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Chapter 20: Problem 481

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

Expert verified
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

01

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: \(R-COOH + H_{2}SO_{4} \rightleftharpoons R-COOH_{2}^{+} + HSO_{4}^{-}\)
02

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.
03

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.
04

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.

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Most popular questions from this chapter

For each of the following pairs tell which acid is the stronger and why: (a) \(\mathrm{CH}_{3}-\mathrm{COOH}\) or \(\mathrm{H}-\mathrm{COOH}\) (b) \(\mathrm{CH}_{3}-\mathrm{COOH}\) or \(\mathrm{CL}-\mathrm{CH}_{2}-\mathrm{COOH}\) (c) \(\mathrm{F}-\mathrm{CH}_{2}-\mathrm{COOH}\) or \(\mathrm{Br}-\mathrm{CH}_{2}-\mathrm{COOH}\) (d) \(\mathrm{CL}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{COOH}\) or \(\mathrm{CH}_{3}-\mathrm{CHCL}-\mathrm{COOH}\) (e) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{OH}\) or \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH}\) (f) \(\mathrm{CH}_{3}-\mathrm{OH}\) or \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{COOH}\)

Draw structures for the following: (a) Propionic acid; (b) 2-Chlorobutanoic acid; (c) \(\mathrm{m}\) -Nitrobenzoic acid; (d) Trifluoroacetic acid; (e) 4-Methoxypentanoic acid; (f) a-Bromopropionic acid (g) Isobutyric acid; (h) 2,2-Dibromoheptanoic acid; (i) Formic acid; (i) 3-Benzyl-7-cyclopropyl-heptanoic acid.

Synthesize butyric acid from any organic compound, using inorganic reagents.

Explain why the chemical shift of the acidic proton of a carboxylic acid, dissolved in a nonpolar solvent like carbon tetrachloride, varies less with concentration than that of the OH proton of an alcohol under the same conditions.

Draw structures for the following compounds. (a) a-aminopropionic acid (alanine) (b) vinylacetic acid (c) para-methoxybenzoic acid
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