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Variable temperature i.r. spectra of aliphatic and aromatic 1,2-diketones
Spcarocbimica Aaa.Vol.32A,pP.1675 to 1679.Perg~onPm&
Variable
temperature
1976.Printed inNortkm lrebd
ix. spectra of aliphatic diketones
and aromatic
1,2-
M.JUAREZ and M. MAR~~-LOMAS Laboratorio de Quimica Biol6gica. Institute de Productos Lkteos C.S.I.C. Arganda de1 Rey. Madrid. Spain
and J.
BELLANATO
Instituto de Optica. C.S.I.C.-Serrano,
121. Madrid-6. Spain.
(Received 26 October 1975) Abstmc-The
variable temperature i.r. spectra of a series of aliphatic (4-16) and aromatic (17-25) 1,2diketones are reported. Changes in the appearance of bands assignable to the c=O and Q-H stretchina vibrations, at diflerent temperatures, are interpreted in terms of conformational equilibria between-tigh and low energy forms. _ INTltODUCI’ION
1,2-Diketones may exist in s-trans (1) and s-cis (2) conformation. Electron difraction measurements [l] on biacetyl(4) have suggested that this molecule exists in a planar s-trans conformation. Measurements of dipole moments, refractivities and molar Kerr constant [2] have later indicated that the effective conformation of 4 is non-planar (quusitrans) with intercarbonyl azimuthal angle of ca. 160”. The s-mans conformation (1) of aliphatic 1,Zdiketones has also been suggested by i.r. and Raman studies [3-51, the appearance of only a band attributable to the C=O stretching vibration in the i.r. spectra of these compounds being taken as indication of the coplanarity of the carbonyl groups in the s-trans (1) form. The failure to detect any band assignable to the s-cis (2) form in the i.r. spectrum of biacetyl (4) has been discussed [4] and it has been estimated that dipoledipole repulsion between the carbonyl groups and van der Waals interactions between methyl groups should destabilize the s-cis form (2) by 4.7 Kcal/mol [6]. Furthermore, it has been anticipated [4] that the s-cis form (2) present, if any, would enolize and the energy of the intramolecular hydrogen bonding in the enolic form (3) would considerably reduce the energy difference which favors the s-tram form (1). According to X-rays diffraction [7], dipole moment [ 1,8,9] and paracor [lo] measurements the effective conformation of benzil (19) is non-planar *Present address: Instituto de Quimica Orgtica General-C.S.I.C. Juan de la Cierva, 3. Madrid-6, Spain.
g
!
?
R-co-CO-R’ 12 : R = R=f-
4 : R=R’=CHJ
C3”,
1_6: R i I#‘= w-l-
R=U.n-C*Hg
p.
C&H9
5 : R=R’.n-C3H7
tz
: R=u.C&-l~
?:
R=R’=n-C,,Hg
lj
: R=R’=p-CH3-C#,,
5:
R=R’=n-C+,,,
lj
: R=R’=p-CI-CgHb
p:
I?=~=‘;--C,H,g
Z_O
: R=R’=p-CH30-CgH4
R=R’=n-CgH,g
2j
: R.R’=p-N02-C6H4
22
: R=R’=m-NO2-C&,
10. Q
: RIR’.“-C,,H~~
12
:Ri
12 1s
R’. n-
C,3H27
22
:Ri
:Ri
IS’= n-
C,&,
23
: R=C&;
:Ri
CH3
25
: R=~-CH~-CsHL;R’=o.o.~-CH2-C~H2
; R’.
C~H,S
R’= O.O,P - CH2 - CgH2 Rko.o.p-CH3-CsH2
with intercarbonyl dihedral angle of ca. 90”, each carbonyl group being coplanar to the adjacent phenyl group. Consequently, the i.r. spectrum of 19 shows two bands assigned to the asymmetric and symmetric stretching vibrations of the non-planar dicarbonyl system [ 111. It has been reported that the high temperature (lOO-180°C) i.r. spectra of biacetyl (4) and benxil (19) show new bands attributed to the presence of higher energy forms [12]. We have recently published a general spectroscopic study (i.r., u.v., NMR, m.s.) of a series of aliphatic (4-16) and
M.JuAREz,M.
1676
MAFCIN-LOMAS
aromatic (17-25) 1,2-diketones and this paper is concerned with the effect of temperature on the i.r. spectra of these compounds. The results outlined below show that the i.r. spectra of the aliphatic 1,2-diketones 5-15 at temperature between 80 and 200°C present new bands which can be assigned to the presence of a mixture of s-tram (l), s-cis (2) and s-cis enolic (3) forms. We have not observed, however, the reported [12] changes in the spectrum of biacetyl (4). The variable temperature i.r. spectra of the aromatic substances (17-25) also suggest the presence of several forms. The enthalphy difference between high and low energy forms has been calculated from band intensity measurements.
andJ.
BELLANATO
from the corresponding ethyl (5-8, 15-16) hutyl (9-10) or methyl esters (11-13) by acyloin condensation and subsequent oxidation of the resulting acyloins. Compound 14 was synthesized from heptylidene acetoacetate [14]. Benzil (17) was purchased from Pluka and recrystallised from CC&. p,p’-Dimethylbenzil (18), p,p’-dichlorobenzil (19) and p,p’-dimethoxybenxil (20) were prepared from p-methyl, p-chloro and p-methoxy benzaldehyde respectively by benxoin condensation followed by oxidation with HNOs or CuSO,-pyridine [ 16-181. p,p’-Dinitrobenzil (21) was prepared through 4,5diphenylglyoxalone [19] and mm’-dinitrobenzil (22) by nitration of benzil [15]. 2,4,6-Trimethyl-benzil (24), 2,4,4’,6-tetramethylbenzil (25), 2,2’,4’,6,6’-hexamethylbenzil (23) were obtained by reaction of mesityl glyoxal with benzene, toluene and mesitylene respectively [20,21]. All compounds were pure by g.1.c. and/or t.1.c.
EXPERIMENTAL
RESULTS AND
Spectra were recorded on a Perkin-Elmer 457 spectrophotometer having a spectral band width of 2.5 cm-’ at 175Ocm-’ and 3.8cm-’ at 35OOcn-‘. Indene, polystyrene and water vapour were used for instrument calibration, and the reported frequencies are estimated to be accurate to within *3 cm-‘. 0.008-0.6 M solutions in 1.0 and 0.1 mm cells were studied. Spectra for very diluted solutions were measured in the O-H stretching region with 1 cm and 4cm quartz cells. Nujol, decalin, chloroform, carbon tetrachloride, tetrachloroethylene and o-dichlorobenzene (reagent grade) were used as solvents. A Beckman-R.I.I.C. variable temperature unit VLT-2 with power unit PS-1 or a cell heater J-l were used for variable temperature measurements. The temperature was measured by a sheated iron-constantan thermocouple for which a Beckman loin. recorder potentiometer was calibrated in terms of various standards. Temperature measurements are expected to be accurate to within *3”C.
DISCUMIONS
Spectral changes with temperature were studied in the range 1800-1600 cm-’ and 3700-3000 cm-’ for compounds 4-16 and 1800-1600 cn-’ for compounds 17-25. The spectra of compounds 5-15, 17-19, 21, 22, 24, and 25 showed variation with temperature in the regions examined. These variations were important when nujol or decaline solutions were used and when the samples were submitted to previous heating for several hours. As expected no changes with temperature were observed in the spectra of dipivaloyl (16) and mesitil (23). In contrast to an earlier publication [12] no variation was detected in the variable temperature spectrum of biacetyl (4). The effect of temperature on the i.r. spectra of compounds 5-15 is illustrated in Figs. l-3. At room temperature the 1800-1600 cm-’ spectral region showed a band at 1700-1720 cm-’ attributed
Materials Biacetyl (4) was supplied by Fluka and purified by repeated distillations. Diketones 5-16 were prepared [13]
r/
I;
Room temp.
J
L
cm-’
Fig. 1. Variable temperature
i.r. spectra in nujol (region 1800-16OOcm-‘) c = 0.62 M, 1 = 0.08 mm.
of octane-4,5-dione
(6);
1677
Variable temperature i.r. spectra of ahphatic and aromatic 1,2-diketones
Fig. 2. Variable temperature Lr. spectra in nujol (region 18~~~~-‘) of tetracosane-12,13dione (11) submitted to previous heating at 80”; c = 0.25 M, 1= 0.08 mm. to the vC=O allowed vibration of the dicarbonyl system in the S-trans conformation (1). The spectra of nujol and decaline solutions of 5-15 recorded from room temperature to 2OtYC showed the appearance of a shoulder at ca. 1760 cm-’ and two weak bands at 1675 and 165Ocm-‘. The shoulder disappeared by slow cooling but the bands at 1675 and 165Ocm-’ slightly increased with this treatment and only disappeared by keeping the samples in the cold for several days (Fig. 1). The spectra at room temperature of nujol and decahne solutions of compounds 5-15 submitted to previous heating to 80°C for several hours before recording, showed
the two bands at 1675 and 1650 cm-’ besides the 1700-1720cm-1 band. The spectra of these samples at temperatures between 80 and 200°C displayed two new bands at 1760 and 1735cm-‘. These bands disappeared by cooling down to room temperature and, as in the above experiments, the bands at 1675 and 165Ocm-’ first increased and later disappeared (Fig 2). The effect of temperature on the spectra of chloroform, carbon tetrachloride and te~ac~or~~ylene solutions of 5-15 was less marked probably due to the limitation imposed by the low boiling point of these solvents. The 3600-3OOOcm-’ region of the spectra of
J
I cm-’
Fig. 3. Variable temperature i-r. spectra in nujol (region 36~3380~-‘) (9); c = 0.24 M, 1= 1.OOmm.
of he~~e-8,9-~one
1678
M. JUAREZ,M. MARTIN-COMAS and J. BELLANATO
these substances (5-15) showed, at room temperature, a weak band at 341Ocm-’ attributed to the
first harmonic of the vC=O vibration and a shoulder at 3480 cm-’ tentatively assigned to the O-H stretching vibration of the intermoleculary bonded enolic form of the diketones on the s-trans conformation [13]. The effect of temperature on this spectral region is shown in Fig. 3. The 3480cm-’ band decreased with temperature simultaneously appearing a new band at 3540 cm-‘, which increased with temperature. The 3540cm-’ band completely disappeared on cooling the samples but the 3410 cm-’ band increased, broadened and shifted towards 3420 cm-’ with this treatment; this increasing and broadening of the absorption at 3410 cm-’ coincided with the appearance of the bands at 1675 and 1650 cm-’ in the 18001600 cm-’ region. As in the above experiments, the original spectra were obtained after several days at room temperature. In this case these effects were also more remarkable when the samples were submitted to heating before the experiments. The i.r. spectra of the aromatic 1.2-diketones 17-25 showed in solution a strong band between 1670 and 1700 cm-‘; this band was generally asymmetric and in the case of benzil was clearly split. The spectra of nujol and decaline solutions of these compounds recorded from 80” to 200°C showed also variations in the 1800-1600 cm-’ spectral region. The spectra of benzil (17) and p,p’-dimethylbenzil (18) and p,p’-dichlorobenzil (19) showed new bands at ca. 1745 and 1720 cm-‘. p,p’-Dinitrom,m’-dinitrobenzil (22), 2,4,6benzil (21), trimethylbenzil (24) and 2,4,4’,6-tetramethylbenzil (25) behaved similarly only when the samples were previously heated for several days. As in the above experiments all new bands disappeared on cooling the original spectra being resumed in all cases; in some experiments, however, as in the case of benzil (17) the band at 1720 cm-’ did not completely disappear which may be attributed to partial decomposition as deduced from other spectral regions. In the spectra of p,p’-dimethoxybenzil (20) and mesitil (23) no apparent changes were observed even after prolongated heating. In the case of the aliphatic 1,2-diketones 5-15 the results just described seem to indicate that at room temperature these substances exist almost exclusively in a s-tram or quasi-trans conformation. The appearing of new bands at 1760 and 1735 cm-’ upon heating can be interpreted assuming the presence of a s-cis or non-trans rotational isomer (2) both bands being assigned to the asymmetric and symmetric stretching vibrations of the
dicarbonyl system in this form. From the apparent intensities of the bands assigned to the s-trans or quasi-tram (1) and non-trans (2) forms at different temperatures, an approximate enthalpy difference between these forms could be calculated by standard methods [22]. The calculated values were in the range 2-3.5 Kcal/mol for all compounds studied (5-14). This destabilization arising from strong steric and polar interactions in such a conformer (2) would explain the rapid enolization to form 3 which slowly reverts to form 1. The presence of 3 in the conformational equilibrium can be inferred from the simultaneous appearance of bands at 1675, 1650 and the increasing of the absorption at 3410 cm-’ which could be assigned to the C=O, C=C and O-H stretching vibrations respectively. All the aliphatic 1,2-diketones in which R and R were n-alkyl groups studied in this paper with the exception of biacetyl (4) behave similarly. Bands attributable to the presence of conformers other than the s- trans (1) have not been detected in the spectrum of biacetyl(4) studied at different temperatures. In this respect our results differ from ‘those reported by STERK [ 121 who claimed the presence of a band assignable to a form other than the
s-trans in the spectrum of biacetyl (4) recorded as net liquid.* In the aromatic series the appearing of new bands with the increasing of temperature might also be attributed to the presence of different conformers with different intercarbonyl dihedral angles. Whether these angles vary from the normal value of around 90” to 180” or from 90” to 0” can not be obviously inferred from the i.r. spectra at the present stage of our study. In the case of benzil (17) our results are also in disagreement with STERK’S [12]. According high
1720cm-‘. 1720
to this author the i.r. spectrum of 17 at
temperature
displays
However
and 1740 cm-‘.
a
new
we observed
band
at
two bands at
In any case, the results we
obtained
in this series are difficult to interpret structural terms.
in
REFERENCES [l]
J. E. Lu VALLE and V. SC.,61,352O (1939).
SCHOMAKER,
J. Am.
Chn.
[2] P. H. CURETON,C. G. LE F&RE and R. J. W. LE F&RE, J. Chem. Sot., 4447 (1961). [3] R. S. RASMUSSEN, D. D. TUNICLIFF and R. R. BRATTAIN,J. Am. Chem. Sot., 71, 1068 (1949). [4] K. NOACKand R. N. JONES,Z. Electrochem.,64,707 (1960). [.5] H. W. F. KOHLRAUSCH and A. PONGRATZ,Ber., 67B, 976 (1934). * No experimentaldetails are given
in this paper.
Variable temperature i.r. spectra of aliphatic and aromatic 1,2diketones [6] T. h'bYAZAWA. J. Chem. Sot. Jawn 74.743 (1953). K. bNSDALi, N&e i4j, 1023 (1939). 181 C. C. CALDWELL and R. J. W. LE F&v=, J. Chcm. Sec. 1614 (1939). [91 C. C. CALDWELL and R. J. W. LE F~VRE, Nature 143, 803 (1939). DOI T. W. GIBLING, J. Chem. Sot. 661 (1942). I. BERNAL, Nature 200, 1318 (1963). 1:;; H. STERK, Monatsh. Chcm 99, 999 (1968). [I31 M. JUAREZ, M. MARTIN L~MAS and J. BELLANATO, An&s Quint. 72 (1976). 1141 M. IGARISHIand H. MIDORIKAWA,J. Org. Chem 29, 3313 (1964).
[71 I. E. KNAccs’and
I151 F. D. CHA~AWAY
&
<
Sot. 577 (1927).
1679
and E. A. COULSON. J. Chem
[16] M. GOMBERG and F. J. VAN NA~A, J. Am. Chem. Sec. 51, 2238 (1929). [17] R. STIERLIN,Ber. 22, 376 (1889). [18] H. B. NISBERT,J. Chem. Sec. 3121 (1928). [19] F. D. CHA~AWAY and E. A. COULSON, J. Chem. Sot. 1361 (1928). [20] A. GRAY and R. C. FUSON, J. Am. Chcm. Sot. 56, 739 (1934). [21] R. C. FUSON,W. S. EMERSONand H. H. WINSTOCK, J. Am. Chem Sec. 61, 412 (1939). [22] A. J. BOWLES,W. 0. GEORGE and W. F. MADDAMS, J. Chem. Sec. (B) 810 (1969).
Variable
temperature
1976.Printed inNortkm lrebd
ix. spectra of aliphatic diketones
and aromatic
1,2-
M.JUAREZ and M. MAR~~-LOMAS Laboratorio de Quimica Biol6gica. Institute de Productos Lkteos C.S.I.C. Arganda de1 Rey. Madrid. Spain
and J.
BELLANATO
Instituto de Optica. C.S.I.C.-Serrano,
121. Madrid-6. Spain.
(Received 26 October 1975) Abstmc-The
variable temperature i.r. spectra of a series of aliphatic (4-16) and aromatic (17-25) 1,2diketones are reported. Changes in the appearance of bands assignable to the c=O and Q-H stretchina vibrations, at diflerent temperatures, are interpreted in terms of conformational equilibria between-tigh and low energy forms. _ INTltODUCI’ION
1,2-Diketones may exist in s-trans (1) and s-cis (2) conformation. Electron difraction measurements [l] on biacetyl(4) have suggested that this molecule exists in a planar s-trans conformation. Measurements of dipole moments, refractivities and molar Kerr constant [2] have later indicated that the effective conformation of 4 is non-planar (quusitrans) with intercarbonyl azimuthal angle of ca. 160”. The s-mans conformation (1) of aliphatic 1,Zdiketones has also been suggested by i.r. and Raman studies [3-51, the appearance of only a band attributable to the C=O stretching vibration in the i.r. spectra of these compounds being taken as indication of the coplanarity of the carbonyl groups in the s-trans (1) form. The failure to detect any band assignable to the s-cis (2) form in the i.r. spectrum of biacetyl (4) has been discussed [4] and it has been estimated that dipoledipole repulsion between the carbonyl groups and van der Waals interactions between methyl groups should destabilize the s-cis form (2) by 4.7 Kcal/mol [6]. Furthermore, it has been anticipated [4] that the s-cis form (2) present, if any, would enolize and the energy of the intramolecular hydrogen bonding in the enolic form (3) would considerably reduce the energy difference which favors the s-tram form (1). According to X-rays diffraction [7], dipole moment [ 1,8,9] and paracor [lo] measurements the effective conformation of benzil (19) is non-planar *Present address: Instituto de Quimica Orgtica General-C.S.I.C. Juan de la Cierva, 3. Madrid-6, Spain.
g
!
?
R-co-CO-R’ 12 : R = R=f-
4 : R=R’=CHJ
C3”,
1_6: R i I#‘= w-l-
R=U.n-C*Hg
p.
C&H9
5 : R=R’.n-C3H7
tz
: R=u.C&-l~
?:
R=R’=n-C,,Hg
lj
: R=R’=p-CH3-C#,,
5:
R=R’=n-C+,,,
lj
: R=R’=p-CI-CgHb
p:
I?=~=‘;--C,H,g
Z_O
: R=R’=p-CH30-CgH4
R=R’=n-CgH,g
2j
: R.R’=p-N02-C6H4
22
: R=R’=m-NO2-C&,
10. Q
: RIR’.“-C,,H~~
12
:Ri
12 1s
R’. n-
C,3H27
22
:Ri
:Ri
IS’= n-
C,&,
23
: R=C&;
:Ri
CH3
25
: R=~-CH~-CsHL;R’=o.o.~-CH2-C~H2
; R’.
C~H,S
R’= O.O,P - CH2 - CgH2 Rko.o.p-CH3-CsH2
with intercarbonyl dihedral angle of ca. 90”, each carbonyl group being coplanar to the adjacent phenyl group. Consequently, the i.r. spectrum of 19 shows two bands assigned to the asymmetric and symmetric stretching vibrations of the non-planar dicarbonyl system [ 111. It has been reported that the high temperature (lOO-180°C) i.r. spectra of biacetyl (4) and benxil (19) show new bands attributed to the presence of higher energy forms [12]. We have recently published a general spectroscopic study (i.r., u.v., NMR, m.s.) of a series of aliphatic (4-16) and
M.JuAREz,M.
1676
MAFCIN-LOMAS
aromatic (17-25) 1,2-diketones and this paper is concerned with the effect of temperature on the i.r. spectra of these compounds. The results outlined below show that the i.r. spectra of the aliphatic 1,2-diketones 5-15 at temperature between 80 and 200°C present new bands which can be assigned to the presence of a mixture of s-tram (l), s-cis (2) and s-cis enolic (3) forms. We have not observed, however, the reported [12] changes in the spectrum of biacetyl (4). The variable temperature i.r. spectra of the aromatic substances (17-25) also suggest the presence of several forms. The enthalphy difference between high and low energy forms has been calculated from band intensity measurements.
andJ.
BELLANATO
from the corresponding ethyl (5-8, 15-16) hutyl (9-10) or methyl esters (11-13) by acyloin condensation and subsequent oxidation of the resulting acyloins. Compound 14 was synthesized from heptylidene acetoacetate [14]. Benzil (17) was purchased from Pluka and recrystallised from CC&. p,p’-Dimethylbenzil (18), p,p’-dichlorobenzil (19) and p,p’-dimethoxybenxil (20) were prepared from p-methyl, p-chloro and p-methoxy benzaldehyde respectively by benxoin condensation followed by oxidation with HNOs or CuSO,-pyridine [ 16-181. p,p’-Dinitrobenzil (21) was prepared through 4,5diphenylglyoxalone [19] and mm’-dinitrobenzil (22) by nitration of benzil [15]. 2,4,6-Trimethyl-benzil (24), 2,4,4’,6-tetramethylbenzil (25), 2,2’,4’,6,6’-hexamethylbenzil (23) were obtained by reaction of mesityl glyoxal with benzene, toluene and mesitylene respectively [20,21]. All compounds were pure by g.1.c. and/or t.1.c.
EXPERIMENTAL
RESULTS AND
Spectra were recorded on a Perkin-Elmer 457 spectrophotometer having a spectral band width of 2.5 cm-’ at 175Ocm-’ and 3.8cm-’ at 35OOcn-‘. Indene, polystyrene and water vapour were used for instrument calibration, and the reported frequencies are estimated to be accurate to within *3 cm-‘. 0.008-0.6 M solutions in 1.0 and 0.1 mm cells were studied. Spectra for very diluted solutions were measured in the O-H stretching region with 1 cm and 4cm quartz cells. Nujol, decalin, chloroform, carbon tetrachloride, tetrachloroethylene and o-dichlorobenzene (reagent grade) were used as solvents. A Beckman-R.I.I.C. variable temperature unit VLT-2 with power unit PS-1 or a cell heater J-l were used for variable temperature measurements. The temperature was measured by a sheated iron-constantan thermocouple for which a Beckman loin. recorder potentiometer was calibrated in terms of various standards. Temperature measurements are expected to be accurate to within *3”C.
DISCUMIONS
Spectral changes with temperature were studied in the range 1800-1600 cm-’ and 3700-3000 cm-’ for compounds 4-16 and 1800-1600 cn-’ for compounds 17-25. The spectra of compounds 5-15, 17-19, 21, 22, 24, and 25 showed variation with temperature in the regions examined. These variations were important when nujol or decaline solutions were used and when the samples were submitted to previous heating for several hours. As expected no changes with temperature were observed in the spectra of dipivaloyl (16) and mesitil (23). In contrast to an earlier publication [12] no variation was detected in the variable temperature spectrum of biacetyl (4). The effect of temperature on the i.r. spectra of compounds 5-15 is illustrated in Figs. l-3. At room temperature the 1800-1600 cm-’ spectral region showed a band at 1700-1720 cm-’ attributed
Materials Biacetyl (4) was supplied by Fluka and purified by repeated distillations. Diketones 5-16 were prepared [13]
r/
I;
Room temp.
J
L
cm-’
Fig. 1. Variable temperature
i.r. spectra in nujol (region 1800-16OOcm-‘) c = 0.62 M, 1 = 0.08 mm.
of octane-4,5-dione
(6);
1677
Variable temperature i.r. spectra of ahphatic and aromatic 1,2-diketones
Fig. 2. Variable temperature Lr. spectra in nujol (region 18~~~~-‘) of tetracosane-12,13dione (11) submitted to previous heating at 80”; c = 0.25 M, 1= 0.08 mm. to the vC=O allowed vibration of the dicarbonyl system in the S-trans conformation (1). The spectra of nujol and decaline solutions of 5-15 recorded from room temperature to 2OtYC showed the appearance of a shoulder at ca. 1760 cm-’ and two weak bands at 1675 and 165Ocm-‘. The shoulder disappeared by slow cooling but the bands at 1675 and 165Ocm-’ slightly increased with this treatment and only disappeared by keeping the samples in the cold for several days (Fig. 1). The spectra at room temperature of nujol and decahne solutions of compounds 5-15 submitted to previous heating to 80°C for several hours before recording, showed
the two bands at 1675 and 1650 cm-’ besides the 1700-1720cm-1 band. The spectra of these samples at temperatures between 80 and 200°C displayed two new bands at 1760 and 1735cm-‘. These bands disappeared by cooling down to room temperature and, as in the above experiments, the bands at 1675 and 165Ocm-’ first increased and later disappeared (Fig 2). The effect of temperature on the spectra of chloroform, carbon tetrachloride and te~ac~or~~ylene solutions of 5-15 was less marked probably due to the limitation imposed by the low boiling point of these solvents. The 3600-3OOOcm-’ region of the spectra of
J
I cm-’
Fig. 3. Variable temperature i-r. spectra in nujol (region 36~3380~-‘) (9); c = 0.24 M, 1= 1.OOmm.
of he~~e-8,9-~one
1678
M. JUAREZ,M. MARTIN-COMAS and J. BELLANATO
these substances (5-15) showed, at room temperature, a weak band at 341Ocm-’ attributed to the
first harmonic of the vC=O vibration and a shoulder at 3480 cm-’ tentatively assigned to the O-H stretching vibration of the intermoleculary bonded enolic form of the diketones on the s-trans conformation [13]. The effect of temperature on this spectral region is shown in Fig. 3. The 3480cm-’ band decreased with temperature simultaneously appearing a new band at 3540 cm-‘, which increased with temperature. The 3540cm-’ band completely disappeared on cooling the samples but the 3410 cm-’ band increased, broadened and shifted towards 3420 cm-’ with this treatment; this increasing and broadening of the absorption at 3410 cm-’ coincided with the appearance of the bands at 1675 and 1650 cm-’ in the 18001600 cm-’ region. As in the above experiments, the original spectra were obtained after several days at room temperature. In this case these effects were also more remarkable when the samples were submitted to heating before the experiments. The i.r. spectra of the aromatic 1.2-diketones 17-25 showed in solution a strong band between 1670 and 1700 cm-‘; this band was generally asymmetric and in the case of benzil was clearly split. The spectra of nujol and decaline solutions of these compounds recorded from 80” to 200°C showed also variations in the 1800-1600 cm-’ spectral region. The spectra of benzil (17) and p,p’-dimethylbenzil (18) and p,p’-dichlorobenzil (19) showed new bands at ca. 1745 and 1720 cm-‘. p,p’-Dinitrom,m’-dinitrobenzil (22), 2,4,6benzil (21), trimethylbenzil (24) and 2,4,4’,6-tetramethylbenzil (25) behaved similarly only when the samples were previously heated for several days. As in the above experiments all new bands disappeared on cooling the original spectra being resumed in all cases; in some experiments, however, as in the case of benzil (17) the band at 1720 cm-’ did not completely disappear which may be attributed to partial decomposition as deduced from other spectral regions. In the spectra of p,p’-dimethoxybenzil (20) and mesitil (23) no apparent changes were observed even after prolongated heating. In the case of the aliphatic 1,2-diketones 5-15 the results just described seem to indicate that at room temperature these substances exist almost exclusively in a s-tram or quasi-trans conformation. The appearing of new bands at 1760 and 1735 cm-’ upon heating can be interpreted assuming the presence of a s-cis or non-trans rotational isomer (2) both bands being assigned to the asymmetric and symmetric stretching vibrations of the
dicarbonyl system in this form. From the apparent intensities of the bands assigned to the s-trans or quasi-tram (1) and non-trans (2) forms at different temperatures, an approximate enthalpy difference between these forms could be calculated by standard methods [22]. The calculated values were in the range 2-3.5 Kcal/mol for all compounds studied (5-14). This destabilization arising from strong steric and polar interactions in such a conformer (2) would explain the rapid enolization to form 3 which slowly reverts to form 1. The presence of 3 in the conformational equilibrium can be inferred from the simultaneous appearance of bands at 1675, 1650 and the increasing of the absorption at 3410 cm-’ which could be assigned to the C=O, C=C and O-H stretching vibrations respectively. All the aliphatic 1,2-diketones in which R and R were n-alkyl groups studied in this paper with the exception of biacetyl (4) behave similarly. Bands attributable to the presence of conformers other than the s- trans (1) have not been detected in the spectrum of biacetyl(4) studied at different temperatures. In this respect our results differ from ‘those reported by STERK [ 121 who claimed the presence of a band assignable to a form other than the
s-trans in the spectrum of biacetyl (4) recorded as net liquid.* In the aromatic series the appearing of new bands with the increasing of temperature might also be attributed to the presence of different conformers with different intercarbonyl dihedral angles. Whether these angles vary from the normal value of around 90” to 180” or from 90” to 0” can not be obviously inferred from the i.r. spectra at the present stage of our study. In the case of benzil (17) our results are also in disagreement with STERK’S [12]. According high
1720cm-‘. 1720
to this author the i.r. spectrum of 17 at
temperature
displays
However
and 1740 cm-‘.
a
new
we observed
band
at
two bands at
In any case, the results we
obtained
in this series are difficult to interpret structural terms.
in
REFERENCES [l]
J. E. Lu VALLE and V. SC.,61,352O (1939).
SCHOMAKER,
J. Am.
Chn.
[2] P. H. CURETON,C. G. LE F&RE and R. J. W. LE F&RE, J. Chem. Sot., 4447 (1961). [3] R. S. RASMUSSEN, D. D. TUNICLIFF and R. R. BRATTAIN,J. Am. Chem. Sot., 71, 1068 (1949). [4] K. NOACKand R. N. JONES,Z. Electrochem.,64,707 (1960). [.5] H. W. F. KOHLRAUSCH and A. PONGRATZ,Ber., 67B, 976 (1934). * No experimentaldetails are given
in this paper.
Variable temperature i.r. spectra of aliphatic and aromatic 1,2diketones [6] T. h'bYAZAWA. J. Chem. Sot. Jawn 74.743 (1953). K. bNSDALi, N&e i4j, 1023 (1939). 181 C. C. CALDWELL and R. J. W. LE F&v=, J. Chcm. Sec. 1614 (1939). [91 C. C. CALDWELL and R. J. W. LE F~VRE, Nature 143, 803 (1939). DOI T. W. GIBLING, J. Chem. Sot. 661 (1942). I. BERNAL, Nature 200, 1318 (1963). 1:;; H. STERK, Monatsh. Chcm 99, 999 (1968). [I31 M. JUAREZ, M. MARTIN L~MAS and J. BELLANATO, An&s Quint. 72 (1976). 1141 M. IGARISHIand H. MIDORIKAWA,J. Org. Chem 29, 3313 (1964).
[71 I. E. KNAccs’and
I151 F. D. CHA~AWAY
&
<
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1679
and E. A. COULSON. J. Chem
[16] M. GOMBERG and F. J. VAN NA~A, J. Am. Chem. Sec. 51, 2238 (1929). [17] R. STIERLIN,Ber. 22, 376 (1889). [18] H. B. NISBERT,J. Chem. Sec. 3121 (1928). [19] F. D. CHA~AWAY and E. A. COULSON, J. Chem. Sot. 1361 (1928). [20] A. GRAY and R. C. FUSON, J. Am. Chcm. Sot. 56, 739 (1934). [21] R. C. FUSON,W. S. EMERSONand H. H. WINSTOCK, J. Am. Chem Sec. 61, 412 (1939). [22] A. J. BOWLES,W. 0. GEORGE and W. F. MADDAMS, J. Chem. Sec. (B) 810 (1969).