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

Question

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

Step-by-Step Solution

Answer

The most acidic compound is (c) benzoic acid.

Step 1: Understand the Chemical Structures

First, we need to understand the chemical structures provided in the SMILES format. The structures are variations of benzoic acid derivatives. For (b), the structure corresponds to o-toluic acid (or 2-methylbenzoic acid). For (c), it is benzoic acid. For (d), the structure is m-toluic acid (or 3-methylbenzoic acid).

Step 2: Assess the Electron Withdrawing and Donating Effects

To determine acidity, evaluate the effect of substituents on the benzene ring. Electron withdrawing groups increase acidity by stabilizing the conjugate base, while electron donating groups decrease it. The structures in (b) and (d) have a methyl group, which is electron donating, whereas (c) lacks substituents except the carboxylic acid group.

Step 3: Compare the Effect on Acidity

Benzoic acid (c) is more acidic than o-toluic acid (b) and m-toluic acid (d) because it has no electron donating alkyl group attached to the ring, which means its conjugate base is more stable compared to the other two acids.

Step 4: Conclude with Acidity Ranking

Based on the electron effects, benzoic acid (c) is the most acidic due to the lack of any additional electron donating groups like methyl groups present in the other options.

Key Concepts

Benzoic Acid

Benzoic acid, represented by the structure \( \text{C}_6\text{H}_5\text{COOH} \), is a simple aromatic carboxylic acid. It consists of a benzene ring attached to a carboxyl group. This arrangement imparts certain stability and acidity characteristics to the molecule. Benzoic acid is often used in foods as a preservative due to its acidity, which helps to inhibit bacterial growth.
The acid dissociation constant, or \( pK_a \) value, of benzoic acid is around 4.2, which makes it a relatively strong acid among simple aromatic acids. This is because the benzene ring can help delocalize and stabilize the negative charge that forms on the conjugate base when the hydrogen ion is released. This type of stabilization makes the release of hydrogen ions more favorable, thus increasing acidity.

Electron Withdrawing Effects

Understanding electron withdrawing effects is crucial to understanding why certain acids are stronger than others. Electron withdrawing groups (EWGs) pull electron density away from other parts of the molecule.
In benzoic acid, the carboxyl group also acts as an electron withdrawing group. It pulls electron density from the benzene ring and contributes to the stability of the conjugate base. This stabilization enhances acidity, as a stable conjugate base will favor the dissociation of hydrogen ions.

• Poor electron sharing leads to strong electron withdrawing.
• Stabilization of the conjugate base increases.
• Results in stronger acidity.

In this way, benzoic acid without additional electron donating groups is more acidic than its alkyl-substituted derivatives.

Conjugate Base Stability

The stability of a conjugate base is a key determinant of an acid's strength. When an acid releases a hydrogen ion, it forms its conjugate base.
A stable conjugate base implies that the negative charge that results from the loss of a hydrogen ion is well accommodated within the molecule structure. For benzoic acid, this stability is provided by the resonance in the benzene ring, where the charge is delocalized.

• Charge delocalization and resonance stabilize the base.
• Unperturbed benzene rings contribute to this effect.

The presence of electron donating groups, such as methyl groups, reduce this stability by increasing electron density, thereby making acids weaker than their unsubstituted counterparts.

SMILES Format

SMILES, or Simplified Molecular Input Line Entry System, is a notation that encodes molecular structures using short strings of text. It is popular in chemical informatics and computational chemistry.
For instance, benzoic acid is represented in SMILES as "O=C(O)c1ccccc1". This notation includes symbols for the elements along with structural information, such as the arrangement of atoms and bonds.

• "O=C(O)" indicates a carboxyl group attached to the benzene ring.
• The sequence "c1ccccc1" denotes the benzene ring, showing cyclic, aromatic carbon atoms.

Learning to read and write SMILES strings can drastically improve one’s ability to work with chemical data and databases.

Methyl Group

The methyl group \(\text{CH}_3\) is a common substituent found in various organic compounds. It is considered to be an electron donating group, which means it can push electron density towards other parts of the molecule.
This electron donating characteristic can decrease the acidity of a carboxylic acid by destabilizing the conjugate base. For acids like o-toluic acid and m-toluic acid, the methyl group interferes with the resonance stabilization of the conjugate base.

• Methyl groups reduce acidity due to electron donation.
• Interference with base stabilization weakens acidic strength.
• Substituent position affects electron density shifts.

Understanding how methyl groups affect acidity is important in predicting the behavior of substituted benzoic acids.