Wasi Ahmed1 State University of New York at Binghampton Binghampton, NY 13901
Classification of Solvents Interaction Mechanisms
It has been common among- students either to ignore the effect of solvents as being minor and insignificant or to consider solvents a continuous media characterized by a parameter called the dielectric constant, f . With the enormous amount of recent literature, however, on solvolysis reactions and the effect of solvents on chemical reactivity and on physical properties, the importance of solvents is being increasingly recognized. Unfortunately, even to this day the understanding of solvent effects has been mainly qualitative, and the problem of rigorous quantitative treatment has remained elusive. This paper is an attempt to show that a study of solvent effects on well-chosen systems designed to separate different modes of interactions can provide a deeper insight into this fascinating area of chemistry. It is noteworthy that, in general, numerous attempts to interpret solvent effects of different kinds (e.g., simultaneous action of both nonspecific and specific solvent-solute interactions) on chemical reactivity and physical properties on the basis of the dielectric approach alone have failed ( I ) . Also the same is true of the apolications of various statistical theories of solutions, e.g., the theory of regular solutions ( I ) . The eeneral failure of fundamentally classical theories, derived From nonspecific solvent-solute interaction models, does not mean that chemical specific solvation theories have escaped a similar fate. Hence, one can understand why many physical organic chemists wish to search for some new approach to the quantitative treatment of solvation prohlems. Out of such attempts have emerged several empirical solvent polarity from sol~atochromicshifts in electronic spectra, such as (1) The Kosower hlue-shift solvent polarity parameter called Z (2-3), which is the transition energy (keallM)of the charge transfer absorption hand of the 1-ethyl-4-earbaethoxy-pyridiniumiodide ion
pair.
However, all such polarity scales are formally based on the assumption that it is necessary to take into account only one mechanism of solvent-solute interaction. The same assumption was made in the solvent polarity parameters devised on the basis of ir stretching frequencies, i.e., the G-parameters of Allerhand and Schleyer (8)who studied the dependence on the solvent of X=O and X-H- - - B ( X may be C, S , N o r P; B is a proton acceptor) ir stretching frequencies; also in the nmr solvent polarity P scale which was introduced by R. W. Taft e t al. (9)on the basis of the I9F chemical shifts of substituted fluorohenzenes, and finally in the study of solvation of sodium ions in nonaqueous solvents by W a nmr by Erlich and Pooov oointed out hv. Konoel . (10). . . But..as . .. and Palm (1). . ., single-parameter correlations are not universal when a great varietv of orocesses or solvents is to be considered. The work of la& and Mohler (11) on the effect of solvents on the excitation enerav of the orotonated Schiff base of all-transretinal indicated that skvera~specific modes of interactions must be taken into account. Failure in the past to take these into account explains the rather limited range of application of the several single parameter terms as mentioned above. Models
It has been postulated by Krygowski and Fawcett (12) that the solvent effect on a physiochemical quantity Q can be represented by a linear function of two independent hut complementary parameters descrihing the Lewis acidity A, and Lewis basicity B of the given solvent. Q=Qo+oA+bB
-
(2) The ET p o l ~ ~ iSC& t y of Dimloth et al. ( 4 4 ) reflects the dependence (blue shift) of a n* transition energies (kcsl1M)of the N-phenol pyridinium betaines on the solvent.
(1)
where a and b are constants describing the sensitivity of the property Q to acidic and basic solvent properties. The Dimroth parameter, ET,mentioned above is chosen as a measure of Lewis acidity A. The Guttman donor number, DN (13,141 is chosen as a measure of solvent basicity B , where DN is the negative of the enthalpy of formation of adducts between the uncharged Lewis acid SbC16 and a given solvent molecule as a Lewisbase in dilute 1,2-dichloroethane solutions. AH
SbCls + solvent -+SbCls. solvent
-
(3) The dependence (red and blue shifts, respectively) an the solvent ofthe a a* transition energies of merocyanine dyes as model
compounds was used by Brooker et al. to establish solvent polarity parameters, X R and XB (6). (4) A so-called universal polarity scale of solvents has been introduced by Zelinskii (7) and is based on the red-shift dependence on the solvent of then a*electronic transitions of N-methyl-d-aminophthalimide
-
'Part of this paper was presented in the Missouri Academy of Science meeting at Rolla in May 1976.
(2)
Althoueh the authors claim that the model offers an acceotable empirical method of analyzing solvent effects, the fHct that onlv 55%of the cases interoreted show fairlv good fit is far froi satisfactory. I t might be noted that theaddition of refractive index, n and dielectric constant. f . terms does not improve the fit. The very limited success of the above model seems to stem from the fact that two different systems are used to define the parameters A and B. A more valuable insiaht into solvent effects emerges from a most recent classificat~onof solvents based on the interesting observation (15) that a linear relationship exists between the excitation energy of all-trans-N-Retinylidenmethyl-n-hutylammonium iodide salt (NRBAM'I-) and dielectric constant of a series of aromatic and aliphatic solvents in the range Volume 56. Number 12 December 1979 / 795
chain. The electrostatic interaction energy between the cation and its counter anion causes a stabilization of the -mound state as shown in Figure 2, such that
AE=AEO+AEi
(3)
Therefore the excitation energy of the salts in solvents which give rise to the linear relationship can he looked upon as the sum of three terms: the excitation energy of the polyene a system, A E O , the electrostatic interaction energy between the cation and the anion, AEi, and the interaction energy between the ion-pair and the solvents, AESi,which is proportional to the dielectric constant c, i.e.,
AE=AEO+AE,-kc
F gure I Plor of exc rat~onenmglas ot w e t nyl~denememyl-sbwlammonn~m d o e as a hnct on al solvent d olech c constant
EXCITED
(4)
The negative sign results from the experimental fact that the excitation energy decreases as the solvent dielectric constant increases. This occurs because the ground state energy of the ion-pair goes up as the solvent dielectric constant increases (Fie. 2). I t should he noted that in this treatment the excited state enerk? is assumed to remain constant which is justified on thr h i s of [ruantum merhanical calculations bv Hlatz et al. (16). The linear relationship observed for 14 solvents, 9 aromatic, and 5 aliphatic is not surprising because of the following reason. The overall energy of the system is determined by electron distribution on the polyene chain. Now, it has been shown that the total energy is dependent upon the extent of electrostatic interaction between the positive charge on nitrogen and the negative charge on the anion. This, in turn, depends upon the extent of electrostatic interaction between the anion and the solvent dipole. In other words, the solvent dipoleanion interaction energy term, AE,i, is a function of the charge on the anion, Q., and of the positive charge on the solvent, Q,, which are themselves functions of the dielectric constant of the solvents, i.e.,
Q. = f ( 4 Figure 2. Anion and solvent perturbationon the ground state energies of N-retinylidenemethyl-sbutylammaniumiodide.
(5)
Qc = f k )
(6)
k'r X k " ~ AE,i = -= kc
(7)
f
of dielectric constant c = 2-10.5. This classification is given in the table and Figure 1shows the linear relationship (15). All solvents which produce a regular linear relationship on the excitation energy are grouped together as class R solvents in the table. In order to understand this classification, the following discussion is necessary. Now, it is reasonable to depict the ground state of the salt as a resonance hybrid of several contributing structures as shown below.
Therefore, the loaic that emerees is that the nositive end of the solvent dipole:. whose nlaxnitude is propurtiunal to the dielectric constant o i t h ~ ~ s n l v r ninteracts t. with H fri~ctiunof the negative charge on the anion. The interacting fractional charge on the anion is proportional to the dielectric constant of the solvent; the remaining decreased charge on the anion is then free to engage in ion-ion bonding with a fraction of positive charge on~nitrogenand the remakder of the positive charge is available for delocalization over the polyene chain. In other words. a solvent with hieher dielectric constant in~-~ teracts more st'rongly with the a&n. This causes a decrease in the electrostatic interaction between the nositivelv chnreed ----nitrogen and the anion. Thus, there is more delocalization of positive charge and hence a decrease in the excitation energy. An i m ~ o r t a noutcome t of the linear relationshin is that it has pnwidcd new insight not only into the effect of solvents on ion- airs but also the rrlatimshin has ~rovidedfresh insight iito the nature of the dielectri'c cons'tant itself. In this connection it might be mentioned that in an attempt to modify the Dehye-Clausius-Mossotti expression involving the dielectric constant, Ousager (17) pointed out that a portion of the field (the "reaction field," induced by the dipole in the polar or polarizable liquid surrounding the dipole) would follow the dipole. Using a model based on this idea, an attempt was made to account for the dielectric constant of polar liquids in terms of the dipole moment measured in dilute solution in nonpolar liquids. Kirkwood (18) extended the Onsager theory h y trying to take intu account the short-range local interorlions which nfftct the reurientation the dipoles. Howe\w, as Cole (19)~ointcrlout. "someu here in such orolifer3tion of correction terms, one may well begin to questibn whether the ~
~
~
~~~~~~~
.
.
I t is evident from these structures that a fraction of the positive charge is held on the nitrogen by electrostatic interaction while the remainder is delocalized over the polyene 796 1 Journal of Chemical Education
~
~~~~~
treatments are warranted for a model in which even the immediate neighbors of a molecule are represented by a macroscopic continuum." The above-mentioned results indicate that at least in the range of L10.5 the dielectric constant can to the oartial charge simolv . " he considered to be ~rooortional . . on the solvent dipole. ~~~~
Class H Solvents
From the table it is clear that not all solvents even in the range of E = 2-10.5 fall on the linear plot. Solvents like chloroform and bromoform give rise to excitation energies much lower than expected from the linear relationship and fall below the linear plot. Such solvents are known to be capable of hydrogen bonding with the anion. Hydrogen honding solvation of the anion is expected to reduce its electrostatic interaction with the cation and hence cause a lowering of the excitation energy. On calculating this lowering one obtains the Hhonding interaction energy as 2.42 kcal for hromoform and 1.74 kcal for chloroform. It is interestine that one can make ~~~~-~ a rough estimate of the number of solvent molecules around the iodide ion in each case. For examole. the literature value for the interaction energy between chloroform and iodide in carhon tetrachloride is 0.60 f 0.1 kcal/mole (20) obtained from equilibrium constant measurements by considering changes in proton nmr chemical shifts. Assuming that carhon tetrachloride does not make any appreciable difference to the equilibrium constant one can obtain approximately 3 chloroform molecules around iodide ion on dividing 1.74 Kcal hy 0.60 kcallmole (Fig. 3). It is interesting to note that hromoforin produces more lowering than does chloroform, indicating that it has better hydrogen-bonding capability, although bromine is less electronegative than chlorine. This is in keeping with the results ~
Table 1. Observed Amax (nm). and Calculated Excitatlon Energies, AE (kCallM) of All-trans-N-Retinylidenemethyl-nbutylammonium Iodide in Three Different Classes of Solvents Dielectric Constant (elt
Solvents Class R 1. Carbon Tetrachloride 2. Benzene 3. Thiophene 4. Furan 5. Anisole 6. 0-Chiorotoiuene 7 Chiombenzene 8 pChiorotoiuene 9a. Benzyi Chloride 9b. Methvi Iodide Methyiene Chloride a-Dichlarabenzene 1.1-Oichioroethane 1.2-Dichioraethane
AE (kCalIM)
reported by Klemperer et al. (21) who have found the same order of hydrogen-bonding by measuring the infrared stretching frequency of a carhonyl compourld in these solvents. This result can be explained if one considers that there is some double bond character in the carbon-halogen bond which is greater with bromine than with chlorine. This would make the C-H bond of bromoform more acidic. Class N Solvents
By studying the spectra of all-trans-N-Retinylidene-nbutylammonium (i.e., the protonated compound) chloride, bromide, and iodide in different solvents, Baltz and Mohler (11) had found that nonnucleophilic hydrogen bonding solvents caused a bathochromic shift (higher wavelength or lower energy) whereas nucleophilic solvents (both H-bonding and non-H-bonding) like alcohols, ethers or ketones produced a hypsochromic shift (lower wavelength or higher energy) with r&nect to the snectrum in a relativdvinert solvent like carhon tetrachloride. he important hypoihesis emerged that nonnucleoohilic H-bondine solvents were causine withdrawal of the anion and thus aliowing more de~ocaliz&~ of positive charee on the oolvene chain which was oroducina a bathochromic shift; cut solvents qhich passess pronounced nucleoohilic centers were interacting directlv with the ~ositive chargeor nitrogen and thus allowing less oithe charge to delocalize, which was oroducina a hv~sochromicshift. Essentially the same resuit is observed with all-trans-N-retinylidene methyl-n-hutylammonium iodide, (the methyl compound). Hence, arises the necessity of classifying solvents into two clear-cut groups H and N in addition to the R-group (which produces bathochromic shift by anion withdrawal due to anion-solvent dipole interaction). I t is important to note that since specific interaction of the hydrogen-bonding and nucleophilic type becomes important i t is not unreasonable that only class R solvents produce the linear relationship; whereas, class H and class N do not. Inclusion of an additional term in eqn. (4) can be used to account for the behavior of such solvents, i.e.,
.~~~~ ~~
~
~~
~
Observed (nm)
,A,
C reoresents the interaction enerev -" of either the hvdroeen" bonding type or nucleophilic type. C can he evaluated since the previously defined terms have already been evaluated. Anomalous Solvents
Nucleophilic solvents are not the only type which fall above the linear plot. Solvents like methylchloroform, tertiaryhutyl chloride, and n-hutylchloride also fall above the linear plot, but their effect is not as much as that of the nucleoohilic solvents. These solvents were tried in order to test theextent of aoolicabilitv of the linear relations hi^ in case of a l i ~ h a t i c .. solvt.nts. ('oniidering the facr that in the conventional st!nsr these solvents are nonnucleophilic and nmhgdrogen trmding.
Class H Bromoform Chior~form Ciass N Dioxane Diethyl Ether 1.2-Dimethoxyethane Tetrahydrofuran Acetonitrile Acetone Methanol Benmnitriie Anomalous
A,
n 8 u i y Chloride Methyl Chloroform t-Bulyl Chloride
9.962
.. .
*
values are accurate to 1 nm. a Dielemlc constant values were obtained from references 1 and 22. mese vaiues are at 20°C except those in parenmess.
Figure 3. The number of chloroform molecules around iodide in NRBAM'I-.
Volume 56, Number 12, December 1979 / 797
it was reasonable to expect these to fall on the linear plot. The rlrar cxprrimental fait that they du not suggests;hnr irn*. must take intu account the detailed strurturnl ft~itureof the solvents in order to make exact predictions about their hehavior. The anomalous solvents appear to behave like carbon tetrachloride, essentiallv having no effect on the excitation energy in spite of the fact that they have higher dielectric constants. This may be because in such solvents the positive end of the dinole is relativelv diffused over methvl " erouos - . as in t-butyl c'hloride. a here fore, there is little or no interaction with the anion, thereby causing no effect on the excitation energy. Correlation with Known Solvent Polarity Parameters t of the finding of a linear relaAn i m ~ o r t a nconsequence tionshipAhetween the Eation excitation energy and the dielectric constant of solvents in the region e = 2-10.5 is that an attempt can he made to correlate this with previously used solvent polarity parameters. Dimroth eta]., (4-5) as already pointed out in the introduction, have proposed a solvent polarity parameter, ET which is simply the i~ a* excitation energy for the pyridinium phenol betaine as determined in a selected series of solvents. In the light of the data on NRBAM+I- nresented herein. it is enliehtenine to comnare them with tke ET values of ~ i m r o t h i. n fact,as is evkent from Figure 4 which is a plot of the AE values for NRBAM+Iagainst Dimroth's ET values for solvents common to both studies there is a good linear relationship between the two. I t is obvious that for the seven solvents plotted, a linear relationship exists between E r and the solvent dielectric constant. This relationship went unrecognized by the authors. Thus, the conclusion that emerges is that any system with a positively charged nitrogen atom that is either a part of a conjugated polyene chain or aromatic rings such that a fraction of the charge is delocalized and the remainder interacts with an anion must give rise to the above relationship. Another solvent polarity parameter, called 2, is the excitation energy (kcal/mole) of the so-called charge transfer absorption hand of the l-ethyl-4-carbomethoxy pyridinium iodide studied by Kosower (2-3). He found that a plot of ET against Z is fairly linear over the wide range of solvents from benzene to water. It is important to note, however, that all the solvents excent benzene and methvlene chloride used in his study are strungly nucleophilic. I.'nforrunntely, sinre the l~near relationshir~with 1 E values are limited tosulvents ofdielectric constants in the range of e = 2-10.5, a direct comparison with Z is not ~ossible.But Kosower's characterization of the bands due to Dimroth's system as intramolecular charge transfer band needs to he reevaluated in the light of the linear relationship observed with NRBAM+I-. The retinylic system has been characterized as i~ a* transition based on spectroscopic properties and the results of various semi-empirical calculations that have been made. Furthermore, charge transfer al)sorption is ruled out t ~ r r a ~ lrht. s r prrrhhmte salt whirh is knuirn to tn: incapuhle of giving rise to charge transfer absorption produces the s&ne band at approximately the same wavelength as iodide. In view of the correlation between the findings presented here and those of Dimroth, therefore, it is unlikely that the Dimroth's system produces a charge transfer band for which a diradical must he produced in the excited state. However, Kosower's system, the pyridinium iodide may give rise to charge transfer band because the positive charge here is not as extensively delocalized and therefore the pyridinium ion may act as a good acceptor of electrons. The fact that a linear relationship between ET and Z is observed may
-
-
798 1 Journal of Chemical Education
t
r
I 32
36 I.:!
I
I 40
I
44
(Kral/M)
Figure 4. Plat of the excitation energy of ail-frans-N-retinylidene-+butylammanium iodlde as a function of E ~ v a l u e sdetermined by Dimroth in s e l e c t e d solvents.
be because for highly nucleophilic solvents there is a similar type of ion-dipole interaction in both cases. Summary Well-known solvent polarity parameters like those of Kosower and Dimroth and a recent model bv Krvgowski and Fawcett have failed to make reliable predictions of solvent effects on physicochemical properties measured in solution. This paper presents a model which is based on the interesting observation that the excitation energy of All-trans-N-Retinvlidenemethvl-n-hutvlammoniumiodide is directlv related to the dielectric constant of a series of aromatic and aliphatic solvents in the ranee of dielectric constant e = 2-10. Althoueh still not rigorousl;quantitative, this model presents solvek effects in terms of different modes of interactions like dielectric constants, hydrogen-bonding, and nucleophilic effects. Solvents common to this study and Dimroth's polarity studies show good correlation which clearly indicates that this model can also he used to interpret Dimroth's polarity parameter. Literature Cited (1) Koppel, i. A..snd Pdm.VYAAAA'Advmdd iiLiiiiF~eeEn~'g~R~iatititihi~.IIPIen~ Prela, New York. 1972. p. 210. (21 Ko.owec, E., J Amer Chem. Soc. 80,3253,3261,3267 (1958). (31 Kosower. E., "An Introduction to Phyeical Organic Chernistry.(l Wiley, Nev York, , O M
(4) Reiehardt, C., "Lurungnmitlel-Effect in der O~ganiachenChemie: Verlag Chemie. Weinheim. 1969. (5) Dirnroth, K.. Reichmdt, C.. Siepmsnn, T., and Bohlrnsnn, F., Ann. Chem., 661.1 (19631. (61 Brooker,L.G.S.,Claig,A.C..HeseItine.D. W., Jenkin%P. W..and Linm1n.L.L.. J. Amer. Chem. Soi. 87.2443 (1965). (71 Zhmyrevs, I.'A.,Zelinskii,V. V.. Knlwkov,V. P.,andKrssnitakaya N. o.,Dokl.Akad, NoukSSSR, 127,1089 (19591: EE(Phys. Chem.), 1091. (81 Allarhand,A.,sndSchley~r,P.von R., J.Amer Chem. Soc., 85,371 (1963). 19) Tail. R. W.,KlingensmithG.B..Price, E..snd For. I.R.. "Sympnmiumon1,inear Free Energy C~rrleafions,~ 1964. p. 265. (10) Erlich. R. H.,and Popov, A.I.. J. Amer Cham. Soe.. 93.562 (1971). (11) Blstz, P. E.,and M0hler.J. H.,Biochem&lry, iI.3240 (1972). (12) Krygawski, T. M., Fawmtf. W. R., J. Amer. Chpm Soc., 97,2143 (1975). (13) Guirnann, L a n d Wychera,E.,lnorgNvel. Chem. Left., 2,257 (1966). 114) Gutmann. V.,andSchmied.R.. Caord. Chrm Rsu.. 12.263 11974). (15) la) Ahmed, W., Ph.D. Thesis. Univerrity of Missouri. Kansan City, 1976. lbi Ahmed, W., B i s h , P. E., '"Curroiation of Excitation Energies of all-frons-N-Retinylide~ of Soluen~,"Missouri nernethyl-n-hutyl amrnonim Cation with DielfftncCanstant Academy ofScienee, Rolla, 1976. (16) B1sh.P. E.. Mohiar, J. H., Navangul, H.V.,Biorhem&fry, 11,648l1972). 117) Onsager,L.,J. Amar Chem. Soc.. 58,1486 (19361. 118) Kirkwo0d.J.G.. J. Chem Phys.. 7.911 (19891. (19) Cde, R. H..Ann. Raus P h y s Chem.. 11,149 iL960). (20) Groon, R. D.,and Martin, J. S., J. Amcr Chsm Soc, 90. 3659 (1968). (21) Kiernperer, W.,Goayn, M. W.,Mari.A.H..andPimenel.G.C., J A m e r Chem Soc.. 76.5846 (1954). I221 Weissberger. A,, Pmsbauer, E., Riddick. J., and T o o p , Jr.. E., "Organic SnluentP." lnterrcienee Publisherr. Inc, New York. 1955.
