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 OH hydrogen in acetic acid is more acidic than the CH₃ hydrogens because it is attached to a more electronegative atom (oxygen), and when removed, it forms a more stable conjugate base (CH₃COO⁻) due to resonance.

Step-by-step solution

Understanding Acidity

Acidity refers to the ability of a molecule to donate a hydrogen ion (H⁺) in a chemical reaction. In other words, a molecule is more acidic if it can easily lose a hydrogen atom to form a stable ion. This is related to the strength of the bonds and stability of the resulting ion after hydrogen removal.

Structural Analysis of Acetic Acid

Acetic acid has the structural formula CH₃COOH, where a carboxylic acid (COOH) functional group is attached to a methyl group (CH₃). The hydrogens to compare are the hydrogen in the hydroxyl group (OH) of the carboxylic acid and the hydrogens in the methyl group (CH₃).

Electronegativity and Bond Strength

When comparing the acidic hydrogen atoms, we should consider the electronegativity of the atoms directly attached to the hydrogens. In acetic acid, the OH hydrogen is attached to an oxygen atom, while the CH₃ hydrogens are attached to a carbon atom. Oxygen is more electronegative than carbon, which means it attracts electrons more strongly. This makes the bond between the OH hydrogen and oxygen more polar, and thus more prone to break and release a hydrogen ion (H⁺).

Stability of Conjugate Base

Removing the acidic hydrogen from a molecule results in the formation of a conjugate base. After the OH hydrogen is removed from acetic acid, we are left with a CH₃COO⁻ ion, while removal of a CH₃ hydrogen results in a CH₂COOH radical. The CH₃COO⁻ ion is more stable due to resonance, which allows the negative charge to be delocalized over the oxygen atoms in the carboxylic acid group. The CH₂COOH radical, on the other hand, has an unpaired electron on the carbon, making it less stable.

Resonance Effect

The resonance effect further stabilizes the conjugate base of acetic acid (CH₃COO⁻). The negative charge on the oxygen atom can be shared between the two oxygen atoms through resonance, which distributes the negative charge and stabilizes the ion.

Conclusion

Overall, the OH hydrogen in acetic acid (CH₃COOH) is more acidic than the CH₃ hydrogens because it is attached to a more electronegative atom, and removing it results in a more stable conjugate base due to resonance. These factors make the OH hydrogen more prone to be released as a hydrogen ion, making it more acidic.