Problem 129 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Choose the correct statement regarding acidic character of acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH}\) and peroxyacetic acid, \(\mathrm{CH}_{3} \mathrm{COOOH}\). (a) Peroxyacetic acid is stronger acid than acetic acid since the former has one extra oxygen, an electronegative element. (b) Peroxyacetic acid is stronger than acetic acid because its conjugate base is a weaker base than acetate. (c) Peroxyacetic acid is weaker than acetic acid because its conjugate base is less stable than that of acetate ion. (d) Both are equally strong.

Step-by-Step Solution

Answer

(b) Peroxyacetic acid is stronger than acetic acid.

Step 1: Understand Acid Strength

The strength of an acid is determined by the stability of its conjugate base. A more stable conjugate base corresponds to a stronger acid.

Step 2: Analyze Acetic Acid

Acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH}\), releases a proton to form the acetate ion, \(\mathrm{CH}_{3} \mathrm{COO}^-\). The acetate ion is stabilized by resonance, where the negative charge is delocalized over two oxygen atoms.

Step 3: Analyze Peroxyacetic Acid

Peroxyacetic acid, \(\mathrm{CH}_{3} \mathrm{COOOH}\), releases a proton to form the peroxyacetate ion, \(\mathrm{CH}_{3} \mathrm{COOO}^-\). While the presence of an extra oxygen atom suggests potential stability, the peroxyacetate ion lacks the similar resonance stabilization found in acetate.

Step 4: Compare the Stability of Conjugate Bases

The acetate ion is more stable than the peroxyacetate ion due to resonance stabilization. Peroxyacetate does not enjoy the same resonance, as its structure leads to less effective charge delocalization.

Step 5: Choose the Correct Statement

Given the stability comparison of the conjugate bases, peroxyacetic acid is indeed stronger than acetic acid as its conjugate base (peroxyacetate) is less stable than acetate. This supports statement (b).

Key Concepts

Conjugate Base Stability

The concept of conjugate base stability is crucial in understanding acid strength. When an acid donates a proton, it forms a conjugate base. A key rule of thumb is: the more stable this conjugate base, the stronger the acid. Stability often depends on factors like charge dispersion and the structure of the conjugate base. If the conjugate base is stable, it won't readily re-acquire a proton, making the original acid weaker. Conversely, if the conjugate base is unstable, it will more eagerly attract a proton, resulting in a stronger acid.

Resonance Stabilization

Resonance stabilization dramatically affects the stability of molecules, including conjugate bases. In chemistry, resonance refers to the delocalization of electrons within certain molecules or ions, allowing for stability through multiple contributing structures.
For acetic acid, when it loses a proton, the acetate ion is formed. This ion is notably stable due to resonance, where the negative charge can be shared or spread out over two oxygen atoms.
On the other hand, the peroxyacetate ion from peroxyacetic acid lacks this level of resonance. Although it has an extra oxygen, the structure doesn't allow for the same effective delocalization, making it less stable than acetate.

Acetic Acid

Acetic acid, with the chemical formula \(\mathrm{CH}_{3} \, \mathrm{COOH}\), is a well-known weak acid. Its acid strength is primarily determined by the stability of its conjugate base, acetate \(\mathrm{CH}_{3} \, \mathrm{COO}^-\).
When acetic acid releases a proton, it forms this acetate ion. Thanks to its resonance stabilization, the acetate ion can distribute the negative charge effectively over the two oxygen atoms. This increased stability, achieved through resonance, means that acetate is less likely to recombine with a proton, signifying that acetic acid is a relatively weaker acid compared to stronger acids with less stable conjugate bases.

Peroxyacetic Acid

Peroxyacetic acid \(\mathrm{CH}_{3} \, \mathrm{COOOH}\) is a derivative of acetic acid, distinguished by an extra oxygen atom. While one might initially think the additional electronegative oxygen could stabilize its conjugate base, the actual outcome is different.
When peroxyacetic acid donates a proton, it forms the peroxyacetate ion \(\mathrm{CH}_{3} \, \mathrm{COOO}^-\). However, the positioning and bonding of that extra oxygen don't afford the same resonance effects present in acetate. As a result, the negative charge on peroxyacetate is less effectively delocalized, making the ion less stable. This lack of stability in the conjugate base effectively makes peroxyacetic acid a stronger acid than acetic acid.