14.4  Structures of Acids and Their Conjugate Bases

Acidic hydrogens are removed by bases as protons (H⁺). So the more positively charged the hydrogen is, the more acidic it will be. Hydrogens are typically not fully charged but rather have partial charges, indicated with the symbol δ+. This can occur if hydrogens are bonded to positively charged atoms. This is why ammonium (NH₄⁺) is much more acidic than its neutral analog NH₃. Even if the hydrogens are not bonded to positively charged atom, they can still be δ+ if it is participating in polar bonds. For example, in acetic acid, there are hydrogens bonded to a carbon and another bonded an oxygen. Since the C–H bonds are nonpolar while O–H bonds are polar, the hydrogen bonded to the oxygen is more δ+ and thus more acidic (Figure 14.11).

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Another way to think about the acidity of compounds is to consider the stability of the conjugate base. The atom that the acidic hydrogen is bonded to becomes negatively charged when the acidic hydrogen is removed. By evaluating how stable this structure is, we can determine how strong of a conjugate base is formed: a stable conjugate base makes it a weak one and a less stable conjugate base a strong one. How does this relate to the acidity? The stronger the acid is, weaker the conjugate base is. Let’s go back to acetic acid. We will remove each possible acidic hydrogen then evaluate the stability of the resulting conjugate base. Let’s first start with one of the hydrogens bonded to the carbon (in blue in Figure 14.12). This results in a conjugate base where the negative charge is on a less electronegative carbon. This negative charge on the carbon results in an unstable conjugate base, i.e. a stronger (conjugate) base. Compare this to removing the hydrogen bonded to the oxygen (in red). This results in a conjugate base where the negative charge is on a much more electronegative oxygen, forming a much more stable conjugate base. The more stable conjugate base means weaker conjugate base, which comes from the stronger acid. So the hydrogen in red, bonded to the more electronegative oxygen, is more acidic.

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A base removes an acidic hydrogen from an acid. It does so by forming a bond with its lone pair. So an atom will act as a base, only if it has lone pairs. Since it is grabbing a δ+ hydrogen, the more negatively charged the atom is, the stronger the attraction to the acidic hydrogen. After all, opposites attract! This is why hydroxide (OH⁻), with its negative charge, is a stronger base than water, where the oxygen is only partially negatively charged (Figure 14.13). 

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Now let’s compare ammonia and water (Figure 14.14). The nitrogen in the ammonia and the oxygen in water have lone pairs, making these atoms the basic one in each compound. Based on the size of the charge, we might expect water to be the stronger base. However, ammonia is the stronger base. What’s going on? Since neither of the atoms have particularly large charges, we need to consider their electronegativities. Since oxygen is more electronegative, it does not want to share its lone pair with the acidic hydrogen. It would rather keep those lone pair electrons for itself. But, ammonia with the less electronegative nitrogen is more willing to share its lone pair electrons and is the stronger base.

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Another way to think about this is to consider the stability of the conjugate acids. Once the oxygen in water and the nitrogen in ammonia are protonated, they will end up with a positive charge (Figure 14.14). And it is better to have a positive charge on a less electronegative atom like nitrogen. In other words, ammonium (NH₄⁺) is more stable or a weaker conjugate acid than hydronium (H₃O⁺). And weaker the conjugate acid is, stronger the base it’s from.

As an acid (HA) and base (B) react, the acidic hydrogen is passed from the acid to the base (Figure 14.15). In the acid, the electrons used to retain the acidic hydrogen remains with the acid, now the conjugate base (A⁻). This decreases the charge of the conjugate base from +1 to 0, 0 to −1, etc. Meanwhile, the base uses a lone pair to bond with the acidic hydrogen to become the conjugate acid (HB⁺). As it does so, its charge increase from −1 to 0, 0 to +1, etc. 

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So how can we use this to predict the products of an acid-base reaction? After identifying which compound is the acid and which one the base, the most acidic hydrogen and most basic atom must be identified. Then the acidic hydrogen should be transferred from the acid to the base. We can see this illustrated in Example 14.17.

This section introduced you to the basics of identifying the most acidic or basic atom in a molecule. You will see this again next year in Organic Chemistry, where they are an important component of organic reactions!