Question

Order the compounds in each set in order of increasing acidity: (a) Acetic acid, oxalic acid, formic acid (b) \(p\) -Bromobenzoic acid, \(p\) -nitrobenzoic acid, 2,4 -dinitrobenzoic acid (c) Fluoroacetic acid, 3 -fluoropropanoic acid, 4 -fluorobutanoic acid

Short answer

(a) Acetic acid < Formic acid < Oxalic acid; (b) p-Bromobenzoic acid < p-Nitrobenzoic acid < 2,4-Dinitrobenzoic acid; (c) 4-Fluorobutanoic acid < 3-Fluoropropanoic acid < Fluoroacetic acid.

Step-by-step solution

Understand the factors affecting acidity

Acidity in organic compounds is influenced by the stability of the conjugate base. Typically, a more stable conjugate base corresponds to a stronger acid. Electron-withdrawing groups increase acidity by stabilizing the conjugate base via resonance or induction.

Analyze set (a): carboxylic acids

In set (a), we consider acetic acid, oxalic acid, and formic acid. Formic acid (HCOOH) has one less alkyl group compared to acetic acid (CH₃COOH), thus making formic acid more acidic as alkyl groups donate electrons, reducing acidity. Oxalic acid (HOOC-COOH) has two carboxyl groups, which can stabilize the conjugate base through resonance and inductive effects, making it the most acidic of the three.

Order set (a) by increasing acidity

Order: Acetic acid < Formic acid < Oxalic acid.

Analyze set (b): substituted benzoic acids

In set (b) we have different substituents on benzoic acid. Electron-withdrawing groups like nitro (−NO₂) significantly increase acidity due to resonance and inductive effects. 2,4-Dinitrobenzoic acid, having two nitro groups, is the most acidic. The p-nitrobenzoic acid follows with only one nitro group, and p-bromobenzoic acid is the least acidic as bromine is less electron-withdrawing.

Order set (b) by increasing acidity

Order: p-Bromobenzoic acid < p-Nitrobenzoic acid < 2,4-Dinitrobenzoic acid.

Analyze set (c): fluoro-substituted carboxylic acids

Fluorine is an electron-withdrawing group, so its proximity to the carboxylic group affects acidity. In fluoroacetic acid, the fluorine is closest to the carboxyl group, causing a strong inductive effect and making it the most acidic. 3-Fluoropropanoic acid has fluorine further away, reducing its acidity compared to fluoroacetic acid. 4-Fluorobutanoic acid has the fluorine atom the farthest, making it the least acidic.

Order set (c) by increasing acidity

Order: 4-Fluorobutanoic acid < 3-Fluoropropanoic acid < Fluoroacetic acid.

Key concepts

Conjugate Base Stability

When we talk about the acidity of a compound, what we’re really discussing is how easily it can donate a proton (H⁺). Once it loses a proton, it forms what's called a conjugate base. The stability of this conjugate base is key to understanding acidity. A more stable conjugate base means a stronger acid.

• If a base is stable, the proton is more readily given up, which increases acidity.
• Stability often comes from distributing or delocalizing the negative charge across a molecule.

In the case of organic acids like carboxylic acids, the conjugate base can be stabilized by resonance and inductive effects, both of which play a significant role in determining acidity.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are like magnets that pull electron density away from a structure. This is crucial in organic chemistry as they enhance acidic character by stabilizing the negatively charged conjugate base.

• Common EWGs include nitro ( O₂), halogens (like F, Cl), and carbonyl groups (C=O).
• These groups pull electrons through the bond network, either by resonance or induction, lowering energy levels and stabilizing the molecular structure.

In substituted benzoic acids, for example, nitro groups stand out because they significantly boost acidity, making these compounds more likely to donate protons.

Inductive Effects

Inductive effects refer to the transmission of charge through a chain of atoms in a molecule, leading to further electron stability or instability.

• Electron-withdrawing inductive effects occur when electronegative atoms, such as fluorine, are nearby.
• This effect diminishes as the distance between the electronegative atom and the acidic proton increases.

In fluoro-substituted acids, the closer the fluorine is to the carboxyl group, the stronger the inductive effect, thereby increasing the acidity. This is why fluoroacetic acid is more acidic than its counterparts like 3-fluoropropanoic acid and 4-fluorobutanoic acid.

Resonance Effects

Resonance effects occur when a molecule can distribute its charge over two or more atoms. This delocalization of electrons helps stabilize the molecule, and in turn, can strengthen an acid by stabilizing its conjugate base.

• Molecules with resonance can support a negative charge on an atom, such as oxygen, more efficiently.
• This is what we observe in multi-functional organic acids like oxalic acid.

Resonance allows these molecules to balance and stabilize charges more effectively, enhancing their ability to act as acids. In multi-substituted benzoic acids, nitro groups not only act via inductive effects but also through resonance to stabilize the conjugate base.

Carboxylic Acids Analysis

Carboxylic acids are a key topic in understanding organic chemistry acidity. They feature a carboxyl group (-COOH) which serves as their acidic center.

• The carboxyl group's -OH bond is weak due to the electronegative oxygen, which draws electron density away.
• Groups like OH can resonate, distributing the negative charge over the carbonyl group when deprotonated.

Analysis of carboxylic acids usually focuses on factors like substituents' electron-withdrawing strength, positions, and resulting inductive and resonance effects—the greater these effects, the stronger the acidity.