3.3 pKa of Organic Acids and Application of pKa to Predict Acid-Base Reaction Outcome

As we mentioned before, all organic compounds could be acids because they all have hydrogen atoms that could potentially be donated. Most organic acids are weak acids with a small K_a. For example, acetic acid CH₃COOH has a K_a of 1.8×10^-5. Many other organic acids are even weaker than acetic acid, and it is this weak acidity that makes it difficult to realize that some organic compounds are actually acids.

However, this weak acidity is very important in Organic Chemistry. Since it is inconvenient to have to say or to remember K_a values like 1.8×10^-5, pK_a is used more often in Organic Chemistry to refer to the relative acidity of different acids. The definition of pK_a is:

pK_a = -logK_a

The smaller the pK_a value, the larger the K_a, and the stronger the acidity is.

The pK_a of most organic acids ranges between 5 ~ 60. While it is impossible to know the pK_a of every organic compound, it is very useful to understand the pK_a (and acidity) based on the functional groups involved because the same functional groups usually have similar pK_as. The approximate ranges of pK_a values for seven major functional groups are listed in Table 3.1, which serves as a valuable starting point for us to predict and understand the acidity of any organic molecule. The strongest organic acid listed here is carboxylic acid with a pK_a of about 5; the weakest organic acids are the alkanes with pK_a values of over 50. Since approximate ranges of pK_a values are listed in the table, the exact pK_a value of a group varies for different compounds because of the structural differences. Fortunately, however, it is usually not necessary to know the exact pK_a values for most cases in organic chemistry, and the approximate range is good enough.

Notes for the pK_a values in Table 3.1:

• Acidity is the ability of a compound to donate H⁺, so when we talk about the acidity (K_a and pK_a) of an organic compound, it must be about a specific H atom (highlighted in blue in the table). For different H atoms in the same compound, the acidity and pK_a are different. As for the example of methanol:

• It is useful to memorize the approximate ranges of pK_a listed in Table 3.1.
• The acidity of the functional groups in the table decreases from top to bottom, and the basicity of the conjugate bases in the last column increases from top to bottom because the stronger the acid, the weaker the conjugate base is.

 

Predict the Outcomes of Organic Acid-Base Reactions — Use pK_a as a Criterion

With our knowledge of acidity and pK_a, we are now ready to see how to apply this information to the understanding of organic reactions from an acid-base perspective.

The following reaction is an example in Section 3.2. If you take a closer look at the reactants and products, you will find that the “product” side also contains an acid (ammonia NH₃) and a base (methoxide CH₃O^–). Now the question is, how can we be so sure that the reaction proceeds to the “product” side as written? The question can also be asked in a different way: if an equilibrium is established for the reaction mixture, which side will the position of the equilibrium predominantly favour? Left or right?

 

 

To answer that question, we will learn about a general rule for acid-base reactions: Acid-base reactions always favour the formation of the weaker acid and the weaker base. This is because the equilibrium always favours the formation of more stable products, and weaker acids and bases are more stable than stronger ones. 

 

With pK_a values available at hand, the relative acidity of reactants vs products can be compared by comparing their pK_a values, and the reaction will proceed to the side of the acid with a larger pK_a (larger pK_a means smaller K_a, and thus a weaker acid).

So for this reaction, the pK_a check indicates that ammonia NH₃ is a weaker acid than methanol CH₃OH, so the reaction does proceed to the right side with CH₃O^– and NH₃ as the major products.