Problem 205 - A Complete Resource Book in Chemistry for JEE Main

Question

The strongest acid amongst the following compounds is: (a) \(\mathrm{HCOOH}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Cl}) \mathrm{CO}_{2} \mathrm{H}\) (c) \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) (d) \(\mathrm{CH}_{3} \mathrm{COOH}\)

Step-by-Step Solution

Answer

Compound (b) has the strongest acid strength due to the presence of a chlorine atom.

Step 1: Understand the Acid Strength

The strength of an acid is determined by its ability to donate a proton (H+). Generally, the presence of electronegative atoms (such as chlorine) increases acid strength due to inductive effects, which stabilize the negative charge on the conjugate base.

Step 2: Analyze Compound (a)

The compound \( \mathrm{HCOOH} \) is formic acid, which has no electronegative atoms directly attached to the carbon of the carboxylic group, therefore it will have a moderate acid strength.

Step 3: Analyze Compound (b)

The compound \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Cl}) \mathrm{CO}_{2} \mathrm{H} \) has a chlorine atom attached to a carbon atom adjacent to the carboxylic group. This increases acid strength through the electron-withdrawing inductive effect.

Step 4: Analyze Compound (c)

The compound \( \mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH} \) has the chlorine atom attached further away from the carboxylic group compared to compound (b), which reduces the inductive effect and consequently the acid strength.

Step 5: Analyze Compound (d)

The compound \( \mathrm{CH}_{3} \mathrm{COOH} \) is acetic acid, which also lacks highly electronegative atoms adjacent to the carboxylic group, resulting in a relatively lower acid strength.

Step 6: Compare Compounds and Determine Strongest Acid

Analyzing the compounds, it is evident that compound \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Cl}) \mathrm{CO}_{2} \mathrm{H} \) (b) has the strongest acid strength due to a chlorine atom directly enhancing the stability of its conjugate base via inductive effects.

Key Concepts

Inductive Effect

The inductive effect is a key concept in understanding acid strength. It refers to the transmission of charge through a chain of atoms in a molecule. This occurs due to the electronegative nature of certain atoms. When an atom like chlorine, which is highly electronegative, is attached to a carbon chain next to a carboxylic group, it can pull electron density away from the carboxylic group.
This pulling effect is called the electron-withdrawing inductive effect. It stabilizes the negative charge that forms on the conjugate base when the acid donates a proton (H+).

• Compounds with electronegative atoms near the acid group tend to be stronger acids.
• This is because the conjugate base becomes more stable thanks to the inductive effect.

In our example, compound (b), \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Cl}) \mathrm{CO}_{2}\mathrm{H}\), has chlorine directly adjacent to the acid group, enhancing its acid strength significantly.

Conjugate Base Stability

The stability of a conjugate base is pivotal in determining the strength of an acid. When an acid donates a proton, it leaves behind a conjugate base. The more stable this conjugate base is, the stronger the acid.
A stable conjugate base does not easily re-associate with the proton, meaning that the acid readily proceeds to donate \(\text{H}^+\).
Factors that contribute to conjugate base stability include:

• Presence of electronegative atoms, which can stabilize the negative charge through inductive effects.
• Resonance, which allows the negative charge to be dispersed over more than one atom.

In our compounds, those like (b) with highly electronegative atoms such as chlorine develop stable conjugate bases, making them overall stronger acids.

Electronegative Atoms

Electronegative atoms play a significant role in acid strength. They can be seen as atoms that "love" electrons. Chlorine is a prime example of an electronegative atom that can dramatically influence acidity.
When electronegative atoms are situated close to the acidic hydrogen, they enhance the acid's ability to donate that hydrogen atom (proton).

• By attracting electron density, electronegative atoms make the proton more positive and easily separable.
• This results in a stronger acid as the remaining ion, the conjugate base, becomes more stable.

In essence, the electronegativity effect makes compound (b) stronger than the others. It shows how proximity and position of electronegative atoms, such as chlorine, are critical in boosting acid strength.