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Molecular spectroscopy of symmetric fragmentary exchange in N-oxypyridinium salts
Joumal of Molecular Structure, 80 (1982) 261-256 Elsevier Scientific Publishing Company, Amsterdem - Printed in The Netherlenda
251
MOLECULAR SPECTROSCOPY OF SYMMETRIC FRAGMENTARY EXCHANGE IN N-OXYPYRIDINIUM SALTS V.I. RYBACHENKO, C.Ju. CHOTIY and E.V. TITOV Institute of Physical Organic Chemistry and Coal Chemistry, Academy of Sciences, Ukrainian SSR, Donetsk (USSR)
ABSTRACT A number of N-acetyloxypy-ridiniumsalts has been studied, using molecular spectroscopy methods. These compounds exist in solutions of bipolar solvents as ions and ion pairs of various structure. The association to ion pairs effects the frequences and intensities of characteristic vibrations, the chemical shifts of proton peaks, and the reaction rate of symmetric exchange of acetyl groups. Based on the spectrochemical correlations, data on the structure of the salts, the acetyl exchange reaction mechanism, and the influence of the solvent nature, the reagent structures, the additions of the base electrolyte and crown-ether on its kinetic characteristics have been obtained.
INTRODUCTION Molecular spectroscopy method8 have been widely and successfully used to study thoroughly not only the structures of chemical compounds, but also their reactivities, The present communication presents some results of the spectroscopic studies of the reaction of symmetr$c fragmentary exchange discovered by us earlier (refs. 1,2): R,X- + ousing N-acetyloxypyridiniu salts (I), R =_-N(CH3j2, -OCH3, -CH3, -H, -Cl, -COOC2H5; X-= Cl-, Br-, ClOi, BPhd, SbCl;. The aims-of that work were to establish the composition of the particles participating in the reaction, to find out its mechanism, and to study the influence of the structure8 of the participant8 and the medium on its kinetic characteristics (1). 0022~2860/82/0666-6000/$02.76 0 1982 Elsevier
Scientific Publishing Company
252
RESULTS Consider the obtained results successively. The salt-like composition of the products (I) was proved by registration in the far infrared region of ionic bond stretching bands which are in the antibate relation to the mass of the vibrating particles (Fig. I).
400
300
200
100
‘4 ,
cm-1
Fig. 1. Infrared spectra of the N-acetyloxypyridiniu salts (nujol mull). That the salts (I) are able to decompose to ions was found out by the electric conductivity method. The degree of electrolytic dissociation alters from 0.9 - 0.4 for CH3CN solutions to 10B3- low5 for CH2C12, CHC13 and C5H5N solutions. The dissociation constants of some salts calculated by the Fuoss-Kraus method (ref. 3) and the distances of closest approach of ions calculated according to (ref. 4) are presented in Table 1. Thus, the salts in point are electrolytes, have ionic structures, and are associated under the conditions of our measurments. TABLE 1 Dissociation constants (Kd) and distances of closest approach of ions (a) for theoseries of N-acetyloxypyridinium salts in CH3CN solutions (25 C) R in (I)
X-
OCH3 OCH3 0CH3 N(CH32 ) CH3
BrClO, BPh; BPhi BPh-
Kd, mol/l
a, II
0.0082 0.0184 0.0378 0.0152 0.0236
1.62 1.93 2.28 1.70 1.98
253
Of significant interest was to locate the coordination of the anion in the ion pair (I) and the character of the cation-anion association
in solutions (contact, solvate-separated
ion pairs).
The frequences and intensities of characteristic vibrations
in CH3CN
solutions are presented in Table 2. The data for the ions were obtained by extrapolation
to zero concentration, the data for ion
pairs were calculated with regard to the degree of dissociation. TABLE 2 Infrared spectroscopic characteristics of N-acetyloxypyridinium salts in solutions of CH3CN (J in cm -1 , A in l/mol.cm2) iu‘os. R in (I) X-
Ions 'C=O AC=0
1.
N(CX3)2
2. 3. 4. 5 _. 6.
Ion pairs 'C=O
AC=0
Ions and ion pairs J 8a '8, A8a A8b
Br-
1825 9950 1822 10000 1642 1573 22000 9200
OCH3
cl-
1832 9500 1826
9700 1631 1574
9700 2000
OCH
Br-
1832 9650 1829
9650 1631 1574
9560 2160
OCH3
ClO,
1832 9400 1830
9450 1631 1574
9800 2200
OCH;
BPh;; 1832 9550 1831
9700 1631 1574
9680
OCR
8bCl; 1832 9400 1831
9700 1631 1574
9600 2100
-
7.
CH33
Br-
1834 9060 1831
9300 1629 1577
2900
110
8.
H
3r
1837 9340 1833
9340 1614 1582
1260
40
9. IO.
Cl
Br-
1838 8460 1834
9000 1615 1570
4500
120
COOC2H
Br-
1840 7510 1835
8200 1616 1577
425
30
The integral intensities of ring skeletal vibrations Aaa and (assignment according to ref. 5) for the ions are linearly A8b dependent on substituent constants 4,', i.e. an increase in their electron donor properties contributes to intramolecular electron transfer from the substituent to the acetyloxy group by the direct polar conjugation mechanism.
In this case the double-bond character of the carbonyl bond decreases, and its polarity increases (see 5 c=. and AC,0 in Table 2). The association to ion pairs does not seem to effect the transfer of electronic effects by intramolecular mechanisms:
'8a' '8b' A8a and A8b do not change. Therefore a decrease in &=o and some increse in AC,0 for ion pairs are indicative of polarization of the carbonyl bond in an anion field. The effect is decreased in the order Cl) that is as the distance of closest approach in the ion pair increases (Table 1). Br->ClOi>BPhi,
264 Significant differences in the spectra of ions and ion pairs are registered for NCH deformation vibrations of the aromatic ring. We calculate the vibrational spectra of (I) to reliably interpret these data. Figure 2 presents the chemical shifts of the signals for acetyl and ring protons of the cations of (I) as dependent on d -constants of the substituents. It can be seen that electron-accepting substituents decrease the screening of the corresponding nuclei.
-0.8
-0.4
0.0
Fig. 2. Plots of the chemical shifts for the protons of cations of N-acetyloxypyridiniu salts vs. e-constants of substituents (solutions in CH3CN at 25 C; the numbers of the substances in Table 2). The formation of ion pairs with the anions Rr- and ClO- results 4 in shifting the peaks of the ring O(-protons to a weak field seeming. ly due to the polarization of the C-H-bonds (ref. 6). The reversal shift of the signals for the BPh, ion pairs is due to the contribution of the local field of the anion benzene rings in the ion nair which is possible when there is a contact cation-anion interaction (ref. 7). The concentration nounced for
J-protons;
dependence of the signals is less pro-
acetyl protons and those of substituents
are practically not sensitive to association. The total combination of the data obtained allows us to maintain with confidence that the most probable site of the coordination of the anion is the N-O-bond region, and contact ion pairs are formed in CH3CN solutions. Solvate-separated ion pairs were detected only in solutions of pyridine, in which solvent, for instance, chemical shifts of protons of (I) are independent of the anion nature
255
in an ion pair. The above results allows us to pass over to the discussion of the kinetic data obtained by the dynamic NMR method. Reaction (1) proceeds by the bimolecular mechanism; in low-polar media (CH2C12, CHC13, C5H5N), it has the first order with respect to each reagent; in solutions of CH3CN, the reaction order with respect to salt is less than unity, i.e. the ions and ion pairs have different reactivities. TABLE 3 Rate constants of ions and ion pairs acetyl exchange for the series of N-acetyloxypyridinium salts (25'C)* R in (I)
X-
Solvent
OCH3 OCH3 0CH3 :::3)2
BrClO, BPhBr_ Br-4
CH3CN CH3CN CH3CN CH2C12
CH33
Br-
OCII3
ClO,
CH2C12 C5H5N
l/mol*set ki, l/mol*sec k. iP' 8950
(1000
9400
9250
(1000
pKBH+
77500
3.88
14200 6400
2.05 1.29
60000
* The accuracy of determination is +lO%. The basicities of N-oxides are from (ref. IO). Table 3 presents the rate constants of acetyl exchange for a number of salts in various solvents. For the solutions in CH Cl? 3 the observed rate constants have been treated according to the Acree equation (ref. 8): kohs= kiOc + kip(l-d) (2) where kobs is the observed rate constant, ki is the rate constant of ions, k. is the rate constant of ion pairs. As it has turned iP out (Table 3), ions are more reactive than ion pairs. In our case the accurate estimation of kin according to eq. (2) is difficult because of errors, since kizkobs *k. . To determine k. in CH3CN kinetic experiments have been perform:Pdadding the base':lectrolyte (LiC104). Then oC+O, kobs-)k. , and for N-acetgloxy-4-methoxypyridinium perchlorate k. =30+2.6 l/z1 sec. Nevertheless it should be noted that the k. val:: determined under the conditions of excess LiC104 iP is somewhate low due to the complexing between N-oxide and cation Ii+.
256
The rate of acetyl exchange considerably increases (Table 31, as the polarity of medium is decreased. This may be caused by solvation effects, a change in the structure of ion pairs, substituent effects The role of the first factor in reaction (I) is demonstrated by the experiments with dibenzo-18-crown-6
which, as is knorm, solvates
cations selectively. Adding the crown ether decreases the exchange rate significantly.
The EMI? spectra showed that the complexes of (1)
with dibenzo-18-crown-6
have a sandwich structure. The stability
constant of the complex ?iith the crown ether is 8.2*1C-2 nol/l at 25'C for N-acetyloxy-4-dimethylaminopyridinium
bromide in CH2C12.
The high exchange rates in pyridine solutions (Table 3) are connected with the fact that solvate-separated
ion pairs rather than
contact ones exist in this solvent in distinction to other solvents. An increase in the exchange rate is also caused by introducing electron donor substituents into molecules a proportional relation between the k.
of rea.::ents.There is
values and the basicities
of Ii-oxides (Table 3) which shows tbatpthe reaction course of (I) is more dependent on the electron donor properties of H-oxide than on the electron accepting ability of (I). Thus, using molecular
spectroscopg methods, by reference to the
symmetric fragmentary exchange reaction, the influence of ionic association on the structure and reactivity of a number of organic electrolytes has been shown quantitatively. The reported data were obtained on PE-180FIR and Tesla 467C instruments. The synthesis of the substances and the experimental techniques are described in (refs. 2,9). REFERENCES E.V. Titov and V.I. Rybachenko, J.Mol.Structure, 60(1980)67. V.I. Rybachenko et all, Theor.Exp.Chem. (USSR), 13(1977)410. R.M. Fuoss and C.A. Kraus, J.Am.Chem.Soc., 55(1933)2387. R.M. Fuoss, J.Am.Chem.Soc., 80(1958)5059. Physical Methods in Heterocyclic Chemistry, Ed. by A.R. Katritzky, Acad. Press, N-Y and London, 1963, 6. R.G. Anderson and M.C.R. Symons, Trans.Faraday Sot., 65(1969) 2537. 7. G.P. Schiemenz and H. Rast, Tetrahedron Letters, 26(1969)2165. P. Beronius and L. Pataki, J.Am.Chnm.Soc., 92(1970)4518. 9":E.V. Titov C.Ju. Chotiy and V.I. Rybachenko, Zhurn.Cbsch. Khim.(USSR!, 51(1981)4518. IO. D.D. Perrin, Dissociation constants of organic Bases in aqueous solutions, Butterworths, 1965, pp. 181-782. :: 3. 4. 5.
251
MOLECULAR SPECTROSCOPY OF SYMMETRIC FRAGMENTARY EXCHANGE IN N-OXYPYRIDINIUM SALTS V.I. RYBACHENKO, C.Ju. CHOTIY and E.V. TITOV Institute of Physical Organic Chemistry and Coal Chemistry, Academy of Sciences, Ukrainian SSR, Donetsk (USSR)
ABSTRACT A number of N-acetyloxypy-ridiniumsalts has been studied, using molecular spectroscopy methods. These compounds exist in solutions of bipolar solvents as ions and ion pairs of various structure. The association to ion pairs effects the frequences and intensities of characteristic vibrations, the chemical shifts of proton peaks, and the reaction rate of symmetric exchange of acetyl groups. Based on the spectrochemical correlations, data on the structure of the salts, the acetyl exchange reaction mechanism, and the influence of the solvent nature, the reagent structures, the additions of the base electrolyte and crown-ether on its kinetic characteristics have been obtained.
INTRODUCTION Molecular spectroscopy method8 have been widely and successfully used to study thoroughly not only the structures of chemical compounds, but also their reactivities, The present communication presents some results of the spectroscopic studies of the reaction of symmetr$c fragmentary exchange discovered by us earlier (refs. 1,2): R,X- + ousing N-acetyloxypyridiniu salts (I), R =_-N(CH3j2, -OCH3, -CH3, -H, -Cl, -COOC2H5; X-= Cl-, Br-, ClOi, BPhd, SbCl;. The aims-of that work were to establish the composition of the particles participating in the reaction, to find out its mechanism, and to study the influence of the structure8 of the participant8 and the medium on its kinetic characteristics (1). 0022~2860/82/0666-6000/$02.76 0 1982 Elsevier
Scientific Publishing Company
252
RESULTS Consider the obtained results successively. The salt-like composition of the products (I) was proved by registration in the far infrared region of ionic bond stretching bands which are in the antibate relation to the mass of the vibrating particles (Fig. I).
400
300
200
100
‘4 ,
cm-1
Fig. 1. Infrared spectra of the N-acetyloxypyridiniu salts (nujol mull). That the salts (I) are able to decompose to ions was found out by the electric conductivity method. The degree of electrolytic dissociation alters from 0.9 - 0.4 for CH3CN solutions to 10B3- low5 for CH2C12, CHC13 and C5H5N solutions. The dissociation constants of some salts calculated by the Fuoss-Kraus method (ref. 3) and the distances of closest approach of ions calculated according to (ref. 4) are presented in Table 1. Thus, the salts in point are electrolytes, have ionic structures, and are associated under the conditions of our measurments. TABLE 1 Dissociation constants (Kd) and distances of closest approach of ions (a) for theoseries of N-acetyloxypyridinium salts in CH3CN solutions (25 C) R in (I)
X-
OCH3 OCH3 0CH3 N(CH32 ) CH3
BrClO, BPh; BPhi BPh-
Kd, mol/l
a, II
0.0082 0.0184 0.0378 0.0152 0.0236
1.62 1.93 2.28 1.70 1.98
253
Of significant interest was to locate the coordination of the anion in the ion pair (I) and the character of the cation-anion association
in solutions (contact, solvate-separated
ion pairs).
The frequences and intensities of characteristic vibrations
in CH3CN
solutions are presented in Table 2. The data for the ions were obtained by extrapolation
to zero concentration, the data for ion
pairs were calculated with regard to the degree of dissociation. TABLE 2 Infrared spectroscopic characteristics of N-acetyloxypyridinium salts in solutions of CH3CN (J in cm -1 , A in l/mol.cm2) iu‘os. R in (I) X-
Ions 'C=O AC=0
1.
N(CX3)2
2. 3. 4. 5 _. 6.
Ion pairs 'C=O
AC=0
Ions and ion pairs J 8a '8, A8a A8b
Br-
1825 9950 1822 10000 1642 1573 22000 9200
OCH3
cl-
1832 9500 1826
9700 1631 1574
9700 2000
OCH
Br-
1832 9650 1829
9650 1631 1574
9560 2160
OCH3
ClO,
1832 9400 1830
9450 1631 1574
9800 2200
OCH;
BPh;; 1832 9550 1831
9700 1631 1574
9680
OCR
8bCl; 1832 9400 1831
9700 1631 1574
9600 2100
-
7.
CH33
Br-
1834 9060 1831
9300 1629 1577
2900
110
8.
H
3r
1837 9340 1833
9340 1614 1582
1260
40
9. IO.
Cl
Br-
1838 8460 1834
9000 1615 1570
4500
120
COOC2H
Br-
1840 7510 1835
8200 1616 1577
425
30
The integral intensities of ring skeletal vibrations Aaa and (assignment according to ref. 5) for the ions are linearly A8b dependent on substituent constants 4,', i.e. an increase in their electron donor properties contributes to intramolecular electron transfer from the substituent to the acetyloxy group by the direct polar conjugation mechanism.
In this case the double-bond character of the carbonyl bond decreases, and its polarity increases (see 5 c=. and AC,0 in Table 2). The association to ion pairs does not seem to effect the transfer of electronic effects by intramolecular mechanisms:
'8a' '8b' A8a and A8b do not change. Therefore a decrease in &=o and some increse in AC,0 for ion pairs are indicative of polarization of the carbonyl bond in an anion field. The effect is decreased in the order Cl) that is as the distance of closest approach in the ion pair increases (Table 1). Br->ClOi>BPhi,
264 Significant differences in the spectra of ions and ion pairs are registered for NCH deformation vibrations of the aromatic ring. We calculate the vibrational spectra of (I) to reliably interpret these data. Figure 2 presents the chemical shifts of the signals for acetyl and ring protons of the cations of (I) as dependent on d -constants of the substituents. It can be seen that electron-accepting substituents decrease the screening of the corresponding nuclei.
-0.8
-0.4
0.0
Fig. 2. Plots of the chemical shifts for the protons of cations of N-acetyloxypyridiniu salts vs. e-constants of substituents (solutions in CH3CN at 25 C; the numbers of the substances in Table 2). The formation of ion pairs with the anions Rr- and ClO- results 4 in shifting the peaks of the ring O(-protons to a weak field seeming. ly due to the polarization of the C-H-bonds (ref. 6). The reversal shift of the signals for the BPh, ion pairs is due to the contribution of the local field of the anion benzene rings in the ion nair which is possible when there is a contact cation-anion interaction (ref. 7). The concentration nounced for
J-protons;
dependence of the signals is less pro-
acetyl protons and those of substituents
are practically not sensitive to association. The total combination of the data obtained allows us to maintain with confidence that the most probable site of the coordination of the anion is the N-O-bond region, and contact ion pairs are formed in CH3CN solutions. Solvate-separated ion pairs were detected only in solutions of pyridine, in which solvent, for instance, chemical shifts of protons of (I) are independent of the anion nature
255
in an ion pair. The above results allows us to pass over to the discussion of the kinetic data obtained by the dynamic NMR method. Reaction (1) proceeds by the bimolecular mechanism; in low-polar media (CH2C12, CHC13, C5H5N), it has the first order with respect to each reagent; in solutions of CH3CN, the reaction order with respect to salt is less than unity, i.e. the ions and ion pairs have different reactivities. TABLE 3 Rate constants of ions and ion pairs acetyl exchange for the series of N-acetyloxypyridinium salts (25'C)* R in (I)
X-
Solvent
OCH3 OCH3 0CH3 :::3)2
BrClO, BPhBr_ Br-4
CH3CN CH3CN CH3CN CH2C12
CH33
Br-
OCII3
ClO,
CH2C12 C5H5N
l/mol*set ki, l/mol*sec k. iP' 8950
(1000
9400
9250
(1000
pKBH+
77500
3.88
14200 6400
2.05 1.29
60000
* The accuracy of determination is +lO%. The basicities of N-oxides are from (ref. IO). Table 3 presents the rate constants of acetyl exchange for a number of salts in various solvents. For the solutions in CH Cl? 3 the observed rate constants have been treated according to the Acree equation (ref. 8): kohs= kiOc + kip(l-d) (2) where kobs is the observed rate constant, ki is the rate constant of ions, k. is the rate constant of ion pairs. As it has turned iP out (Table 3), ions are more reactive than ion pairs. In our case the accurate estimation of kin according to eq. (2) is difficult because of errors, since kizkobs *k. . To determine k. in CH3CN kinetic experiments have been perform:Pdadding the base':lectrolyte (LiC104). Then oC+O, kobs-)k. , and for N-acetgloxy-4-methoxypyridinium perchlorate k. =30+2.6 l/z1 sec. Nevertheless it should be noted that the k. val:: determined under the conditions of excess LiC104 iP is somewhate low due to the complexing between N-oxide and cation Ii+.
256
The rate of acetyl exchange considerably increases (Table 31, as the polarity of medium is decreased. This may be caused by solvation effects, a change in the structure of ion pairs, substituent effects The role of the first factor in reaction (I) is demonstrated by the experiments with dibenzo-18-crown-6
which, as is knorm, solvates
cations selectively. Adding the crown ether decreases the exchange rate significantly.
The EMI? spectra showed that the complexes of (1)
with dibenzo-18-crown-6
have a sandwich structure. The stability
constant of the complex ?iith the crown ether is 8.2*1C-2 nol/l at 25'C for N-acetyloxy-4-dimethylaminopyridinium
bromide in CH2C12.
The high exchange rates in pyridine solutions (Table 3) are connected with the fact that solvate-separated
ion pairs rather than
contact ones exist in this solvent in distinction to other solvents. An increase in the exchange rate is also caused by introducing electron donor substituents into molecules a proportional relation between the k.
of rea.::ents.There is
values and the basicities
of Ii-oxides (Table 3) which shows tbatpthe reaction course of (I) is more dependent on the electron donor properties of H-oxide than on the electron accepting ability of (I). Thus, using molecular
spectroscopg methods, by reference to the
symmetric fragmentary exchange reaction, the influence of ionic association on the structure and reactivity of a number of organic electrolytes has been shown quantitatively. The reported data were obtained on PE-180FIR and Tesla 467C instruments. The synthesis of the substances and the experimental techniques are described in (refs. 2,9). REFERENCES E.V. Titov and V.I. Rybachenko, J.Mol.Structure, 60(1980)67. V.I. Rybachenko et all, Theor.Exp.Chem. (USSR), 13(1977)410. R.M. Fuoss and C.A. Kraus, J.Am.Chem.Soc., 55(1933)2387. R.M. Fuoss, J.Am.Chem.Soc., 80(1958)5059. Physical Methods in Heterocyclic Chemistry, Ed. by A.R. Katritzky, Acad. Press, N-Y and London, 1963, 6. R.G. Anderson and M.C.R. Symons, Trans.Faraday Sot., 65(1969) 2537. 7. G.P. Schiemenz and H. Rast, Tetrahedron Letters, 26(1969)2165. P. Beronius and L. Pataki, J.Am.Chnm.Soc., 92(1970)4518. 9":E.V. Titov C.Ju. Chotiy and V.I. Rybachenko, Zhurn.Cbsch. Khim.(USSR!, 51(1981)4518. IO. D.D. Perrin, Dissociation constants of organic Bases in aqueous solutions, Butterworths, 1965, pp. 181-782. :: 3. 4. 5.