Rank the following compounds in order of acidity, and briefly explain your reasoning. Strength of an acid is inversely proportional to the strength of the conjugate base. Because a base is a nucleophile that shares electrons with a proton, we can analyze base strength by analyzing the base's ability or driving force to share these electrons. In lecture, we discussed four factors that should be considered in every case. A proton is hard, so it will bond most strongly with hard atoms.
Hard atoms are smaller, and have higher electronegativity than softer atoms.
In this case, we must consider the polarizability of the atom of the conjugate base that is sharing electrons with a proton. These atoms are sulfur and oxygen. Oxygen is smaller and has a higher electronegativity than sulfur, so an oxygen atom is harder than a sulfur atom. Having decided ethoxide is the strongest base, we can now focus on the two thiols. Because the atoms are identical, there is no electronegativity difference to influence basicity.