18.2 Properties of Amines Amines contain polar N — H bonds, which allow primary and secondary amines to form hydrogen bonds with each other, while all amines can form hydrogen bonds with water. Learning Goal Describe the boiling points and solubility of amines; write equations for the dissociation and neutralization of amines.

Hydrogen Bonding in Amines Primary and secondary amines can form hydrogen bonds with water molecules. Tertiary amines can only form hydrogen bonds with water molecules. Nitrogen is not as electronegative as oxygen, so hydrogen bonds in amines are weaker than the hydrogen bonds in alcohols.

Boiling Points of Amines Amines have boiling points that are higher than those of alkanes but lower than those of alcohols. Primary (1°) amines can form more hydrogen bonds and have higher boiling points than secondary (2°) amines of the same mass. Tertiary (3°) amines cannot form hydrogen bonds with each other and have lower boiling points than primary or secondary amines of the same mass.

Solubility of Amines in H2O Amines that contain a polar N — H bond can form hydrogen bonds with water. Amines with one to six carbon atoms, including tertiary amines, are soluble in water. As the number of carbon atoms in an amine increases in the nonpolar alkyl portions, the effect of hydrogen bonding is diminished.

Solubility of Amines in H2O

Solubility of Amines in Water In amines with more than six carbon atoms, the effect of hydrogen bonding is diminished. the nonpolar hydrocarbon chains of the amine decrease its solubility in water.

Study Check Which of the following compounds are soluble in water? A. CH3—CH2—CH2—NH2 B. CH3—CH2—NH—CH3 C. CH3—CH2—CH2—CH3 D.

Solution Which of the following compounds are soluble in water? A. CH3—CH2—CH2—NH2 soluble in water B. CH3—CH2—NH—CH3 soluble in water C. CH3—CH2—CH2—CH3 alkane, not soluble in water D. more than six carbons, not soluble in water

Amines React as Bases in Water Ammonia (NH3), 1°, and 2°amines act as a Brønsted–Lowry base. Nitrogen’s lone electron pair accepts H+ from water to produce an ammonium ion (NH4+) and a hydroxide ion (OH–). NH3 + H2O NH4+ + OH– ammonia ammonium ion hydroxide ion CH3—NH2 + H2O CH3—NH3+ + OH– methylamine methylammonium ion hydroxide ion CH3—NH—CH3 + H2O CH3—NH2—CH3+ + OH– dimethylamine dimethylammonium ion hydroxide ion

Neutralization of Amines An amine salt forms when amines such as those responsible for the odor in fish are neutralized by an acid. amines react as a Brønsted– Lowry base in a neutralization reaction with citric acid. The “fishy” odor from amines is removed by converting them to their ammonium salts.

Neutralization of Amines In a neutralization reaction, an amine acts as a base and reacts with an acid to form an ammonium salt. the lone pair of electrons on the nitrogen atom accepts H+ from an acid to give an ammonium salt; no water is formed. An ammonium salt is named by using its alkylammonium ion name followed by the name of the negative ion.

Neutralization, Ammonium Salts

Quaternary Ammonium Salts In a quaternary ammonium salt, a nitrogen atom is bonded to four carbon groups, which classifies it as a quaternary (4°) amine. the nitrogen atom has a positive charge and is not bonded to an H atom. Choline, a component of glycerophospholipids, is a quaternary ammonium ion.

Properties of Ammonium Salts Ammonium salts are ionic compounds with strong attractions between the positively charged ammonium ion and an anion, usually chloride. solids at room temperature, odorless, and soluble in water and body fluids. Amines are therefore usually converted to their ammonium salt before being used as drugs.

Ammonium Salts, Medications Ammonium salts are used as the active ingredients in medications, including ephedrine, used as a bronchodilator in the decongestant Sudafed. diphenylhydramine found in Benadryl, used for relief of itching and pain from skin irritations and rashes.

Ammonium Salts, Cocaine The salt of cocaine (cocaine hydrochloride) is smuggled and used illegally on the street. The solid product, a free amine, is rapidly absorbed when smoked and gives stronger highs than the cocaine hydrochloride, which makes crack cocaine more addictive.

Ammonium Salts, Cocaine Cocaine hydrochloride, an ammonium salt, can be converted back to its free amine or free base form, “crack cocaine,” by treating it with a strong base.

Study Check Write the structural formula for A. methylpropylamine. B. 2-chloroaniline.

Solution Write the structural formula for A. methylpropylamine. CH3—CH2—CH2—NH—CH3 B. 2-chloroaniline.