Question Video: Understanding the Association between Carboxylic Acid Molecule Length and Solubility | Nagwa Question Video: Understanding the Association between Carboxylic Acid Molecule Length and Solubility | Nagwa

Question Video: Understanding the Association between Carboxylic Acid Molecule Length and Solubility Chemistry • First Year of Secondary School

Why is pentanoic acid insoluble in water, whereas ethanoic acid is soluble? [A] Pentanoic acid does not react with water. [B] The hydration energy is too high. [C] The shape of the molecule prevents hydrogen bonding. [D] Pentanoic acid has a high boiling point. [E] Pentanoic acid has a long hydrocarbon chain.

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Video Transcript

Why is pentanoic acid insoluble in water, whereas ethanoic acid is soluble? (A) Pentanoic acid does not react with water. (B) The hydration energy is too high. (C) The shape of the molecule prevents hydrogen bonding. (D) Pentanoic acid has a high boiling point. (E) Pentanoic acid has a long hydrocarbon chain.

In this question, we are told that pentanoic acid is insoluble in water and that ethanoic acid is soluble in water. Solubility refers to the maximum amount of a solute that can dissolve in a given amount of solvent at a specific temperature. In more general terms, we can think of solubility as the tendency of a solute to dissolve in a specific solvent. In this question, pentanoic acid and ethanoic acid are solutes, and water is the solvent. If a substance is soluble in a solvent, then it is just able to dissolve in that solvent. So ethanoic acid is able to dissolve in water.

On the other hand, if a substance is insoluble in a solvent, then it is unable to dissolve in that solvent. Therefore, pentanoic acid is unable to dissolve in water. Dissolving is the physical process by which a solution forms. In order for dissolving to occur, all of the solute–solute interactions must be broken and some of the solvent–solvent interactions must be broken. By breaking the intermolecular attractions, the particles become more separated. To complete the formation of a solution, the solute and solvent particles mix and new solute–solvent interactions form.

A solute tends to dissolve in a solvent if the solute–solvent interactions are similar in strength to the solvent–solvent interactions. In liquid water, there are strong hydrogen bonds between water molecules. A solute that can form strong hydrogen bonds with water molecules is more likely to dissolve in water. A hydrogen bond is an electrostatic interaction between a hydrogen atom that is covalently bonded to a fluorine, oxygen, or nitrogen atom and a lone pair of electrons found on a fluorine, oxygen, or nitrogen atom. Hydrogen bonds occur between water molecules because water molecules are strongly polar and have electric dipole moments.

This is because the oxygen atom has a much greater electronegativity value than the hydrogen atoms and attracts most of the electron density, which leads to oxygen having a partial negative charge and the hydrogen atoms having partial positive charges. So the hydrogen bond is the electrostatic attraction of these opposite charges. Pentanoic acid and ethanoic acid are both carboxylic acids. The molecular structure of a carboxylic acid contains a hydrocarbon chain and a carboxyl group, which is the functional group of a carboxylic acid. The hydrocarbon chain is composed of only carbon and hydrogen atoms, which form nonpolar covalent bonds. Therefore, this part of the molecule is nonpolar.

In contrast, the carboxyl group is composed of three polar bonds: a carbon-to-oxygen double bond, a carbon-to-oxygen single bond, and a oxygen-to-hydrogen single bond. The oxygen atoms attract most of the electron density and have partial negative charges, while the carbon and hydrogen atoms of the carboxyl group have partial positive charges. Let’s take a look at a partially condensed formula to better show the shape of the carboxylic acid molecule.

The presence of the oxygen-to-hydrogen single bond and lone pairs of electrons on the oxygen atom allow a carboxylic acid molecule to form hydrogen bonds with nearby water molecules. So if both pentanoic acid and ethanoic acid have a carboxyl group, why is it that pentanoic acid is not soluble in water? To help us answer this question, we’ll need to closely compare the molecular structures of the two carboxylic acids. Let’s start by sketching the displayed formula of ethanoic acid. The prefix eth- indicates that there are a total of two carbon atoms in the molecule. At one end of the molecule is the polar carboxyl group, and the other part of the molecule is the nonpolar hydrocarbon chain.

Now let’s sketch the displayed formula of pentanoic acid. The prefix pent- indicates there are five carbon atoms in the molecule. When comparing the two displayed formulas we drew, the main difference between the molecules is that pentanoic acid has a longer nonpolar hydrocarbon chain. In general, as the length of the hydrocarbon chain in the carboxylic acid increases, its solubility in water decreases. We might expect that the polar carboxyl group of pentanoic acid would form strong hydrogen bonds with water molecules. But the problem is that the large hydrocarbon chain also needs to fit among the water molecules.

To do so, some of the strong hydrogen bonds between the water molecules would need to be replaced with very weak interactions with the hydrocarbon chain. This is not energetically favorable, so pentanoic acid does not dissolve in water. Why is pentanoic acid insoluble in water, whereas ethanoic acid is soluble? The correct answer is answer choice (E), pentanoic acid has a long hydrocarbon chain.

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