Question

Phenols \((\mathrm{ArOH})\) are relatively acidic, and the presence of a substituent group on the aromatic ring has a large effect. The \(\mathrm{p} K_{\mathrm{a}}\) of unsubstituted phenol, for example, is \(9.89,\) while that of \(p\) -nitrophenol is \(7.15 .\) Draw resonance structures of the corresponding phenoxide anions and explain the data.

Short answer

Nitro group stabilizes p-nitrophenoxide anion via additional resonance.

Step-by-step solution

Understand Phenol as a Phenoxide Anion

Phenol can lose a hydrogen ion (H⁺) to form a phenoxide ion ( ArO⁻). The negative charge in the ion is delocalized over the aromatic ring through resonance, increasing its stability.

Draw Resonance for Unsubstituted Phenol

For unsubstituted phenol, draw the phenoxide anion, ArO⁻. Indicate three main resonance structures: (1) the charge localized on oxygen, (2) delocalization into the ortho position, and (3) delocalization into the para position on the benzene ring.

Consider Electron-Withdrawing Effects

Identify that the nitro group (NO₂) is an electron-withdrawing group. Electron-withdrawing groups help stabilize the negative charge of the phenoxide ion through resonance, making the ion more stable and the acid stronger.

Draw Resonance Structures for p-Nitrophenol

For p-nitrophenol, again draw the phenoxide ion. Show additional resonance structures with the negative charge on the ortho and para positions relative to the nitro group. Highlight how the nitro group can accept electrons and resonate, further stabilizing the structure. This extra stability corresponds to a lower pKₐ value.

Explain Data with Resonance Stabilization

With the nitro group present, extra resonance structures are possible that include the nitro group and negatively charged oxygen. The additional resonance structures further stabilize p-nitrophenoxide anion, explaining why p-nitrophenol has a lower pKₐ (7.15) compared to phenol (9.89).

Key concepts

Phenoxide Anion

Phenoxide anions are formed when phenols lose a hydrogen ion, resulting in the formation of a negatively charged ion, denoted as \( ext{ArO}^{-}\). This negative charge is not static but is distributed over the aromatic ring via a concept we call resonance. Resonance involves the shifting of electrons in a molecule which allows the negative charge to be shared, rather than being concentrated in one area. In the case of the phenoxide ion, this distribution occurs over different positions on the benzene ring. Here’s why this matters:

• Resonance increases stability. In phenoxide ions, this means the negative charge isn't just on the oxygen but shared across the ortho and para positions on the ring.
• Stable ions mean stronger acids. The ability of a hydrogen ion to leave the molecule easily relates to the overall stability of the resulting ion.

This sharing of the charge across the aromatic system implies that phenols act as relatively stronger acids compared to alcohols, which do not benefit from such delocalization.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) play a crucial role in modulating the acidity of phenols. These groups, such as the nitro group \(( ext{NO}_2)\), pull electron density away from the rest of the molecule. But why is this important? Let's explore.

• Stabilizing the charge: EWGs like nitro can help stabilize the negative charge on the phenoxide ion. The presence of such groups in proximity to a negative charge allows for additional resonance structures.
• Acid strength enhancement: By stabilizing the ion, EWGs effectively enhance the acid strength. More stable conjugate bases (like the phenoxide ion) mean the related acid will donate its hydrogen ion more readily.
• Practical Example: In p-nitrophenol, the nitro group's electron-withdrawing nature increases the molecule's acidity relative to regular phenol.

This fascinating interplay between electron-withdrawing groups and the phenoxide ion demonstrates how molecular structure can directly influence chemical properties such as acidity.

pKₐ Values

Understanding \(pK_{a}\) values is key to grasping the relative acidity of various phenols. The \(pK_{a}\) is a numerical scale used to specify the strength of an acid, with lower values representing stronger acids. Here’s how it works with our phenols:

• Phenol vs. p-Nitrophenol: The \(pK_{a}\) of unsubstituted phenol is 9.89, while for p-nitrophenol, it drops to 7.15. The lower \(pK_{a}\) value for p-nitrophenol indicates it is a stronger acid.
• Resonance and \(pK_{a}\): The additional electron-withdrawing nitro group allows for more resonance structures in the phenoxide ion, further stabilizing it and thus lowering the \(pK_{a}\).
• Effect of Structure: The presence or absence of these substituent groups on the benzene ring showcases how small structural changes can have significant chemical impact.

By analyzing \(pK_{a}\) values and resonance in phenols, we gain a deeper insight into the molecular mechanisms behind acidity and stability, which is crucial for understanding broader organic chemistry concepts.