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- Deboleena Roy (American River College)
- Cañada College
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Learning Objectives
- Recognize the weak acid property of carboxylic acids.
- Recognize the weak base property of amines.
The proteins in all living species, from bacteria to humans, are polymers constructed from the same set of 20 amino acids, so called because each contains an amino group attached to a carboxylic acid. The amino acids in proteins are α-amino acids, which means the amino group is attached to the α-carbon of the carboxylic acid. Humans can synthesize only about half of the needed amino acids; the remainder must be obtained from the diet and are known as essential amino acids.
Amino Acid
Every amino acid contains an amino group, (–NH2), a carboxyl group, (–COOH), and a side chain or R group, which are all attached to the alpha-carbon (the one directly bonded to the carboxyl functional group). Therefore, amino acids are commonly called alpha-amino acids. The figure 9.2.1 shows the structure of a generic α-amino acid.
Figure \(\PageIndex{1}\): An α-amino acid. The three parts to an amino acid include the amino group, the carboxyl group, and the side-chain or R group all attached to the α-carbon. The R group is the part that distinguishes one amino acid from the next.
Properties of Carboxylic Acids and Amines
We will start by looking at the properties of each of these functional groups.
Carboxylic Acids are Weak Organic Acids
Carboxylic acids are named such because they tend to be more acidic than other functional groups in organic chemistry. In dilute aqueous solutions, they act as weak acids that partially dissociate to produce the corresponding carboxylate anion and hydronium cation (H3O+). Carboxylate anions are named by replacing the -ic acid ending from the carboxylic acid with -ate, see example below.
The extent of dissociation of these weak acids in water is described by \(K_a\) values. Remember that a compound with a smaller \(K_a\) value will be a weaker acid.
Note
The anion of the carboxylic acid is more soluble in water compared to the carboxylic acid. The salts of long-chain carboxylic acids are called soaps. The fatty acid palmitic acid is classified as hydrophobic. Sodium palmitate on the other hand is amphipathic.
Note To Your Health: Organic Salts as Preservatives
Some organic salts are used as preservatives in food products. They prevent spoilage by inhibiting the growth of bacteria and fungi. Calcium and sodium propionate, for example, are added to processed cheese and bakery goods; sodium benzoate is added to cider, jellies, pickles, and syrups; and sodium sorbate and potassium sorbate are added to fruit juices, sauerkraut, soft drinks, and wine. Look for them on ingredient labels the next time you shop for groceries.
Amines as Weak Organic Bases
Ammonia (NH3) acts as a base because the nitrogen atom has a lone pair of electrons that can accept a proton. Amines also have a lone electron pair on their nitrogen atoms and can accept a proton from water to form substituted ammonium (NH4+) ions and hydroxide (OH−) ions:
As a specific example, methylamine reacts with water to form the methylammonium ion and the OH− ion. Amines accept H+ from water and form a 4o ammonium ion.
Nearly all amines, including those that are not very soluble in water, will react with strong acids to form salts soluble in water. Amines accept H+ from acids to form a 4o ammonium ion.
Amine salts are named like other salts: the name of the cation is followed by the name of the anion. The 4o ammonium salts have a greater water solubility compared to amines.
Note To Your Health: Amine Salts as Drugs
Salts of amines are ionic, and their solubility is characteristic of salts. Many drugs that are amines are converted to salts to increase their solubility in aqueous solution. The decongestant ephedrine is sold as ephedrine hydrochloride or ephedrine sulfate. The amine salt drugs are easier to administer due to their greater water solubility.
