Question

The most acidic compound among the following is (A) Salicylic acid (B) Acetyl salicyclic acid (aspirin) (C) Benzoic acid (D) Carbolic acid

Short answer

The most acidic compound among the given options is Salicylic Acid (A), as its conjugate base has resonance stabilization across the phenyl ring and the carboxyl group, enhanced by the -I (inductive) effect of the hydroxyl group.

Step-by-step solution

Identify the acidic sites

In each molecule, the acidic site is the hydrogen atom bonded to an oxygen atom, as in the carboxylic acid group or the hydroxyl group.

Analyze the conjugate base stability

The more stable the conjugate base is, the more acidic the compound. To determine the stability of each conjugate base, we must consider resonance, inductive effect, and aromaticity. - Salicylic Acid: The conjugate base has resonance stabilization across the phenyl ring and the carboxyl group. Moreover, the hydroxyl group has a -I (inductive) effect that increases the stability of the conjugate base. - Acetyl Salicylic Acid (Aspirin): The conjugate base has resonance stabilization within the phenyl ring, and the carboxyl group is now bonded with an ester group. The O-C(=O)-CH3 technically shows a +M(resonance) effect on the conjugate base which is reducing the stability of the conjugate base compared to Salicylic Acid. - Benzoic Acid: The conjugate base has resonance stabilization across the phenyl ring and the carboxyl group. However, it lacks any additional -I (inductive) effect of the Hydroxyl group as in Salicylic Acid. - Carbolic Acid (Phenol): The conjugate base has resonance stabilization across the phenyl ring, but it lacks the carboxyl group, which reduces its acidity compared to carboxylic acids (Salicylic Acid, Aspirin, and Benzoic Acid).

Compare acidity and select the most acidic compound

Based on our analysis, the acidity order is (from most acidic to least acidic): Salicylic Acid > Benzoic Acid > Acetyl Salicyclic Acid (Aspirin) > Carbolic Acid (Phenol). Thus, among the given options, Salicylic Acid (A) is the most acidic compound.

Key concepts

Conjugate Base Stability

Understanding the stability of a conjugate base gives you a big clue about the acidity of a compound. A conjugate base is what remains after an acid donates a proton (H⁺). The more stable this base, the easier it is for the acid to release that proton, making the acid stronger. Several factors affect the stability of a conjugate base:

• Resonance: When a conjugate base can distribute its negative charge over several atoms, it is more stable.
• Inductive Effects: Electronegative atoms or groups can stabilize a negative charge through electron withdrawal.
• Hybridization and Electronegativity: The type of hybrid orbital and the electronegativity of the atom holding the charge can influence stability.

In the exercise, the emphasis is on resonance and inductive effects. Salicylic acid exhibits resonance stabilization when its conjugate base disperses charge over the aromatic ring. It also benefits from an inductive effect due to its hydroxyl group, enhancing the stability of its conjugate base more than the other acids considered.

Resonance Stabilization

Resonance stabilization is a key concept in determining the acidity of compounds. When the conjugate base of an acid has resonance structures, it can share its negative charge over multiple atoms, reducing energy and increasing stability.
For example, the conjugate base of salicylic acid benefits from resonance across its aromatic ring and carboxyl group, leading to significant charge distribution. This makes the negative charge less localized and the entire structure more stable. Increased stability of the conjugate base makes salicylic acid more willing to donate its proton, thus increasing its acidity.
In contrast, while benzene derivatives like benzoic acid also take advantage of resonance stabilization, they may lack additional factors, like the hydroxyl group's influence in salicylic acid that further contribute to this stability enhancement.

Inductive Effect

The inductive effect refers to the phenomenon where electronegative atoms or groups pull electron density through sigma bonds. This effect is critical in understanding acidity, as the presence of such groups can influence the stability of a compound's conjugate base.
For instance, in salicylic acid, the hydroxyl group exhibits an 'inductive effect' by withdrawing electron density through the sigma bond. This withdrawal enhances the stability of the conjugate base. The increased stability boosts the acid strength by making it easier for the acid to lose its proton.
This effect is absent in simpler acids like benzoic acid, where such electron-withdrawing groups are not present. Consequently, without this stabilizing factor, benzoic acid's conjugate base is less stable compared to its derivative, salicylic acid.