Problem 158 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following acids: (1) \(\mathrm{O}-\mathrm{HOC}_{6} \mathrm{H}_{4} \mathrm{COOH}\) (2) \(\mathrm{O}-\mathrm{CH}_{3} \mathrm{OC}_{6} \mathrm{H}_{4} \mathrm{COOH}\) (3) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\) Arrange these acids in the decreasing order of their acidities. (a) \(1>3>2\) (b) \(3>2>1\) (c) \(1>2>3\) (d) \(2>3>1\)

Step-by-Step Solution

Answer

The correct order is \(1>3>2\), option (a).

Step 1: Identify the Functional Group

All of the given compounds are benzoic acid derivatives with the functional group \(-COOH\) attached to a benzene ring.

Step 2: Analyze the Influence of Substituents

Look at the groups attached to the aromatic ring. In (1), the hydroxyl group (\(-OH\)) is ortho to the carboxyl group. In (2), a methoxy group (\(-OCH_3\)) is in the ortho position. In (3), the benzoic acid has no additional substituents, so its structure is simply \((C_6H_5COOH)\).

Step 3: Understand the Electron Donating/Withdrawing Effects

The \(-OH\) group in compound 1 has an electron-donating effect through resonance but can also stabilize the conjugate base by hydrogen bonding. The \(-OCH_3\) group in compound 2 has a strong electron-donating effect through resonance and no hydrogen bonding. Compounds with electron-withdrawing groups are generally stronger acids as they stabilize the conjugate base.

Step 4: Assess Acid Strength Based on Substituents

The electron-donating effect of the \(-OCH_3\) group in compound 2 makes it the least acidic due to increased electron density on the benzene ring. Compound 3 is more acidic than compound 2 because it has no electron-donating groups. Compound 1 is the most acidic because the \(-OH\) group allows for additional stabilization via hydrogen bonding.

Step 5: Arrange the Compounds in Decreasing Order

Based on the previous analysis, the order of acidity is compound 1 being the most acidic, followed by compound 3, and then compound 2 being the least acidic. This gives the order: \(1 > 3 > 2\).

Key Concepts

Electron Donating Effects

The electron donating effects in organic chemistry mainly involve how substituents attached to a molecule influence its chemical properties. In the context of benzoic acids, substituents can either donate electrons to the benzene ring or withdraw them. For benzoic acids, substituents that are electron-donating generally decrease acidity.

How electron donating effects decrease acidity is quite straightforward. When a substituent donates electrons to the ring, it increases the electron density on this ring. Increased electron density means that the carboxylate ion (the conjugate base formed after the acid donates a proton) is less stable. A less stable conjugate base corresponds to a weaker acid.

In our examples, the methoxy group ( -OCH_3 ) in compound 2 is a classic example of an electron-donating group. It donates electrons through resonance, strongly increasing electron density, thereby decreasing the acidity of the compound. The hydroxyl group ( -OH ) in compound 1 also donates electrons, but its effect is less pronounced due to other stabilizing forces.

Conjugate Base Stabilization

Conjugate base stabilization is an essential concept in understanding acid strength. When an acid donates a proton, it forms a conjugate base. The stability of this conjugate base largely determines the acid's strength. More stable conjugate bases lead to stronger acids.

Stability can be enhanced by factors such as resonance and hydrogen bonding. Resonance involves the delocalization of electrons across several atoms, spreading out any negative charge more evenly. Meanwhile, hydrogen bonding can provide additional stabilization by forming weak interactions with other parts of the molecule or with solvent molecules.

In the benzoic acid examples, compound 1, which includes a hydroxyl group ( -OH ), benefits from both electron donating through resonance and hydrogen bonding. This additional hydrogen bonding provides extra stability to the conjugate base, [C_6H_4(OH)COO^-] . Therefore, compound 1 emerges as the strongest acid due to superior conjugate base stabilization.

Functional Groups in Organic Chemistry

Functional groups are specific groups of atoms within molecules that determine the characteristic chemical reactions of those molecules. In benzoic acids, the notable functional group is the carboxyl group ( -COOH ), which is inherently acidic.

In organic chemistry, the presence and position of additional functional groups can significantly alter a compound's properties, including its acidity. Substituents on the benzene ring influence the acid strength based on their position and electronic nature. Ortho, meta, and para positions refer to how substituents are arranged relative to the carboxyl group, affecting the interplay of electron donating/withdrawing effects and subsequent acidity.

For instance, in compound 1, the ortho-positioned -OH allows for both electron donating and added conjugate base stabilization through hydrogen bonding, increasing the acidity. Conversely, the methoxy group ( -OCH_3 ) in compound 2, though also at the ortho position, does not offer similar stabilization, making it less acidic than compound 3, which lacks additional functional groups. Understanding these dynamics is crucial for predicting and manipulating chemical behavior in organic synthesis.