309

Unit 3
Elements of Organic Chemistry

3-9. AMINES

Amines result from the replacement of one or more of the hydrogen atoms of ammonia with hydrocarbons and have the general formula R–-NH2. There are four classifications of amines: primary, secondary, tertiary, and quaternary. Primary amines result from replacing one of the hydrogens of ammonia by a hydrocarbon, as in CH3–NH2; secondary amines result from the replacement of two hydrogens of ammonia by two hydrocarbons, as in CH3–NH–CH3 ; and tertiary amines result from the replacement of all three hydrogens of ammonia by hydrocarbon groups. The fourth classification of amines is sometimes encountered in drug structures. This classification is the quaternary amine that is formed by replacing the four hydrogens of the ammonium ion (NH4+) by hydrocarbon groups. Whenever one of the hydrocarbon groups connected to the nitrogen atom contains a benzene ring, the compound is referred to as an aromatic hydrocarbon.

Properties of Amines. The low-molecular-weight amines are all volatile liquids, and those having up to five carbons are soluble in water. The element nitrogen is in the same period of the periodic table as oxygen and has some similar properties--the most significant being the ability to form hydrogen bonds. The formation of hydrogen bonds between amines, and between amines and water, accounts for their higher boiling points (than alkanes) and their water solubility.

Reactions of Amines. Since amines are derivatives of ammonia, they are bases as defined by the Bronsted-Lowry theory. The nitrogen of the amine can accept a proton to form a substituted ammonium ion.

CH3–CH2–NH3 + H+ CH3–CH2–NH3+

Amines will thus react with inorganic acids to form salts. (Amines react with organic acids to form amides, a class of organic compounds discussed later in this subcourse.)

CH2–NH2 + HCl     CH3–NH3 + Cl-

The reaction in the example above results in a hydrochloride salt of the amine and is a very important reaction in pharmacy. Many drugs contain an amine functional group, and if they contain many carbon atoms, they are not very soluble in water. The salts formed from amines, however, are very soluble in water. Therefore, if we wish to use a water solution of an amine drug that is insoluble, we can make it soluble by forming the salt of the amine.

Use of Amines. As already stated, the amine functional group is contained in many different drugs that have quite different actions in the body. Generally, these drugs are very complex and you would never be expected to draw or know the structure for these drugs. You should, however, recognize the -NH2 group of an amine and be cognizant of its basic properties.