Thiols are much more acidic than alcohols.

When comparing the acidity of molecules, we often look at their pKa values, a numerical scale used to describe the strength of an acid. The pKa value represents the acid dissociation constant, which indicates the extent to which an acid can donate a proton. A crucial point to remember is that a lower pKa value signals a stronger acid, as it implies a higher tendency for that molecule to give up a hydrogen ion (H+).

In the context of thiols and alcohols, thiols have an average pKa in the range of 10-12, whereas alcohols fall into a higher range, typically between 16-18. This considerable gap indicates that thiols are significantly more acidic than alcohols. The distinct pKa values underpin that the conjugate bases of thiols are more stabilized than that of alcohols, which is a key determinant in the acidity of these compounds.

The stability of conjugate bases is instrumental in defining the strength of an acid. After losing a proton, the negative charge that arises is taken up by the remaining part of the molecule, which is the conjugate base. In our case, thiols form a thiolate ion (R-S-), and alcohols form an alkoxide ion (R-O-).

The thiolate ion tends to be more stable than the alkoxide ion. This is because the negative charge on a thiolate ion is spread over a larger volume due to sulfur's size, and thus is more delocalized. This delocalization diminishes the energy and makes the base, and thus the original acid (thiol), more stable. In contrast, the alkoxide ion has a localized negative charge that is not as well accommodated, rending the alcohol relatively less acidic.

Two major factors that influence acidity are atom size and electronegativity. These properties impact how a molecule accommodates negative charge. After deprotonation, the sulfur atom in a thiol is larger than the oxygen atom in an alcohol. Due to sulfur's larger size, it can spread out the negative charge over a greater volume, leading to a more stabilized, less energy-dense ion.

Meanwhile, electronegativity is the ability of an atom to attract electrons. Oxygen is more electronegative than sulfur, which means in alcohols, the oxygen pulls electrons closer to itself, making the negative charge more localized and hence the alkoxide ion less stable. The larger, less electronegative sulfur in thiols can better accommodate the negative charge, contributing to the greater acidity of thiols compared to alcohols.

Understanding the nature of the ions formed upon deprotonation helps clarify why thiols are more acidic than alcohols. The thiolate ion (R-S-) benefits from the larger radius of sulfur, which helps distribute the negative charge over a wider area. This distribution is not as effective in the alkoxide ion (R-O-) due to oxygen's smaller size, resulting in a more condensed negative charge.

This difference in the handling of the negative charge has significant implications. It means thiolate ions are inherently more stable than alkoxide ions. The increased stability of the conjugate base reflects the acid's ability to readily give off a proton. Therefore, the nature of the thiolate and alkoxide ions directly contributes to why thiols are stronger acids than alcohols.