Question

Account for the fact that nitroacetic acid, \(\mathrm{O}_{2} \mathrm{NCH}_{2} \mathrm{COOH}\left(\mathrm{p} K_{\mathrm{a}} 1.68\right)\), is a considerably stronger acid than acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH}\left(\mathrm{p} K_{\mathrm{a}} 4.76\right)\).

Short answer

Answer: Nitroacetic acid is a stronger acid than acetic acid because the nitro group (O2N) present in nitroacetic acid stabilizes its conjugate base (O2NCH2COO-) by delocalizing the negative charge via resonance. This increased stability of the conjugate base makes it more likely for the nitroacetic acid to donate a proton compared to acetic acid.

Step-by-step solution

Understand the pKa values

For any acid, the strength of the acid is represented by the pKa value (acid dissociation constant). The lower the value of pKa, the stronger the acid is. In our case, nitroacetic acid has a pKa value of 1.68, and acetic acid has a pKa value of 4.76. As nitroacetic acid has a lower pKa value, it is a stronger acid than acetic acid.

Analyze the structures of the acids

Acetic acid is CH3COOH, which contains a carboxylic acid group (COOH) linked to a methyl group (CH3). Nitroacetic acid is O2NCH2COOH, which also contains a carboxylic acid group (COOH) but is linked to a nitro group (O2N) instead of a methyl group.

Deprotonation of the acids

When the COOH group of both acids loses a proton (H+), it forms their respective conjugate bases: - Deprotonation of Acetic Acid: CH3COOH → CH3COO- + H+ - Deprotonation of Nitroacetic Acid: O2NCH2COOH → O2NCH2COO- + H+

Compare the stability of the conjugate bases

The strength of an acid depends on the stability of the conjugate base formed after deprotonation. A more stable conjugate base means that the acid is more likely to donate a proton and would be a stronger acid. In the case of acetic acid, the negative charge on the conjugate base (CH3COO-) is localized on the oxygen atom. In the case of nitroacetic acid, the negative charge on the conjugate base (O2NCH2COO-) can be delocalized via resonance throughout the nitro group (O2N). This resonance effect leads to the distribution of the negative charge to both the oxygen atoms, increasing the stability of the O2NCH2COO- ion.

Conclusion

The nitroacetic acid is a stronger acid than acetic acid (having a lower pKa value) because the nitro group (O2N) present in nitroacetic acid stabilizes its conjugate base (O2NCH2COO-) by delocalizing the negative charge via resonance. This increased stability of the conjugate base makes it more likely for the nitroacetic acid to donate a proton compared to acetic acid.