Chemical Reactions in Solvents and Melts

526 Pages·1969·9.265 MB·English

by  G. Charlot and B. Trémillon (Auth.)

CHEMICAL REACTIONS IN
SOLVENTS AND MELTS
G. CHARLOT
Professor in the Faculty of Science, Paris
AND
B.TREMILLON
Assistant Professor in the Faculty of Science, Paris
TRANSLATED BY
P. J. J. HARVEY, B. Sc, Ph. D.
Petro Carbon Developments Limited, Manchester
3&?
*
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Copyright © 1969
Pergamon Press Ltd.
First English edition 1969
This book is a translation of Les
Reactions Chimiques dans les Solvants
et les Sels Fondus. Copyright © 1963
Gauthier-Villars Editeur, Paris
Library of Congress Catalog Card No. 68-8529
Printed in Germany
08 012678 2
PREFACE
THE use of inorganic and organic compounds and of melts as solvents becomes
more and more important each year because of the immense possibilities of new
reactions. Applications in inorganic and organic chemistry and electrochemistry
become more numerous as knowledge in these fields progresses.
The logical arguments found to work so well in the case of dilute aqueous so-
lutions can be easily applied to polar solvents analogous to water, and, with
certain precautions, to all other solvents including melts. This is what we have
mainly endeavoured to set out clearly in this book.
With this aim in mind, we have limited ourselves almost exclusively to reac-
tions in solution, leaving aside preparative chemistry by precipitation or crys-
tallization and simple reactions in the immense field of organic chemistry.
We have considered only the cases where equilibrium is reached, and have
considered carefully the important cases of slow reactions. Physico-chemical con-
siderations have only been developed in so far as they give results which can be
used directly by chemists. Similarly, problems of structure have scarcely been
touched.
Only the chemical reactions are described. The electrochemical properties are
used only when they allow us to predict oxidation-reduction properties; thus
electrometallurgy, particularly in melts, has not been discussed.
In the field thus defined we have selected the most interesting properties, es-
pecially for the analytical chemist. The plan of the book is as follows:
In Part I we describe general properties, considering successively the various
types of reactions: acid-base reactions, considered purely as proton transfer,
complexes, oxidation-reduction and precipitation, differentiating between two
classes of solvents, namely those in which ionic dissociation can take place and
those in which it is negligible.
In Part II we have described the properties in each solvent in so far as it has
been possible to co-ordinate these properties. We have given all the equilibrium
constants which we have been able to gather together, grouping them in tables;
for analysts we have indicated the titrations possible in different^olvents and
mixtures of solvents.
We have gathered together and given the corresponding bibliography (about
5000 references).
We would like to thank those in our laboratory who have made their contri-
bution to research in organic solvents—Mmes C. Bertin-Batsch, J.Badoz-
Lambling, MM. J.P.Wolff, J.Desbarres, J.P.Billon; and in melts, MM.G.De-
larue, R.Molina and M.Leroy, and also our colleague, J.Saulnier, who has
helped us with the literature search.
G.CHARLOT and B.TREMILLON
vn
INTRODUCTION
THE use of various solvents offers numerous possibilities as the chemical pro-
perties can be modified when the solvent is changed. Not only are acid-base
equilibrium constants and the constants appropriate to oxidation-reduction,
complexes and solubility changed, but also the chemical species which exist
in water may cease to exist in other solvents and, on the other hand, other
species can appear; new oxidation states, acids and bases, different complexes.
This offers considerable possibilities.
The number of pure solvents used is already large if one considers the sub-
stances which are liquids at ordinary temperatures; but it is possible to work at
higher temperatures, for example in melts, or at low temperatures, for example
in liquid ammonia.
In another way, the number of solvents can be made infinite by using mix-
tures, and thus the properties can be varied in a continuous manner.
In the case of each solvent a chemistry (and an electrochemistry) as important
as that for water can be set up.
POLAR AND NON-POLAR SOLVENTS AND IONIZED SOLVENTS
Ionization and Ionic Dissociation
First of all we recall, in a very simplified form, some general ideas on solva-
tion, then on the ionization of substances in solution.
The solvation of a dissolved substance, ion or molecule, is the action of the
solvent on this substance. The solvation (solution) equilibrium of a molecule M
in a solvent S is:
M + pS ^ M, pS
Similarly, the solution of a substance by ionic dissociation gives in solution sol-
vated ions, such as Na+,pH 0 and Cl",qH 0 for NaCl in water.
2 2
From the practical point of view it is not necessary to know the exact formula
of the solvated species. The solvation is completely characterized by a certain
reaction energy, the energy of solvation, which is related to all the bonds of the
solvated molecule (or ion) with the solvent molecules which surround it. But,
in general, to simplify the text, we will represent the solvated species by simple
indication of the particle: thus we will write M, Na+, Cl~ instead of M,pS,
Na+,pH 0, Cl-,qH 0.
2 2
The combination of solvent molecules with ions or molecules depends on
their nature and we know that, notwithstanding structural considerations, it is
3
4 CHEMICAL REACTIONS IN SOLVENTS AND MELTS
not always possible to predict the stability of different substances. Similarly,
the solubility of different substances is, in general, not predictable. It is only in
the case of solvents of similar structure that one can predict that the combination
will be analogous. Thus the properties of a single substance can vary with the
nature of the solvent due to the solvation phenomena.
The action of solvent on a substance dissolved in it can be arbitrarily divided
into two. In one part, the solvent molecules can attack the bonds uniting the
constituent elements of the substance. Thus, if the bond of a molecule A—B is
covalent it can be made partially or totally ionic A+~B, if the solvent stabilizes
A (or B) (solvolysis reaction); the formation of a link between A (or B) and the
solvent modifies that between A and B. The action of the solvent can therefore
involve on a large or small scale the ionization of the substance if it is not already
totally ionic in form:
AB + «S^SA+B"S or AB^A +B"
[AB]
= &
[A+] [B-]
where SA+B"S represents the solvent action on AB and K is the solvolysis
s
constant.
In the other part, the dielectric power of the solvent environment has an effect
on the relative freedom of the ions present. This is ionic dissociation.
SA+B"S ^ SA+ + B"S,
which can be written.
A+B- = A+ + B~
and
[A+] [B-]
[A+B~] = AD
K is the ionic dissociation constant.
D
The overall dissociation constant of the substance AB is defined by
[A+] [B-] K
D
Ar = [AB] + [A+B-] 1 + K
s
Ion-Pairs and Complexes
ION-PAIRS. Let us consider a compound AB, composed of two ions A + and
B" held together by purely electrostatic attraction (pure ionic bond), and let us
put the substance into surroundings of dielectric constant e. The energy of the
bond between A+ and B", which is here the attraction energy between the two
charges of opposite sign, is a decreasing function of e; the separation of the two
ions is easier the greater the dielectric constant of the solvent.
The dissociation equilibrium set up is
A+B"^A+ + B",
INTRODUCTION 5
A+B", A+ and B~ representing the solvated species. The purely ionic com-
pound A+B~, which we suppose to be soluble, now consists of the electrostatic
association of the two solvated ions A+ and B"; it is called an "ion-pair". The
ion-pair is more easily dissociated into free ions the larger the dielectric constant
of the solvent; consequently the constant of ionic dissociation:
_ [A+][B-]
K
[A+B"]
is greater the larger the dielectric constant of the solvent.
For solvents of low dielectric constant, practically no free ions exist in
solution. On the other hand, for solvents of high dielectric constant there are
practically no ion-pairs, these being almost completely dissociated into free
ions.
A theory, developed by Bjerrum, later completed and amended, allows the
calculation of the values of the dissociation constant of an ion-pair (where
pK = —log K ) as a function of e provided the minimum approach distance
D D
of the two solvated ions constituting the ion pair is known. The curves in
Fig. 1 represent the variation of pK calculated by this method as a function
D
of e for three values of the minimum approach distance of the ions. On the same
figure are values of pK, experimentally determined for some salts (see the tables
0 5 10 15 20 25 30 35 e
Dielectric constant
FIG. 1. Variation of the ionic dissociation constant of tetrabutylammonium picrate
(□) and tributyl ammonium picrate ( x ) as a function of the dielectric constant of
the solvent. The curves represent the theoretical variation of pK calculated for ion
D
pairs having sums of ionic radii of: 4 A (—), 5-6 A (—), 8 A ( ) (see the dissocia-
tion constants for each solvent).
6 CHEMICAL REACTIONS IN SOLVENTS AND MELTS
in Part II for a more complete collection of experimentally determined ionic
dissociation constants).
A simpler relationship than that of Bjerrum, which has been verified ap-
proximately in a large number of cases, was established by Denison and Ram-
sey21 and Gilkerson23:
«JT nJT 4. 0A3Ne2 Z ^ l
pA = pA + — —
D 0
RT fi + f e
2
where z and z are the absolute values of the respective charges on the two
x 2
ions; r and r their radii (r + r = minimum distance of approach); e, the
1 2 ± 2
charge on an electron; N Avogadro's number; R the ideal gas constant; Tthe
absolute temperature; and pK a constant (very often omitted)
0
REAL COMPOUNDS, COMPLEXES. In fact, the ion pair is an ideal case because
it is rare that the bond between A and B is purely electrostatic. Consequently,
the substance is less dissociated than the corresponding ideal ion pair. The value
of the overall dissociation constant pK is generally higher than the value pre-
c
dicted for an ion pair. If pK is close to the theoretical value pK , the compound
c D
can be practically considered as an ion-pair; this is generally the case for
quaternary ammonium salts for example. But if the experimental value is
significantly greater, the compound must be considered as a "complex" between
A and B.
According to experimental data, it is established that alkali metal salts are
clearly less dissociated than quaternary ammonium salts, indicating a partially
covalent bond in the salt. We will therefore consider the alkali metal salts and, to
an even greater extent, the other metal salts, as capable of behaving as complexes
in certain solvents. The stability of the complex depends, as stated above, on
the influence of the solvent because of its solvation properties.
Equally it is established that amine salts, e.g. R NHX, are often clearly less
3
dissociated into R NH+ and X than ion pairs of comparable dimensions, prob-
3
ably due to a hydrogen bond holding the two ions together and making the
associated compound more stable than a simple ion pair.24
To summarize, the ideal case of ion-pairs allows us to estimate which com-
pounds are the most strongly (ionically) dissociated (smallest values of pK ) in
D
each solvent, each compound being less dissociated or, at the most, as disso-
ciated as can be predicted by this method.
Non-Polar and Polar Solvents
Apart from the specific chemical action of each solvent on the compounds
which dissolve in it, the usual solvents can be placed in two categories.
Generally, ionic dissociation is very low in solvents with a low dielectric con-
stant. For practical purposes in solvents with a dielectric constant less than
about 10 to 15, the fraction of a dissolved compound which has dissociated
into ions can be neglected. Simplifying the argument considerably we will call
non-polar those solvents which are in this category: aliphatic and aromatic
INTRODUCTION 7
hydrocarbons, chloro- anddichloro-benzenes, chloroform, carbon tetrachloride,
dioxan, acetic acid, dichloroethanes, pyridine, etc.
We will call polar those solvents where the dissociation of non-complex
compounds is practically complete. Their dielectric constant is greater than
about 40: formic acid, amides, water, sulphuric acid, etc.
For solvents with a dielectric constant between 15 and 40, the dissociation
of ionic or ionized compounds is partial, important without being complete:
methanol, ethanol, nitrobenzene, nitromethane, acetonitrile, acetone, dimethyl-
formamide, liquid ammonia, etc. The simple rules which hold in the case where
dissociation is practically complete must be extended to deal with these solvents.
Note. In non-polar solvents, the existence not only of ion pairs but also of more important
ion aggregates, triplets and quadruplets has been proved.
{ +- +- +
AB + A+ ^ ABA
+ - - + -
AB + B~ ^ BAB
+ - +-
2AB^(AB), etc.
2
But the dissociation constants of these aggregates are generally large enough for us to
neglect their formation to a first approximation, their concentration being small compared to
that of the "ion pairs". Their formation takes place only in solvents with a very low dielectric
constant and generally non-polar (benzene, etc.). Constants are given for several solvents.
Other phenomena (hydrogen bonding) can bring about the association of molecules in non-
polar solvents (see Chapter 1, Acid-Base Reactions in Non-polar Solvents).
Ionized Solvents
Certain solvents, some melts (halides, sulphates, nitrates, carbonates, alkalis,
etc.) are almost completely ionized, e.g. the eutectic melt LiCl + KC1 consists
of the ions Cl", Li+ and K+. We will call these particular solvents ionized
solvents.
The rules worked out above for ion pairs do not apply in this case as they
are only valid for surroundings in which the ionic concentration is sufficiently low.
Although the dielectric constant of ionized melts is generally small (of the order
of 2 to 3) they behave as polar solvents due to the possibility of exchange of
ions of a solute with ions of the same sign provided by the solvent. In effect, the
electrical neutrality remains the same everywhere, whatever the nature of the
ions present.
Concentrated solutions of salts, acids or bases, notably in water, behave as
ionized solvents.
REFERENCES
Association of Ions in Solvents with Low Dielectric Constants
Ion pairs
1. C.A.KRAUS and W.C.BRAY, /. Am. Chem. Soc. 35, 1315 (1913).
2. A.SCHANOV, Z. Physik. Chem. 83, 129 (1913).
3. N.BJERRUM, Kgl. Danske Videnske. Selskab 7, No. 9 (1926).
8 CHEMICAL REACTIONS IN SOLVENTS AND MELTS
4. A.R.MARTIN, /. Chem. Soc. 3270 (1928).
5. P. WALDEN, Z. Physik. Chem. A174,1 (1930).
6. R.M.Fuoss and C.A.KRAUS, J. Am. Chem. Soc. 55, 476, 1019 (1933).
7. R.M.Fuoss and C.A.KRAUS, /. Am. Chem. Soc. 55, 2387 (1933).
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10. R.M.Fuoss, Physik. Z. 35, 59 (1934).
11. R.M.Fuoss, Trans. Faraday Soc. 30, 967 (1934).
12. R.M.Fuoss, Chem. Revs. 17, 27 (1935).
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14. A. R. MILLER, Trans. Faraday Soc. 35, 691 (1939).
15. C.A.KRAUS, /. Phys. Chem. 43, 231 (1939).
16. C.A.KRAUS, Ann. N.Y. Acad. Sci. 51, 789 (1949).
17. R.M.Fuoss and T.SHEDLOVSKY, /. Am. Chem. Soc. 71, 1496 (1949).
18. J.G.KIRKWOOD, /. Chem. Soc. 18, 380 (1950).
19. E.GRUNWALD, Anal. Chem. 26, 1696 (1954).
20. R.M.Fuoss and L.ONSAGER, Proc. Nat. Acad. Sci. N.Y. 41, 274, 1010 (1955).
21. J.T.DENISON and J.B.RAMSEY, /. Am. Chem. Soc. 11, 2615 (1955).
22. T.SHEDLOVSKY and R.L.KAY, /. Phys. Chem. 60, 151 (1956).
23. W.R.GILKERSON, /. Chem. Phys. 25, 1199 (1956).
24. C.A.KRAUS, /. Phys. Chem. 60, 129 (1956).
25. S. A. RICE, /. Am. Chem. Soc. 78, 5247 (1956).
26. R.M.Fuoss, /. Am. Chem. Soc. 79, 3301 (1957).
27. R.M.Fuoss and C.A.KRAUS, /. Am. Chem. Soc. 79, 3304 (1957).
28. C.W.DAVIES, in The Structure of Electrolytic Solutions, W.J.Hamer, ed., Wiley, 1957.
29. H.S.HARNED and B.B.OWEN, The Physical Chemistry of Electrolytic Solutions, Reinhold,
3rd edition, 1958.
30. P.H.FLAHERTY and K.H.STERN, /. Am. Chem. Soc. 80, 1034, 2615 (1958).
31. R.M.Fuoss, /. Am. Chem. Soc. 80, 5059 (1958).
32. K.H.STERN and E.S.AMIS, Chem. Revs. 59, 1 (1959).
33. R.M.Fuoss, Proc. Nat. Acad. Sci. N.Y. 45, 807 (1959).
34. R.A.ROBINSON and R.H.STOKES, Electrolyte Solutions, Butterworths, 2nd edition,
1959.
35. R.M.Fuoss and F.ACCASCINA, Electrolytic Conductance, Chs. XVI-XVIII, Interscience,
1959.
36. E.A.RICHARDSON and K.H.STERN, /. Am. Chem. Soc. 82, 1296 (1960).
37. K.H.STERN and E.A.RICHARDSON, /. Phys. Chem. 64,1901 (1960).
38. E.C.BAUGHAN, /. Phys. Chem. 64, 1951 (1960).
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Ion triplets.2*7'9*29'3*'3*
Ion quadruplets.2'13'29'36'37
Conductimetric determination of the dissociation constants of ion pairs, triplets and quadru-
plets—see conductimetric measurements in Chapter 1.
Values of the dissociation constants of ions pairs, triplets and quadruplets—see tables for
each solvent.
THE FUNDAMENTAL TYPES OF REACTIONS
Quantitative predictions can be made only when comparing reactions which
involve the same particle (same type of bond). This is why we will examine
successively the following:
INTRODUCTION 9
1. ACID-BASE REACTIONS, i.e. all the reactions involving only the proton H+.
2. COMPLEX FORMATION REACTIONS with different species Cl", Br~~, O2", etc.
3. OXIDATION-REDUCTION REACTIONS involving the electron. These can be
brought about chemically or electrochemicallyt. The particular action of
each solvent will be considered according to its behaviour vis-a-vis the
species considered. Real reactions can simultaneously involve several mech-
anisms: acidity and complex formation; complex formation; and oxida-
tion-reduction, etc.
Notes. 1. Activities and concentrations. Activity coefficients. We will adopt, as is customary,
the definition of the activity of a substance in a solvent, analogous to the definition used in
water: the activity coefficient tends to 1 when the ionic force tends to zero. For dilute solu-
tions, activities are the same as concentrations although the activity coefficients often change
very quickly in certain solvents with increase in concentration. To simplify the arguments, we
shall suppose, in what follows, that this approximation is valid.
Consequently we will generally represent both the concentration and the activity by the sign
[].
2. The solute concentration is generally expressed as its molarity (number of gram-molecules
of solute per litre of solution). The molality (number of gram-molecules of solute in 1000 g
of solution) or the mole fraction (number of gram-molecules of solute per gram-molecule of
mixture of solvent and solutes) can be used as well. In the book all the concentrations are
expressed as molarities unless otherwise indicated.
t For electrochemical reactions, refer to G. Chariot, J.Badoz-Lambling and B.Tremillon,
Electrochemical Reactions (Masson, 1959), and Electrochemical Reactions (Elsevier, 1962),
where the use of various solvents in electrochemistry is dealt with in Chapter 13.
la CRS