Problem 185 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The correct order is (d) \(\mathrm{ii} > \mathrm{iii} > \mathrm{iv} > \mathrm{i}\).

Step 1: Understand the Influencing Factor

Acidity of carboxylic acids is influenced by the electron-withdrawing or electron-donating substituents present on the benzene ring. Electron-withdrawing groups stabilize the conjugate base, increasing the acid strength.

Step 2: Identify the Substituents

The given acids have a nitro group (\(\mathrm{NO_2}\)) at different positions relative to the carboxy group. \(\mathrm{o}\)-nitro (ortho), \(\mathrm{m}\)-nitro (meta), and \(\mathrm{p}\)-nitro (para). Nitro groups are strong electron-withdrawing groups.

Step 3: Consider Ortho Effect

An ortho substituent can have a resonance as well as an inductive effect, which is often stronger than para and meta positions because of proximity to the carboxyl group. Therefore, \(\mathrm{o}-\mathrm{NO}_2\mathrm{C}_6\mathrm{H}_4\mathrm{COOH}\) is the strongest acid among the given options.

Step 4: Evaluate Para and Meta Effects

The para position allows for resonance stabilization, while the meta position does not participate in resonance. \(\mathrm{p}-\mathrm{NO}_2\mathrm{C}_6\mathrm{H}_4\mathrm{COOH}\) is a stronger acid than \(\mathrm{m}-\mathrm{NO}_2\mathrm{C}_6\mathrm{H}_4\mathrm{COOH}\) due to this resonance effect.

Step 5: Compare the Acidity Against Unsubstituted Acid

Without any substituents, \(\mathrm{PhCOOH}\) is less acidic than the nitro-substituted carboxylic acids, because it lacks the strong electron-withdrawing effect of \(\mathrm{NO}_2\). Hence, it has the least acidity among the options.

Key Concepts

Acidity

In the realm of carboxylic acids, acidity is a pivotal characteristic. It refers to the ability of a compound to donate a proton (\( ext{H}^+ \)) to a base. More acidic carboxylic acids dissociate more easily in solution, releasing hydrogen ions. This property is influenced by the substituents attached to the acid structure.
Carboxylic acids with strong electron-withdrawing groups enjoy heightened acidity. This is because they stabilize the negative charge on the conjugate base, making it more favorable for the acid to release a proton. Consequently, the strength of an acid is closely tied to its molecular structure and the nature of its substituents.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) play a crucial role in influencing the acidity of carboxylic acids. These groups pull electrons away from other parts of the molecule by their inductive or resonance effects. Nitro groups (\( ext{NO}_2 \)), prominent in our discussion, are classic examples of strong electron-withdrawing groups.
When placed in proximity to the carboxyl group, nitro groups increase acidity by stabilizing the conjugate base.

• Ortho position: Enhances both resonance and inductive effects due to proximity.
• Para position: Primarily stabilizes the conjugate base through resonance.
• Meta position: Limited resonance impact, relies more on inductive effects.

Thus, the placement of EWGs significantly impacts the acid's dissociation tendency, with ortho positions often leading to stronger acids due to combined effects.

Ortho Effect

The ortho effect is a fascinating aspect of substitution in aromatic compounds. It particularly affects carboxylic acids by increasing their acidity when substituents are positioned ortho to the carboxyl group.
At the ortho position, substituents can both resonate with and inductively withdraw electrons from the carboxylic acid. This dual action enhances the ability of the acid to release a proton. The close spatial arrangement facilitates stronger electronic interactions.
In our example, the \( ext{o}- ext{NO}_2 \) substitution maximizes these effects, thereby making \( ext{o}- ext{NO}_2 ext{C}_6 ext{H}_4 ext{COOH} \) the most acidic among the given carboxylic acids. The ortho effect combines inductive power and resonance to boost acidity considerably.

Resonance Stabilization

Resonance stabilization is a key concept that significantly influences the acidity of a molecule. It involves the delocalization of electrons across adjacent atoms, resulting in more stable structures.
With respect to carboxylic acids, resonance is especially valuable when electron-withdrawing substituents are in positions that allow for electron delocalization.

• Para-positioned substituents can extend resonance throughout the aromatic ring and to the carboxyl group.
• Meta-positioned substituents are less effective in contributing to resonance stabilization.

In the exercise example, the para-nitro carboxylic acid (\( ext{p}- ext{NO}_2 ext{C}_6 ext{H}_4 ext{COOH} \)) benefits from resonance, making it more acidic than its meta counterpart despite the lack of ortho effects. Understanding resonance can greatly aid in predicting the relative acidities of aromatic carboxylic acids.