Problem 58 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following orders of relative strengths of acids is correct? (a) \(\mathrm{ClCH}_{2} \mathrm{COOH}>\mathrm{FCH}_{2} \mathrm{COOH}>\mathrm{BrCH}_{2} \mathrm{COOH}\) (b) \(\mathrm{ClCH}_{2} \mathrm{COOH}>\mathrm{BrCH}_{2} \mathrm{COOH}>\mathrm{FCH}_{2} \mathrm{COOH}\) (c) \(\mathrm{BrCH}_{2} \mathrm{COOH}>\mathrm{ClCH}_{2} \mathrm{COOH}>\mathrm{FCH}_{2} \mathrm{COOH}\) (d) \(\mathrm{FCH}_{2} \mathrm{COOH}>\mathrm{ClCH}_{2} \mathrm{COOH}>\mathrm{BrCH}_{2} \mathrm{COOH}\)

Step-by-Step Solution

Answer

Option (d) is the correct order: \( \mathrm{FCH}_{2} \mathrm{COOH}>\mathrm{ClCH}_{2} \mathrm{COOH}>\mathrm{BrCH}_{2} \mathrm{COOH} \).

Step 1: Understanding Acid Strength

The strength of carboxylic acids is largely influenced by the electron-withdrawing effects of substituents attached to the carbon atom adjacent to the carboxyl group. Stronger electron-withdrawing groups increase the acidity by stabilizing the negative charge on the conjugate base.

Step 2: Electron-Withdrawing Power of Halogens

Among halogens, the order of electron-withdrawing ability is fluorine (F) > chlorine (Cl) > bromine (Br) due to the difference in electronegativities. The greater the electronegativity of the halogen, the stronger the acidity of the acid to which it is attached.

Step 3: Comparing Given Options

Now, compare each given option based on the strength of their electron-withdrawing groups.- (a) \( \mathrm{ClCH}_{2} \mathrm{COOH}>\mathrm{FCH}_{2} \mathrm{COOH}>\mathrm{BrCH}_{2} \mathrm{COOH} \)- (b) \(\mathrm{ClCH}_{2} \mathrm{COOH}>\mathrm{BrCH}_{2} \mathrm{COOH}>\mathrm{FCH}_{2} \mathrm{COOH} \)- (c) \(\mathrm{BrCH}_{2} \mathrm{COOH}>\mathrm{ClCH}_{2} \mathrm{COOH}>\mathrm{FCH}_{2} \mathrm{COOH} \)- (d) \(\mathrm{FCH}_{2} \mathrm{COOH}>\mathrm{ClCH}_{2} \mathrm{COOH}>\mathrm{BrCH}_{2} \mathrm{COOH} \)

Step 4: Identifying Correct Order

Based on the electron-withdrawing abilities, the correct order of acid strength is determined by which has the strongest electron-withdrawing substituent. Hence, fluorine (highest electronegativity) makes \(\mathrm{FCH}_{2} \mathrm{COOH} \) the strongest, followed by chlorine, then bromine.

Key Concepts

Electron-Withdrawing Groups

In chemistry, substituents like halogens attached to a molecule can have a significant impact on the properties of that molecule. This occurs due to the concept of electron-withdrawing groups.
An electron-withdrawing group is an atom or a group of atoms that attracts electrons towards itself from a neighboring atom, creating a dipole within the molecule.
This resulting dipole can stabilize a negative charge elsewhere in the molecule. This is crucial in acid-base chemistry, especially with carboxylic acids like

• Fluorine (\(\mathrm{F}\))
• Chlorine (\(\mathrm{Cl}\))
• Bromine (\(\mathrm{Br}\))

These halogens alter the acidity of carboxylic acids by increasing the stability of the conjugate base. The more electron-withdrawing a substituent is, the stronger the acid becomes.
This is why understanding the role and influence of electron-withdrawing groups is important in determining acid strength.

Halogen Electronegativity

The concept of electronegativity is key to understanding why certain halogens make acids stronger. Electronegativity is the ability of an atom to attract electrons.
Among the halogens, there is a general order of electronegativity:

• Fluorine (\(\mathrm{F}\)) is the most electronegative.
• Chlorine (\(\mathrm{Cl}\)) follows.
• Bromine (\(\mathrm{Br}\)) is the least electronegative among them.

A more electronegative atom pulls electrons away from other parts of the molecule, increasing the molecule's reactivity.
In the context of acid strength, a higher electronegativity of the attached halogen means a greater ability to stabilize the negative charge on the conjugate base.
This stabilization makes acids with more electronegative halogens stronger than those with less electronegative halogens.

Carboxylic Acids

Carboxylic acids are a class of organic compounds that are characterized by the presence of a carboxyl group (\(\mathrm{COOH}\)). This functional group is highly important in determining the properties of the compound, including its acidity.
The stability of the carboxylate anion, which is the conjugate base of the acid, is crucial for understanding acid strength.
The presence of electron-withdrawing groups, such as halogens, influences the stability of the carboxylate anion by stabilizing the negative charge that forms when the acid donates a proton (\(\mathrm{H}^+\)).

• When a substituent like fluorine is attached to a carboxylic acid's \(\mathrm{CH}_2\) group, it enhances the acid's strength.
• This effect occurs because the halogen increases the stability of the carboxylate anion, as it assists in dispersing the negative charge.

This makes fluorinated carboxylic acids stronger than those with other halogens like chlorine or bromine. Understanding the way carboxylic acids behave is essential to grasp the fundamental concepts of organic chemistry related to acidity.