Non-Aqueous Solvents in Inorganic Chemistry

146 Pages·1965·3.56 MB·English

by  A. K. Holliday, A. G. Massey, Robert Robinson, H. M. N. H. Irving and L.A.K. Staveley (Auth.)

Non-aqueous Solvents
in Inorganic Chemistry
by
A. K. HOLLIDAY, Ph.D., D.Sc, F.R.I.C.
Reader in Inorganic Chemistry, The University of Liverpool
and
A. G. MASSEY, B.Sc, Ph.D., A.R.I.C.
Lecturer in Inorganic Chemistry, Queen Mary College,
University of London
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Copyright © 1965 Pergamon Press Ltd.
First edition 1965
Library of Congress Catalog Card No. 65-24224
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PREFACE
BY THE end of the nineteenth century, two non-aqueous inorganic
solvents — liquid ammonia and liquid sulphur dioxide — had
already been investigated. Today, a large number and great
variety of such solvents are known, but the early work has had a
great influence on the way in which our knowledge of these newer
solvents has developed. The investigations on ammonia and
sulphur dioxide were notable in being concerned experimentally
with the manipulation of low-boiling, reactive liquids and
theoretically with the extension of the simple acid-base concepts
of the original Ionic Theory to non-aqueous systems. These
trends can be seen very clearly in Franklin's classical monograph
on liquid ammonia chemistry — The Nitrogen System of Com­
pounds (1935). The influence of this early viewpoint can be
realized when one recognizes how many of the newer non-aqueous
solvents are low-boiling reactive liquids and how often acid-base
concepts have been invoked to explain the observations made.
Much of this later work has been very fruitful, but until recently
attention has not been directed to the use of substances with a
higher-temperature liquid range, or to the use of non-aqueous
solvents for (e.g.) oxidation or synthetic reactions. Only in the last
decade has work on solvents such as pure sulphuric acid and fused
salts begun to show valuable results, and this has been due in
great measure to the development of newer methods — and
especially spectroscopic methods — of studying solute-solvent
systems.
In writing this book, our aim has been to give a concise treat­
ment of the important inorganic non-aqueous solvents, empha­
sizing why they do in fact exhibit solvent power, how they are
prepared and handled experimentally, how they can be used as
vii
viii PREFACE
media for the synthesis or analysis of inorganic and organo-
metallic compounds, and how far the various acid-base concepts
can be useful in accounting for many (but not all) of the reactions
observed. It has not been easy to achieve this aim in the final
chapter, on high temperature solvents, because the latter (mainly
fused salts) have as yet been little used for synthesis or analysis in
the laboratory, although their use on a large scale has been known
for many years. However, a considerable amount of physico-
chemical information is now available for fused salt systems, and
we have endeavoured to show the relevance of this to the investi­
gation of reactions in these systems. This field of study is likely
to be of great importance in inorganic chemistry in the future.
This book is intended primarily for the undergraduate reader —
both for the intending Chemistry Honours or R.I.C. graduate and
the non-specialist student of chemistry. We have therefore tried
to present the subject-matter in a simple and readable form,
without the inclusion of elaborate tables of properties and with
the minimum of detail necessary for comprehension. Therefore,
those working for the A- and S-level chemistry examinations
for the G.C.E. could read much of the book with profit; and the
research student who aspires to work in the field of non-aqueous
solvents will, it is hoped, find this book a useful introduction to a
fascinating branch of inorganic chemistry.
A. K. H.
A. G. M.
CHAPTER I
THE NATURE
AND SCOPE OF INORGANIC
NON-AQUEOUS SOLVENTS
WATER is such a common and therefore readily obtainable sub­
stance that it was an obvious choice as a solvent by the very early
chemists. The extraordinary versatility of water as a solvent was
soon recognized and the solubilities of many substances were
determined over the range 0-100°C. It was not surprising that
other solvents were almost completely neglected until the develop­
ment of organic chemistry produced, simultaneously, organic
substances which were often insoluble in water and organic
liquids which could be used as solvents instead of water.
The process by which a simple organic molecule, e.g. a paraffin
hydrocarbon, dissolves in (say) benzene is comparatively simple.
The relatively weak intermolecular forces between the solute
hydrocarbon molecules permit dissolution and are replaced by
solvent-solute interactions which, again, are weak; the "driving
force" leading to solution is here the change to a state of higher
entropy which the solute molecules attain by solution and hence
the solubility usually increases markedly with temperature.
By contrast, the process of solution in water is always com­
plicated, and even now it is not in general possible to make
quantitative predictions about solubilities. The complications
arise because in water there are already relatively strong, specific
and directed intermolecular forces — hydrogen bonds — which
give to the liquid water some semblance of an ordered crystalline
structure. The mechanism by which, for example, an ionic solid
1
2 NON-AQUEOUS SOLVENTS IN INORGANIC CHEMISTRY
such as sodium chloride dissolves, is not then simply a matter of
the reduction of the strong inter-ionic attractions in the crystals
by a continuous medium of high dielectric constant. The
reorientation of the solvent consequent upon solvation of both
cation and anion plays an important role in the energetics of
solution. However, covalent solids can sometimes dissolve in
water and here hydrogen bonding between solute and solvent is
generally the factor favouring solubility; solutes containing —OH
or —NH2 groups, such as alcohols, carbohydrates, amines and
amino-compounds are typical examples of this type of behaviour.
Although many non-ionic substances undergo hydrolysis in
water by an essentially bimolecular process in which a water
molecule "attacks" the solute molecule, apparent hydrolysis —
appearing as a decrease in pH — is not uncommon in solutions of
salts of multivalent ions. Here, however, the change in pH is due
to an increased dissociation of water molecules co-ordinated
around highly charged cations, i.e. the process is essentially
unimolecular.
The frequent occurrence of hydrolysis, real or apparent, does
limit the usefulness of water as a solvent; the other limitation is of
course the rather narrow liquid range which does not permit the
study in solution of substances of low thermal stability or of
species stable only at high temperatures. The use of organic
solvents to overcome either of these limitations is itself subject
to the severe limitation imposed by the very low solubility of
many inorganic substances in such solvents; for this reason,
organic liquids have not been widely investigated as solvents for
inorganic systems, although solvents such as dimethyl formamide,
dimethyl sulphoxide and the "glymes" (e.g. "diglyme", 1,2-
dimethoxyethane) are now used to an increasing extent.
The approach to the problem of finding suitable non-aqueous
solvents for inorganic chemistry has not in general been made
systematically because of the difficulties, already mentioned, which
attend any quantitative theoretical approach to the problem of
solubility. In practice, two considerations have often influenced the
choice of suitable solvent systems ; the possibility of dissociating
NATURE AND SCOPE OF INORGANIC NON-AQUEOUS SOLVENTS 3
the solute into ions and the possibility of setting up acid-base
systems in which the solvent might participate. Hence solvents
possessing a dielectric constant which is greater than about 10
and an electrical conductance, possibly due to some degree of
self-ionization, have often been chosen for study. Even a low
degree of self-ionization does not, however, preclude the use of
a solvent for ionic substances, thus, for example, dinitrogen
tetroxide, N204, whilst having a low degree of self-ionization can
still be used as an effective solvent for many salts.
It is the application of the acid-base concept which has been
the dominant theme in the development of non-aqueous solvents
in inorganic chemistry. Franklin, who did much pioneering work
with liquid ammonia, postulated the following self-ionization
equilibrium for this solvent:
+
2NH3 ^ NH4 + NH2-
compare
2H20 ^ H30+ + OH-
He observed acidic behaviour by the ammonium salts (for
example NH4C1) and the basic behaviour of alkali amides (such
as potassium amide KNH2) in liquid ammonia and was led to
formulate new acid and base definitions; viz. an acid was a sub­
stance giving a cation characteristic of the solvent and a base was a
substance giving an anion characteristic of the solvent. Otherwise,
acids and bases had their characteristic properties, for example, an
acid plus a base gave a salt and solvent; acids dissolved metals
to produce salts, and so on.
It is important to note that Franklin's definitions are not
restricted to hydrogen-containing substances; thus acid-base
behaviour arising from the following possible self-ionization
equilibria are covered by them :
Acid Base
+
liquid N 20 4 ^ NO + i l ­
liquid 2BrF3 ^ BrF2+ + BrF4~
2 2
liquid 2S02 ^ S0 + + S03 -
4 NON-AQUEOUS SOLVENTS IN INORGANIC CHEMISTRY
Whilst there is good evidence for the first two equilibria, the
third is very doubtful, but the important point is that investiga­
tions in all three liquids as non-aqueous solvents have been
influenced and often aided by Franklin's definitions as have other
investigations of hydrogen-containing solvents, for example liquid
hydrogen fluoride (Chapter IV). The weakness of the Franklin
definitions is that they are so wide as to be incapable of quanti­
tative application — we cannot use them to compare one solvent
with another in any quantitative sense.
If we consider those hydrogen-containing solvents which are
protonic, then the familiar Lowry-Bronsted acid-base theory
can be applied. Consider again the equilibria
2NH3 ^ NH4+ + NH2-
2H20 ^ H3O+ + OH-
Here the Lowry-Bronsted definition concerns the processes
NH3 + H+ ^ NH4+
H20 + H+ ^ H3O+
and the position of equilibrium in each process is determined by
the proton affinity of ammonia and water respectively. Since the
proton affinity of ammonia is greater than that of water, addition
of ammonia to an aqueous solution of a strong acid HA (present
+
as H30+ and A" ions) produces the ions NH4 and A~
NH3 + H30+ ^ NH4+ + H20
and we say that ammonia is a more basic solvent than water. But
addition of water to a solvent such as glacial acetic acid gives
immediately the reaction
HOAc + H20 ^ H30+ + OAc-
Hence here the water is the stronger base or, putting it another
way, pure acetic acid is a more acidic solvent than water. So far as
protonic non-aqueous solvents are concerned, these differentia­
tions from water — more basic or more acidic — have proved
NATURE AND SCOPE OF INORGANIC NON-AQUEOUS SOLVENTS 5
very useful guides to behaviour with inorganic solutes (see further
Chapters III and IV).
Two other acid-base concepts require mention. The protonic
solvents water and ammonia, as we have seen, can behave as
bases by uniting with a proton. This, in terms of electron-pair
bonding, requires donation of an electron pair from base to
proton. G. N. Lewis proposed an extension of this idea from
protonic acids and bases to the definition of any base as an
electron donor, and any acid as an electron acceptor. Hence, for
example, boron trifluoride is a "strong" Lewis acid because the
position of equilibrium in the reaction
BF3 + NMe3 ^ Me3N.BF3
is well to the right. The Lewis definitions are clearly applicable to
a wide range of substances, but they are not capable of quanti­
tative application in many cases. Further reference to the Lewis
definitions is made in Chapter VI.
In the Lux-Flood acid-base concept, oxide ions are regarded as
the transferable species corresponding to protons in the Lowry-
Bronsted scheme, e.g.
2 2
SO«, + O - ^= S03 ~
acid base
Note that the base gives up oxide ions, and the acid gains oxide
ions. This idea is discussed further in Chapter VI.
Experimentally, studies in a non-aqueous solvent require, first,
preparation and purification of the solvent, then means of
manipulating it, and finally methods for the observation of
phenomena in it. Purity is of prime importance, and methods of
purification (and of preparation where appropriate) will be
mentioned briefly as each solvent is discussed. Means of mani­
pulation depend upon physical properties, particularly the
melting point to boiling point temperature range, and therefore
short lists of the relevant properties of each solvent will be given.
Observations of phenomena in the "older" solvents (e.g. liquid
6 NON-AQUEOUS SOLVENTS IN INORGANIC CHEMISTRY
ammonia, sulphur dioxide) were made by classical methods;
in[the "newer" solvents (i.e. pure sulphuric acid, fused salts) such
methods are insufficient, and a wide variety of physical methods
must be used. It is worth noting, however, that the application of
the newer methods to a classical problem — the nature of metal-
ammonia solutions — has not been completely successful in
elucidating the structure of these systems, which are, nevertheless,
extraordinarily useful in preparative [inorganic and organic
chemistry.