Solvent Effects on Chemical Phenomena

479 Pages·1973·6.06 MB·English

by  Edward Amis (Auth.)

SOLVENT EFFECTS ON
CHEMICAL PHENOMENA
Edward S. Amis
James F. Hinton
Department of Chemistry
University of Arkansas
Fayetteville. Arkansas
VOLUME I
ACADEMIC PRESS New York and London 1973
A Subsidiary of Har court Brace Jovanovich, Publishers
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United Kingdom Edition published by
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Library of Congress Cataloging in Publication Data
Amis, Edward Stephen, DATE
Solvent effects on chemical phenomena.
Includes bibliographical references.
1. Solution (Chemistry) 2. Solvation. 3. Sol
vents. I. Hinton, James F., joint author. II. Title.
QD541.A54 541'.348 72-9983
ISBN 0-12-057301-6 (v. 1)
PRINTED IN THE UNITED STATES OF AMERICA
To Dr. E. A. Moelwyn-Hughes
retired general of the Solvation Army
PREFACE
The influences of the solvent on chemical phenomena in solution have been
observed over a number of years by the authors, who have been astonished at
some of the effects noted. It is intriguing that, for example, a reaction rate can
be changed by several powers of ten in magnitude by merely changing the
solvent medium in which the reaction occurs. The oscillatory behavior with
solvent composition in mixed solvents of phenomena has been encountered
numerous times, the maxima and minima in the wavelike curves for many
phenomena often occurring in the same general regions of solvent composition.
The marked influence of a trace of a second solvent on many chemical
phenomena in solution has continued to amaze the authors.
Some of these manifestations of solvent effects have been satisfactorily
explained on the basis of electrostatics, solvation of solutes, internal cohesions
of the solvents, protic or dipolar aprotic nature of solvents, viscosity, and other
understandable properties of solvent and solute. But many of these effects have
obscure origins that cannot be elucidated by measuring microscopic properties
using macroscopic probes. What is the extent of solvation of a given ion in a
given solvent ? The solvation of a particular ion depends not only on the extent
of solvation of some given reference ion in the pertinent solvent but also on the
method of measurement. What is the microscopic dielectric constant in the
neighborhood of a given ion? These and other questions on the properties of
solutions are in the "dark ages" of progress as to the correct answers. The tack
of elucidation of solvent influence on phenomena in solution suffers not only
from obscurity as to causes of such effects, but also from the enormity of the
undertaking. Nevertheless, the authors, like the proverbial fools, have rushed
IX
X PREFACE
in where angels fear to tread, and the results are recorded in this volume and
are also to be published in a second volume. We hope this effort on the
presentation of selected solvent effects on selected phenomena in solution will
at least be a beacon on the oceans of solvent effects to serve as a guide to other
sailors on the obscure and boundless mains of solution chemistry.
CONTENTS OF VOLUME II (Tentative)
1 THEORIES OF THE STRUCTURE OF SOLUTIONS
2 SOLVENT EFFECTS ON CONDUCTIVITY
3 SOLVENT EFFECTS ON ELECTROMOTIVE FORCE
4 SOLVENT EFFECTS ON POLAROGRAPHY
5 SOLVENT EFFECTS ON COMPLEX FORMATION
Chapter I
INTRODUCTION
Writing anything approaching a complete discussion of the theory of
solvation and the influences of solvents on chemical phenomena is almost an
impossible task due to its enormity, the lack of knowledge of and the agree
ment on the fundamental nature of the phenomena, and the variation in the
magnitude and specificity of the solvation or solvent effect in any one particular
phenomenon. Thus the extent of the solvation of the lithium ion may range
from zero to many molecules in solvent water depending on the nature of
the measurement used. The seeming conflict in this one phenomenon results
from the circumstance that different methods of measurement detect different
properties of the lithium ion in its relationship to water. Each of these
relationships has its own significance. For example, transference measure
ments indicate the average number of water molecules that accompany an
ion on its movement through water, even though there is constant exchange
of water molecules among the various solvent layers and between the outer
loosely bound layer and the free solvent. With respect to solvation, another
problem is that all solvation numbers of ions are relative to the solvation
of some particular reference ion which is allocated from some reasonable
assumptions a given number of molecules of solvent of solvation. These
and other related phenomena are discussed in Chapter 3.
1
2 1 INTRODUCTION
As for solvent effects on chemical phenomena, these are so numerous and
varied for any particular phenomenon that complete discussion of each effect
would lead to an enormous volume of material, as shown in Chapter 5,
where an attempt is made to give a rather extensive presentation of a variety
of solvent effects on chemical reactions rate and mechanisms.
Notwithstanding the great magnitude of the task, there should be an
organized, comprehensive, and to a reasonable extent, detailed compilation
of data and discussion on solvation and the effects of solvent on chemical
phenomena. This is true because solution chemistry is so important in life
processes, both in the plant and animal world, and because so many
laboratory and industrial chemical processes take place in solution.
For solubility and other reasons, solvents other than water prove useful in
certain chemical processes. Thus for reaction rates the solvent effect on the
rate is determined by the difference in the free energies, enthalpies and entropies
of solvation of the reactants and of the transition states. The degree of
solvation of the reactants and activated complex can influence reaction rates
markedly. As an illustration, the rates of quatenary salt formations are over
104 times as great in nitrobenzene as in benzene since the intermediate com
plexes in these reactions are highly solvated by nitrobenzene but not by
benzene, the activity coefficient of the complex is much less, and the entropy
of its formation much greater in nitrobenzene than in benzene. The individual
solvent activity coefficients of anions and cations, like individual solvation
numbers, cannot be determined directly. The problem has been approached
by making certain extrathermodynamic assumptions based on reasonable
foundations. This problem will be discussed fully in Chapter 5. Added to
these thermodynamic and solvation effects of the solvent on reaction rates are
ionization, hydrogen bonding, solvalysis, electrostatics, viscosity, internal
pressure, cage, and various other effects. In fact the solvation effects themselves
are complicated by inner-shell solvation, outer-shell solvation, negative-
solvation, dynamic solvation, and other phenomena such as the replacement
of molecules in the solvation shells of solute particles of the molecules of one-
solvent component by another solvent component in the case of mixed
solvents, and the specific solvation of solute particles by one component in a
mixed solvent.
In mixed solvents, periodicity of chemical phenomena often occur. Various
explanations of such observations are extant. In particular maxima and
minima are observed in many chemical phenomena in the neighborhoods of
15 and 80 weight % of the organic component in aqueous-organic solvents.
Sometimes solvent mixtures containing no solute show periodicity in certain
properties with change in solvent composition. Again explanations are in
the literature.
The water proton chemical shift in aqueous jV-methylacetamide, N-
INTRODUCTION 3
methylformamide and 7V-ethylformamide has been observed to undergo a
periodic shift depending upon the amide concentration. The initial addition
of the amide to water produces a small shift to low field. This low field shift
is felt to be the result of the amide molecules behaving as predominently
interstitial species causing the water-water hydrogen bonds to bend less and
appear to be stronger. The low field shift reaches a maximum at about 0.2
mole fraction amide. The subsequent addition of the amide to water produces
a net high field shift in the water proton resonance position indicative of the
breaking of solution structure. At about 0.8 mole fraction of amide the water
proton resonance shifts rather drastically downfield accompanied by the
broadening of the water resonance line width. These last two effects would be
indicative of a restructuring process in which water appears to be tightly bound.
The oscillatory effect observed in the relative solvation of the two ions in
binary electrolytes when water-alcohol is the solvent is explained by the
successive replacement of the water in the different solvation layers of the two
ions beginning with the outermost solvation layer of the more highly solvated
ion. These and other oscillatory effects in chemical phenomena are presented
extensively in Chapter 4.
Abrupt changes in phenomena are sometimes manifested when even
minuscule amounts of one solvent component is added to a solution con
taining another solvent component. When the particular property is plotted as
ordinate versus the solvent composition, there is almost a verticle drop in the
property which may amount to over 30 %. This is the case with the equivalent
conductance of perchloric acid in anhydrous ethanol when as little as 0.3
weight % of water is added to the solution. For processes in galvanic cells
similar abrupt changes occur in ion-size parameters, salting-out coefficients,
and thermodynamic functions when minuscule amounts of water are added
to the anhydrous methanol solvents in such galvanic cells as those represented:
Pt, H |HI(m), X%CH OH, Y%H 0| Agl-Ag
2 3 2
Pt, H |HBr(/n),X%CH OH, Y%H 0| AgBr-Ag
2 3 2
Such phenomena were explained on the basis of the breaking down of solvent
methanol chains and the selective solvation by solvent water of the protons.
Chapter 4 gives the details of such observations.
Another intriguing result of mixing solvents is the catalytic effect exhibited
by certain substances on certain reaction rates in mixed solvents which are not
manifested when the reaction is observed in one of the pure solvent com
ponents. Thus Cu2+ and Hg2+ ions show no catalytic effect on the electron
exchange reaction
U(IV) + Tl(III) -> U(IV) + T1(I) (1.1)
when the solvent is pure water, but these ions produce about a sevenfold
4 1 INTRODUCTION
increase in the rate of this reaction whfcn the solvent is 25% water-75% by
weight methanol. This enormous increase of the rate of this reaction in this
solvent is not common to all cations. Even without the presence of the catalyst
the rates, the mechanism, and the thermodynamic quantities for this reaction
were changed in the 25% water-75% methanol as compared to the water
solvent. The catalytic effect of the Cu2 + and Hg2 + ions on the rate were
explained by a stepwise mechanism for the reaction in the 25% water-75%
methanol solvent. Difference in the solvolytic processes resulting in the
formation of different complex intermediates was proposed to explain the
differences in the rates and mechanisms of the reaction in the different solvents.
This reaction is discussed in Chapter 5, and the mechanisms assumed in the
two solvents are presented. However, the stepwise mechanism for the catalytic
effect of the Cu2+ and Hg2+ ions are not present in Chapter 5, and is, there
fore, illustrated using Hg2+ ion as the example:
U(IV) + Tl(III) -+ U(V) + Tl(II) (1.2)
U(V) + Hg(II) -> U(VI) + Hg(I) (1.3)
Tl(II) + Hg(I) -> T1(I) + Hg(II) (1.4)
These cation effects indicate that the reaction proceeds in two one-electron
steps in 25% water-75% methanol, instead of one two-electron step as was
found in water. This could be due to a solvent cage effect in which the hydrogen
bonding in the water-methanol is weaker than in the water solvent thus
permitting the reactants to diffuse out of the cage more readily in the former
solvent for reaction with other species.
Many aspects of solvent effects are yet to be satisfactorily explained, though
some of the phenomena have been verified by laboratory experiments, and
have been amenable to theoretical treatment. Empirical approaches have
been used to supplement theoretical methods and to explain specific effects not
accounted for by theory. These empirical approaches are not to be ridiculed
since they in many cases correlate much data in useful manners. Differences of
opinion exist as to the correct interpretation of many solvent effects. To
separate, even in a qualitative manner, the various influences at work in many
instances is quite frequently impossible and the methods used may be
questionable. In some cases one of many concurrent effects dominates and a
particular theory can be successfully applied. One dominant effect, among
several concurrent ones that tend to allay or moderate each other, may be
the explanation of why some theories apply as often as they do. Much is yet
ufirevealed about solutes and solvents in the solution state, or about solvent
components in mixed solvents. "Now we see through a glass darkly."
Especially is there on the edge of night, or even the dark of night, micro
scopic regions around the solute particles. What is really known about