Problem 64 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is most acidic in nature? (a) (b) Cc1ccccc1C(=O)O Cc1cccc(C(=O)O)c1 (c) (d) O=C(O)c1ccccc1 Cc1ccc(OC(=O)O)cc1

Step-by-Step Solution

Answer

(d) is the most acidic since it is pure benzoic acid.

Step 1: Understand the Structure

The SMILES strings represent chemical structures. We need to interpret each SMILES to figure out the molecular structure and identify acidic groups. Here, we focus on carboxylic acids which contain the group -COOH.

Step 2: Identify the Acidic Group

Look for the presence of the -COOH (carboxylic acid) group in each compound. Compounds containing this group may exhibit acidic behavior, (i.e., they can donate a proton, H⁺).

Step 3: Determine Structural Effects on Acidity

Consider resonance and inductive effects. In benzoic acid derivatives, if an electron-withdrawing group is attached, it increases acidity by stabilizing the negative charge on the carboxylate ion after losing a proton.

Step 4: Analyze Each Option

(b) represents "CH₃-C6H₄-COOH", and (d) represents "Phenyl-COOH". Both have the carboxylic acid group, but (d) is simpler because it lacks the additional methyl group that can have an inductive electron-donating effect.

Step 5: Compare Acidity

The presence of extra substituents can influence acidity. Generally, pure benzoic acid (phenyl-COOH) without extra electron-donating groups (like methyl) is more acidic than toluic acids due to the lack of an electron-donating methyl group.

Key Concepts

Carboxylic Acid Group

The carboxylic acid group, represented as -COOH, is the hallmark of many acidic organic compounds. This functional group is key to the acid’s ability to donate a proton (H⁺) easily, which defines its acidic nature. In the carboxylic group, the hydrogen atom is loosely bound and therefore, easily lost as a proton.
A carboxylic acid can be recognized by the combination of a carbonyl (C=O) and a hydroxyl (O-H) group attached to the same carbon atom.

• The carbonyl group helps stabilize the resulting carboxylate ion upon losing a proton.
• The molecule can undergo resonance, balancing out the charge after the proton is donated.

Carboxylic acids are often more acidic than alcohols or phenols, owing to this unique functionality and the potential for resonance stabilization.

Resonance Effects

Resonance effects play a critical role in determining the acidity of organic compounds, particularly those containing the carboxylic acid group. When a carboxylic acid donates a proton, it forms a carboxylate ion. This ion benefits from resonance stabilization.

• The negative charge can be spread between the two oxygen atoms in the carboxylate ion.
• This resonance-induced distribution of charge increases overall stability, making the compound more acidic.

In compounds where resonance effects are stronger, the acidity is generally higher because the loss of a proton results in a more stable carboxylate ion. Benzoic acids capitalize on this effect due to the benzene ring that can further delocalize the negative charge.

Inductive Effects

Inductive effects involve the transmission of charge through a chain of atoms in a molecule, which significantly impacts the acidity of a compound. It refers to the ability of atoms or groups to either donate or withdraw electron density through sigma bonds.
In the context of acidity, electron-withdrawing groups increase acidity by stabilizing the negative charge on the carboxylate ion after proton loss.

• Electron-donating groups, such as alkyl groups, decrease acidity by introducing additional electron density.
• The inductive effect can be fine-tuned by the substituents on the aromatic ring, as seen in benzoic acid derivatives.

Understanding these effects is crucial in predicting the behavior of benzoic acid and toluic acid concerning their acidity.

Benzoic Acid Derivatives

Benzoic acid derivatives are compounds where the benzoic acid structure is modified with various substituents. These modifications can influence the acidity of the compound significantly. The standard molecular structure of benzoic acid consists of a benzene ring attached to a carboxylic acid group.
Adding different substituents can either enhance or reduce its acidic nature.

• Substituents like nitro groups, which withdraw electrons, will increase the acidity due to better stabilization of the carboxylate ion.
• Methyl and other alkyl groups donate electrons through the inductive effect, thus reducing the acidic nature of the compound.

The balance between these effects can be intuitively reasoned to predict which derivative would be more acidic. For instance, unsubstituted benzoic acid is purely acidic, whereas substituted derivatives may show decreased acidity if they contain electron-donating groups.