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) \(\mathrm{BrCH}_{2} \mathrm{COOH}>\mathrm{ClCH}_{2}^{2} \mathrm{COOH}>\mathrm{FCH}_{2} \mathrm{COOH}\) (d) \(\mathrm{FCH}_{2} \mathrm{COOH}>\mathrm{ClCH}_{2} \mathrm{COOH}>\mathrm{BrCH}_{2} \mathrm{COOH}\)

Step-by-Step Solution

Answer

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

Step 1: Understand the Problem

We need to determine the correct order of acid strengths for the given compounds based on their halogen substituents (Cl, Br, F) affecting the acetic acid derivative. The strength of the acids is influenced by the electronegativity of the substituent.

Step 2: Consider the Effect of Electronegativity

Electronegativity of the substituents affects the acid’s strength. The more electronegative the substituent, the greater the electron-withdrawing effect, stabilizing the negative charge on the carboxylate ion after losing a proton, and thus increasing the acidity.

Step 3: Evaluate the Electronegativity of Halogens

The halogens in question have different electronegativities: Fluorine (F) is the most electronegative, followed by Chlorine (Cl), and then Bromine (Br). The more electronegative the halogen, the stronger the acid due to better stabilization of the conjugate base.

Step 4: Rank the Acids According to Substituent Influence

Applying the concept of electronegativity, the acid strength should be ranked as follows: - FCH}_{2} COOH (strongest due to F), - ClCH}_{2} COOH, - BrCH}_{2} COOH (weakest due to Br).

Step 5: Choose the Correct Answer Based on Ranking

The correct order of acidity based on the substituents is option (d): FCH}_{2} COOH > ClCH}_{2} COOH > BrCH}_{2} COOH, as it reflects the correct descending order of strength based on electronegativity.

Key Concepts

Electronegativity

Electronegativity is an essential concept in chemistry that helps to explain why certain substances, like acids, vary in strength. It refers to the tendency of an atom to attract electrons towards itself. The more electronegative an atom is, the stronger its pull on electrons. This plays a crucial role in the chemistry of organic molecules, especially regarding acids.
In the context of acid strength, substituents with high electronegativity, such as halogens, can withdraw electron density from the rest of the molecule. This enhances the stability of the conjugate base formed after the acid donates a proton (H⁺).
For example, in compounds like fluorinated acetic acids, the electronegativity of fluorine plays a pivotal role. As the most electronegative element, fluorine greatly stabilizes the conjugate base by withdrawing electrons, thus making the acid stronger.

Carboxylic Acids

Carboxylic acids are a class of organic acids characterized by the presence of a carboxyl group \(-\mathrm{COOH}\). They are known for their range of chemical reactions and are fundamental in various biological and chemical processes. Their acidity level is a critical parameter in understanding their behavior.
The acidity of carboxylic acids is influenced by their ability to donate a proton \(-\mathrm{H}^+\), which depends largely on the stabilization of the resulting conjugate base, the carboxylate ion \(-\mathrm{COO}^-\). The substitution of different groups can either increase or decrease their acidity. Halogen atoms, particularly those that are highly electronegative, can enhance the acidity by stabilizing the negative charge on the conjugate base, making the acid a stronger proton donor.

• Examples include acetic acids substituted with halogens like fluorine, chlorine, and bromine, where their position and electronegativity affect the overall acidity.

Halogen Substituents

Halogen substituents have a significant impact on the strength of acids like carboxylic acids. These are atoms from the halogen group, including fluorine, chlorine, and bromine, known for their electron-withdrawing capability due to high electronegativity.
When halogens are substituted onto a carboxylic acid, they affect the acid's strength by enhancing the stability of the conjugate base. The greater the electronegativity of the halogen, the stronger its effect. For instance, substituting a hydrogen by a fluorine in acetic acid will result in a stronger acid compared to substitutions made by chlorine or bromine.

• Fluorine, being the most electronegative, increases the acid strength the most.
• Chlorine and bromine have progressively lesser effects due to their lower electronegativity compared to fluorine.

Conjugate Base Stabilization

The concept of conjugate base stabilization is crucial in understanding why some acids are stronger than others. When an acid donates a proton, it forms its conjugate base. The stability of this base is a major factor in determining the acid's strength—the more stable the conjugate base, the stronger the acid.
Halogens, due to their electronegativity, can stabilize this negative charge on the conjugate base through an electron-withdrawing effect. This stabilization makes it easier for the acid to release a proton, thereby increasing its acidic strength.
For example, in the case of \(\text{FCH}_2\text{COOH}\), the fluorine atom stabilizes the \(\text{CH}_2\text{COO}^-\) ion effectively due to its high electronegativity. This makes \(\text{FCH}_2\text{COOH}\) a stronger acid compared to its chlorinated or brominated counterparts. Understanding these stabilizing effects is crucial for predicting and explaining the behavior of different acids.