Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Acid—Base Reactions in Organic Chemistry
7.4 Predicting the Relative Strengths of Acids and Bases
The pKa table is very helpful in predicting the acidity of individual compounds, but if an acid were to be mixed with a base, the question now becomes would the equilibrium favor the right or the left? To fully appreciate this concept, consider mixing hydrochloric acid (HCl) with water (a base); the equilibrium is shown in Reaction (7-10), along with the pKa values of the acids involved.
(7-10)
Based on these pKa values of the acids of this equilibrium, one has a pKa value more negative (more acidic) than the other acid, which is less acidic. The position of the equilibrium is always towards the weaker acid. Thus, in this case, HCl is a stronger acid, compared to the conjugate acid of water, and as a result, the equilibrium will favor the right (the weaker acid), as represented in Reaction (7-11).
(7-11)
Problem 7.4
Predict the position of the following equilibria (pKa values are shown below the acids).
This concept will be used continuously throughout this course since a large percentage of organic reactions involves the use of an acid or a base. As a result, a basic knowledge of the position of equilibria is needed in order to decide on the best acid (or base) to use for various reactions.