The Lewis structure of acetic acid (CH3COOH) is shown on the right

The Lewis structure of acetic acid (CH3COOH) is shown on the right. Its aqueous solution is described by an equilibrium:

CH3COOH(aq)CH3COO (aq) + H(aq)

Which of the four numbered Hydrogens in the Lewis structure yields H'(aq) appearing in the above chemical equilibrium?

(A) 1 (B) 2 (C)3 (D) 4

(E) Any one of the equivalent 1-3 position

The Correct Answer and Explanation is :

To answer this question, let’s carefully analyze the structure of acetic acid (CH₃COOH) and its dissociation in aqueous solution.

Lewis Structure of Acetic Acid:

Acetic acid (CH₃COOH) consists of a methyl group (-CH₃) and a carboxyl group (-COOH). The carboxyl group consists of a carbon atom double-bonded to an oxygen atom (C=O) and single-bonded to a hydroxyl group (-OH). The other part of the molecule is the methyl group (-CH₃), which is composed of a carbon atom single-bonded to three hydrogen atoms.

The structure looks like this:

   H     O
   |     ||
H-C-C-O-H
   |
   H

The numbered hydrogens (H₁, H₂, H₃, and H₄) refer to the four hydrogen atoms in the molecule, three of which are attached to the methyl group (H₁, H₂, and H₃), and one attached to the oxygen atom in the hydroxyl group (H₄).

Dissociation of Acetic Acid in Aqueous Solution:

In an aqueous solution, acetic acid dissociates according to the equilibrium:

[
CH₃COOH(aq) \rightleftharpoons CH₃COO⁻(aq) + H⁺(aq)
]

Here, the acetic acid molecule donates a proton (H⁺) from the carboxyl group (the -OH part of the carboxyl group).

Which Hydrogen Donates H⁺?

In this dissociation, it is the hydrogen attached to the oxygen of the hydroxyl group (H₄) that dissociates to form H⁺. This is because the O-H bond is polar and relatively weak compared to the C-H bonds in the methyl group. Therefore, it is more likely that the hydrogen from the hydroxyl group will be released as a proton (H⁺).

Conclusion:

The correct answer is (D) 4, as it is the hydrogen in the hydroxyl group (-OH) that dissociates to yield H⁺ in the equilibrium reaction.

In summary:

• The hydrogens in positions 1, 2, and 3 are part of the methyl group and are not involved in the dissociation.
• The hydrogen in position 4 is the one that dissociates to form H⁺ in the aqueous solution, which is a key characteristic of the behavior of carboxylic acids like acetic acid in water.

Thus, H₄ is the hydrogen that yields H⁺(aq).