Question

In acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH}\), the \(\mathrm{OH}\) hydrogen is more acidic than the \(\mathrm{CH}_{3}\) hydrogens. Explain.

Short answer

Answer: The hydrogen attached to the oxygen (OH) in acetic acid is more acidic than the hydrogen attached to the carbon (CH3). This is because the stability of the conjugate base formed when the OH hydrogen is removed (acetate ion) is greater than the stability of the conjugate base formed when one of the CH3 hydrogens is removed (methyl carbanion). The greater stability of the acetate ion is due to resonance, which allows the negative charge to be delocalized between the two oxygen atoms, thereby increasing its stability.

Step-by-step solution

Identify the acidic hydrogens

In acetic acid, there are two types of hydrogens: one attached to the oxygen (OH) and three attached to the carbon atom (CH3). We need to analyze the stability of the conjugate bases when these hydrogens are removed during ionization.

Investigate the ionization process

When a molecule loses a hydrogen ion (H+), it forms a negatively charged ion called a conjugate base. In the case of acetic acid, when it loses the \(\mathrm{OH}\) hydrogen, it forms the acetate ion (\(\mathrm{CH}_{3}\mathrm{COO}^- \)), and when it loses one of the \(\mathrm{CH}_3\) hydrogens, it forms a less common conjugate base "methyl carbanion" (\(\mathrm{CH_{2}}\mathrm{COOH}^-\)).

Compare the stability of formed conjugate bases

The stability of a conjugate base is related to the acidity of its parent molecule. A more stable conjugate base corresponds to a more acidic parent molecule. 1. Acetate ion stability: The \(\mathrm{CH}_3\mathrm{COO}^-\) is stabilized due to resonance, as the negative charge can be delocalized between the two oxygen atoms, decreasing the charge density. This resonance effect increases the stability of the acetate ion. 2. Methyl carbanion stability: The \(\mathrm{CH}_{2}\mathrm{COOH}^-\) ion (methyl carbanion) does not possess any stabilization by resonance or inductive effect. In this ion, the negative charge is localized on a single carbon atom, resulting in lower stability compared to the acetate ion.

Relate the stability to acidity

Since the acetate ion (\(\mathrm{CH}_3\mathrm{COO}^-\)) is more stable than the methyl carbanion (\(\mathrm{CH}_{2}\mathrm{COOH}^-\)) due to resonance, the parent molecule, acetic acid, has a more acidic OH hydrogen. Therefore, the \(\mathrm{OH}\) hydrogen in acetic acid is more acidic than the \(\mathrm{CH}_3\) hydrogens.