Concerning Amines. Their Properties, Preparation and Reactions

214 Pages·1967·4.11 MB·English

by  David Ginsburg and Robert Robinson (Auth.)

CONCERNING
AMINES
Their Properties, Preparation and Reactions
by
DAVID GINSBURG
Israel Institute of Technology, Haifa
PERGAMON PRESS
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Copyright © 1967 Pergamon Press Ltd.
First edition 1967
Library of Congress Catalog Card No. 66-19082
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PREFACE
WE have evidence that at least one of the physical properties
of amines was known to the Ancient Egyptians as early as
1226 B.C.* In the subsequent centuries more knowledge has been
accumulated about the physical properties of these interesting
substances, their chemical properties and methods for their
preparation. This book is an attempt to describe this knowledge.
Hopefully, the reader will not suffer nearly as much as the
Egyptians who were subjected to a concentrated dose of amines.
Haifa, DAVID GINSBURG
October 1964
* Exodus, vii. 21.
ix
A*
EDITORIAL PREFACE
PROFESSOR GINSBURG'S contribution to this series of
volumes, designed to be welded into a comprehensive
text, covers the field of amine chemistry apart from some
specialized sections such as the alkaloids. The didactic
treatment originally contemplated has been maintained,
but the scope has been enlarged so as to bring together
all the essential chemistry of the amines no matter to
what part of the complex edifice of organic chemistry it
may apply. There are certain lacunas, but these are
deliberate and due to the intention to deal with these
matters elsewhere in the course.
R. ROBINSON
XI
THE NITROGEN ATOM
NITROGEN having atomic number 7 has the structure ls22s22p39
the three 2/? electrons disposed singly in the 2p, 2p and 2p
x y 2
orbitals. The ground state of a nitrogen atom is represented as
follows :
Nitrogen \±J\ \Γ±\ ΓΠΊΓΓΠ
Is 2s 2/7
It is therefore not surprising that the nitrogen atom may form
three covalent bonds with each of three hydrogen atoms with the
resulting stable ammonia molecule NH in which the nitrogen
3
atom still owns a lone pair of electrons.
• I·
H
TYPES OF AMINES
We can consider various organic compounds containing a
nitrogen atom at the same oxidation level as in ammonia, ai
derivatives of ammonia in which, formally, one or more hydrogen
1
THE NITROGEN ATOM
NITROGEN having atomic number 7 has the structure ls22s22p39
the three 2/? electrons disposed singly in the 2p, 2p and 2p
x y 2
orbitals. The ground state of a nitrogen atom is represented as
follows :
Nitrogen \±J\ \Γ±\ ΓΠΊΓΓΠ
Is 2s 2/7
It is therefore not surprising that the nitrogen atom may form
three covalent bonds with each of three hydrogen atoms with the
resulting stable ammonia molecule NH in which the nitrogen
3
atom still owns a lone pair of electrons.
• I·
H
TYPES OF AMINES
We can consider various organic compounds containing a
nitrogen atom at the same oxidation level as in ammonia, ai
derivatives of ammonia in which, formally, one or more hydrogen
1
2 CONCERNING AMINES
atoms have been replaced by carbon-containing groups. Because
of this way of looking at these derivatives these substances have
been given the name amines.
H H R' R'
I I I I
H—N: R—N: R—N: R—N:
I I I I
H H H R"
Ammonia Primary amine Secondary amine Tertiary amine
This formal representation is useful only insofar as it emphasizes
the relationship of the various classes of amines to ammonia;
it does not mean necessarily that the only way of preparing amines
is by substitution of the hydrogen atoms in ammonia by carbon-
containing groups. Quite the contrary, we shall discuss many
methods of preparing amines in which ammonia is not one of the
reactants.
The above four formulae show that any organic substance in
which the —NH group is present is called a primary amine;
2
this group is attached to a carbon-containing group R which may
be as simple as CH or quite a bit more complex in structure.
3
Whatever the case, it should be noted that the nitrogen atom still
owns, of itself, a lone pair of electrons. (This last statement will
be somewhat qualified in our later discussion.)
An organic substance which contains an )>NH group attached
to two carbon-containing groups of varying complexity is called
a secondary amine and one in which three carbon-containing
groups are attached to a nitrogen atom is called a tertiary amine.
Here also it should be emphasized that the lone pair of electrons
is present on the respective nitrogen atoms.
BASICITY OF AMINES
The presence of the lone pair cannot be overemphasized since
it is the lone pair which confers upon the amines, whether they
be primary, secondary or tertiary, their basic character. The
2 CONCERNING AMINES
atoms have been replaced by carbon-containing groups. Because
of this way of looking at these derivatives these substances have
been given the name amines.
H H R' R'
I I I I
H—N: R—N: R—N: R—N:
I I I I
H H H R"
Ammonia Primary amine Secondary amine Tertiary amine
This formal representation is useful only insofar as it emphasizes
the relationship of the various classes of amines to ammonia;
it does not mean necessarily that the only way of preparing amines
is by substitution of the hydrogen atoms in ammonia by carbon-
containing groups. Quite the contrary, we shall discuss many
methods of preparing amines in which ammonia is not one of the
reactants.
The above four formulae show that any organic substance in
which the —NH group is present is called a primary amine;
2
this group is attached to a carbon-containing group R which may
be as simple as CH or quite a bit more complex in structure.
3
Whatever the case, it should be noted that the nitrogen atom still
owns, of itself, a lone pair of electrons. (This last statement will
be somewhat qualified in our later discussion.)
An organic substance which contains an )>NH group attached
to two carbon-containing groups of varying complexity is called
a secondary amine and one in which three carbon-containing
groups are attached to a nitrogen atom is called a tertiary amine.
Here also it should be emphasized that the lone pair of electrons
is present on the respective nitrogen atoms.
BASICITY OF AMINES
The presence of the lone pair cannot be overemphasized since
it is the lone pair which confers upon the amines, whether they
be primary, secondary or tertiary, their basic character. The
PROPERTIES, PREPARATION AND REACTIONS 3
amines are the important bases of organic chemistry. They may
be stronger or weaker bases but the fact that they are bases is due
to the lone pair on nitrogen. If delocalization of the lone pair
occurs through its involvement with other parts of the molecule
due to resonance, the basicity of the amine may be very low
indeed and we shall exemplify this in our discussion of certain
aromatic and heterocyclic amines. However, in principle, the
basicity of the amines may be represented in the following
equations :
H H
I I
R _N : + H + +± R—N^-H
I I
H H
R' R'
I L
R_N: + H+ «± R—N^H
I I
H H
R' R'
R_N: + H+ <± R—N±-H
I I
R" R"
The above equations fit the Lowry-Bronsted concept of acid-
base relationships. Any compound which may accept a proton
is a base; any compound which can donate a proton is an acid.
The positive ions on the right-hand side of each equation are, in
accordance with the Lowry-Bronsted view, acids as may be seen
from the part of the equilibrium flowing from right to left.
The ionization of primary amines in aqueous solution may be
represented by the following equation :
RNH + H 0<±RNH + OH-
2 2 3
4 CONCERNING AMINES
The ionization constant K is obtained as follows:
b
+
, [RNH3HOH-]. ^ , . _
K = = K[U20]vKb = logKb
[RNH ] [H 0]
2 2
+
The substituted ammonium ion RNH is the conjugate acid of
3
the base RNH ; equally the base RNH is the conjugate base of
2 2
+
the acid, RNH which ionizes as shown above:
3
RNH «±RNH + H+
3 2
The ionization constant of this acid is K , defined as follows:
a
[RNH3]
Since K = [H+] [OH~] = 10"14 the general relationship
w
pK = 14—pK holds also for equilibria involving amines and
a b
their conjugate acids, the corresponding substituted ammonium
ions.
Analogous equilibrium constants may, of course, be obtained
for secondary and tertiary amines.
We revert to first principles to demonstrate that the positive
charge of the cations in the above equilibria resides on the
nitrogen atom. These positive ions may be formally viewed as
derived from the ammonium ion NH 4 in which the positive charge
also resides on the nitrogen atom.
H H
I I
H—N: + H+ +>H—N+— H
I I
H H
In doing these simple arithmetical calculations we consider that