Structure and Classification
Amines are classified as 1°, 2°, or 3° depending on
the number of carbon groups bonded to nitrogen.
Aliphatic amine: All carbons bonded to nitrogen are
derived from alkyl groups. See the three above.
Aromatic amine: One or more of the groups bonded to
nitrogen are aryl groups.
Structure and Classification
• Heterocyclic amine: An amine in which the nitrogen
atom is part of a ring.
• Heterocyclic aliphatic amine: A heterocyclic amine in
which the ring is saturated (has no C=C bonds).
• Heterocyclic aromatic amine: The amine nitrogen is
part of an aromatic ring.
Nomenclature
IUPAC names
• We derive IUPAC names for aliphatic amines just as we
did for alcohols.
• Drop the final -e of the parent alkane and replace it by
-amine.
• Use a number to locate the amino group on the parent
chain.
Nomenclature
Common names
• For most aliphatic amines, list the groups bonded to
nitrogen in alphabetical order in one word ending in the
suffix -amine.
Nomenclature
Amine salts
• When four atoms or groups of atoms are bonded to a
nitrogen atom, as for example CH3NH3+, nitrogen bears
a positive charge and is associated with an anion as a
salt.
• Name the compound as a salt of the corresponding
amine.
• Replace the ending –amine (or aniline or pyridine or the
like) by -ammonium (or anilinium or pyridinium or the
like) and add the name of the anion.
Physical Properties
Like ammonia, low-molecular-weight amines have very
sharp, penetrating odors.
• Trimethylamine, for example, is the pungent principle
in the smell of rotting fish.
• Two other particularly pungent amines are 1,4butanediamine (putrescine) and 1,5-pentanediamine
(cadaverine).
Physical Properties
Figure 16.1 Amines are polar compounds:
• Both 1° and 2° amines have N-H bonds, and can form
hydrogen bonds with one another.
• 3° Amines have no N-H bond and cannot form
hydrogen bonds with one another.
Physical Properties
• An N-H---N hydrogen bond is weaker than an O-H---O
hydrogen bond, because the difference in
electronegativity between N and H (3.0 - 2.1 = 0.9) is
less than that between O and H (3.5 - 2.1 = 1.4).
• We see the effect of hydrogen bonding between
molecules of comparable molecular weight by
comparing the boiling points of ethane, methanamine,
and methanol.
Physical Properties
• All classes of amines form hydrogen bonds with water
and are more soluble in water than are hydrocarbons of
comparable molecular weight.
• Most low-molecular-weight amines are completely
soluble in water.
• Higher-molecular-weight amines are only moderately
soluble in water or are insoluble.
Basicity of Amines
Like ammonia, amines are weak bases, and aqueous
solutions of amines are basic.
• The acid-base reaction between an amine and water
involves transfer of a proton from water to the amine.
Basicity of Amines
• Aliphatic amines have about the same base strength,
and are slightly stronger bases than NH3.
• Aromatic and heterocyclic aromatic amines are
considerably weaker bases than aliphatic amines.
• Note that while aliphatic amines are weak bases by
comparison with inorganic bases such as NaOH, they
are strong bases among organic compounds.
Basicity of Amines
• Given the basicities of amines, we can determine which
form of an amine exists in body fluids, say blood.
• In a normal, healthy person, the pH of blood is
approximately 7.40, which is slightly basic.
• If an aliphatic amine is dissolved in blood, it is present
predominantly as its protonated (conjugated acid)
form.
Basicity of Amines
• Assume that the amine, RNH2, has a pKb of 3.50 and
that it is dissolved in blood, pH 7.40 (pOH 6.60).
• We first write the base dissociation constant for the
amine and then solve for the ratio of RNH3+ to RNH2.
• Substituting values for Kb and OH- gives:
Reactions of Amines
The most important chemical property of amines is their
basicity.
• Amines, whether soluble or insoluble in water, react
quantitatively with strong acids to form water-soluble
salts.
Basicity of Amines
Example: Complete each acid-base reaction and name the
salt formed.
Basicity of Amines
Example: Complete each acid-base reaction and name the
salt formed.
Solution: