Thermochemical and Theoretical Study of Some Quinoxaline 1,4-Dioxides and of Pyrazine 1,4-Dioxide Page: 3,722
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J. Org. Chem. 1997, 62, 3722-3726
Thermochemical and Theoretical Study of Some Quinoxaline
1,4-Dioxides and of Pyrazine 1,4-Dioxide
W. E. Acree, Jr.,*, Joyce R. Powell,t Sheryl A. Tucker,t Maria D. M. C. Ribeiro da Silva,*
M. Agostinha R. Matos,t J. M. Gongalves,* L. M. N. B. F. Santos,* V. M. F. Morais, and
G. PilcherT
Department of Chemistry, University of North Texas, Denton, Texas, 76203-0068, Departamento de
Quimica, Faculdade de Ciencias, Universidade do Porto, P-4150, Porto, Portugal, Instituto de Ciencias
Biomedicas Abel Salazar, Universidade do Porto, P-4050 Porto, Portugal, and Department of Chemistry,
University of Manchester, Manchester M13 9PL, UK
Received November 18, 1996
The following standard molar enthalpies of formation in the gaseous state at 298.15 K were
determined from the enthalpies of combustion of the crystalline solids, and their enthalpies of
sublimation and the mean (N-O) bond dissociation enthalpies were derived. AfR%(g); (D(N-O))
(kJ mol-1): quinoxaline 1,4-dioxide, 227.1 + 2.4; 255.8 + 2.0; 2-methylquinoxaline 1,4-dioxide, 169.9
+ 7.2; 268.3 + 4.9; 2-methyl-3-acetylquinoxaline 1,4-dioxide, 33.1 + 5.0; 251.6 + 4.2; 2-phenyl-3-
benzoylquinoxaline 1,4-dioxide, 355.2 + 7.1; 227.3 + 5.4; 2-methyl-3-carbomethoxyquinoxaline 1,4-
dioxide, -148.7 + 3.2; 242.3 + 3.9; pyrazine 1,4-dioxide, 186.5 + 1.9; 254.0 + 2.3; 2-methyl-5-
pyrazinecarboxylic acid, -213.6 + 1.7. Unconstrained geometry optimizations by ab initio
calculations showed the effect of steric hindrance on changes in extended delocalizations and were
in accord with the trends in the mean bond dissociation enthalpies.Introduction
A reaction scheme has been employed by Holm and his
colleagues,1'2 to order various oxygenated species, includ-
ing N-oxides, in terms of their abilities to transfer oxygen
atoms in molybdenum(IV) to molybdenum(VI) conver-
sions, by considering the enthalpy of the reaction
RN(g) + 1/202 - RN+ - 0_
These particular transfers are important in the bio-
physical chemistry of molybdenum complexes, and the
order of decreasing enthalpy of reaction provides a
method for predicting the feasibility of such conversions.
Limited enthalpies of formation have restricted the scale
to a "scarce" selected compounds. The dissociation en-
thalpy of the (N+-O-) dative covalent bond shows large
variations depending on the molecular environment in
the immediate vicinity of the bond. The present paper
reports the mean molar (N+-O-) bond dissociation en-
thalpies, (D(N-O)), in pyrazine di-N-oxide and in the
quinoxaline di-N-oxide derivatives shown in Scheme 1.
The standard molar enthalpies of formation of the
compounds 1-7 in the gaseous state were derived from
the enthalpies of combustion of the crystalline solids and
the enthalpies of sublimation. To derive the mean (N+-
O-) bond dissociation enthalpies, the enthalpies of forma-
tion of the corresponding quinoxaline derivatives and of
pyrazine are required. Unfortunately, quinoxaline de-
rivatives are so difficult to obtain in a sufficiently pure
condition for precise combustion measurements that only
two experimental values are available: for quinoxaline3
University of North Texas.
$ Departamento de Quimica, Universidade do Porto.
Instituto de Ciencias Biomedicas, Universidade do Porto.
University of Manchester.
Abstract published in Advance ACS Abstracts, May 15, 1997.
(1) Holm, R. H. Chem. Rev. 1987, 87, 1401.
(2) Caradonna, J. P.; Reddy, P. R.; Holm, R. H. J. Am. Chem. Soc.
1988, 110, 2139.
(3) Ribeiro da Silva, M. A. V.; Matos, M. A. R.; Morais, V. M. F. J.
Chem. Soc., Faraday Trans. 1995, 91, 1907.N
0
(1)Scheme 1
0
yNCH3
N
0
(2)0
N CH3
N CO2CH3
0
(4)
NCH3
HO2C N
(7)0
C(N CH3
N COCH3
0
(3)0
N %C6H5
N COC6Hs
N
(6)and for 2,3-dimethylquinoxaline.4 The measurement of
2-methyl-5-pyrazinecarboxylic acid was undertaken to
facilitate the thermochemical estimation of the required
enthalpies of formation of the quinoxaline derivatives.
To assist in the interpretation of the trends in the
mean (N+-O-) dissociation enthalpies, ab initio calcula-
tions were made to investigate the geometric structures
of some of these molecules.
Experimental Section
Materials. Quinoxaline 1,4-dioxide (1) was prepared by
(sodium tungstate + hydrogen peroxide) oxidation of quinoxa-
line as described by Kobayashi et a1.5 2-Methylquinoxaline
(4) Ribeiro da Silva, M. A. V.; Morais, V. M. F.; Matos, M. A. R.;
Rio, C. M. A.; Piedade, C. M. G. S. Struct. Chem. 1996, 7, 329.
(5) Kobayashi, Y.; Kumadaki, I.; Sato, H.; Sekine, Y.; Hara, T. Chem.
Pharm. Bull. 1974, 22, 2097.S0022-3263(96)02149-4 CCC: $14.00 1997 American Chemical Society
3722
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Acree, William E. (William Eugene); Powell, Joyce R.; Tucker, Sheryl A. (Sheryl Ann); Silva, Maria D. M. C. Ribeiro da; Matos, M. Agostinha R.; Gonçalves, Jorge M. et al. Thermochemical and Theoretical Study of Some Quinoxaline 1,4-Dioxides and of Pyrazine 1,4-Dioxide, article, May 30, 1997; [Washington, D.C.]. (https://digital.library.unt.edu/ark:/67531/metadc333020/m1/1/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.