Question

Benzoic acid is not esterified by the procedure that is useful for mesitoic acid because, when benzoic acid is dissolved in sulfuric acid, unlike mesitoic acid, it gives the conjugate acid and no acy 1 carbonium ion. Explain why the mesitoy 1 carbonium ion might be more stable relative to the conjugate acid of mesitoic acid than benzoyl carbonium ion is relative to the conjugate acid of benzoic acid. (Among other factors, consider the geometries of the various species involved.)

Short answer

In conclusion, the mesitoyl carbonium ion is more stable than the benzoyl carbonium ion due to the presence of electron-donating methyl groups and its planar, conjugated geometry. This results in mesitoic acid forming an acylium ion when dissolved in sulfuric acid, whereas benzoic acid only forms its conjugate acid due to the relative instability of the benzoyl carbonium ion.

Step-by-step solution

Understand the reaction of carboxylic acids with sulfuric acid

In the presence of sulfuric acid, carboxylic acids can form acylium ions (R-C≡O+) and their conjugate acids (R-COOH2+). The process involves the donation of a proton (H+) from sulfuric acid to the oxygen atom in the carboxylic acid group. The reaction can be represented as follows: \(R-COOH + H_{2}SO_{4} \rightleftharpoons R-COOH_{2}^{+} + HSO_{4}^{-}\)

Compare the stability of mesitoyl carbonium ion and benzoyl carbonium ion

In order to understand why mesitoic acid forms an acylium ion while benzoic acid doesn't, we need to analyze the stability of the mesitoyl carbonium ion and the benzoyl carbonium ion. Mesitoyl carbonium ion (formed from mesitoic acid) is more stable than the benzoyl carbonium ion (formed from benzoic acid) due to the presence of three electron-donating methyl groups which contribute to the overall stability of the ion. These methyl groups provide an electron-donating inductive effect, thus stabilizing the positive charge of the mesitoyl carbonium ion.

Consider the geometry of the molecules

The other factor to consider is the geometry of the molecules. The mesitoic acid molecule has a planar, conjugated structure. Due to the presence of the electron-donating methyl groups, the positive charge on the mesityl carbonium ion is delocalized, making it more stable than the benzoyl carbonium ion, which does not have a similar electron-donating group to stabilize the positive charge.

Conclude the comparison and explain the observation

In conclusion, mesitoic acid can form mesitoyl carbonium ion by protonation in sulfuric acid because the mesitoyl carbonium ion is more stable relative to the conjugate acid of mesitoic acid as a result of the electron-donating methyl groups and the molecule's planar and conjugated geometry. On the other hand, benzoic acid gives only the conjugate acid when dissolved in sulfuric acid because the benzoyl carbonium ion is less stable relative to the conjugate acid of benzoic acid due to the absence of electron-donating groups and the different molecular geometry.

Key concepts

Acylium Ion Stability

When carboxylic acids react with sulfuric acid, they can transform into acylium ions. Acylium ions are represented as \(R-C\equiv O^+\). Whether or not an acylium ion forms depends greatly on the stability of these ions. Their stability is influenced by several factors. One crucial aspect is the presence of electron-donating or withdrawing groups adjacent to the ion.

• **Electron-Donating Groups**: Groups such as methyl can donate electrons, stabilizing the positive charge on the acylium ion.
• **Electron-Withdrawing Groups**: Such groups can increase the reactivity and decrease the stability of acylium ions by drawing electrons away.

These interactions affect how the positive charge on the carbon is balanced. If the charge is not well-stabilized, the formation of the acylium ion becomes less favorable. Hence, comparing the stability of different acylium ions involves analyzing the electronic environment around the ion.

Mesitoyl Carbonium Ion

The mesitoyl carbonium ion is an interesting species formed from mesitoic acid in sulfuric acid. It exhibits greater stability compared to other similar ions due to specific structural features. A major contributing factor is the presence of three methyl groups attached to the aromatic ring.
These methyl groups act as electron-donating entities, facilitating the stabilization of the positive carbon atom in the carbonium ion. The stabilization occurs via an inductive effect, where the electron-rich methyl groups push electrons towards the positively charged carbon.

• **Stabilization through Inductive Effect**: The electron donation process helps neutralize the positive charge, decreasing the energy and making the ion more stable.
• **Aromatic Conjugation**: Additionally, within the mesitoyl ion, the electron cloud of the aromatic ring can delocalize further surrounding the positive charge, enhancing the stability even more.

Hence, the structure and electron-donating characteristics of the mesitoyl carbonium ion make it more stable relative to other carbonium ions like the benzoyl carbonium ion.

Molecular Geometry and Stability

The molecular geometry of a compound plays a significant role in determining its stability, especially in ion form. For carboxylic-derived ions, like the mesitoyl carbonium ion and the benzoyl carbonium ion, the geometric arrangement of atoms can critically affect their behavior.
Mesitoic acid's planar, conjugated geometry provides a favorable condition for the formation and stabilization of the mesitoyl carbonium ion. The planarity and conjugation allow for the better overlap of orbitals, facilitating charge delocalization.

• **Planar Structure**: In mesitoic acid, the planar structure helps in dispersing the positive charge across the ion, aiding in its stabilization.
  
• **Lack of Stabilizing Groups in Benzoyl Ion**: In contrast, the benzoyl carbonium ion, arising from benzoic acid, lacks significant electron-donating groups, leading to less charge delocalization. This makes it less stable.

Thus, mesitoic acid's better geometric and electronic arrangement contributes to the stability and reactivity of its respective carbonium ion in sulfuric acid.