Which of the following carboxylic acids has the highest boiling point? (A) Methanoic acid, (B) ethanoic acid, (C) propanoic acid, (D) butanoic acid, or (E) pentanoic 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 we can write in a structural formula as COOH. Carboxylic acids differ by their R group, which we can deduce from the names. The A-N tells us that the R group will contain an alkane, and the prefix indicates how many carbons are present. So, if we were to write the structural formula of methanoic acid, we know it would contain COOH, since as a carboxylic acid it contains a carboxyl group. And since methanoic acid contains only one carbon, the R group would be a single hydrogen. The structural formula would read HCOOH.
We can do the same for ethanoic acid, changing only the R group to reflect the number of carbons and how many hydrogens would be needed to give each carbon four bonds. We can do this using the names of the remaining answer choices. We are asked to determine of these five carboxylic acids which has the highest boiling point.
The boiling point of a substance is the temperature at which it changes from a liquid state to a gas state. The process of boiling is an endothermic process, which means it requires energy to be added to the system for it to occur. When the boiling point of a substance is high, this means large amounts of energy are required to overcome the attractions between neighboring molecules. These interactions are called intermolecular forces, which are the attractive and repulsive interactions between neighboring molecules or nonbonded atoms. Substances with strong intermolecular forces have high boiling points.
One of the predominant types of interactions occurring between separate molecules of carboxylic acids is London dispersion forces. London dispersion forces are electrostatic interactions between temporarily induced dipoles and cause even nonpolar molecules to be attracted to each other. When molecules are small, like molecules of methanoic acid, the temporary dipole, and thus London dispersion force, is not as strong. Larger molecules, such as molecules of pentanoic acid, experience stronger London dispersion forces. So larger carboxylic acid molecules have stronger London dispersion forces and, therefore, stronger intermolecular forces. This means they require more energy to reach their boiling point. This would result in a higher temperature at which boiling would occur.
We can see that the largest carboxylic acid from our answer choices is pentanoic acid. Therefore, the carboxylic acid that has the highest boiling point is answer choice (E), pentanoic acid.