1616
16-1© 2003 Thomson Learning, Inc.All rights reserved
General, Organic, and General, Organic, and Biochemistry, 7eBiochemistry, 7e
Bettelheim,Bettelheim,
Brown, and MarchBrown, and March
1616
16-2© 2003 Thomson Learning, Inc.All rights reserved
Chapter 16Chapter 16
AminesAmines
1616
16-3© 2003 Thomson Learning, Inc.All rights reserved
Structure & ClassificationStructure & Classification• Amines are classified as 1°, 2°, or 3° depending on the
number of carbon groups bonded to nitrogen
• aliphatic amine:aliphatic amine: all carbons bonded to nitrogen are derived from alkyl groups
• aromatic amine:aromatic amine: one or more of the groups bonded to nitrogen are aryl groups
CH3-NH2 CH3-N-CH3
H
CH3-N-CH3
CH3
Methylamine(a 1° amine)
Dimethylamine(a 2° amine)
Trimethylamine(a 3° amine)
NH2 N-CH3
HCH2-N-CH3
CH3
Aniline(a 1° aromatic amine)
N-Methylaniline(a 2° aromatic amine)
Benzyldimethylamine(a 3° aliphatic amine)
1616
16-4© 2003 Thomson Learning, Inc.All rights reserved
Structure & ClassificationStructure & Classification• heterocyclic amine:heterocyclic amine: an amine in which the nitrogen
atom is part of a ring• heterocyclic aliphatic amine:heterocyclic aliphatic amine: a heterocyclic amine in
which the ring is saturated (has no C=C bonds) • heterocyclic aromatic amine:heterocyclic aromatic amine: the amine nitrogen is part
of an aromatic ring
PiperidinePyrrolidine Pyridine(heterocyclic aliphatic amines) (heterocyclic aromatic amines)
NH
NH
N N
N
Pyrimidine
N
N
Imidazole
N
N
N
N
HPurine
H
1616
16-5© 2003 Thomson Learning, Inc.All rights reserved
NomenclatureNomenclature• 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 --amineamine
• use a number to locate the amino group on the parent chain
CH3CHCH3
NH2
NH2
H2NNH2
1,6-HexanediamineCyclohexanamine2-Propanamine
1616
16-6© 2003 Thomson Learning, Inc.All rights reserved
NomenclatureNomenclature• IUPAC names (cont’d)
• IUPAC nomenclature retains the common name aniline for C6H5NH2, the simplest aromatic amine
• name simple derivatives of aniline by using numbers to locate substituents or, alternatively, use the prefixes ortho (o), meta (m), and para (p)
• several derivatives of aniline have common names that are still widely used; among them is toluidine
NH2
CH3
NH2
NO2
NH2
3-Methylaniline (m-Toluidine)
Aniline 4-Nitroaniline(p-Nitroaniline)
1616
16-7© 2003 Thomson Learning, Inc.All rights reserved
NomenclatureNomenclature• IUPAC names (cont’d)
• name unsymmetrical secondary and tertiary amines as N-substituted primary amines
• take the largest group bonded to nitrogen as the parent amine
• name the smaller group(s) bonded to nitrogen, and show their location on nitrogen by using the prefix N (indicating that they are bonded to nitrogen)
NCH3
CH3
NHCH3
N,N-Dimethyl- cyclopentanamine
N-Methylaniline
1616
16-8© 2003 Thomson Learning, Inc.All rights reserved
NomenclatureNomenclature• Common names
• for most aliphatic amines, list the groups bonded to nitrogen in alphabetical order in one word ending in the suffix -amineamine
Diethylmethylaminesec-Butylamine
NNH2
NH2
Cyclohexylamine
NH2
Propylamine
1616
16-9© 2003 Thomson Learning, Inc.All rights reserved
NomenclatureNomenclature• 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-amine (or aniline or pyridine or the like) by -ammonium-ammonium (or anilinium or pyridinium or the like) and add the name of the anion
(CH3CH2)3NH+Cl-
Triethylammonium chloride
1616
16-10© 2003 Thomson Learning, Inc.All rights reserved
Physical PropertiesPhysical 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 1,4-
butanediamine (putrescine) and 1,5-pentanediamine (cadaverine)
1616
16-11© 2003 Thomson Learning, Inc.All rights reserved
Physical PropertiesPhysical Properties• 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
1616
16-12© 2003 Thomson Learning, Inc.All rights reserved
Physical PropertiesPhysical 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
CH3OHCH3CH3 CH3NH2
-6.3 65.032.031.1MW (amu)
bp (°C)
30.1
-88.6
1616
16-13© 2003 Thomson Learning, Inc.All rights reserved
Physical PropertiesPhysical 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
1616
16-14© 2003 Thomson Learning, Inc.All rights reserved
Basicity of AminesBasicity 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
CH3-NH
HH-O-H CH3-N-H
H
HO-H
Methylammoniumhydroxide
Methylamine(a base)
++
-: :
::
::
1616
16-15© 2003 Thomson Learning, Inc.All rights reserved
Basicity of AminesBasicity of Amines• The base dissociation constant, Kb, for the
reaction of an amine with water has the following form, illustrated for the reaction of methylamine with water to give methylammonium hydroxide • pKb is defined as the negative logarithm of Kb
[CH3NH3+][OH
- ]
[CH3NH2]Kb = = 4.37 x 10-4
pKb = - log 4.37 x 10-4 = 3.360
1616
16-16© 2003 Thomson Learning, Inc.All rights reserved
Basicity of AminesBasicity 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
C6H5NH2
CH3CH2NH2Aliphatic
Ammonia
Aromatic
3.0 - 4.0
4.74
8.5 - 9.5
Class pKb Example Name
Ethanamine
Aniline
Stronger base
Weaker base
1616
16-17© 2003 Thomson Learning, Inc.All rights reserved
Basicity of AminesBasicity 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) formHO
HO
NH2
Dopamine
HO
HO
NH3+
Conjugate acid of dopamine(the major form present
in blood plasma)
1616
16-18© 2003 Thomson Learning, Inc.All rights reserved
Basicity of AminesBasicity 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
RNH2 H2O RNH3+ OH-+ +
[RNH3+][OH-]
[RNH2]Kb =
[RNH3+]
[RNH2][OH-]Kb =
[RNH3+]
[RNH2]
4.0 x 10-4
2.5 x 10-7 = = 1600
1616
16-19© 2003 Thomson Learning, Inc.All rights reserved
Reactions of AminesReactions 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
HO H
NH2HO
HOHCl
H2O
HO
HO NH3+Cl-
HHO
(R)-Norepinephrine hydrochloride(a water-soluble salt)
+
(R)-Norepinephrine(only slightly soluble in water)
1616
16-20© 2003 Thomson Learning, Inc.All rights reserved
Reactions of AminesReactions of Amines• example:example: complete each acid-base reaction and name
the salt formed
(CH3CH2)2NH HCl
NCH3COOH
+(a)
(b) +
1616
16-21© 2003 Thomson Learning, Inc.All rights reserved
Reactions of AminesReactions of Amines• example:example: complete each acid-base reaction and name
the salt formed• solution:solution:
(CH3CH2)2NH HCl
NCH3COOH
NH
(CH3CH2)2NH2+Cl-
CH3COO-
Diethylammoniumchloride
+Pyridinium acetate
+(a)
(b) +
1616
16-22© 2003 Thomson Learning, Inc.All rights reserved
End End Chapter 16Chapter 16
AminesAmines