Download V a.) \

15.2AmineBasicity 465
Chlorophyll is the green pigment
found in the chloroplast of plants.
The structure ofthe pyrrole ring
system in chlorophyll is very similar to that found in hemoglobin. In
photoslmthesis, chlorophyll absorbs light energy,which is eventually stored as chemical potential
energy in carbohydrate and other
nutrient molecules.
tant components of the nucleic acids, the molecules of heredity. Caffeine,
the stimulant in tea, coffee,cocoa, and many soft drinks, is a purine derivative. The hemoglobin of blood is an example of a naturally occurring molecule that contains the pyrrole ring system.
o
'ryZ-r\
'ryZ\
til)
ol^i
rl
ttl
\\
Pyrimidine
N
I
H
Purine
/
oz\N-r-
I
H
I
N
n.c-lri/\1-
//
\*/
(r-'l
CH.
CH,
Caffeine
Pl'rrole
PRACTICE
EXERCISE
I5.4
\tVhich structure represents an aromatic heterocyclic amine?
(b)
tat,r'\1
a.) \-,V
V
NHz
Niphatic
(c)
ra11Nuz
T
,o,f-\
\>
\-"
diamines
Aliphatic amines that contain two amino groups in the same molecule are
called aliphatic diamines. Two aliphatic diamines produced by decaying
flesh are putrescine and cadaverine. These amines have characteristic
odors.You may judge from their names what the odors are like.
H2N-CHzCH2CH2CHz-NHz
H2N-CH2CH2CH2CH2CH2-NHZ
Putrescine
Cadaverine
15.2Aminehosicity
AIM: Toshow with equations,how ominesoct os weok bases.
Focus
Amines areweakbases.
As we discussed in Chapter 9 for the weak base ammonia, the unshared
electron pair on the nitrogen of primary, secondary,or tertiary amines can
accept a hydrogen ion, or proton, from acids. Amines are weak bases of
base strength similar to that of ammonia. The general reaction for the protonation of an amine is
H
H
I
R-N:
I
+
H*
l*
R-ry-H.
I
H
Free amine
(ulprotonated amineJ
--
H
Proton
Ammoniumion
(protonated amine)
An amine that has accepteda hydrogen ion and acquired a positiue chargeto
466
15 Aminesand Amides
CHAPTER
becomea cation is called aprotonated amine. Thecations produced by protonation of aliphatic and aromatic amines are calledallcylammonium ions
and arylarnmonium ions, respectiuely.An electrically neutral, unprotonated amine is called afueeamine.
Amines are bases,so they can reactwith acidsto form salts.Sinceamines
are derivativesof ammonia, the saltsformed from the reactionsof aminesand
aci.dsare calledammonium salts. Here are three specific examplesof the formation of ammonium saltsfrom amines and hydrochloric acid:
H
H
I
+
CH,-N:
I
l*
.-
HCI
+
CH3-N-H
Cl-
I
H
H
Hydrochloric
acid
Methylamine
(free amine)
Methylammonium
ion
Chloride
ion
Methylammonium
chloride
H
H
I
(cH3cHt2N: + HCI i-
l*
(CH3CH)2N-H
Cl
Diethylamine
Diethylammonium chloride
r\
r\
ll
l + H C r. \nZ
ll
\*/
lcr
*l
H
Pyridine
Pyridinium hydrochloride
The methylammonium chloride formed in the reaction of methylamine
and hydrochloric acid is the salt of a weak base and a strong acid just as
ammonium chloride is. The methylammonium ion undergoes slight dissociation in water in the sameway aswe discussedfor ammonium chloride in
Section9.9:
H
l*
CH*-N-H
'l
H
+
Hzo
I
H
H
Methylammonium ion
(proton donor)
I
Water
(proton acceptor)
Methylamine
(free base)
Hydronium ion
(makesthe
solution acidic)
The ammonium saltsformed between amines and strong acids make slightly
acidic aqueoussolutions.Becauseof their ability to accepta proton and retain
it most of the time, amines can act as buffers, or proton sponges,to protect against drastic changesin pH (seeSec.9.10).Many naturally occurring
amines serveas pH buffers in the fluids of living organisms,such asblood.
Like alcohols, the solubilities of free amines in water depend on their
molecular structure. Amines with large hydrocarbon groups on the amine
nitrogen tend to be insoluble. Those with small hydrocarbon substituents,
such as methylamine, are very soluble in water. Protonated amines are
almost always soluble in water because they are ionic. Many medicinal
15.5 Preparation
of Amines
467
drugs that contain the amino group are supplied as the hydrochloride salts.
Becauseof its greater solubility in water, a hydrochloride salt is usually easier to administer and more readilv absorbed into the bloodstream than the
free amine.
PRACTICE
EXERCISE
I5.5
Complete the following reactions, and name the products.
(a) (CH3)2NH+ HCI
(b) CH3NH2+ HNO3
(c) _Z:- -NH,
(d) CH3CH2NH2+ H3PO4
('
I
ir
ll
+HCl
\--'
l5.I Preporotionof amines
AIMS: To write on equotionfor the prcporotion of on omine
from a holocarbonond ommonioor on omine nitrogen.
Tonome ond write the structurefor o quoternory
ommoniumsolt.
Focus
Simplealiphaticaminescanbe
prepared by displacement
reactions.
There are several ways to prepare amines. One common way to prepare
aliphatic amines is to displace the halogen in a halocarbon with ammonia
or an amine nitrogen. This reaction produces an ammonium salt.
H
l*
+
H-N:
+
R-I
I
R-NH"
I
H
Ammonia
'
Alkyliodide
(other halides
maybe used)
Alkylammoniumiodide
The solution containing the ammonium salt can be treated with a base
such as sodium hydroxide to produce the free amine.
+
R-NH3
+
I
Ammonium
salt
NaOH
-R-NHz*
Sodium
hydroxide
Free
amine
NaI
Sodium
iodide
+
H2O
Water
It maybe difficult to control the number of alkyl groups attaching to the
amine nitrogen. Treatment of ammonia with an alkyl halide usually gives a
mixture of primary, secondary and tertiary amines.
NH3 + R-X
_-
R-NH2
Primary(1') amine
R
R-NH,
R-I-H
+ R-X
+ R-X
I
."".- R-NH
Secondary(2") amine
.+
Tertiary (3.) amine
R-N-R
Because of its basicity, the nitrogen of a tertiary amine can displace a halo.
468
CHAPTER
15 Aminesand Amides
gen from a fourth alkyl halide molecule. A nitrogen with forrr R
attached has a positive charge and is called a quaternary amnxoniunx
Choline, an important component of some constituents of cell me
(seeSec.17.3),is a quaternary ammonium ion.
CHt
cn -NLcrScHroH
CH.
Choline
Quaternary
quator (Latin):
a quaternary
Salts containing
fourtimes
Quaternary ammoniurn salts
having one long a\l group function as soaps.One quaternary
ammonium salt, cetyltrimethylamrnonium chloride, is found in
mouthwashes and is used as a
germicide for sterilizing medical
instruments.
ammonium
ion are called
ammonium salts. The formation of a quaternary ammonium salt can
illustrated bv the reaction of trimethvlamine with bromomethane:
cI{3
CH3-N:
cHo
+
CFI3BT +
H3C-NLCH,
cI{3
Trimethylamine
Br-
cH3
Bromomethane
Tetramethyl
'(methvrbromide)
ffi##,l"
=
EXERGISE
T5.6
-= PRAGTIGE
An aqueous solution of ammonia is heated with l-chlorobutane.
=
=
-== (a)lVhat is the organic product? (b) Give the structure and name of the
: free amine that is liberated when the product in part (a) is treated with
=_
7=sodium hydroxide.
15.4Amides
AIM: To nomeond write strucf;ures
of simpleomides.
Amides are ammonia or amine deriuatiues of organic acids. Amides may be
simple, monosubstituted, or disubstituted. For example:
Amidesare arrimonia and
aruiue derivativesof carborylic
acids.
o
tl
R-C-OH
o
tl
R-C-NI{2
Carboxylic acid
Simple amide
OH
il-f
R-C-N-R
Monosubstitutedamide
OR
lll,
R-C-N-R
Disubstiflrtedamide
Although the amides of carboxylic acids are the focus of this chapter,
the acid part of the amide need not be a carboxylic acid. The amides of
fi{