Acid and base strength is extremely important in organic chemistry. In fact, most mechanisms can be determined by understanding various acid/base reactions (especially when considering Lewis acids you can see a video of Lewis acids here). This post will discuss a wide range of acids including common inorganic acids, predicting the equilibrium direction, Ka, and pKa.

Determining Acid Strength

The strength of an acid is measure using its acidity constant, or Ka value. The Ka value tells us how much of an acid will dissociate in water. Larger Ka values indicate a high level of dissociation which in turn, indicates a stronger acid. For an example, we will use HA as a generic acid that is placed in water. To find the Ka we need to know the concentration of each species in solution. Remember that brackets are used to denote concentration. So, our Ka expression would look like this:

Here, we can see how the calculation of Ka works. Note that water is omitted from the expression because there is an excess of water on each side of the equation. So, what kind of values are we typically talking about when dealing with Ka's? Well, values of Ka are all over the place depending on what acid you are talking about. For typical inorganic acids (these are the ones you might generally think of as strong like sulfuric acid, hydrochloric acid, nitric acid) the values are in a range 10^2 to 10^9. There are other acids, which will be discussed shortly, that are called organic acids and these are usually much less acidic having Ka values ranging from 10^-5 to 10^-15.

The pKa Value

Since the Ka values are often rather large or small and writing a number in scientific notation is not often convenient, and alternate way to expressing acid strength is using the negative log of the Ka or pKa. It is important that you begin to familiarize yourself with these values as they are commonly used when discussing acid strength. So, a large Ka value will have a small pKa value. For instance, the Ka of HCl is approximately 10^7 and therefore, the pKa would be -7. This indicates that HCl is of course, a very strong acid.

Organic Acids

When considering acids in organic chemistry, it is important to note that there are many more acids than the typical list that is given in a general chemistry course. For instance, water can act as both an acid and a base! So, should you spend hours of time committing specific pKa values to memory? No!! Instead, begin by generalizing pKa values of functional groups. For example, see the following chart.

Another aspect that you should consider about organic acids is that their acidity is always relative. For instance, it may not seem normal to think of an alkyne as being a strong acid but when it is in a group with alkanes and amines, the alkyne H is the most acidic and therefore the strongest acid. Just be sure to keep that in mind. Let's dive a little deeper into organic acids.

Predicting the Direction of Acid/Base Reactions

Often, we are asked to consider if an acid/base reaction will either go to complete, towards the products, or if it will not under the given conditions. This can be easily predicted using pKa values. Just remember this statement: THE EQUILIBRIUM WILL LIE IN THE DIRECTION OF THE WEAKEST ACID. In other words, the stronger acid will always donate its proton. Another concept to remember is regarding conjugate bases. Remember that the stronger the acid, the weaker the conjugate base (that is just something to keep in your head). Let's look at some examples.

Which direction does the equilibrium favor in the following reaction?

In the above example, the direction of the equilibrium can be determined by determining which molecule in the reactants is acting as an acid and then finding which is the conjugate acid in the products. So, in the reactants, there is acetic acid (that is the carboxylic acid) and a methoxide anion. This makes our decision rather easy since the names give them away but also the methoxide has a negative charge and therefore is not our acid. Using the pKa chart above, you can see the carboxylic acids have pKa values around 5. Next, we need to determine which is our conjugate acid in the products and we can see that it is methanol. Again, using our general values from the chart above, we can see that alcohols have a pKa of around 16. Now, all of our work is done! Since the equilibrium will lie in the direction of the weaker acid (lower pKa indicates the stronger acid) we can clearly see that the equilibrium will favor the products, or lie to the right. Try another one.

In this example, we again need to pick the acid and base in the starting materials. Here we can see that we have a hydroxide, which is a strong base, and methanol which is acting as our acid. On the products side, we see a methoxide anion and water. Water is our conjugate acid. Now, all we need to do is to prepare our pKa values. So, alcohols have a pKa of approximately 16 and water has a pKa of 15.74. So which is the weakest acid? Well, the answer is that they are really too close. This means that the reaction is not favored and both products and starting materials will be present in the equilibrium. Ok, one more.

In this reaction, we can see that acetylene (the alkyne, also ethyne) is acting as our acid. The conjugate acid in this case would be water. The pKa for alkynes is approximately 25 while that of water is 15.74. Therefore, this reaction will not proceed and the equilibrium will lie to the left. What this means is that a hydroxide anion is not a strong enough base to deprotonate the alkyne.

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