Question

Account for the fact that \(p\)-nitrophenol \(\left(K_{\mathrm{a}} 7.0 \times 10^{-8}\right)\) is a stronger acid than phenol $$ \left(K_{\mathrm{a}} 1.1 \times 10^{-10}\right) $$

Short answer

Question: Explain the factors that contribute to the increased acidity of p-nitrophenol as compared to phenol. Answer: The increased acidity of p-nitrophenol can be attributed to the presence of an electron-withdrawing nitro group in its structure, which stabilizes the negative charge on the phenoxide ion due to the inductive and resonance effects. This makes it easier for p-nitrophenol to lose a hydrogen ion, thus exhibiting a higher acidity compared to phenol.

Step-by-step solution

Recall the definition of Ka and acidity

The acidity constant, Ka, is a quantitative measure of an acid's strength. The higher the Ka value, the stronger the acid. This means that p-nitrophenol, with a Ka of \(7.0\times10^{-8}\), is a stronger acid than phenol, which has a Ka of \(1.1\times10^{-10}\).

Identify the difference in the chemical structure of both compounds

In order to analyze the cause behind the difference in acidity, we need to look at the structures of both compounds. p-nitrophenol has a nitro group (NO2) attached para to the hydroxyl group in the phenol molecule, while phenol only has a single hydroxyl group (OH) attached to the benzene ring.

Explain how the nitro group affects the acidic strength

The presence of the nitro group in p-nitrophenol can increase the compound's acidity due to the electron-withdrawing effect of the nitro group on the benzene ring. This effect stabilizes the negative charge on the oxygen atom of the phenoxide ion (conjugate base) after the phenol loses its H+ ion, making it easier for the phenol to lose the hydrogen ion and thus be more acidic. The presence of the nitro group also induces resonance effects, which further stabilize the negative charge on the oxygen atom. Overall, this results in p-nitrophenol being a stronger acid compared to phenol.

Conclusion

In summary, the difference in acidity between p-nitrophenol and phenol can be attributed to the presence of an electron-withdrawing nitro group in the p-nitrophenol molecule, which stabilizes the negative charge on the phenoxide ion due to the inductive and resonance effects, making it easier for p-nitrophenol to lose its hydrogen ion and demonstrate higher acidity. This is reflected by the higher Ka value of p-nitrophenol compared to phenol.

Key concepts

p-nitrophenol

p-nitrophenol is an aromatic compound characterized by the presence of a nitro group \(\text{-NO}_2\) and a hydroxyl group \(\text{-OH}\) on a benzene ring. The term "p" stands for "para," indicating that the nitro group and hydroxyl group are positioned opposite each other on the benzene ring. This structural orientation significantly impacts the chemical properties of p-nitrophenol, particularly its acidity.

Due to this arrangement, the p-nitrophenol molecule displays enhanced acidic properties owing to the electron-withdrawing power of the nitro group. This means it can release hydrogen ions \(\text{H}^+\) more readily, making it a relatively stronger acid compared to other phenolic compounds.

Phenol

Phenol is a simple aromatic compound with a hydroxyl group \(\text{-OH}\) attached directly to a benzene ring. This specific attachment gives phenol its characteristic acidic nature but does not make it as strong an acid as p-nitrophenol.

The simplicity of the phenol structure means there are no additional groups, like nitro groups, that might influence its acidity through electronic effects such as electron-withdrawing or resonance stabilization. Consequently, phenol has a weaker ability to donate a hydrogen ion compared to p-nitrophenol, reflected by its lower acidity constant.

Nitro Group Effect

The nitro group has a significant influence on the acidic strength of a compound. As an electron-withdrawing group, it pulls electron density away from the benzene ring where it is attached. This effect is crucial for understanding why p-nitrophenol is a stronger acid than phenol.

By withdrawing electrons, the nitro group effectively stabilizes the negative charge that forms on the oxygen atom of the phenoxide ion when the hydrogen ion is lost. This stabilization makes it easier for p-nitrophenol to donate a hydrogen ion. Additionally, the nitro group participates in resonance effects that further distribute and stabilize the negative charge, enhancing the acidic properties of p-nitrophenol.

Acidity Constant (Ka)

The acidity constant, represented as \(K_a\), is a measure of the strength of an acid. It quantifies how well an acid dissociates in water to form its corresponding anion and a proton \(\text{H}^+\). A higher \(K_a\) value indicates a stronger acid, meaning the acid dissociates more readily to donate a hydrogen ion.

For example, the \(K_a\) of p-nitrophenol is \(7.0 \times 10^{-8}\), which is significantly higher than phenol's \(K_a\) of \(1.1 \times 10^{-10}\). This notable difference underscores the greater ease with which p-nitrophenol releases its hydrogen ion, largely due to the impact of the nitro group.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are atoms or groups of atoms that draw electron density away from other parts of a molecule. This effect plays a significant role in the acidity of aromatic compounds like phenol and p-nitrophenol.

When an EWG, such as the nitro group, is present, it stabilizes the negative charge on the conjugate base formed after the acid donates a proton. This stabilization makes it more favorable for the acid to release its hydrogen ion, contributing to higher acidity.

In the case of p-nitrophenol versus phenol, the presence of the nitro group as an electron-withdrawing entity profoundly influences acidity by enhancing the formation of the phenoxide ion. Thus, compounds with strong EWGs are generally stronger acids.