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Studies of hydrogen bonding—Part XI
Research notes
1698
Studies of hydrogen bonding-Parf; XI The hydrogen bonding ability of 2,6-dim&hyl-4-pyrone* GLUXSER and THOMPSON [Z] have shown that t)he infrared spectrum of 2,6-dimethyl-4-pyrone changes completely on complex formation with iodine. These changes have been explained [3] as being due to delocalization of the unshared electron pairs on the ring-ether oxygen atom to give a pseudoaromatic structure. In conncxion with these findings we found it of interest to study the hydrogen bonding ability of 2,6-dimethyl-4-pyrone since hydrogen bond formation through the carbonyl group would favour a pseudoaromatic structure. Hence it W&Eexpected that 2,6-d~ethyl-4-pyro~e would readily take part in hydrogen bonding. Our results, using Ccl, as solvent, are tabulated below (the method and concentrations used were as described
2,6-dimethyl-4-pyrone A%a (cm-‘) Phmlol Pentaohlorophenol
350
459
196.4 186.1
SOa Easl (l/mole)
Av,,o (cm-r)
Avc=c (cm-r)
73.3 691
22 27
6 6
elsewhere [4]). As can be seen, 2,6-dimethyl-4pyrone forms very strong hydrogen bonds with phenol and pentachlorophenol in comparison with other carbonyl compounds [d, 51. N-methylpyridone-2 also forms a hydrogen bond with phenol of comparable strength &ox = 345 crnwt, If%@= a = 202.4, Ez = 77-S l/mole) [4]* This may be similarly explained as being due to the formation of pseudoaromatic ring, in this case by the delocalization of the unshared pair on the nitrogen atom. In agreement with the results of COOR [6] we have localized the C=O stretching frequency in 2,6-dimethyl-4.pyrone at 1639 om-l and the C==C stretching in the ring at 1679 cm-l (in Ccl,, 063 M). The frequency shifts of these bands upon hydrogen bondings are shown in the table above. ~~rthe~ore~ the C=O and C==C stretching frequencies are shifted from 1639 and 1679 cm-1 in carbon ~tra~hloride solution to 1612 and 1674 in solid &ate [as KBr disk). This strongly indicates an intermolecular association and may be attributed to a dipole-dipole association, e.g. \&
-
at/’ . . ..o
/ \ The infrared spectrum of 2,6-dimethyl-4-pyr~ne hydrochloride has a strong, broad band in the region 2~0~30#0 cm-r which is assigned to a hydrogen stretching motion of the protona~d carbonyl group IS]. The C=O and C=C stretching frequencies appear at 1492 and 1646 cm-r (as HBr disk) indicating a very strong interaction of the C--0 group and consequently great changes in the electron distribution in the ring, TRORGRAMSTAD
* For Part X see Ref. [I] [I] T. GRANSTAD, Spectrockim. Acta 19, 1931 (1963). 12lD.L. GLUSKER andH.W. THOMPSON,J.C~~~.SOC.~~~ f3] L. J. BE~AMY and P. E. ROCASH, Spectrochim Acta f47T. GXA&EJTAD andW.J. ~CLE,VIg,ActaGilena,S~~. [5] T. GRABSTAD,~~~~~~h~~. A& 32, 497 (1963). IS] D. COOK,Can. J. Chem. 39, 1164 (1960).
(I%%).
16, 30 (1960). IS,1369f1962).
1698
Studies of hydrogen bonding-Parf; XI The hydrogen bonding ability of 2,6-dim&hyl-4-pyrone* GLUXSER and THOMPSON [Z] have shown that t)he infrared spectrum of 2,6-dimethyl-4-pyrone changes completely on complex formation with iodine. These changes have been explained [3] as being due to delocalization of the unshared electron pairs on the ring-ether oxygen atom to give a pseudoaromatic structure. In conncxion with these findings we found it of interest to study the hydrogen bonding ability of 2,6-dimethyl-4-pyrone since hydrogen bond formation through the carbonyl group would favour a pseudoaromatic structure. Hence it W&Eexpected that 2,6-d~ethyl-4-pyro~e would readily take part in hydrogen bonding. Our results, using Ccl, as solvent, are tabulated below (the method and concentrations used were as described
2,6-dimethyl-4-pyrone A%a (cm-‘) Phmlol Pentaohlorophenol
350
459
196.4 186.1
SOa Easl (l/mole)
Av,,o (cm-r)
Avc=c (cm-r)
73.3 691
22 27
6 6
elsewhere [4]). As can be seen, 2,6-dimethyl-4pyrone forms very strong hydrogen bonds with phenol and pentachlorophenol in comparison with other carbonyl compounds [d, 51. N-methylpyridone-2 also forms a hydrogen bond with phenol of comparable strength &ox = 345 crnwt, If%@= a = 202.4, Ez = 77-S l/mole) [4]* This may be similarly explained as being due to the formation of pseudoaromatic ring, in this case by the delocalization of the unshared pair on the nitrogen atom. In agreement with the results of COOR [6] we have localized the C=O stretching frequency in 2,6-dimethyl-4.pyrone at 1639 om-l and the C==C stretching in the ring at 1679 cm-l (in Ccl,, 063 M). The frequency shifts of these bands upon hydrogen bondings are shown in the table above. ~~rthe~ore~ the C=O and C==C stretching frequencies are shifted from 1639 and 1679 cm-1 in carbon ~tra~hloride solution to 1612 and 1674 in solid &ate [as KBr disk). This strongly indicates an intermolecular association and may be attributed to a dipole-dipole association, e.g. \&
-
at/’ . . ..o
/ \ The infrared spectrum of 2,6-dimethyl-4-pyr~ne hydrochloride has a strong, broad band in the region 2~0~30#0 cm-r which is assigned to a hydrogen stretching motion of the protona~d carbonyl group IS]. The C=O and C=C stretching frequencies appear at 1492 and 1646 cm-r (as HBr disk) indicating a very strong interaction of the C--0 group and consequently great changes in the electron distribution in the ring, TRORGRAMSTAD
* For Part X see Ref. [I] [I] T. GRANSTAD, Spectrockim. Acta 19, 1931 (1963). 12lD.L. GLUSKER andH.W. THOMPSON,J.C~~~.SOC.~~~ f3] L. J. BE~AMY and P. E. ROCASH, Spectrochim Acta f47T. GXA&EJTAD andW.J. ~CLE,VIg,ActaGilena,S~~. [5] T. GRABSTAD,~~~~~~h~~. A& 32, 497 (1963). IS] D. COOK,Can. J. Chem. 39, 1164 (1960).
(I%%).
16, 30 (1960). IS,1369f1962).