Let’s first define the meaning of a strong acid. According to the Arrhenius theory, acids dissociate in water to form protons (H⁺). The general equation for acid dissociation can be shown as:

For a specific example, we can look at the dissociation of hydrochloric acid:

In this case, the conjugate base of the acid is the chloride ion. Hydrochloric acid is a strong acid because it completely ionizes in aqueous solutions.

So, one measure of acid strength is the extent of its dissociation, which indicates the amount of protons or hydronium ions that it produces in the solution.  

The more the acid dissociates into a proton, the stronger it is. We can also say, the stronger the tendency of a compound to donate a proton, the stronger acid it is.

The same principle applies to the acidity of organic molecules. However, most organic compounds are weak acids, and their dissociation is shown with a double arrow like we do in equilibrium reactions. 

For example, let’s consider the dissociation of ethanol and ethyl amine:

Given equal concentration, whichever produces more H⁺ is the stronger acid and, it turns out that ethanol is a stronger acid than the ethyl amine because it dissociates to a greater extent than ethyl amine does. 

The pKa

Aside from the qualitative comparison of the acid strength, we need a quantitative definition for the acid strength. This is where the pK_a comes into play. Acid-dissociation is a reversible reaction described by an equilibrium constant (K_eq), which, for the ionization of acids, is denoted as K_a. The pKa is derived from the K_a by taking its negative log. Let’s see how that works:

Before bringing in the numbers, let’s mention that another way of showing the acid-dissociation reaction (which is more accurate) is to include the water and formation of the hydronium ion (H₃O⁺):

The equilibrium constant for this reaction would be:

Since the concentration of the water is constant and very large compared to the acid, it is included in the formula of K_a:

Therefore, the [H₂O] term does not appear in the K_a formula, and we have:

Just like in any equilibrium reaction, the stronger the acid, the larger the Ka.

Now, let’s go back to the dissociation of the alcohol and the ethylamine. The K_a values of ethyl alcohol (1) and ethyl amine (2) are shown below:

The alcohol is a stronger acid because its Ka is higher. To simplify the numbers, -log K_a is used instead of K_a, getting rid of the exponent.

pKa = – logKa

The pKa is the quantitative indicator of the acid strength. For our two compounds, we have

pKa₁ = – log10^-16  = 16      <    pKa₂ = – log10^-38  = 38

pKa (EtOH) = 16,  pKa (EtNH₂) = 38

What we notice is that the alcohol is a stronger acid but has a lower pKa value, and this is true for any molecule. Just memorize this!

The following table summarizes the pKa values of most functional groups in organic chemistry:

Download the PDF file of the pKa Table below here, and this is a shorter version of the pKa table as well:

You can use the table to look up the pK_a value of a given compound based on the functional group(s) it contains. For example, if you need to find the pK_a of isopropanol and it is not in the pK_a table, you can go based on the ethanol. Since it is an alcohol and has approximately the same pKa value:

Same for the amines, they all have about the same pK_a as ammonia since they are the organic derivatives of ammonia (functional groups).

The pK_a value does vary for the same functional group in different compounds, but it is within an accepted range of approximation to describe the acid strength and predict acid-base reactions.

Notice also that the pKa scale is logarithmic, so even a small difference, let’s say pKa₁ = 16 and pK_a2 = 19, represents 3 pK_a units, which means the first compound is 10³ or 1,000 times more acidic than the second compound. For example:

 Predicting the Strength of Bases

So far, we have talked about the strength of the acid. However, it is also very important to understand the strength of the base. Let’s look at the dissociation of ethanol and ethylamine one more time:

We have determined that ethanol is a stronger acid, and another way to look at this is to say that the equilibrium is shifted more to the right side (more dissociation) compared to the dissociation of the ethylamine. This, in turn, means that the ethoxide ion is not as reactive as the EtNH^– ion because otherwise the equilibrium would have been shifted to the left as much as it is for the amine. So, the ethoxide is more stable, less reactive/active than the EtNH^– ion. All of this, in one word, means that it is a weaker base.

This correlation is true for any acid and conjugate base pair:

The stronger the acid, the weaker its conjugate base.

 And vice versa:

 The weaker the acid, the stronger its conjugate base.

Predicting the More Acidic Compound

If both molecules contain one functional group, then you simply need to look up the pKa values. The one with a lower pKa value is more acidic. For example, which of the following two molecules is more acidic?

The pKa of carboxylic acids is ~5, and the pKa of phenol is ~10. Therefore, propionic acid is 10⁵ times more acidic than phenol.

What if there are more functional groups in the molecule?

Here, you need to identify the most acidic proton in each molecule. In this case, molecule A is more acidic than molecule B.

How come alcohols aren’t more acidic than ketones?

Yes, they are. However, molecule A has two sets of protons. The ones next to the methyls and the ones in between the two methyl groups, which are a lot more acidic:

This is a dicarbonyl compound, which, because of its unusual acidity, is also called C-H acids.

Look in the table and determine the pK_a values for these two protons.

One is ~19, and the other is ~9. That is a huge difference – 10¹⁰ times more acidic.

Finally, the pK_a of alcohols is about 16, and comparing these values, you can see that molecule A is a million times more acidic than molecule B.

Notice that we did not even look at the pKa values of the other protons in molecule B. These are connected to carbons and range at pK_a ~ 50, and obviously, we won’t refer to them for comparing the acidity of alcohols.

We also have a dedicated post on predicting the most acidic proton in a molecule or a pair of compounds, so check that out as well for more examples and factors affecting acidity. 

Here is also a shorter pK_a table for the most common functional groups in organic chemistry:

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