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Short range solvation by cyclic thioethers
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 381 (1996) 101-105
Short range solvation by cyclic thioethers 1 C.Q. Zhu, N.Z. Zhou, S.E. Schullery, R.M. Scott* Department of Chemistry, Eastern Michigan University, Ypsilanti, MI, 48197, USA Received 11 September 1995; accepted in final form 1 November 1995
Abstract
Short range solvation by thioethers in benzene bulk solvent is studied by observing the change in proton transfer equilibrium constant (Ker) that occurs for reactions between 2,4-dinitrophenol and diethylamine. Solvation stoichiometries and binding constants were determined by fitting Ker vs. thioether concentration data to binding isotherms. These results indicate that 1,4-thioxane hydrogen bonds to the amine proton such that the ring oxygen and the ring sulphur contribute electrons to the proton, and that two thioxane molecules solvate the proton. 1,4-Dithiane contributes electrons from both sulfurs to the proton, but only one dithiane molecule solvates a given proton.
Keywords: Hydrogen bonding; Proton transfer; Solvation constants; Thioethers
1. Introduction
Solvation, the interaction of solutes with their solvent, affects solute properties and reactions. Solvation effects have been subdivided into two components: long range solvation and short range solvation. Long range solvation is the effect of solvent polarity on charge separation components o f a chemical reaction, whereby an increase in solvent polarity favors charge separation. Short range solvation is the effect on solute reactivity of direct interaction between solvent and solute molecules, most commonly by formation of hydrogen bonds. These effects were observed * Corresponding author. i Presented at the Xlth International Workshop 'Horizons in Hydrogen Bond Research', Bir~tonas, Lithuania, 9-14 September 1995.
at an early stage in the reactivity of amines in a variety of pure solvents [1]. The order of reactivity for simple alkyl amines is 3 ° > 2 ° > 1° in non-polar solvents due to the inductive effect of the alkyl group. Dissolving these amines in strongly electron donating solvents (e.g. dimethylsulfoxide) reverses this order of reactivity as the solvent hydrogen bonds protons attached to amine nitrogens [2]. Such hydrogen bonding increases electron density on the amine nitrogen, increasing its basicity. Studies of Taft and coworkers [3,4] evaluate the potential of a number of solvents for solvation effects by observing the influence of each particular solvent on reference reactions, then deriving constants based on these results to be used to predict solvation effects. Taft's pi value [3] describes the effectiveness of solvents in long range solvation. The Taft alpha and beta values [4] describe the effect on reactivity of solutes of proton donating and electron donating (Lewis base) solvents,
0022-2860/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-2860(95)09192-0
C.Q. Zhu et al./Journal of Molecular Structure 381 (1996) 101 105
102
once the proton transfer complex is formed. We have found reaction at the unreacted amine proton to be negligible [5]. Detailed analysis of the influence of short range solvation requires knowledge of three aspects of
respectively, as they form the hydrogen bonds of short range solvation. As a probe of solvation we use the interactions of phenols and amines. Here is the overall reaction scheme: Khb P
+
~
A
Kt ~ P" "A
.._,
KS '
KI "
P-A+ .,~ ~
P(S) n'
"
P-
+ A+
KS
P-A + (S) n Scheme
Where: P = p h e n o l , A=amine, P..A=Hbonded adduct, P - A + = proton transfer complex, P - , A + = phenolate, ammonium ions, S = Lewis base solvent. Forming separated phenolate and ammonium ions requires that the solvent is very polar, a circumstance not found in our studies, so our reactions effectively stop at formation of the proton transfer complex. In the absence of solvation we experimentally determine the equilibrium constant for formation of proton transfer complex from phenol and amine K~OT (where K~,T = Khb " Kt). The solvation binding constant K s represents bonding of all n solvent molecules, and is convertible to a binding constant for a single solvent molecule, Ks, by the relation Ks = (KJ". Similarly, K~ = (Ks') "'. In the presence of short range solvation, the apparent proton transfer constant, Kpr, is given by Eq. 1 [5]. K p T = K ~ T 1 --~ ( K s ) n ( s ) n
(1)
1 + (/c)"' (s)"' Usually adding a Lewis base solvent at least doubles K e r involving a secondary amine [5] or a primary amine [6], and sometimes increases it dramatically more than that. In the study reported here, the phenol is 2,4-dinitrophenol (DNP) and the amine is diethylamine (DEA). In this system there are three possible sites for short range solvation by a Lewis base solvent: the unreacted D N P phenol group proton, the proton attached to the nitrogen of unreacted DEA, and that same proton
the solute-solvent relationship: the strength of solute-solvent bonding (Ks, Kj), the stoichiometry of the interaction (n, n'), and the geometry of the solute-solvent complex. Our studies focus directly on the first two of these aspects, and indirectly on the third. We have previously applied this analysis to solvation by oxygen-containing Lewis bases [5]. In the majority of cases best fit determinations of K s and Ks' were obtained for n = n' = 2, corresponding to the hydrogen bonding of two Lewis base solvent molecules per solute proton, both for the unreacted phenol and the proton transfer complex. The objective of these studies was to determine how well a thioether would serve as electron donor in short range solvation of proton transfer complex formation. The sulfur in a thioether should be significantly less effective as an electron donor in solvation than is an oxygen, as evidenced by the Taft beta value of 0.45 for diethyl ether versus only 0.29 for diethyl thioether [4].
2. Materials and methods
D N P was recrystallized twice from benzene and stored in a desiccator over calcium sulfate. DEA was distilled and stored in a dark bottle under dry nitrogen until use. Benzene, the bulk solvent, was spectrograde, was assayed for water, and in the absence of water was stored over molecular sieve.
103
C.Q. Zhu et al./Journal of Molecular Structure 381 (1996) 101 105
1,4-Dithiane and 1,4-thioxane were purified by distillation. Solutions were prepared in a series o f mixed solvents ranging f r o m 0 to 0.33 M Lewis base solvent in benzene. In a given solvent mixture a series o f solutions are m a d e in which the D N P concentration is constant at a b o u t 4 x 10 -5 M, and D E A c o n c e n t r a t i o n varies f r o m 0 to a b o u t 4 x 10-4 M. U V spectra o f these solutions were scanned using a Perkin Elmer L a m b d a 7 s p e c t r o p h o t o m e t e r equipped with a temperature control set at 25.0°C. Values o f the equilibrium constant for formation o f K e r were determined from the spectra as described earlier [7]. Values for the equilibrium constant for h y d r o g e n b o n d f o r m a t i o n between the a m m o n i u m ion p r o t o n o f the p r o t o n transfer complex and the electron d o n a t i n g solvent (Ks), for the equilibrium constant for h y d r o g e n b o n d f o r m a t i o n between the free phenol g r o u p p r o t o n and the electron d o n a t i n g solvent (Ks'), for the n u m b e r o f solvent molecules per p r o t o n transfer complex p r o t o n (n), and for the n u m b e r o f solvent molecules per unreacted phenol g r o u p p r o t o n (n') were determined by fitting K e r versus concentration Table 1 Values of Ker for experimental concentrations of electron donating solvents Molarity of solvent
0 0.0075 0.0151 0.0226 0.0319 0.0377 0.0452 0.0532 0.0603 0.0753 0.0798 0.1064 0.151 0.213 0.226 0.301 0.319
K?r
Thioxane
Dithiane
1200
1200 1300 1350 1375
1600 1500 1550
o f electron d o n a t i n g solvent data to Eq. 1, as described earlier [5]. The degree o f solvation o f the unreacted amine is too slight to be significant.
3. Results The values for K e r at various concentrations o f 1,4-thioxane and at various concentrations o f 1,4-dithiane are presented in Table 1. These values p r o d u c e curves that initially rise at increasing low concentrations o f electron d o n a t i n g solvent, but at higher concentrations the slope reduces, tending t o w a r d a plateau. Experience has shown that this pattern is only obtained when n = n ~ [5]. The closeness o f fit o f the data to curves obtained by optimizing values for K, and Kst for assumption o f a given c o m m o n whole n u m b e r value o f n and n' is estimated by s u m m i n g the squares o f the deviations o f the data points f r o m the calculated best fit curve (SS). These values are presented for b o t h 1,4-thioxane and 1,4-dithiane for values o f n and n ~ equal to 1, 2, 3, and 4 on Table 2, and are displayed in Figs. 1 and 2, respectively. Both the calculated sums o f squares and visual inspection o f Figs. 1 and 2 establish that n = 2 for 1,4-thioxane Table 2 Curve fitting comparisonsa for thioxane and dithiane solvating components of a DNP/DEA proton transfer complex formation reaction for various possible solvation stoichiometries Thioether
2050 2150 1900 2300 2000 2050 2300
SS/103 b
n,n t
Best-fit binding constants Ks
K~'
Thioxane
1 2 3 4
20.2 4.92 15.2 53.9
37.1 31.1 28.7 28.0
17.2 22.3 23.2 24.0
Dithiane
1 2 3 4
6.54 19.9 53.6 85.4
20.8 25.7 24.7 24.0
10.7 19.8 20.9 21.1
1850 1675 1750
Stoichiometry
a Method of Ref. [5]. b SS is the sum of the squares of deviations between experi-
mental data and the best fit curve calculated using the indicated stoichiometry parameters.
104
C.Q. Zhu et al./Journal of Molecular Structure 381 (1996) 101-105 3 5 0 0 - •
o
o
3
[]
[]
~ioxan~
•
3000
•
t
2
2500 v
2000 1500 1000 0.00
0.11 Thioxane
0.22
0.00
0.33
0.11
0.22
0.33
Molarity
Molerity
Fig. 1. Best fit curves to Eq. 1 for the thioxane data assuming (top to bottom) n = n' = 4, 3, 2 and 1. The Y axis has common dimensions for the various curves, but each is displaced upwards from the previous curve.
and n = 1 for 1,4-dithiane. Corresponding values for K, are 31.1 and 20.8 and for K" are 22.3 and 10.7 for 1,4-thioxane and 1,4-dithiane respectively.
4. Discussion The unmistakable shifts in Ker establish that both sulfur compounds solvate these solutes. For both Lewis bases, the steep rise followed by the plateau (Fig. 3) suggests a qualitatively similar
Fig. 3. Data of Table 1 plotted for thioxane and dithiane. The corresponding plot for 1,4-dioxane is reproduced [5] for comparison.
interaction to that of the oxygen Lewis bases. With only sulfur atoms as electron donors, the dithiane data shows clear evidence that a thioether is capable of short range solvation in our system. The values for thioxane are higher, supporting the reasonable assumption that the oxygen is involved in hydrogen bond formation. The Ks, Ks, n, and n' values for 1,3-dioxolane and various six-membered ring cyclic ethers and thioethers are reported in Table 3. Values for n and n' are 2 in every case but 1,4-dithiane, and where n = 2 we are dealing with nonlinear (bifurcated) hydrogen bonds. Such hydrogen bonds are found in solids, but reports of their occurrence in the liquid state are infrequent
[81. Using K s and Ks' values we have built a case [5,9] for all ring oxygens in the six-membered rings being involved in electron donation to the hydrogen bonds with solute. In review of this argument, 1,3-dioxolane, which sterically cannot involve a
2_o
•
4
•
Table 3 Solvation by cyclic ether and thioether structures
0.00
0.11 Dithiane
0.22
Compound
Ks
K~
n, n'
Tetrahydropyran 1,3-Dioxolane 1,3-Dioxane 1,4-Dioxane 1,4-Thioxane 1,4-Dithiane
20 26 60 86 31 21
11 18 38 53 22 11
2 2 2 2 2 1
0.33
Molarity
Fig. 2. Best fit curves to Eq. 1 for the dithiane data assuming (bottom to top) n = n' = 4, 3,2 and 1. The Y axis has common dimensions for the various curves, but each is displaced upwards from the previous curve.
C.Q. Zhu et al./Journal of Molecular Structure 381 (1996) 101-105
second ring oxygen in hydrogen bonding with a single proton, shows a small effect from the presence of a second electronegative atom in the ring when compared to tetrahydropyran. 1,3Dioxane shows sizable increases in Ks and K" over tetrahydropyran, explained by assuming both ring oxygens, which are adjacent when the ring is in the chair conformation, contribute electrons to the hydrogen bond. 1,4-Dioxane with higher yet Ks and Ks' values can direct even more electron density at the proton in the boat conformation and the oxygens at bow and stern,1 and 1,3,5-trioxane with the highest Ks and Ks~ contributes electron density from all three oxygens to the hydrogen bond when in the chair conformation. 1,4-Thioxane should be slightly lower in K s and Kst values than 1,3-dioxolane if only the oxygen is contributing electron density to the solvation hydrogen bond, but instead it has somewhat higher values. This supports a model in which the 1,4thioxane is in the boat conformation and both the sulfur and the oxygen contribute to the solvation hydrogen bond. Sulfur has a much lower Taft beta value than does oxygen. If in the case of 1,4dithiane only one sulfur were bonding, Ks and Ks~ values should be significantly lower than those of 1,3-dioxolane, but in fact they are slightly higher. The conclusion is that 1,4-dithiane too is in a boat conformation and is contributing electron density from both sulfurs. It remains to explain the n and n' values of 1 each displayed by 1,4-dithiane. We propose that the much larger radius of sulfur (1.02 A) compared to oxygen (0.73 .&) simply fills the volume available for solvation such that only one solvating molecule can be accommodated. We are aware that these solvation structures are crowded [10].
5. Summary Studies of short range solvation of cyclic
105
thioethers 1,4-thioxane and 1,4-dithiane reveal that these molecules hydrogen bond to protic solutes with patterns in which the six-membered rings assume the boat conformation and electrons from both electronegative atoms are directed at the solute protons, a pattern previously observed for 1,3- and 1,4-dioxane and their derivatives. Two molecules of 1,4-thioxane solvate a single proton, but only one molecule of the larger 1,4-dithiane is found per proton.
Acknowledgments An author (R.M.S.) wishes to acknowledge support for this research from W.K. Kellogg Co. and Katherine Donvig.
References [1] R.G. Pearson and D.C. Vogelsong, J. Am. Chem. Soc., 80 (1957) 1038; J.P. Briggs and R. Yamdagni, J. Am. Chem. Soc., 87 (1965) 2332; LI. Bravman, J.M. Riveros and L.K. Blair, J. Am. Chem. Soc., 93 (1971) 3914. [2] E.D. Berman, R. Thomas, P. Stahl and R.M. Scott, Can J. Chem., 65 (1987) 1594. [3] M.J. Kamlet, LL.M. Abboud and R.W. Taft, Prog. Phys. Org. Chem., 13 (1981) 485. [4] M.J. Kamlet, J.L.M. Abboud, M.J.H. Abraham and R.W. Taft, J. Org. Chem., 48 (1983) 2877. [5] S.E. Schullery and R.M. Scott, J. Mol. Struct., 322 (1994) 287. [6] S. Guo and R.M. Scott, J. Mol. Struct., 237 (1990) 307. [7] Z. Ye, S. Yazdani, R. Thomas, G. Walker, D. White and R.M. Scott, J. Mol. Struct., 177 (1988) 513. [8] P.A. Giguere and M. Pigeon-Gosselin, J. Soln. Chem., 17 (1988) 1007. [9] A. Chen, S. Schullery and R.M. Scott, J. Mol. Struct., 322 (1994) 321. [10] J.A. Tayh and R.M. Scott, J. Mol. Struct., 237 (1990) 297.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 381 (1996) 101-105
Short range solvation by cyclic thioethers 1 C.Q. Zhu, N.Z. Zhou, S.E. Schullery, R.M. Scott* Department of Chemistry, Eastern Michigan University, Ypsilanti, MI, 48197, USA Received 11 September 1995; accepted in final form 1 November 1995
Abstract
Short range solvation by thioethers in benzene bulk solvent is studied by observing the change in proton transfer equilibrium constant (Ker) that occurs for reactions between 2,4-dinitrophenol and diethylamine. Solvation stoichiometries and binding constants were determined by fitting Ker vs. thioether concentration data to binding isotherms. These results indicate that 1,4-thioxane hydrogen bonds to the amine proton such that the ring oxygen and the ring sulphur contribute electrons to the proton, and that two thioxane molecules solvate the proton. 1,4-Dithiane contributes electrons from both sulfurs to the proton, but only one dithiane molecule solvates a given proton.
Keywords: Hydrogen bonding; Proton transfer; Solvation constants; Thioethers
1. Introduction
Solvation, the interaction of solutes with their solvent, affects solute properties and reactions. Solvation effects have been subdivided into two components: long range solvation and short range solvation. Long range solvation is the effect of solvent polarity on charge separation components o f a chemical reaction, whereby an increase in solvent polarity favors charge separation. Short range solvation is the effect on solute reactivity of direct interaction between solvent and solute molecules, most commonly by formation of hydrogen bonds. These effects were observed * Corresponding author. i Presented at the Xlth International Workshop 'Horizons in Hydrogen Bond Research', Bir~tonas, Lithuania, 9-14 September 1995.
at an early stage in the reactivity of amines in a variety of pure solvents [1]. The order of reactivity for simple alkyl amines is 3 ° > 2 ° > 1° in non-polar solvents due to the inductive effect of the alkyl group. Dissolving these amines in strongly electron donating solvents (e.g. dimethylsulfoxide) reverses this order of reactivity as the solvent hydrogen bonds protons attached to amine nitrogens [2]. Such hydrogen bonding increases electron density on the amine nitrogen, increasing its basicity. Studies of Taft and coworkers [3,4] evaluate the potential of a number of solvents for solvation effects by observing the influence of each particular solvent on reference reactions, then deriving constants based on these results to be used to predict solvation effects. Taft's pi value [3] describes the effectiveness of solvents in long range solvation. The Taft alpha and beta values [4] describe the effect on reactivity of solutes of proton donating and electron donating (Lewis base) solvents,
0022-2860/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-2860(95)09192-0
C.Q. Zhu et al./Journal of Molecular Structure 381 (1996) 101 105
102
once the proton transfer complex is formed. We have found reaction at the unreacted amine proton to be negligible [5]. Detailed analysis of the influence of short range solvation requires knowledge of three aspects of
respectively, as they form the hydrogen bonds of short range solvation. As a probe of solvation we use the interactions of phenols and amines. Here is the overall reaction scheme: Khb P
+
~
A
Kt ~ P" "A
.._,
KS '
KI "
P-A+ .,~ ~
P(S) n'
"
P-
+ A+
KS
P-A + (S) n Scheme
Where: P = p h e n o l , A=amine, P..A=Hbonded adduct, P - A + = proton transfer complex, P - , A + = phenolate, ammonium ions, S = Lewis base solvent. Forming separated phenolate and ammonium ions requires that the solvent is very polar, a circumstance not found in our studies, so our reactions effectively stop at formation of the proton transfer complex. In the absence of solvation we experimentally determine the equilibrium constant for formation of proton transfer complex from phenol and amine K~OT (where K~,T = Khb " Kt). The solvation binding constant K s represents bonding of all n solvent molecules, and is convertible to a binding constant for a single solvent molecule, Ks, by the relation Ks = (KJ". Similarly, K~ = (Ks') "'. In the presence of short range solvation, the apparent proton transfer constant, Kpr, is given by Eq. 1 [5]. K p T = K ~ T 1 --~ ( K s ) n ( s ) n
(1)
1 + (/c)"' (s)"' Usually adding a Lewis base solvent at least doubles K e r involving a secondary amine [5] or a primary amine [6], and sometimes increases it dramatically more than that. In the study reported here, the phenol is 2,4-dinitrophenol (DNP) and the amine is diethylamine (DEA). In this system there are three possible sites for short range solvation by a Lewis base solvent: the unreacted D N P phenol group proton, the proton attached to the nitrogen of unreacted DEA, and that same proton
the solute-solvent relationship: the strength of solute-solvent bonding (Ks, Kj), the stoichiometry of the interaction (n, n'), and the geometry of the solute-solvent complex. Our studies focus directly on the first two of these aspects, and indirectly on the third. We have previously applied this analysis to solvation by oxygen-containing Lewis bases [5]. In the majority of cases best fit determinations of K s and Ks' were obtained for n = n' = 2, corresponding to the hydrogen bonding of two Lewis base solvent molecules per solute proton, both for the unreacted phenol and the proton transfer complex. The objective of these studies was to determine how well a thioether would serve as electron donor in short range solvation of proton transfer complex formation. The sulfur in a thioether should be significantly less effective as an electron donor in solvation than is an oxygen, as evidenced by the Taft beta value of 0.45 for diethyl ether versus only 0.29 for diethyl thioether [4].
2. Materials and methods
D N P was recrystallized twice from benzene and stored in a desiccator over calcium sulfate. DEA was distilled and stored in a dark bottle under dry nitrogen until use. Benzene, the bulk solvent, was spectrograde, was assayed for water, and in the absence of water was stored over molecular sieve.
103
C.Q. Zhu et al./Journal of Molecular Structure 381 (1996) 101 105
1,4-Dithiane and 1,4-thioxane were purified by distillation. Solutions were prepared in a series o f mixed solvents ranging f r o m 0 to 0.33 M Lewis base solvent in benzene. In a given solvent mixture a series o f solutions are m a d e in which the D N P concentration is constant at a b o u t 4 x 10 -5 M, and D E A c o n c e n t r a t i o n varies f r o m 0 to a b o u t 4 x 10-4 M. U V spectra o f these solutions were scanned using a Perkin Elmer L a m b d a 7 s p e c t r o p h o t o m e t e r equipped with a temperature control set at 25.0°C. Values o f the equilibrium constant for formation o f K e r were determined from the spectra as described earlier [7]. Values for the equilibrium constant for h y d r o g e n b o n d f o r m a t i o n between the a m m o n i u m ion p r o t o n o f the p r o t o n transfer complex and the electron d o n a t i n g solvent (Ks), for the equilibrium constant for h y d r o g e n b o n d f o r m a t i o n between the free phenol g r o u p p r o t o n and the electron d o n a t i n g solvent (Ks'), for the n u m b e r o f solvent molecules per p r o t o n transfer complex p r o t o n (n), and for the n u m b e r o f solvent molecules per unreacted phenol g r o u p p r o t o n (n') were determined by fitting K e r versus concentration Table 1 Values of Ker for experimental concentrations of electron donating solvents Molarity of solvent
0 0.0075 0.0151 0.0226 0.0319 0.0377 0.0452 0.0532 0.0603 0.0753 0.0798 0.1064 0.151 0.213 0.226 0.301 0.319
K?r
Thioxane
Dithiane
1200
1200 1300 1350 1375
1600 1500 1550
o f electron d o n a t i n g solvent data to Eq. 1, as described earlier [5]. The degree o f solvation o f the unreacted amine is too slight to be significant.
3. Results The values for K e r at various concentrations o f 1,4-thioxane and at various concentrations o f 1,4-dithiane are presented in Table 1. These values p r o d u c e curves that initially rise at increasing low concentrations o f electron d o n a t i n g solvent, but at higher concentrations the slope reduces, tending t o w a r d a plateau. Experience has shown that this pattern is only obtained when n = n ~ [5]. The closeness o f fit o f the data to curves obtained by optimizing values for K, and Kst for assumption o f a given c o m m o n whole n u m b e r value o f n and n' is estimated by s u m m i n g the squares o f the deviations o f the data points f r o m the calculated best fit curve (SS). These values are presented for b o t h 1,4-thioxane and 1,4-dithiane for values o f n and n ~ equal to 1, 2, 3, and 4 on Table 2, and are displayed in Figs. 1 and 2, respectively. Both the calculated sums o f squares and visual inspection o f Figs. 1 and 2 establish that n = 2 for 1,4-thioxane Table 2 Curve fitting comparisonsa for thioxane and dithiane solvating components of a DNP/DEA proton transfer complex formation reaction for various possible solvation stoichiometries Thioether
2050 2150 1900 2300 2000 2050 2300
SS/103 b
n,n t
Best-fit binding constants Ks
K~'
Thioxane
1 2 3 4
20.2 4.92 15.2 53.9
37.1 31.1 28.7 28.0
17.2 22.3 23.2 24.0
Dithiane
1 2 3 4
6.54 19.9 53.6 85.4
20.8 25.7 24.7 24.0
10.7 19.8 20.9 21.1
1850 1675 1750
Stoichiometry
a Method of Ref. [5]. b SS is the sum of the squares of deviations between experi-
mental data and the best fit curve calculated using the indicated stoichiometry parameters.
104
C.Q. Zhu et al./Journal of Molecular Structure 381 (1996) 101-105 3 5 0 0 - •
o
o
3
[]
[]
~ioxan~
•
3000
•
t
2
2500 v
2000 1500 1000 0.00
0.11 Thioxane
0.22
0.00
0.33
0.11
0.22
0.33
Molarity
Molerity
Fig. 1. Best fit curves to Eq. 1 for the thioxane data assuming (top to bottom) n = n' = 4, 3, 2 and 1. The Y axis has common dimensions for the various curves, but each is displaced upwards from the previous curve.
and n = 1 for 1,4-dithiane. Corresponding values for K, are 31.1 and 20.8 and for K" are 22.3 and 10.7 for 1,4-thioxane and 1,4-dithiane respectively.
4. Discussion The unmistakable shifts in Ker establish that both sulfur compounds solvate these solutes. For both Lewis bases, the steep rise followed by the plateau (Fig. 3) suggests a qualitatively similar
Fig. 3. Data of Table 1 plotted for thioxane and dithiane. The corresponding plot for 1,4-dioxane is reproduced [5] for comparison.
interaction to that of the oxygen Lewis bases. With only sulfur atoms as electron donors, the dithiane data shows clear evidence that a thioether is capable of short range solvation in our system. The values for thioxane are higher, supporting the reasonable assumption that the oxygen is involved in hydrogen bond formation. The Ks, Ks, n, and n' values for 1,3-dioxolane and various six-membered ring cyclic ethers and thioethers are reported in Table 3. Values for n and n' are 2 in every case but 1,4-dithiane, and where n = 2 we are dealing with nonlinear (bifurcated) hydrogen bonds. Such hydrogen bonds are found in solids, but reports of their occurrence in the liquid state are infrequent
[81. Using K s and Ks' values we have built a case [5,9] for all ring oxygens in the six-membered rings being involved in electron donation to the hydrogen bonds with solute. In review of this argument, 1,3-dioxolane, which sterically cannot involve a
2_o
•
4
•
Table 3 Solvation by cyclic ether and thioether structures
0.00
0.11 Dithiane
0.22
Compound
Ks
K~
n, n'
Tetrahydropyran 1,3-Dioxolane 1,3-Dioxane 1,4-Dioxane 1,4-Thioxane 1,4-Dithiane
20 26 60 86 31 21
11 18 38 53 22 11
2 2 2 2 2 1
0.33
Molarity
Fig. 2. Best fit curves to Eq. 1 for the dithiane data assuming (bottom to top) n = n' = 4, 3,2 and 1. The Y axis has common dimensions for the various curves, but each is displaced upwards from the previous curve.
C.Q. Zhu et al./Journal of Molecular Structure 381 (1996) 101-105
second ring oxygen in hydrogen bonding with a single proton, shows a small effect from the presence of a second electronegative atom in the ring when compared to tetrahydropyran. 1,3Dioxane shows sizable increases in Ks and K" over tetrahydropyran, explained by assuming both ring oxygens, which are adjacent when the ring is in the chair conformation, contribute electrons to the hydrogen bond. 1,4-Dioxane with higher yet Ks and Ks' values can direct even more electron density at the proton in the boat conformation and the oxygens at bow and stern,1 and 1,3,5-trioxane with the highest Ks and Ks~ contributes electron density from all three oxygens to the hydrogen bond when in the chair conformation. 1,4-Thioxane should be slightly lower in K s and Kst values than 1,3-dioxolane if only the oxygen is contributing electron density to the solvation hydrogen bond, but instead it has somewhat higher values. This supports a model in which the 1,4thioxane is in the boat conformation and both the sulfur and the oxygen contribute to the solvation hydrogen bond. Sulfur has a much lower Taft beta value than does oxygen. If in the case of 1,4dithiane only one sulfur were bonding, Ks and Ks~ values should be significantly lower than those of 1,3-dioxolane, but in fact they are slightly higher. The conclusion is that 1,4-dithiane too is in a boat conformation and is contributing electron density from both sulfurs. It remains to explain the n and n' values of 1 each displayed by 1,4-dithiane. We propose that the much larger radius of sulfur (1.02 A) compared to oxygen (0.73 .&) simply fills the volume available for solvation such that only one solvating molecule can be accommodated. We are aware that these solvation structures are crowded [10].
5. Summary Studies of short range solvation of cyclic
105
thioethers 1,4-thioxane and 1,4-dithiane reveal that these molecules hydrogen bond to protic solutes with patterns in which the six-membered rings assume the boat conformation and electrons from both electronegative atoms are directed at the solute protons, a pattern previously observed for 1,3- and 1,4-dioxane and their derivatives. Two molecules of 1,4-thioxane solvate a single proton, but only one molecule of the larger 1,4-dithiane is found per proton.
Acknowledgments An author (R.M.S.) wishes to acknowledge support for this research from W.K. Kellogg Co. and Katherine Donvig.
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