Problem 228 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of increasing acid strength of the compound is (a) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) (b) \(\mathrm{MeOCH}_{2} \mathrm{CO}_{2} \mathrm{H}\) (c) \(\mathrm{CF}_{3} \mathrm{CO}_{2} \mathrm{H}\) (d) \((\mathrm{Me})_{2} \mathrm{CH} \mathrm{CO}_{2} \mathrm{H}\) (a) \(\mathrm{b}<\mathrm{d}<\mathrm{a}<\mathrm{c}\) (b) \(\mathrm{d}<\mathrm{a}<\mathrm{c}<\mathrm{b}\) (c) \(d

Step-by-Step Solution

Answer

The correct order is (a) \(\mathrm{b}<\mathrm{d}<\mathrm{a}<\mathrm{c}\).

Step 1: Identify the Compound Structures

First, identify the structures of the given compounds:- \(\text{(a) } \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \) is acetic acid.- \(\text{(b) } \mathrm{MeOCH}_{2} \mathrm{CO}_{2} \mathrm{H} \) is methoxyacetic acid.- \(\text{(c) } \mathrm{CF}_{3} \mathrm{CO}_{2} \mathrm{H} \) is trifluoroacetic acid.- \(\text{(d) } (\mathrm{Me})_{2} \mathrm{CH} \mathrm{CO}_{2} \mathrm{H} \) is isobutyric acid.

Step 2: Consider Inductive Effects

Evaluate the inductive effect of substituents on acid strength:- Electronegativity of substituents affects the acidity of the compound. - Compound (c) has a strongly electron-withdrawing \(\mathrm{CF}_{3}\) group, significantly increasing its acidity.- Compound (b) has an electron-donating \(\mathrm{MeO-}\) group, reducing acidity.- Compound (d) has electron-donating alkyl groups, which also decrease acidity compared to (a).- Compound (a) is a standard reference for carboxylic acids.

Step 3: Rank the Compounds by Acid Strength

Rank the compounds from least to most acidic based on the analysis:- The inductive effect of the \(\mathrm{CF}_{3}\) group makes \(\mathrm{CF}_{3} \mathrm{CO}_{2} \mathrm{H} \) (compound c) the most acidic.- Compound (a) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \) follows after (c).- Compound (d) \((\mathrm{Me})_{2} \mathrm{CH} \mathrm{CO}_{2} \mathrm{H} \) (isobutyric acid) is less acidic than (a).- Due to the electron-donating \(\mathrm{MeO}-\) group, compound (b) \(\mathrm{MeOCH}_{2} \mathrm{CO}_{2} \mathrm{H} \) is the least acidic.

Step 4: Verify Against Options

Compare the determined order to the provided multiple-choice options:- The correct increasing order of acid strength is \(b < d < a < c\).- Compare this with option (a), which matches our ranking.

Key Concepts

Carboxylic Acids

Carboxylic acids are a group of organic compounds that contain a carboxyl group (\(-COOH\)) as their functional group. This group is key in determining the acidity of these compounds. The carboxylic group consists of a carbonyl group \( (C=O) \) and a hydroxyl group \( (O-H) \), which together make carboxylic acids unique.
Characteristics

• Proton Donors: Carboxylic acids can donate protons (H+) due to the presence of the acidic hydrogen in the hydroxyl group.
• Polarity: The carboxyl group is highly polar, making these compounds relatively more acidic than other organic compounds like alcohols or ketones.

The acidic nature of carboxylic acids stems from their ability to form a resonance-stabilized carboxylate ion \((RCOO^{-})\). This ion is more stable, with negative charge delocalization over the two oxygen atoms, which enhances their acidity. Understanding these properties helps explain why certain substituents can affect the acidity through inductive effects, as discussed in the following sections.

Inductive Effect

The inductive effect is an important concept in understanding how substituents attached to a functional group, like the carboxyl group, can influence the acidity of carboxylic acids. It relates to the transmission of charge through a chain of atoms in a molecule by the electrostatic induction.
How It Works

• Electron-Withdrawing Inductive Effect: When an electronegative substituent, such as \(CF_3\), is attached to the carboxylic acid, it pulls electron density away from the carboxyl group, increasing the acid's ability to donate a proton.
• Electron-Donating Inductive Effect: Conversely, when a less electronegative group, such as \(OCH_3\), is attached, it pushes electron density towards the carboxyl group, decreasing the acid's strength.

This transmission of electron density is essential in determining the relative acidity of carboxylic acids in different molecular environments. By affecting the dissociation of hydrogen ions, the inductive effect can make some carboxylic acids much stronger than others, as seen in trifluoroacetic acid compared to acetic acid.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are key in modulating the acidity of carboxylic acids. These groups attract electron density towards themselves, thereby enhancing the acidity of carboxylic acids by stabilizing the negative charge of the conjugate base (carboxylate ion).
Examples and Effects

• Common EWGs: Halogens like fluorine and functional groups like nitro \((NO_2)\) are typical electron-withdrawing groups.
• Impact on Acidity: For instance, the \(CF_3\) group in trifluoroacetic acid significantly boosts its acidity compared to acetic acid because it pulls electron density away from the carboxyl group, facilitating proton release.

The presence of EWGs is often the reason behind the increased acid strength in carboxylic acids, as they enhance the molecule's ability to stabilize the carboxylate ion, fundamentally altering its acidic properties. This makes understanding EWGs crucial when predicting or comparing acid strengths in compounds.

Electron-Donating Groups

Electron-donating groups (EDGs) are substituents that release electron density towards the attached molecules, reducing the acidity of carboxylic acids. By providing additional electron density to the carboxyl group, EDGs make it harder for the carboxylic acid to release protons.
Characteristics

• Common EDGs: Alkyl groups such as methyl \((CH_3)\) and ethyl \((C_2H_5)\), as well as ether groups like methoxy \((OCH_3)\), serve as EDGs.
• Effect on Acidity: The presence of EDGs, such as in methoxyacetic acid, results in a weaker acid due to the reduced tendency to form a carboxylate ion.

These groups donate electrons through sigma bonds, and their presence often necessitates a reassessment of a carboxylic acid's acidity. Understanding the influence of EDGs prepares students to anticipate changes in acid strength and predict chemical behavior accordingly.