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Chapter 21: Problem 32

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

Expert verified
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

01

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 \(1.1\times10^{-10}\).
02

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.
03

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.
04

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.

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