonance by accepting the delocalized electrons in the system. This dual role, in essence, provides a more substantial stabilization to the phenoxide ion, increasing the acidity of the phenol.

The stability of the phenoxide ion is a fundamental factor in determining the acidity of phenols. A stable phenoxide ion means the corresponding phenol will more readily lose its proton, demonstrating stronger acidity. The stability is influenced by both the electron-withdrawing inductive effect and the resonance effect, as previously discussed.

For a phenoxide ion to be considered stable, its negative charge must be well accommodated. This is achieved when the negative charge is either delocalized over a larger volume of the molecule or is drawn towards electron-withdrawing groups.

Role of Substituent Position and Nature

The position and nature of substituents on the phenol ring can increase this stability. In para-substituted phenols like p-nitrophenol, the substituent can greatly stabilize the phenoxide ion through both the inductive and resonance effects. Given this increased stability due to the specific position and electron-withdrawing nature of the nitro group, p-nitrophenol is decisively more acidic than phenol without such substituents.