Video Transcript
Which of the following carboxylic acids has the highest solubility in water? (A) Methanoic acid, (B) hexanoic acid, (C) heptanoic acid, (D) octanoic acid, or (E) nonanoic acid.
This question deals with various carboxylic acids, which in the name of the substance is indicated by the -oic acid ending. Carboxylic acids contain a carboxyl group, which contains a carbon atom double bonded to an oxygen atom and single bonded to a hydroxy group. Carboxylic acids differ by their R group, which we can deduce from the first portion of their names. The A-N tells us that the R groups will contain alkanes, and the prefixes indicate the number of carbons present. This list includes prefixes for one through 10 carbons. So methanoic acid contains one carbon, hexanoic acid contains six carbon atoms, and so forth.
We are asked to determine which has the highest solubility in water. Solubility can be defined as the maximum amount of solute that can dissolve in a given amount of solvent at a given temperature. The solubility of substances is influenced by their intermolecular forces. The intermolecular forces of substances are the attractive and repulsive interactions between neighboring molecules or nonbonded atoms. The substance in which the carboxylic acids are dissolving is water.
Water molecules are very polar, with partially negative and partially positive portions of the molecule. Neighboring water molecules engage in strong hydrogen bonds with one another. Hydrogen bonds are dipole–dipole interactions that exist between covalently bonded hydrogen atoms and electron lone pairs on strongly electronegative elements. These electronegative elements are generally fluorine, oxygen, and nitrogen. For a substance to be soluble in water, it must also be fairly polar or even able to engage in hydrogen bonding as well so that it may mix and disperse throughout the water. Substances that are nonpolar or not polar enough to disrupt the bonding between water molecules will not mix with water and are considered insoluble.
Let’s have a look at the largest carboxylic acid, nonanoic acid. Here is its displayed formula with its carboxyl group and nine-single-bonded-carbon chain. While a small portion of this molecule would be able to engage in hydrogen bonding with water, the large R group of this molecule is nonpolar. This region of the molecule would not interact strongly with the very polar water molecules. And nonanoic acid would not be soluble in water.
A similar behavior occurs with octanoic acid, which is only very slightly soluble in water. Heptanoic acid is considered poorly soluble in water, while hexanoic acid is only slightly soluble in water. Due to their large nonpolar regions, these four carboxylic acids all have poor or no solubility in water. Generally, if a carboxylic acid contains more than a few carbons, it is insoluble in water. However, molecules of methanoic acid contain only one carbon and do not have long nonpolar regions. Methanoic acid would be very soluble in water and would readily engage in hydrogen bonding with water molecules. Generally, carboxylic acids with only a few carbons are soluble in water.
Therefore, the carboxylic acid that has the highest solubility in water is answer choice (A) methanoic acid.
