Ions and Ion Pairs and their Role in Chemical Reactions. Invited Lectures Presented at the Symposium on Ions and Ion Pairs and Their Role in Chemical Reactions, Syracuse, NY, USA, 30 May–2 June 1978

134 Pages·1979·6.536 MB·English

by  J. Smid (Eds.)

INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY
lUPAC Secretariat: Bank Court Chambers, 2-3 Pound Way,
Cowley Centre, Oxford OX4 3YF, UK
Organizing Committee
Honorary Chairman: G. Smets
Chairman: J. Smid
Secretary: P. Caluwe
Members: D. J. Cram
G. Levin
J. Meyer
P. Rempp
C. Schuerch
K. J. Smith
V. T. Stannett
G. Vincow
S. I. Weismann
International Union of Pure and Applied Chemistry
(Physical Chemistry Division)
in conjunction with
State University of New York College of Environmental Science
and Forestry
National Science Foundation
Ions and Ion Fairs and their Role
in Chemical Reactions
Invited lectures presented at the
Symposium on Ions and Ion Pairs and their Role in
Chemical Reactions,
Syracuse, NY, USA,
30 May - 2 June 1978
Symposium Editor:
J.'Smid
SUNY College of Environmental Science
and Forestry, Syracuse, NY
Dedicated to Professor Michael Szwarc, F.R.S.
PERGAMON PRESS
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The contents of this book appeared in Pure and Applied Chemistry
Vol. 51, No. I (1979)
Printed in Great Britain by A. Wheaton <£ Co. Ltd., Exeter
ISBN 0 08 022355 9
Fuve & Appl. Chem., Vol. 51, p. 49.
Pergamon Press Ltd. 1979. Printed in Great Britain.
PREFACE
This volume contains the invited lectures presented at the International Symposium on
Ions and Ion Pairs and their Role in Chemical Reactions held in Syracuse, New York from
May 30-June 2, 1978.
The conference,jointly sponsored by lUPAC, the National Science Foundation and the State
University of New York College of Environmental Science and Forestry, Syracuse, New York
was dedicated to Professor Michael Szwarc on the occasion of his retirement from the
College, honoring him for his many original and outstanding contributions to a better
understanding of ionic solution phenomena.
The lectures cover a significant part of the broad spectrum of ion pairing phenomena.
They focus on three main areas. Firstly, the structure, energetics and dynamics of ions
and ion pairs and their complexes with solvent molecules and ion coordinating ligands in
the liquid, solid and gaseous state are discussed by Grunwald, Kebarle, Persoons,
Van Beylen, de Boer and Velthorst. A second emphasis is on some aspects related to the
design and properties of neutral ion binding ligands, notably the crown ethers and
cryptands, as well as polymers carrying crown ligands as pendent groups (Popov, Smid).
The lecture of Dr. Lehn on the design of molecular receptors, believed by many participants
to be the highlight of the symposium, is not included in this volume, but a good part of
Lehn's presentation may be found in the lUPAC issue Coordination Chemistry, Volume 17,
1977.
The third area covers aspects of reactivity and stereochemistry of ions and ion pairs in
different types of chemical reactions such as those involving carbocations (Dorfman),
enolates (Arnett), carbanions (Hogen Esch), anion-cation combinations (Ritchie), and
cationic polymerization (Ledwith).
Johannes Smid
Pure & Appl. Chem,^ Vol. 51, pp. 51-52.
Pergamon Press Ltd. 1979. Printed in Great Britain.
DR. MICHAEL SZWARC, F.R.S.
Distinguished Professor, State University of New York
College of Environmental Science and Forestry, Syracuse, New York
As research professor at the College of Environmental Science and Forestry for the past
25 years, Michael Szwarc has had a free hand pursuing his great love: chemical research.
When he announced his retirement from the College few believed that his very productive
scientific career had come to an end. His ideas are still fresh and provocative; there is
still that keen perception of the problem, its careful analysis, the subsequent elegant
design of experimental procedures to solve the problem, and the excitement when the solution
is found and communicated to others.
Being an excellent experimentalist, Michael Szwarc always had a knack of coming up with
systems that would yield the maximum amount of return for the work done, avoiding confusing
complexities that would render data interpretation more difficult. Examples of this are
his early work on bond dissociation energies using the elegant toluene carrier technique
and his extensive investigations on radical addition and abstraction reactions. The work
that truly electrified chemists, especially those in the polymer field, was his studies
on electron transfer-initiated polymerization of monomers leading to living polymers. He
quickly realized the implications of these findings: the possibility of synthesizing
narrow molecular weight polymers as well as tailor-made block and graft copolymers. Those
who worked with Michael Szwarc during the living polymers "era" still remember his excite­
ment as he peered with sparkling eyes into the vacuum tubes of living polymers, watching
the colors come and go as newly added monomers were devoured by the evergrowing and hungry
living ends. Such were the emotions that a new quasi-scientific terminology was created.
52 JOHANNES SMID
Shouts of "let's kill the living polymers" or "I just resurrected a dead polymer" or signs
like "feeding of living polymers only permitted by management" could be heard and seen. The
studies, important from both an academic as well as industrial point of view, stimulated
investigators in laboratories throughout the world to study the properties of the new
products.
The career-long interest of Michael Szwarc in the mechanisms of chemical reactions led him
from the living polymer work into his still ongoing studies on the behavior of ionic solu­
tions in low polarity media. His many and original contributions to an understanding of
the fundamental processes underlying the often complex character of ionic processes in
these solvents become obvious on reading the two volumes Ions and Ion Pairs in Organic
Reactions which he edited. Through the application of various techniques, Michael Szwarc
and his coworkers have provided us with a wealth of information on ion pair reactions that
has ramifications in many branches of chemistry.
At an age when most people call it quits, Michael Szwarc has not given us a hint that he
will. He still gets a kick out of learning new things. His footsteps in the laboratory
are still quicker than those of his younger colleagues, and his face still is the best
barometer as to whether an experiment worked or failed. There is never a dull moment
being around Michael Szwarc. Those who have had the privilege of working with him have
greatly benefited from his enthusiasm for scientific research, his skill as an experi­
mentalist, his broad knowledge and understanding of chemical phenomena and his willingness
to communicate his discoveries and insights to all who wanted to listen. For this we are
deeply grateful to him.
Johannes Smid
Pure & Appl, Chem,^ Vol. 51, pp. 53-61.
Pergamon Press Ltd. 1979. Printed in Great Britain.
STRUCTURE OF SOLVATED ION PAIRS FROM ELECTRIC DIPOLE MOMENTS
Ernest Grunwald
Department of ChemistryBrandeis University, Waltham^MA 02154, USA
and Ting-Po I
Central Research, Pfizer Inc., Groton, CT 06340, USA
Abstract - Theory and complications involved in dipole moment
measurements in slightly conductive polar solvents are dis­
cussed. Experimental results are presented for the following
electrolytes in anhydrous acetic acid: LiCl, KCl, KBr, LiTs,
KTs, CsTs, TlTs, R4NNO3, BiiTs, Ts = p-toluenesulfonate; BH =
p-toluidinium; R4N = tetra-isoamylammonium.
Except for R4NNO3, the dipole moments indicate that solvation
complexes are formed between the respective ion pairs and
acetic acid molecules. Comparison of dipole moments of the
solvation complexes with intrinsic dipole moments of the ion
pairs yields information about the structure of the solvation
complexes.
INTRODUCTION
The value of electric dipole moments for elucidating structure and bonding
in ion pairs is well known from measurements in the gas phase and in non-
polar solvents (Refs. 1-4) . However, because of limitations of volatility
and solubility, the number of ion pair species that can be studied in this
fashion is limited. Measurements in polar solvents offer the advantage of
greater solubility and, furthermore, will detect the fonnation of solvation
complexes.
There are formidable difficulties, however. Ion pair dissociation is no
longer negligible, and the presence of free ions introduces complications
of measurement and interpretation (Ref. 5) . For high-precision measurements
of solution capacitance at audio-frequencies, the conductivity should be
less than about 10"^ ohm"l cm"^ (Ref. 6). High precision is essential be­
cause one needs to interpret the small difference Δε = ε-εο:» where ε is the
dielectric constant of the solution and εο that of the solvent. Moreover,
the presence of ions complicates the interpretation of the electrical capa­
citance of the measuring cell, for two reasons: 1) It becomes more difficult
to subtract the capacitance due to the electrical double layer at the elec­
trode (Ref. 5). 2) The conducting ions contribute significantly to the capa­
citance because the ionic current is out of phase with the applied AC volt­
age (Refs. 7-8) . Correcting for this effect unfortunately introduces an
adjustable parameter for each electrolyte.
If there is solvent dipole correlation, the introduction of any solute (polar
or nonpolar) may interfere with that correlation (Ref. 9) . Thus the dielec­
tric constant ascribable to the solvent component may vary with solute con­
centration. This solute-induced medium effect can be treated rigorously
only in rare cases when the molecular interactions are understood in detail.
In most cases, the solute-induced medium effect must be evaluated by empiri­
cal methods (Ref. 9) .
Finally, in polar solvents, basic theory of dielectrics is not as easy to
apply. Although the Kirkwood theory in its general form (Refs. 10^11) is
54 ERNEST GRUNWALD and TTNG-PO I
accurate, when the theory is applied to a specific solvent, simplifying as­
sumptions must be made. This is because the general theory is a statistical
theory whose application requires detailed microscopic information about the
dielectric medium. In the absence of such information, a microscopic model
must be assumed to provide the needed link with macroscopic reality. In
spite of these difficulties, we hope to show that results of meaningful ac­
curacy can be obtained for ion pair dipole moments in polar solvents.
In the present discussion, we shall be concerned primarily with ion pairs in
anhydrous acetic acid. This solvent is of special interest because isomer­
ism between intimate and solvent-separated or "loose" ion pairs was first
demonstrated by means of kinetic studies carried out in acetic acid (Ref.12).
EXPERIMENTAL VARIABLES
It is convenient to represent the measuring cell by an equivalent AC circuit
consisting of a conductance G in parallel with a capacitance C. The meas­
ured complex AC admittance Y"^ is equal to G + jijuc, where UÜ = 2nf, f = AC
frequency, and = -1. To obtain a quantity that is independent of the
dimensions of the measuring cell one next calculates the admittancy = γ·><·.
t/A. The cell constant ¿/A is the ratio of the current path length to the
cross-sectional area. Eq. (1) represents the admittancy in complex notation,
y^^ = y' + jo^y^' (1)
where y' and γ" denote the respective conductive and capacitive components
of y^^; Y' = G¿/A and γ" = CI/A.
For a conductive dielectric the admittancy is related to the ionic conduc­
tivity L^^ and the dielectric permittivity via Eq. (2) ,
y^^ = L^^ + jαJε^<··8.85 χ ΐο"-*"^ (2)
-14
in which the constant, 8.85 χ 10 , denotes the permittivity of empty space
in practical cgs units of Farad cm'^. Because there is a phase difference
between the AC voltage and the ionic current, L^*" can be written in complex
notation,
= L' + jL'' (3)
where L' is the component of L"^ that is in phase with the voltage. Simi­
larly, because of dielectric loss, the dielectric permittivity can be
written in complex notation.
ε' - je^ (4)
Combining (2), (3) and (4) one obtains,
y^ = (L' + me"'8.85 χ lo'"^"^) + j (L'' + aje'.8.85 χ lo""^^) (5)
Thus the complex component of the admittancy after due correction for double-
layer capacitance (Ref. 5) , links the experimental to theoretical variables
via (6) .
y'' = Cl/A = 8.85·1θ'-'·'^ε' + LVUÜ (6)
At audiofrequencies, ε' is simply the dielectric constant ε.
L'V(8.85 X 10" 1^ uü) has been called "the effect of the moving ions on the
dielectric constant" and denoted by (Refs. 5,7,13).
The component of the ionic conductivity, in solutions containing free
ions and ion pairs, is given by the theory of Onsager and Provencher (Ref.7) .
Under conditions where the free-ion fraction γ of the electrolyte is small,
the general expression reduces to the simple form (7) (Ref. 13), which ap­
plies to the measurements reported here.
Structure of solvated ion pairs from electric dipole moments 55
L''^ 6.20 X 1Q"^ €OI^(1 + a[l + y^]^ - [2aR]^) (7)
^ (£oT)^''^[2aR]*(aV + Π-α]^)
I = ionic strength, Yc2
Go = dielectric constant of pure solvent
α = (1 + 2β)/2; β is a parameter (see below)
y = 8.85 X lO"-"-"^ uüGo/(2L'a)
R = 1 4- (1 + y^)*
Physically, the parameter β represents the ratio k /AJ» where k is the actual
rate constant for ion pair association, and A is theoretical rate constant
for diffusion-controlled combination of opposite univalent point charges
(Refs. 7,8). Because of this, physically plausible values for β lie in the
range from 0 to 1.
In the present work it was sufficient to use a uniform value of β = 0.5 for
all electrolytes (Ref. 13) . With L'VUU thus determined, straightforward ap­
plication of Eq. (6) to audiofrequency data yielded a value of the dielectric
constant e' = e.
CALCULATION OF DIPOLE MOMENTS
In current practice there are two main approaches to the calculation of molec­
ular dipole moments in polar solvents. One is based on the Kirkwood-
Froehlich equation (Ref. 14) ,
ieo-ej {2£o+eJ 4πΝο ^
= gμ (8)
εο(ε^+2)^ 9kTV
Here eo is the static dielectric constant, ε^ is the dielectric constant of
induced polarization. No is Avogadro's number, V is the molar volume, g
is the Kirkwood correlation factor, and μ is the intrinsic dipole moment of
the molecules. For liquids, Eq. (8) is exact if the liquid lattice has
cubic symmetry, if the lattice positions are independent of molecular orien­
tations, and if translational fluctuations may be neglected (Ref. 14) .
In the present work an alternate approach was used which seemed to be par­
ticularly appropriate for dilute solutions. A full derivation has been pub­
lished elsewhere (Refs. 5,15). For the present, we begin by representing
the polarization f(ε) of the solution as a sum of additive terms due to the
individual molecular species,
f(ε) = (ε-1) (2ε+1)/(9ε) = Σο.P. (9)
1 1
where c. = molar concentration and P. = molar polarization of the i-th molec­
ular species. Each P^ depends not only on the intrinsic dipole moment
but also on a specific dipole correlation factor g.(Ref. 15). The final
equation for a single solute (subscript 2) in dilute solution in a polar sol­
vent (subscript 1) is (10) .
2 9kTho dε /2εο + 1 12φo[f(eo) - φο] ^
^2 = '
4πΝο
ν 9ε^ ho[2εo + l] ^ /
4[f(εo) - φo][R2 - φονζΙΓεο - 1 ]^
+ Vzf (εο) - R2 -
[2εο + llho
^^[g. - 1] ^ c?U? (10)
56 ERNEST GRUNWALD and TING-PO I
c? = Vl^; CO = (n^-1)/(n^+2) for the solution; φο= (η?-1) /(n?+2) for the pure
solvent; η = refractive index; Ri = φονι and R2 denote molar refractions;
ho = 1 - 2φο (eo-1)/(2eo+l) .
The first term on the right in (10) can be measured directly and will be de­
noted by ; the second term involves the unknown quantities ga and
dgi/dC2. TÍiui, in the dilute-solution limit (c2-0) ,
2 2 0 2
t^2^app = 92μ2 + οιμι (dgi/dC2)^^^Q (11)
In discussing solvated ion pairs, it is reasonable to introduce a chemical
model (Ref. 15), i.e. assume that the ion pairs form firm solvation com­
plexes :
+ - + -
AB + mS A Β 'mS
Dipole μ. μχ μ^^^^^^
We further assume that dipole correlation between A'^B -mS and solvent mol­
ecules outside the solvation complexes is negligible, i.e. g^ g^^^^ ~ On
introducing this model into (11), we obtain '
.r^r. = ^r.^.r ^ ^^μ^ (dgi/dc2) ^ . (12)
2,app 2,solv C2=0
It should be noted that, in computing μ^ ^ from the first term in (10), V2
and R2 must be reckoned for A'^'B'-mS and ' thus depend on the assumed
value for the solvation number m. Fortunately, this dependence is slight.
Our aim, of course, is to obtain μ^ ^^i^- ^xcept for a small uncertainty due
to the indeterminate solvation num&er m, μ^ ^ is known. This leaves one
further unknown term^ Οχμι (dgi/dC2) . Because (by hypothesis) g^ ^^ly. =
this term can be evaluated by empirical methods (Ref. 9). '
Table I lists dielectric data in acetic acid for a series of nonassociating
solutes for which g2 may be assumed to be unity. The results are represented
satisfactorily by the empirical equation,
' \il = 0.286 μ^ - 0.0362 (13)
2,app 2 2 2
The standard error of fit is 0.77 D^, which is within the combined errors
of measurement. On introducing this formula into (12), one obtains
1.286 μ^ , = μ^ + 0.0362 , (14)
2,solv 2,app 2,solv
Table I Data for Nonassociating Solutes in Acetic Acid at 25°(Ref.9)
2 2 , 2.
Solute^ dG/dC2 (¿.moΓ·^)μ2 (D)
2,app 2
PhN02 1.90 3.93 0.24
PhCN 2.02 3.93 1.12
PhCl -0.54 1.58 -3.44
PhH -0.82 0.00 -3.13
DTB -1.98 0.00 -7.34
CCI4 -1.11 0.00 -4.70
^ Ph = CeHs; DTB = 1,4-di-t-butylbenzene
EXPERIMENTAL RESULTS
Data required for the calculation of electric dipole moments are listed in
Table II (Ref. 13). μ. was calculated for m = 0 and m = 1. Resulting
2, app