ORGANIC CHEMISTRY
CHM 207
CHAPTER 9:
AMINES
NOR AKMALAZURA JANI
• Amines:
- organic derivatives of ammonia with one or more alkyl or aryl
groups bonded to the nitrogen atom.
• Functional group:
• Classification of amines:
Primary amine
Secondary amine
Tertiary amine
• Primary (1o) amine: one alkyl or aryl group attached to the
nitrogen atom.
• Secondary (2o) amine: two alkyl or aryl group attached to the
nitrogen atom.
• Tertiary (3o) amine: three alkyl or aryl group attached to the
nitrogen atom.
• Quaternary (4o) amine: an ion in which nitrogen is bonded to
four alkyl or aryl groups and bears a positive charge
CH3
H3C N CH3
CH3
NAMING AMINES
• Common names:
- formed from the names of the alkyl groups bonded to nitrogen,
followed by the suffix –amine.
- the prefixes di-, tri-, and tetra- are used to decribe two, three or
four identical substituents.
CH3 CH2 NH2
ethylamine
CH3
N
CH3
cyclohexyldimethylamine
CH3 CH2
CH3
N
CH3
ethyldimethylamine
(CH3CH2CH2CH2)4N+ -CI
tetrabutylammonium chloride
• IUPAC names:
- similar to that alcohols.
- the longest continuous chain of carbon atoms determine the root
name.
- the –e in alkane name is changed to –amine, and a number shows
the position of the amino group along the chain.
- other substituents on the carbon chain are given numbers, and
the prefix N- is used for each substituent on nitrogen.
CH3 CH2
3
4
NH2
CH CH3
1
2
2-butanamine
NHCH3
CH3 CH2 CH CH3
4
3
2
1
N-methyl-2-butanamine
CH3
NH2
CH3 CH CH2 CH2
4
3
2
1
3-methyl-1-butanamine
CH3
CH3
CH3 CH2 CH CH CH CH3
N CH3
CH3
2,4, N, N-tetramethyl-3-hexanamine
• The prefix ‘amino’ is used to indicate the presence of an –NH2
group in a molecule containing than one functional group.
• For example,
NH2CH2COOH
H2NCH2CH2OH
aminoethanoic acid
2-aminoethanol
OH
NH2
NH2
2,4-diaminophenol
COH
NH2
3-aminobenzaldehyde
NAMING AROMATIC PRIMARY AMINES
• Aromatic amines have an amine group (-NH2) attached directly
to the aromatic ring.
• Aromatic amines known as arylamines.
• Examples,
NH2
CH3
NH2
2
3
1
4
6
1
NH2
5
phenylamine
(aniline)
2-methylphenylamine
(2-methylaniline)
6
2
5
3
4
NO2
4-nitrophenylamine
(4-nitroaniline)
• Compounds with two –NH2 groups are named by adding
the suffix ‘diamine’ to the name of the corresponding
alkane or aromatic compounds.
H2N
(CH2)6 NH2
hexane-1,6-diamine
(1,6-hexanediamine)
H2N
NH2
benzene-1,4-diamine
(1,4-benzenediamine)
PHYSICAL PROPERTIES OF AMINES
i)
Boiling points:
- the boiling points of amines is increase with increasing relative
molecular mass.
- the lower aliphatic amines are gases or low-boiling liquids.
- amines are polar compounds and both primary and secondary
amines associate by intermolecular hydrogen bonding.
Hydrogen bonding
H
H
R
N
H R
N
H
R
H N
H
* Comparing the boiling points of 1o, 2o and 3o amines
- for isomeric amines, the boiling points decreases in the order,
1° amine > 2° amine > 3° amine
- reason: decrease in intermolecular hydrogen bonding.
- example,
CH3
CH3CH2CH2NH2
1-propanamine
o
(1 amine)
boiling point:
o
48.6 C
molecular formula: C3H9N
molecular mass: 59
CH3CH2N H
N-methylethanamine
o
(2 amine)
o
37.0 C
CH3
CH3 N CH3
N, N-dimethylmethanamine
o
(3 amine)
3.5oC
* Comparing the boiling points of amines with other organic
compounds
- the boiling points of aliphatic amines are higher than
those of alkanes or haloalkanes of similar relative
molecular mass due to intermolecular hydrogen bonding.
- the N-H bond is more polar than the C-H bond but less
polar than O-H bond. Hydrogen bonding in amines are
weaker than that of alcohols or carboxylic acids. Boiling
points of amines are lower than those corresponding
alcohols or carboxylic acids.
Comparison of boiling points of some organic compounds with similar
molecular weight
alkane < ether < alkyl halide < amine < ketone, aldehyde < alcohol < acid
ii)
Solubilities of 1o, 2o and 3o amines:
- all three classes of aliphatic amines are capable of
forming hydrogen bonds with water molecules.
- the lower amines (with chain length up to four
carbon atoms per molecule) are very soluble in
water because they can form hydrogen bonds with
water molecules.
- the solubilities of amines is decrease with
increasing number of carbon atoms in the chain.
- amines are soluble in organic solvents.
THE BASICITY OF AMINES
• Amines can act as:
- a nucleophile (a Lewis base) because its lone pair none bonding
electrons can form a bond with an electrophile.
- a Brønsted-Lowry base because it can accept a proton from a
proton acid.
Reaction of an amine as a nucleophile
H
H
R
N
CH3 I
R
H
nucleophile
electrophile
N
CH3
I
-
H
new N-C bond formed
Reaction of an amine as a proton base
H
H
R
H
N
X
H
base
proton acid
R
N H
H
protonated
-
X
• Amines are fairly strong base and their aqueous solutions are basic.
• An amine can abstract a proton from water, giving an ammonium
ion and a hydroxide ion.
• The equilibrium constant for this reaction is called base-dissociation
constant, symbolized by Kb.
H
R
N
H
Kb
H O H
R
H
Kb = [RNH3+] [-OH]
N H
H
pKb = - log 10 Kb
[RNH2]
Stronger base have smaller values of pKb
-
OH
• The basicity of the amines depends on the ability of the lone pair
none bonding electrons at nitrogen atom to form bond with an acid.
• The more easier the lone pair electrons formed bond with the acid,
will make the amines a stronger base.
• Factors that effect the basicity of the amines:
i) substitution by alkyl groups
- the presence of alkyl groups (electron-donating group) such as (CH3) and (CH3CH2-) will make the amine become more basic.
- for example, methylamine is more basic than ammonia.
ii) substitution by electron-withdrawing groups
- the presence of electron-withdrawing groups or atom will decrease
the basicity.
- for example, nitroaniline is less basic than aniline
Basicity of aromatic amines
* Aromatic amines is less basic than aliphatic amines and
ammonia.
* Reason:
- the lone pair of electrons on the nitrogen atom is delocalised
into the benzene ring.
- As a result, the lone pair of electrons is less available for
donation to an acid.
- The reaction is shifted toward the left and makes aniline a
weaker base than ammonia or aliphatic amines.
REACTIONS OF AMINES
•
•
•
•
Salt formation
Reaction with nitrous acid
Amide formation
Ring halogenation of phenylamine
Salt formation
• Reaction of amines and acid will give amine salt.
• Amine salt:
- composed of two types of ions:
i) the protonated amine cation (an ammonium ion)
ii) anion derived from the acid
• Amine salts are ionic, have higher melting points,
nonvolatile solids, more soluble in water than the parent
amines and slightly soluble in nonpolar organic solvents.
R
NH2
HCl
alkylammonium chloride
primary amine
R2 NH
HCl
secondary amine
R3N
RNH3Cl
HCl
tertiary amine
R2NH2Cl
dialkylammonium chloride
R3NHCl
trialkylammonium chloride
EXAMPLES:
CH3CH2CH2 NH2
HCl
n-propylammonium chloride
n-propylamine
(CH3CH2)3 N
triethylamine
CH3CH2CH2 NH3Cl
HCl
(CH3CH2)3 NH Cl
triethylammonium chloride
Reaction with nitrous acid
• Nitrous acid (HNO2) is unstable and is prepared in situ by the
reaction of dilute HCl or dilute H2SO4 with sodium nitrite in
the absence of heat.
NaNO2 (s) + HCl (aq) → NaCl (aq) + O=N-OH (aq)
nitrous acid
• Nitrous acid can be used to differentiate primary, secondary
and tertiary aliphatic amines.
Primary aliphatic amines
• When aliphatic primary amines react with HNO2, nitrogen is evolved
rapidly and an alcohol is produced.
RNH2 + O=N-OH → R-OH + H2O + N2 (g)
• For example, ethylamine gives nitrogen and a mixture of ethanol
(60%), ethene and other products.
C2H5NH2 + O=N-OH → C2H5-OH + H2O + N2 (g) + other products
• The reaction of propylamine with HNO2 produces nitrogen and a
mixture of 1-propanol (7%), 2-propanol (32%) and propene (28%).
• The reaction of methylamine with HNO2 produces only a little
methanol, and the main products are methoxymethane and
nitrogen.
Secondary aliphatic and aromatic amines
• Aliphatic secondary amines react with HNO2 at room
temperature to form nitrosoamines / nitrosamines (yellow oils).
R2N-H + HO-N=O → R2N-N=O + H2O
nitrosoamine
• Example,
CH3
CH3
N H
dimethylamine
HO N=O
CH3
CH3
N N=O
H2O
N-nitroso-N,N-dimethylamine
Tertiary aliphatic amines
• A tertiary aliphatic amines react with HNO2 will produced
ammonium salts which is dissolve readily in water as a clear
solution.
R3N + HNO2 → [R3NH]+NO2- (aq)
Primary aromatic amines
• A primary aromatic amines react with cold HNO2 and
dissolved in dilute HCl at 0-5oC will produced diazonium
salt. When this cold salts heated at room temperature,
nitrogen gas will evolved.
NH2
HNO2
HCl
5oC
+
N2 Cl
-
2H2O
benzenediazonium chloride
RT = room temperature
RT
N2
mixture
products
Tertiary aromatic amines
• Tertiary aromatic amines reacts with nitrous acid by
undergoing substitution at the para position of the benzene
ring to form nitrosoaniline which is a yellow precipitate.
R
N R
HNO2
< 5oC
R
ON
N R
a nitosoamiline compound (yellow precipitate)
Amide formation
i)
Reaction with acyl chlorides
•
Primary and secondary amines are acylated at room
temperature by acyl chlorides to form N-substituted amides.
RNH2 + CH3COCl → RNHCOCH3 + HCl
R2NH + CH3COCl → R2NCOCH3 + HCl
•
Tertiary amines are NOT acylated because they do not have
hydrogen atom attached to the nitrogen atom.
Examples:
H
O
H O
CH3N H
CH3C Cl
CH3N C CH3
ethanoyl chloride
H
N H
O
CH3C Cl
ethanoyl chloride
HCl
N-methylethanamide
H O
N C CH3
N-phenylbenzamide
HCl
ii)
Reaction with acid anhydrides
•
Primary and secondary amines are readily acylated
by acid anhydrides to yield the corresponding N-alkyl
or N-aryl amides.
For example,
•
H
CH3CH2CH2N H
propylamine
H
CH3CH2N CH2CH3
diethylamine
O
O
CH3 C O C CH3
ethanoic anhydride
O
O
CH3 C O C CH3
ethanoic anhydride
H O
CH3CH2CH2N C CH3
O
HO C CH3
N-propylethanamide
O
CH3CH2N C CH3
CH2CH3
N, N-diethylethanamide
O
HO C CH3
Ring halogenation of phenylamine
• When bromine water is added to phenylamine (aniline) at
room temperature, decolorisation of the bromine water
occurs and a white precipitate of 2,4-6-tribromoaniline
(C6H4Br3N) is obtained.
• This reaction is used as a test for aniline.
NH2
NH2
Br
3Br2 (aq)
Br
room temperature
3HBr
Br
2,4,6-tribromoaniline
(white precipitate)
USES OF AMINES
SYNTHESIS OF NYLON
• Nylons are condensation copolymers formed by reacting
equal parts of a diamine and a dicarboxylic acids, so that
peptide bonds form at both ends of each monomer in a
process analogous to polypeptides biopolymers.
• General reactions:
Dicarboxylic acids
Diamines
Nylon
Basic concepts of nylon production
• The first approach:
- combining molecules with an acid (COOH) group on each
end are reacted with two chemicals that contain amine (NH2)
groups on each end.
- Form nylon 6,6, made of hexamethylene diamine with six
carbon atoms and acidipic acid, as well as six carbon atoms.
• The second approach:
- a compound has an acid at one end and an amine at the
other and is polymerized to form a chain with repeating units
of (-NH-[CH2]n-CO-)x.
- Form nylon 6, made from a single six-carbon substance
called caprolactam.
SYNTHESIS OF DYE
• Primary aromatic amines are used as a starting material for the
manufacture of azo dyes.
• Azo compounds:
- compounds bearing the functional group R-N=N-R', in which
R and R' can be either aryl or alkyl.
- N=N group is called an azo group
- HNNH is called diimide
• Aryl azo compounds have vivid colors, especially reds,
oranges, and yellows
Yellow azo dye
• Amines react with nitric(III) acid to form diazonium salt,
which can undergo coupling reaction to form azo
compound.
• Azo-compounds are highly coloured, they are widely
used in dyeing industries, such as:
i) Methyl orange
ii) Direct brown 138
iii)Sunset yellow FCF
iv)Ponceau
Uses and important of azo dye
• Methyl orange - used as acid-base indicators due to
the different colors of their acid and salt forms
• Artist’s paints – clays, yellow to red range
• Dye in food and textiles
E102: Tartrazine
E107 : Yellow 2G
EXAMPLES OF
AZO DYES USED
IN FOOD
E110 : Sunset Yellow
E122 : Azorubine
E123 : Amaranth
E124 : Ponceau 4R
E129 : Allura Red
E151 : Brilliant Black