Question

Out of these acids, the strongest acid is (a) O-nitrobenzoic acid (b) P - nitrobenzoic acid (c) \(\mathrm{M}\) - nitrobenzoic acid (d) Benzoic acid

Short answer

O-nitrobenzoic acid is the strongest acid.

Step-by-step solution

Understanding Acid Strength

The strength of an acid is influenced by its ability to release protons (H+ ions) in a solution. A key factor is the stabilization of the conjugate base, which occurs when the negative charge is delocalized over the structure. In benzoic acids, electron-withdrawing groups like the nitro group can increase acid strength by stabilizing the conjugate base through resonance and inductive effects. We need to compare the effects of the nitro group in different positions (ortho, meta, para) on benzoic acid.

Evaluating Ortho Position

In O-nitrobenzoic acid, the nitro group is at the ortho position relative to the carboxyl group. This allows the nitro group to strongly withdraw electrons through both resonance (pi bonding) and inductive (sigma bonding) effects, significantly stabilizing the conjugate base. This increases the acid strength.

Evaluating Meta Position

In M-nitrobenzoic acid, the nitro group is at the meta position. The nitro group in this position can stabilize the conjugate base through the inductive effect but not as effectively through resonance, leading to moderate acid strength increase.

Evaluating Para Position

In P-nitrobenzoic acid, the nitro group is at the para position. It can withdraw electrons through both resonance and inductive effects, but generally, the ortho and para positions have similar effects with para being slightly weaker due to less direct interaction compared to ortho.

Evaluating Benzoic Acid

In benzoic acid, there is no electron-withdrawing nitro group to help stabilize the conjugate base, so its acid strength is the weakest among all options provided.

Comparing All Options

Now, we compare all the effects: O-nitrobenzoic acid > P-nitrobenzoic acid ≈ M-nitrobenzoic acid > Benzoic acid. The ortho position has the strongest electron-withdrawing effect, leading to the highest stabilization of the conjugate base, and thus it is the strongest acid.

Key concepts

Electron-withdrawing groups

Electron-withdrawing groups are atoms or groups of atoms that pull electron density away from other parts of a molecule. They are crucial in increasing the strength of acids by stabilizing their conjugate bases. For instance, in nitrobenzoic acids, the nitro group (\(-\text{NO}_2\)) is a classic electron-withdrawing group. When present on a benzoic acid molecule, it enhances the acid's ability to donate a proton, making the acid stronger.How does this work? When the nitro group is present, it pulls electrons away from the acidic proton's vicinity, decreasing the electron density around it. This makes it easier for the proton to break free, thus increasing the acidity. Understanding the role of electron-withdrawing groups is essential for predicting and comparing the strengths of different acids effectively.

Conjugate base stabilization

To understand how a conjugate base is stabilized, consider what happens when an acid donates its proton. The remaining molecule, now negatively charged, is the conjugate base. A stable conjugate base translates to a stronger acid because it can efficiently disperse the negative charge. In the context of nitrobenzoic acids, the stabilization of the conjugate base is achieved through the delocalization of this negative charge. Electron-withdrawing groups like the nitro group assist in this process by pulling the electron cloud away, helping to spread out the charge. The more stable the conjugate base, the more easily the original acid gives up its proton, again enhancing acid strength. Hence, the presence of such groups creates a favorable environment for conjugate base stabilization, contributing significantly to the acid's overall strength.

Resonance and inductive effects

The concepts of resonance and inductive effects are pivotal in understanding the chemical behavior of molecules. Resonance involves the delocalization of electrons across different atoms, while inductive effects involve the transmission of charge through sigma bonds. In nitrobenzoic acids, the nitro group can exert both effects. The resonance effect occurs as the electrons from the conjugate base's negative charge can be spread across the molecule, stabilizing the charge over a larger area. This happens most effectively when the electron-withdrawing group is in resonance positions (such as ortho or para relative to the carboxyl group). The inductive effect, on the other hand, is a bit more straightforward as it involves withdrawing electron density directly through the sigma bond chain. This effect is most pronounced when the group is closer to the site of deprotonation, making the ortho position particularly effective for acid strengthening.

Ortho, meta, para positions

The positioning of an electron-withdrawing group on a benzene ring significantly influences its ability to affect acidity. These are commonly known as ortho, meta, and para positions, referring to the relative spacing between the electron-withdrawing group and the functional group of interest (like the carboxyl group in benzoic acids). In the ortho position, the group is adjacent to the carboxyl group, and this proximity allows for both strong inductive and resonance effects, making the acid very strong. The meta position, while allowing for some inductive influence, does not effectively facilitate resonance, resulting in moderate acid strengthening. The para position allows for some resonance and inductive effects, although typically less potent than ortho due to greater distance from the carboxyl group. These positional influences are crucial when comparing the strengths of acids with different substituents, as seen in the case of o-nitrobenzoic acid being the strongest due to the ortho effect. Understanding these concepts helps in predicting acid strengths based on molecular structure.