Video Transcript
Which of the following correctly compares phenol and ethanol? (A) Ethanol is more highly acidic than phenol. (B) Phenol can react with hydrochloric acid, while ethanol cannot. (C) Phenol can react with sodium hydroxide, while ethanol cannot. (D) Both of them can react with hydrochloric acid. (E) Both of them can react with sodium hydroxide.
When looking at the answer choices, we can see that we are being asked to compare the chemical properties of phenol and ethanol. More specifically, we are being asked to consider their acidity and reactivity with strong acids and bases such as hydrochloric acid and sodium hydroxide. Let’s discuss acidity first. According to the Brønsted–Lowry theory of acids and bases, an acid is a substance that can lose or donate protons, or H+, in a reaction. Acids differ in their ability to donate protons. Phenol and ethanol belong to a class of organic compounds called alcohols.
In general, alcohols are considered weak acids, which means that they do not lose protons easily. Both phenol and ethanol contain one hydroxy, or OH, group, which is the functional group of an alcohol. In an aqueous solution, the phenol molecule releases an H+ ion, or proton, from the hydroxy group and donates it to a water molecule, producing the phenoxide anion and a hydronium ion, which we can shorten here to H3O+. Similarly in aqueous solution, an ethanol molecule donates a proton from its hydroxy group to water, producing the ethoxide anion and H3O+.
Although phenol and ethanol are both weak acids, phenol is considered more acidic than ethanol. The reason for this difference is that the phenoxide anion is more stable than the ethoxide anion. The phenoxide anion is more stable because the negative charge of the ion is spread out over a large volume. In fact, there is delocalization of the negative charge over the entire aromatic ring structure. In the ethoxide anion, most of the negative charge is concentrated on the oxygen atom, so this ion is less stable. Knowing this allows us to eliminate answer choice (A).
As we learned, in general, alcohols are considered weak acids. Because of this, they typically do not react with weak bases, and only some can react with strong bases like sodium hydroxide. Phenol is acidic enough to react with sodium hydroxide. However, ethanol is not and no reaction will take place. Knowing this allows us to eliminate answer choice (E).
It looks like the answer choice (C) is our correct answer. But let’s first quickly confirm that choices (B) and (D) are incorrect. Phenol does not normally react with hydrochloric acid. In contrast, under certain conditions, ethanol can react with hydrochloric acid in a substitution reaction to produce a halogenated alkane, known as chloroethane, and water. So we have confirmed that choices (B) and (D) are incorrect. Therefore, the statement that correctly compares phenol and ethanol is answer choice (C). Phenol can react with sodium hydroxide, while ethanol cannot.
