Problem 197 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of increasing acid strength of the compound: (1) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) (2) \(\mathrm{MeOCH}_{2} \mathrm{CO}_{2} \mathrm{H}\) (3) \(\mathrm{CF}_{3} \mathrm{CO}_{2} \mathrm{H}\) (4) \((\mathrm{Me})_{2}-\mathrm{CO}_{2} \mathrm{H}\) (a) \(2<4<1<3\) (b) \(4<1<3<2\) (c) \(4<1<2<3\) (d) \(1<4<3<2\)

Step-by-Step Solution

Answer

The correct answer is (c) \(4<1<2<3\).

Step 1: Understanding Acid Strength

Acid strength depends on the stability of the conjugate base. More stable conjugate bases usually lead to stronger acids. This stability can be influenced by factors like electronegativity, inductive effects, and resonance.

Step 2: Analyze Each Acid

1) \(\mathrm{CH}_{3}\mathrm{CO}_{2}\mathrm{H}\) - Acetic acid, the methyl group slightly donates electrons, making it weaker.2) \(\mathrm{MeOCH}_{2}\mathrm{CO}_{2}\mathrm{H}\) - Methoxy acetic acid, methoxy group can donate electrons + has a weak inductive effect.3) \(\mathrm{CF}_{3}\mathrm{CO}_{2}\mathrm{H}\) - Trifluoroacetic acid, fluorine atoms have a strong electron-withdrawing inductive effect, making it very strong.4) \((\mathrm{Me})_{2}-\mathrm{CO}_{2}\mathrm{H}\) - Dimethyl acetic acid, two methyl groups tend to donate electrons, making it weaker.

Step 3: Compare Acids Based on Structures

1. \(\mathrm{CF}_{3}\mathrm{CO}_{2}\mathrm{H}\) is the strongest due to the very strong electron-withdrawing effect of the trifluoromethyl group.2. \(\mathrm{CH}_{3}\mathrm{CO}_{2}\mathrm{H}\) comes next, with a simple methyl group.3. \((\mathrm{Me})_{2}-\mathrm{CO}_{2}\mathrm{H}\) is less acidic due to electron donation by two methyl groups.4. \(\mathrm{MeOCH}_{2}\mathrm{CO}_{2}\mathrm{H}\) is the least acidic with an electron-donating methoxy group.

Step 4: Order The Acids

From weakest to strongest, the order is \((\mathrm{Me})_{2}-\mathrm{CO}_{2}\mathrm{H} < \mathrm{MeOCH}_{2}\mathrm{CO}_{2}\mathrm{H} < \mathrm{CH}_{3}\mathrm{CO}_{2}\mathrm{H} < \mathrm{CF}_{3}\mathrm{CO}_{2}\mathrm{H}\). This corresponds to the order 4 < 1 < 2 < 3 in terms of increasing acid strength.

Key Concepts

Conjugate Base Stability

The stability of a conjugate base plays a crucial role in determining the strength of an acid. When an acid donates a proton, it forms its conjugate base. The more stable this conjugate base is, the stronger the corresponding acid will be. This is because a stable conjugate base does not easily re-accept the donated proton, allowing the acid to more effectively donate protons in solution.

Several factors contribute to the stability of a conjugate base, including:

• Electronegativity: More electronegative atoms tend to hold negative charges more effectively, leading to greater stability.
• Resonance: The ability of a conjugate base to delocalize its negative charge through resonance structures enhances its stability.

The presence of electron-withdrawing groups near the negatively charged site also helps stabilize the conjugate base, thus increasing acid strength.

Inductive Effect

The inductive effect refers to the transmission of charge through a chain of atoms in a molecule, mainly due to differences in electronegativity between atoms. Atoms or groups that are more electronegative will pull electron density towards themselves, exerting a negative inductive effect that stabilizes a conjugate base.In this scenario, the presence of groups like \( \text{CF}_3 \), which are highly electronegative, can lead to strong electron-withdrawing inductive effects. This makes the conjugate base more stable by reducing the negative charge density at the site of deprotonation. The stronger the inductive effect, the more stable the conjugate base, leading to a stronger acid. By understanding how different groups affect electron distribution through the inductive effect, we can predict and compare the strengths of various acids.

Electron-Withdrawing Groups

Electron-withdrawing groups are those that pull electron density away from a molecule's acidic proton, stabilizing the conjugate base. These groups enhance acid strength by making the conjugate base less prone to accepting protons—essentially making the base less reactive.For example, the trifluoromethyl group (\( \text{CF}_3 \)) in trifluoroacetic acid has a strong electron-withdrawing ability due to the high electronegativity of fluorine atoms. This group significantly stabilizes the conjugate base by dispersing the negative charge away from the acidic center. In contrast, electron-donating groups, such as methyl (\( \text{Me} \)), decrease acid strength because they increase electron density around the conjugate base.

Resonance Effect

The resonance effect involves the delocalization of electrons in a molecule, which can lead to increased stability of the conjugate base. When a molecule can spread out the negative charge across several atoms, it becomes more stable and thus forms a stronger acid. Let's consider acetic acid as an example. When it loses a proton, the conjugate base ( o{-} C−C(=O)−O− ) can resonate between two structures, which helps delocalize the negative charge over the oxygen atoms, stabilizing the base. Acids with conjugate bases that can participate in resonance typically have increased acid strengths due to this enhanced stability. Understanding resonance effects is crucial for predicting and interpreting acid-base behavior in organic chemistry.