Download Hydrogen Bonding in Amines

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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solubility of Amines in H2O
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
CH3
CH3 —CH2—N—CH2—CH2—CH2—CH2—CH2—CH3
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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
CH3
D. CH3 —CH2—N—CH2—CH2—CH2—CH2—CH2—CH3
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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
CH3
D. CH3 —CH2—N—CH2—CH2—CH2—CH2—CH2—CH3
more than six carbons, not
soluble in water
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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
ammonia
CH3—NH2 + H2O
methylamine
CH3—NH—CH3 + H2O
dimethylamine
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
NH4+
+
ammonium ion
OH–
hydroxide ion
CH3—NH3+
+
OH–
methylammonium ion hydroxide ion
CH3—NH2—CH3+
+
OH–
dimethylammonium ion hydroxide ion
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Neutralization, Ammonium Salts
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Write the structural formula for
A. methylpropylamine.
B. 2-chloroaniline.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Write the structural formula for
A. methylpropylamine.
CH3—CH2—CH2—NH—CH3
NH2
Cl
B. 2-chloroaniline.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.