MCAT Organic Chemistry Review

Carboxylic Acids

8.1 Description and Properties

carboxylic acid contains both a carbonyl group and a hydroxyl group, bound to the same carbon. With three bonds to oxygen atoms, this is one of the most oxidized functional groups encountered in organic chemistry. Carboxylic acids are always terminal groups.

NOMENCLATURE

In the IUPAC system of nomenclature, carboxylic acids are named by adding the suffix –oic acid to the parent root when the carboxylic acid is the highest-priority functional group. When this is true, the carbonyl carbon becomes carbon number 1. Figure 8.1 shows two examples.

Figure 8.1. IUPAC Names of Carboxylic Acids

Like the other functional groups, many carboxylic acids are also named by their common names. Make note of the common prefixes used in the examples in Figure 8.2.

Figure 8.2. IUPAC and Common Names of Carboxylic Acids

MCAT EXPERTISE

The same common-name prefixes are used for both aldehydes and carboxylic acids: form— for one carbon, acet— for two, and propion— for three.

Cyclic carboxylic acids are named by listing the cycloalkane with the suffix carboxylic acid. Salts of carboxylic acids are named beginning with the cation, followed by the name of the acid with the ending oate replacing oic acid. Typical examples are shown in Figure 8.3.

Figure 8.3. Cyclic Carboxylic Acid and Carboxylic Acid Salt

Finally, dicarboxylic acids, which have a carboxylic acid group on each end of the molecule, are common in biological systems. The smallest dicarboxylic acid is oxalic acid, with two carbons. The next five straight-chain dicarboxylic acids are malonicsuccinicglutaricadipic, and pimelicacids. Their IUPAC names have the suffix –dioic acid: ethanedioic acid, propanedioic acid, butanedioic acid, pentanedioic acid, hexanedioic acid, and heptanedioic acid. Figure 8.4 shows several examples.

Figure 8.4. IUPAC and Common Names of Dicarboxylic Acids

PHYSICAL PROPERTIES

Many of the physical properties of carboxylic acids are similar to those of aldehydes and ketones because they both contain carbonyl groups. However, the additional hydroxyl group permits carboxylic acids to hydrogen bond and provides another acidic hydrogen that can participate in reactions.

Hydrogen Bonding

Carboxylic acids are polar because they contain a carbonyl group, and can also form hydrogen bonds because they contain a hydrogen bound to a very electronegative atom (in this case, the hydroxyl oxygen). Carboxylic acids display particularly strong intermolecular attractions because both the hydroxyl oxygen and carbonyl oxygen can participate in hydrogen bonding. As a result, carboxylic acids tend to form dimers: pairs of molecules connected by two hydrogen bonds. Multiple hydrogen bonds elevate the boiling and melting points of carboxylic acids past those of corresponding alcohols. Boiling points also increase with increasing molecular weight.

KEY CONCEPT

Carboxylic acids are polar and can form hydrogen bonds. Their acidity is due to resonance stabilization and can be enhanced by the addition of electronegative groups or a greater ability to delocalize charge.

Acidity

The hydroxyl hydrogen of a carboxylic acid is quite acidic. This results in a negative charge that remains after the hydrogen is removed and resonance stabilization occurs between both of the electronegative oxygen atoms. Delocalization of the negative charge results in a very stable carboxylate anion, which is demonstrated in Figure 8.5.

Figure 8.5. Carboxylate Anion Stability The negative charge from deprotonation is stabilized through resonance.

The more stable the conjugate base is, the easier it is for the proton to leave, and thus, the stronger the acid. Carboxylic acids are relatively acidic, with pKa values on the order of 4.8 for ethanoic acid and 4.9 for propanoic acid. However, keep in mind that although these are quite acidic for organic compounds, they do not compare to strong acids like HCl (pKa = –8.0) or even HSO4 (pKa = 1.99). Remember, lower pKa values indicate stronger acids.

Substituents on carbon atoms near a carboxyl group influence anion stability and therefore affect acidity. Groups like –NO2 or halides are electron-withdrawing and increase acidity. In contrast, –NH2 or –OCH3 are electron-donating groups that destabilize the negative charge, decreasing the acidity of the compound. The closer the substituent groups are to the carboxyl group, the greater the effect will be.

In dicarboxylic acids, each –COOH group influences the other –COOH group. Carboxylic acids are electron-withdrawing due to the electronegative oxygen atoms they contain. The net result is that dicarboxylic acids are more acidic than the analogous monocarboxylic acids. However, when one proton is removed from the molecule, the carboxylate anion is formed, resulting in an immediate decrease in the acidity of the remaining carboxylic acid. This makes sense because if the second group were deprotonated, it would create a doubly charged species with two negative charges repelling each other. Due to this instability, the second proton is actually less acidic (harder to remove) than the analogous proton of a monocarboxylic acid.

β-dicarboxylic acids are dicarboxylic acids in which each carboxylic acid is positioned on the β-carbon of the other carboxylic acid; in other words, there are two carboxylic acids separated by a single carbon. These compounds are notable for the high acidity of the α-hydrogens located on the carbon between the two carboxyl groups (pKa ≈ 9–14). Loss of this acidic hydrogen atom produces a carbanion, which is stabilized by the electron-withdrawing effect of both carboxyl groups, as shown in Figure 8.6.

Figure 8.6. Acidity of the α-Hydrogen in β-Dicarboxylic Acids Note that the α-hydrogen is less acidic than the hydroxyl hydrogens; the hydroxyl groups are left protonated in this example for demonstration purposes only.

KEY CONCEPT

The hydroxyl hydrogen is the most acidic proton on a carboxylic acid. However, in 1,3-dicarbonyls, the α-hydrogen is also quite acidic.

Note that this also applies to the α-hydrogens in a β-diketone, β-ketoacids, β-dialdehydes, and other molecules that share the 1,3-dicarbonyl structure shown in Figure 8.7.

Figure 8.7. General Structure of 1,3-Dicarbonyl Compounds

MCAT Concept Check 8.1:

Before you move on, assess your understanding of the material with these questions.

1.    What causes the relatively high acidity of carboxylic acids?

2.    Between a monocarboxylic acid, a dicarboxylic acid, and a dicarboxylic acid that has been deprotonated once, which will be the most acidic? Why?

3.    What effects do additional substituents have on the acidity of carboxylic acids?