Carboxylic acids usually have a pKa around 4.5. The acidity of carboxylic acids results from both resonance and inductive effects that help stabilize the conjugate base, making it more stable and less likely to undergo reprotonation. The hydrogen of a −COOH shows up around 10–12 ppm in proton nuclear magnetic resonance (1H NMR), and the hydrogen of a carbon next to the carbonyl shows up between 2 and 3 ppm in 1H NMR.
Carboxylic acids melt and boil at higher temperatures than comparably sized hydrocarbons and oxygen-containing compounds, indicating the presence of strong intermolecular forces. Carboxylic acid molecules create hydrogen bonds because the hydroxyl group (−OH) of one molecule acts as a proton donor to the carbonyl (C=O) oxygen of a second molecule and the hydroxyl proton of the second carboxyl interacts with the carbonyl oxygen of the first molecule. In this way, two hydrogen bonds form.
The solubility of carboxylic acids in water is similar to that of alcohols, aldehydes, and ketones. Carboxylic acids with fewer than five carbons dissolve in water, but acids of larger molecular weights are insoluble in water because of their large, hydrophobic hydrocarbon portions.
Hydrogen Bonding between Two Acetic Acid Molecules
Two hydrogen bonds form between two carboxylic acids.
Carboxylic acids have pKaa around 4.5 and are stronger acids than water, alcohol, and most other classes of organic compounds because of structural changes that occur during ionization. A carboxylate is a deprotonated carboxylic acid that has a general RCOO– stoichiometry.
Carbonyl Attracts Electrons from the Oxygen
In an acetate ion, the carbonyl group attracts electrons from the negatively charged oxygen, stabilizing the ion.
First, electron delocalization results in equal sharing of the negative charge by both oxygens. Because of this resonance effect, the carboxylate anion is more stable and less likely to protonate to re-form the acid.
Electrostatic potential maps illustrate resonance effects on an acetate ion when compared to ethoxide, resulting in a more delocalized negative charge and increased stability for the acetate ion.
Electrostatic Potential Maps of Ethoxide and Acetate
Electrostatic potential maps show the relative stability of the acetate ion compared to the ethoxide ion because of resonance and inductive effects present in the carboxylic acid.
Omega-6 and omega-3 fatty acids are carboxylic acids that serve important functions in the human body. These molecules help maintain the cell membrane and perform important metabolic functions. Carboxylic acids are also used as food additives and preservatives.
Naturally Occurring Carboxylic Acids
Malic acid, pyruvic acid, niacin, citric acid, biotin, abietic acid, and cholic acid are examples of naturally occurring carboxylic acids.
In 1H NMR the hydroxyl proton of a −COOH group shows up between 10 and 12 parts per million (ppm), and the hydrogen on a carbon next to the carbonyl shows up between 2 and 3 ppm. In infrared spectroscopy, characteristic stretches occur in the 3,500–2,500 wavenumber region, which is measured in reciprocal centimeters (cm–1), and the carbonyl group shows a band for C=O near 1,680 cm–1.
