Question

Salicylic acid is more acidic than o-methoxybenzoic acid though both have electron releasing groups i.e., \(-\mathrm{OH}\) and \(-\mathrm{OCH}_{3} .\) This is because (a) \(-\mathrm{OH}\) is a stronger electron releasing group compared to \(-\mathrm{OCH}_{3}\). (b) salicylate ion is stabilized by intramolecular hydrogen bonding. (c) salicylic acid has two replaceable hydrogen atoms. (d) \(-\mathrm{OCH}_{3}\) has a stronger steric effect to prevent the stabilization of the carboxylate anion.

Short answer

Answer: Salicylic acid is more acidic than o-methoxybenzoic acid because the salicylate ion (conjugate base of salicylic acid) is stabilized by intramolecular hydrogen bonding, which makes the acid stronger.

Step-by-step solution

Identify the structure of the compounds

Salicylic acid has a carboxylic acid group (-COOH) and a hydroxyl group (-OH) attached to a benzene ring, while o-methoxybenzoic acid has a carboxylic acid group (-COOH) and a methoxy group (-OCH3) attached to a benzene ring. Step 2: Analyzing acidity in terms of electron releasing groups

Analyze acidity based on electron releasing groups

Both salicylic acid and o-methoxybenzoic acid have electron releasing groups (-OH and -OCH3), which donate electron density into the benzene ring. However, we need to analyze their effect on the acidity of the compounds. Step 3: Assessing the reason for salicylic acid's higher acidity

Assess the correct reason for salicylic acid's higher acidity

Option (a) suggests that -OH is a stronger electron-releasing group compared to -OCH3, which isn't a valid reason for higher acidity. Option (b) suggests that the salicylate ion (conjugate base of salicylic acid) is stabilized by intramolecular hydrogen bonding, which is a valid reason for higher acidity as more stable conjugate bases make the corresponding acids more acidic. Option (c) states that salicylic acid has two replaceable hydrogen atoms. This is incorrect, as only the hydrogen in the carboxylic acid group is acidic, while the hydrogen in the -OH group is not acidic due to its attachment to the benzene ring. Option (d) suggests that the steric effect of -OCH3 prevents stabilization of the carboxylate anion, which is not the main reason for the difference in acidity between the two compounds. Step 4: Choosing the correct answer

Choose the correct answer

Based on the analysis, the correct option is (b), which states that the salicylate ion is stabilized by intramolecular hydrogen bonding, making salicylic acid more acidic compared to o-methoxybenzoic acid.

Key concepts

Salicylic Acid

Salicylic acid is a well-known organic compound with a specific structure that influences its chemical properties. It contains a benzene ring with two functional groups attached: a hydroxy group (-OH) and a carboxylic acid group (-COOH). These groups are positioned ortho to each other, meaning they are adjacent on the benzene ring.
This specific arrangement plays a crucial role in the compound's behavior, especially its acidity. The hydroxyl group can form unique interactions, such as hydrogen bonds, which are important for determining the overall stability and reactivity of the molecule.

• This molecule is known not only for its presence in the medical field, such as in anti-acne treatments, but it also serves as a fundamental study element for understanding acidity in organic chemistry.

Understanding salicylic acid's structure helps us appreciate why it behaves more acidic in comparison with other similar compounds, like o-methoxybenzoic acid.

Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding is a pivotal concept in understanding the increased acidity of salicylic acid. In this type of interaction, a hydrogen bond is formed within the same molecule.
In salicylic acid, the hydrogen from the carboxylic acid group forms a bond with the oxygen in the hydroxyl group. This internal hydrogen bonding stabilizes the molecule significantly.

• This stabilization results in the increased acidity because it makes the removal of a proton (H+) from the carboxylic group easier. A stable conjugate base means the original compound, or the acid, effectively releases its proton in solution.

This internal stability is key to why salicylic acid is more acidic compared to its counterparts, which lack such favorable bonding.

Electron Releasing Groups

Electron releasing groups, such as -OH and -OCH₃, influence acidity by donating electron density through resonance and inductive effects. Both salicylic acid and o-methoxybenzoic acid contain these groups, but their influence differs.
The hydroxyl group (-OH) in salicylic acid, although slightly donating, supports the formation of intramolecular hydrogen bonds, enhancing the molecule's stability.

• In contrast, the -OCH₃ group in o-methoxybenzoic acid is not as effective in such stabilizing interactions and can contribute to steric hindrance, affecting the stability of the molecule.

Understanding these subtle differences in electron releasing capacity and their spatial effects helps explain variations in acidity between similar structures.

Carboxylate Anion Stabilization

Carboxylate anion stabilization is essential in determining the acidity of a compound. When a carboxylic acid donates a proton, it forms a carboxylate anion, which must be stable for the acid to be strong.
Salicylic acid yields a salicylate ion upon losing a proton, which is highly stable due to intramolecular hydrogen bonding. This stability favors the formation of the anion and thus the overall acidity of the acid.

• In o-methoxybenzoic acid, the absence of this type of bonding and the possible steric hindrance from the methoxy group mean the carboxylate anion formed is less stable, making it a weaker acid.

By comparing how these ions stabilize, we can directly observe how molecular interactions dictate the strength of acidity in organic compounds.