Question

Acetic acid has a boiling point of \(118^{\circ} \mathrm{C}\), whereas its methyl ester has a boiling point of \(57^{\circ} \mathrm{C}\). Account for the fact that the boiling point of acetic acid is higher than that of its methyl ester even though acetic acid has a lower molecular weight.

Short answer

Answer: The boiling point of acetic acid is higher than its methyl ester due to the presence of strong hydrogen bonding in acetic acid, which leads to stronger intermolecular forces, despite acetic acid having a lower molecular weight.

Step-by-step solution

Identifying the compounds

The first step is to identify the two compounds. The molecule acetic acid has the formula \(\mathrm{CH_3COOH}\), and its methyl ester has the formula \(\mathrm{CH_3COOCH_3}\).

Evaluating the molecular weights

Now, let's calculate the molecular weights of both compounds. This will help us compare their molecular weights and understand the problem statement better. For Acetic acid (\(\mathrm{CH_3COOH}\)): - Carbon (C): 2 atoms, each at 12.01 g/mol - Hydrogen (H): 4 atoms, each at 1.01 g/mol - Oxygen (O): 2 atoms, each at 16.00 g/mol Molecular weight of Acetic acid = \(2\times12.01 + 4\times1.01 + 2\times16.00 = 60.05\, \mathrm{g/mol}\). For Methyl ester (\(\mathrm{CH_3COOCH_3}\)): - Carbon (C): 3 atoms, each at 12.01 g/mol - Hydrogen (H): 6 atoms, each at 1.01 g/mol - Oxygen (O): 2 atoms, each at 16.00 g/mol Molecular weight of Methyl ester = \(3\times12.01 + 6\times1.01 + 2\times16.00 = 74.09\, \mathrm{g/mol}\). As we can see, despite having a lower molecular weight, acetic acid has a higher boiling point.

Analyzing intermolecular forces

Now, we need to analyze the intermolecular forces present in both compounds. Intermolecular forces are responsible for the boiling points of the compounds. Stronger intermolecular forces lead to higher boiling points. In acetic acid (\(\mathrm{CH_3COOH}\)), there's a highly polar O-H bond, allowing it to form strong hydrogen bonds with other acetic acid molecules. This strong hydrogen bonding results in a higher boiling point. In the methyl ester (\(\mathrm{CH_3COOCH_3}\)), though it has a polar C-O bond, it doesn't have any hydrogen bonding due to the absence of the highly polar O-H bond. Instead, it has weaker dipole-dipole and van der Waals forces between the molecules.

Conclusion

Based on the analysis of molecular weight and intermolecular forces, we can conclude that the boiling point of acetic acid (\(\mathrm{CH_3COOH}\)) is higher than that of its methyl ester (\(\mathrm{CH_3COOCH_3}\)) because of the presence of strong hydrogen bonding in acetic acid, which leads to stronger intermolecular forces, despite acetic acid having a lower molecular weight.