Amines
Ammonia derivatives
Specification from OCR
o
o
o
o
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Explain the basicity of amines in terms of proton
acceptance by the nitrogen lone pair.
Describe the reactions of amines with acids to form salts
Describe the preparation of:
i) aliphatic amines by substitution of halogenalkanes with
excess ethanolic ammonia
ii) aromatic amines by reduction of nitroarenes using tin
and conc. hydrochloric acid
Describe the synthesis of an azo-dye
State the use of reactions in the formation of dyestuffs
Amines


Amines are essentially molecules of ammonia
One or more of the hydrogen atoms have been
replaced with an alkyl group.
H
H
N
H
Amines

Replace one hydrogen atom with an alkyl
group = primary amine, replace 2 = secondary
amine etc.
R NH2
Primary amine
R
NH
R'
Secondary amine
R
R
N R''
R'
Tertiary amine
N
R'
+
R'''
R''
Quaternary ammonium salt
The LONE PAIR on the nitrogen atom in
amines makes them ...

BRØNSTED-LOWRY BASES - they can
be proton acceptors
RNH2 + H+ ——> RNH3+

GOOD NUCLEOPHILES – able to attack
the positive end of a polarised bond.
Amines as bases



Bases are proton acceptors.
Amines don’t actually accept protons, they
donate a lone pair to the hydrogen atom to
form a dative bond.
Ammonia and bases can do this with any
suitable acid to give a salt.
H3N
H Cl
NH4+Cl-
Replacing a hydrogen in
ammonia has the following effect
Causes increased electron donation in the
C-N bond
 Becomes polar, and nitrogen becomes
slightly negative
 Lone pair on nitrogen slightly repelled
 Can be donated to a proton more easily
 1° amines are more basic than ammonia

What about secondary and
tertiary?
So following the same argument, 2°
amines will be more basic still, as the lone
pair will be repelled even more.
 In phenylamine, the lone pair becomes
involved in the aromaticity, so it is less
basic. The lone pair as part of the ring’s
delocalised system, it is less readily
donated to a proton.
 A tertiary amine will be more basic still.

Reaction of amines with acids

A standard Acid Base reaction.
CH3CH2NH2 + HCl
+
CH3CH2NH3
-
+ Cl
The preparation of amines
1. Reduction of nitrobenzene to
give phenylamine
NO2
NH2
i) conc. HCl/Sn
+ 6[H]
ii) NaOH(aq)
+ H2O
Conditions are reflux, this is important in the production of compounds
called azo-dyes.
Substitution of haloalkanes with excess
ethanolic ammonia
AMMONIA
Reagent
Conditions
Product
Nucleophile
Equation
Aqueous, alcoholic ammonia (in EXCESS)
Reflux in aqueous , alcoholic solution under pressure
Amine
Ammonia (NH3)
e.g. C2H5Br + 2NH3 (aq / alc) ——> C2H5NH2 + NH4Br
(i) C2H5Br + NH3 (aq / alc) ——> C2H5NH2 + HBr
(ii) HBr + NH3 (aq / alc) ——> NH4Br
Mechanism
Notes
The equation shows two ammonia molecules.
The second one ensures that the HBr is removed.
NUCLEOPHILIC SUBSTITUTION
AMMONIA
Why excess ammonia?
The second ammonia molecule ensures the removal of HBr which would lead to
the formation of a salt. A large excess ammonia ensures that further substitution
doesn’t take place - see below
Problem
Amines are also nucleophiles (lone pair on N) and can attack another molecule of
halogenoalkane to produce a 2° amine. This too is a nucleophile and can react
further producing a 3° amine and, eventually an ionic quarternary ammonium salt.
C2H5NH2 + C2H5Br
——> HBr + (C2H5)2NH
(C2H5)2NH + C2H5Br ——> HBr + (C2H5)3N
(C2H5)3N +
C2H5Br ——> (C2H5)4N+ Br¯
diethylamine, a 2° amine
triethylamine, a 3° amine
tetraethylammonium bromide
a quaternary (4°) salt
Diazonium Salts


Diazonium: there are 2 nitrogen atoms joined
together in the positive ion.
In French, nitrogen is still called by its old name
‘azote’ which means unable to support life.
N N Cl
diazonium chloride
Formation of a Diazonium
salt.
Formed by reacting phenylamine with
sodium nitrite and hydrochloric acid below
10◦C.
 These reagents form in situ nitrous acid
HNO2.

Diazonium Salts


Notice the triple bond between the nitrogen
atoms
The positive charge is on the nitrogen that is
attached to the benzene ring
N N Cl
diazonium chloride
Why are they important? They
look pretty weird!
They are essential in the dye industry.
 A Diazonium salt is produced then reacted
with a phenol. If the correct phenol is
used, almost any colour can be produced.
 OCR specify this as a reaction you need to
know.

Formation of the Diazonium salt.




The Diazonium salt is unstable above 10°C, so
the reaction is normally carried out in ice.
An aliphatic Diazonium salt is very unstable, so
only aromatics are used.
The lone pairs present in the salt can participate
in the benzene ring, making it more stable. More
correctly this is due to overlap of p-orbitals in the
diazo group with the p-system in the ring.
So phenylamine would give benzenediazonium
chloride.
Formation of the Diazonium salt.
NH2
N N Cl
+ HONO(aq) + HCl(aq)
+ 2H2O(l)
Formation of the Diazonium salt.
The conditions are below 10◦C and
remember the HNO2 (nitrous acid) is
prepared in situ by reacting sodium nitrite
with hydrochloric acid.
 The diazonium salt can then do one of two
things depending on the temperature

Reactions of aromatic diazonium
salts
OH
hydrolysis
room temperature
N N Cl
OH
Coupling reaction
5 degrees Celcius
with phenol
N N
Hydrolysis

The following occurs if a solution of a
diazonium salt is warmed up:
C6H5N2+Cl-(aq) + H2O(l) → C6H5OH + N2(g) + HCl(aq)
Coupling reactions

The mechanism is for
interest only, you do
not need to know it.
OH
N
N N
N
H
O H
H
O
H
OH
N
N
General method for synthesis of
azo dyes




Add a cold aqueous solution of sodium nitrite
slowly (with cooling and stirring) to a cold
solution of the amine compound in excess
hydrochloric acid
The temperature must not rise above 5°C.
This solution (still cold) should then be added
slowly with stirring to a solution of the coupling
compound.
This should be kept below 5°C the whole time.
Amino Acids
These are bi-functional compounds. The
contain 2 functions groups:
 A primary amine (in most cases) –NH2
 The carboxylic acid group –COOH
 An amino acid must contain at least both
of these functional groups.

Amino Acids

The simplest amino acid is glycine.
H
H
H
N C
H
O
C
O H
Amino Acids




All the amino acids (the twenty vitally important
ones biologically) are 2-amino acids.
The amine and acid groups are both attached to
the same carbon.
All can be names systematically, but in most
cases the old names are used.
Alanine is also known as 2-aminopropanoic acid,
but alanine is the acceptable name to use.
Alanine
H
H
H
O
N C
C
H C
O
H
H
H
General Formula
H
H
H
N C
R
O
C
O H
Physical Properties
White solids
 With relatively high melting points glycine
(the simplest) has a melting point of
235°C.
 Normally readily soluble in water
 Almost totally insoluble in non-polar
solvents

Acid – Base Properties
They are very largely ionic compounds.
 The carboxyl group can lose a proton
 The amine group can gain a proton
 The result is a ZWITTERION. From the
German for hermaphrodite, hybrid or
mongrel!

Zwitterions

Glycine mainly exists
as.
H3N
+
CH2 COO
-
Zwitterions

The strong attractions in the crystal cause
the high melting point
 In aqueous solution depending on the pH,
they form either the neutral form, or the
carboxylate will lose a proton, or the amino
group will gain a proton.
Zwitterions
H
H3N
+
C C
H
O
+
+ H in strong acid H
O H
H
H
N C C
H
O
+
-H in strong alkali
O H
H
H
H
N C C
H
O
O-
Isoelectric Point
For each amino acid there is a definite pH
– the isoelectric point at which the acid
and basic ionisations are equal.
 The molecule is effectively neutral – it
carries equal and opposite charges
 This is rarely near pH 7 because the
molecule ionisation tendencies are
affected by the other groups in the
molecule.

Isoelectric Point
Aspartic acid – which has 2 –COOH
groups – is acid in aqueous solution.
 Lysine with more amino than carboxyl
groups is alkaline.
 Due to this dual functionality, they are able
to act as buffer solutions (able to maintain
a reasonably constant pH with small
additions of acid or alkali).
 They also have optical activity.

How amino acids join together


Amino acids join together in specific ways to
form specific proteins.
One amino acid can join to another to form a
substituted amide.
H
H
H2N C COOH + H2N C COOH
R'
R''
H O
H
H2N C C N C COOH + H2O
R'
H R''
How amino acids join together

This kind of bond between 2 amino acids is
called a peptide bond or a peptide link.
O
C
N
H
How amino acids join together
H
H
O
N C C
H
R'
H
H O
R'
O
+
N C C
OH
OH H
R'
N C C
H
H
H
H
O
N C C
H R'
OH
How amino acids join together
Two joined amino acids = dipeptide
 Three = tripeptide
 Many = polypeptide
 At some point a polypeptide becomes a
protein. This can be put at 40 amino acids.

Acid Hydrolysis of proteins
Proteins and peptides can be hydrolysed
with hot concentrated (6 mol dm-3) HCl.
 The protein is refluxed for about 24 hours.
 This hydrolysis is the exact reverse of the
formation of the peptide bond.
 A molecule of water is in effect added
across the linkage to regenerate the
original amino acid and carboxyl groups.

Acid Hydrolysis of proteins
O
C
R
O R'
O
N CH C
CH C
H + H2O
N
N
H
H
O
R'
R
C
O
N CH C
H
+
OH
H
O
CH C
N
H
H
N