Question

Consider the acidity of the carboxylic acids: (1) \(\mathrm{PhCOOH}\) (2) \(\mathrm{o}-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH}\) (3) \(\mathrm{p}-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH}\) (4) \(\mathrm{m}-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH}\) Which of the following order is correct? (a) \(1>2>3>4\) (b) \(2>4>3>1\) (c) \(2>4>1>3\) (d) \(2>3>4>1\)

Short answer

Option (d) is correct: 2 > 3 > 4 > 1.

Step-by-step solution

Review Effect of Electron Withdrawing Groups

Understand how electron-withdrawing groups, like the nitro group (NO₂), affect acidity. They stabilize the carboxylate ion by delocalizing the negative charge, increasing the acid's acidity. This effect is strongest in ortho positions due to proximity to the carboxyl group, followed by para and then meta positions.

Analyze ortho Position

The compound with an NO₂ group in the ortho position, (2) o-NO₂C₆H₄COOH, is expected to be the most acidic. The strong electron-withdrawing nitro group is very close to the carboxyl group, thus maximizing stabilization of the carboxylate ion by resonance.

Analyze para Position

The compound (3) p-NO₂C₆H₄COOH has the NO₂ group in the para position. The nitro group can still affect the acidity, but less effectively than in the ortho position. Thus, this compound is less acidic than the ortho but more effective than those without NO₂ directly influencing the carboxylate ion.

Analyze meta Position

The compound (4) m-NO₂C₆H₄COOH, with a nitro group in the meta position, has some electron-withdrawing effect on the carboxylate ion, but this is weaker than the ortho and para positions. Hence, it's less acidic than both ortho and para nitro benzoic acids.

Compare with Benzoic Acid

A simple benzoic acid (1) PhCOOH has no additional electron-withdrawing groups attached to the phenyl ring. Thus, it is the least acidic compared to the substituted benzoic acids with nitro groups because it lacks the additional stabilization for the carboxylate ion.

Determine Correct Order

Based on the positions of the nitro groups and their effects, the correct order of acidity from most to least acidic is: (2) > (3) > (4) > (1). This corresponds to option (d).

Key concepts

Electron-Withdrawing Groups

Electron-withdrawing groups are key players in determining the acidity of carboxylic acids. These are atoms or groups that pull electron density away from other areas of the molecule. One prime example is the nitro group (NO₂).
For carboxylic acids, the presence of such groups can greatly enhance their acidity. They do this by stabilizing the negatively charged carboxylate ion that forms when the acid loses a proton. This stabilization occurs through a process known as delocalization, where the negative charge is spread out over a larger area across the molecule.
In essence, the more effective the electron-withdrawing group, the easier it is for the acid to lose a proton, making the acid stronger. It's important to always consider the position and strength of these groups when comparing acidity levels.

Ortho, Meta, and Para Positions

The terms ortho, meta, and para refer to positions on an aromatic ring relative to a reference group, such as a carboxylic acid group (COOH). Each position interacts differently with substituent groups and influences the overall molecular behavior.

• Ortho Position: This is directly adjacent to the reference group. Substituents in the ortho position have the most immediate effect on the carboxyl group and, when electron-withdrawing, can significantly enhance acidity.
• Meta Position: This is separated by one carbon and is less impactful on the reference group compared to ortho. The effect of electron-withdrawing groups here is weaker but still present.
• Para Position: Opposite on the ring from the reference group, para substituents can still interact via resonance, though less impactfully than ortho positions.

By recognizing these positions, one can predict how a substituent will influence the molecule's properties, particularly acidity.

Nitro Group Effect

The nitro group (NO₂) is a potent electron-withdrawing group, making it a key factor in determining acidity in aromatic carboxylic acids.
When attached to a benzene ring, nitro groups can have a varied impact based on their position (ortho, meta, or para to the acid group). The ortho position benefits the most, as the close proximity to the carboxyl group allows it to strongly stabilize the carboxylate ion via resonance. This results in a more acidic compound.
In para substitution, the nitro group also stabilizes the carboxylate ion, though the effect is slightly less pronounced due to its distance. The meta position exhibits the weakest influence because the electronic communication between the nitro group and the carboxyl group is not as effective.
Understanding the location and function of nitro groups enables a better prediction of acidity levels in substituted benzoic acids.

Carboxylate Ion Stabilization

Carboxylate ion stabilization is crucial for determining the acidity of carboxylic acids. When a carboxylic acid loses a proton, a carboxylate ion \( ext{RCOO}^- \) forms. The stability of this ion directly influences the acid's strength.
Stability is achieved through the delocalization of the negative charge. Electron-withdrawing groups attached to the aromatic ring can stabilize the carboxylate ion. For example, nitro groups aid in spreading out the charge by drawing electron density away from the ion.
This stabilization reduces the energy of the carboxylate ion, making it more stable compared to less acidic acids which don't possess such groups. This attribute is why substituted benzoic acids with strong electron-withdrawing groups can lose protons more easily, displaying greater acidity.