Pyrazines chemical properties

We wish to thank Professor F. W. Bergstrom for the sample of pyrazine, Dr. W. E. Vaughan for that of tetrabromobutane, Dr. K. J. Palmer for his help with experimental work, Dr. Sidney Weinbaum for assistance in the preparation of the figures, and Dr. E. R. Buchman for discussion of the chemical properties of the substances. We are especially indebted to Professor L. O. 

Although much less work has been done on this ring system, as might be expected the chemical properties are generally similar to those of the pyrido[2,3-b]pyrazines. 

The other results in Table III are those of data sets not having noncolinear physical-chemical properties. Log P was highly correlated with these data sets as well. Ethylesters threshold data in air weis linearly related to log P (eq. 4) while 3-alkyl-2-methoxy pyrazines had threshold odor intensity which was parabolically related to log P (eq. 7). The pyrazine data indicates that 3-alkyl-2-methoxy pyrazines having a log P value of 2.43 would have the most intense odor of the series. 

The chemical properties of iodine trifluoride are almost unknown. IF3 forms 1 1 complexes with pyrazine or 2,2 -bipyridyl and reacts with CsF in a 1 3 molar ratio to give CS3IF6 [8]. The ligand exchange reaction of IF3 with trifluoroacetic anhydride leading to iodine(III) trifluoroacetates has been reported [17]. 

Our understanding of the physicochemical properties of pyrazines has deepened. The internal rotation and IR spectrum of 2,5-pyrazinedicarboxamide were studied by quantum chemical calculations <05TC73>, and ab initio MO calculations at the MP2/6-31++G( ) level were used to explain the electronic and vibrational properties of complexes of pyrazine and HX linear acids <05JMS2822>. MM and MO calculations were used to investigate the conformational and electronic properties of odor-active pyrazines <05JMS169>, and NMR, IR, X-ray, and DFT methods were used to examine the structures of pyridol l,2-a pyrazinium bromide <05JMS7>. 

Theoretical calculations have been a successful approach for the description of structures and thermochemistry for a wide variety of chemical substances in fact, the theoretical methods have greatly increased in importance for investigating various properties of pyrazines, quinoxalines, and phenazines <1996CHEC-II(6)233>. In most cases, the calculated results have been discussed in comparison with spectroscopic properties accordingly, in this chapter, such examples are shown in each section on spectroscopy. 

The Maillard reaction has received much attention since the 1950 s as the source of flavor chemicals in cooked foods. Numerous compounds produced by this reaction have been reported in the last two decades. The major flavor chemicals are nitrogen- and sulfur-containing heterocyclic compounds. For example, nitrogen-containing pyrazines contribute a characteristic roasted or toasted flavor to cooked foods. Sulfur-containing thiophenes and thiazoles give a characteristic cooked meat flavor. A striking property of these compounds is their extremely low odor thresholds. 

The redox properties of tris(quinoxaline-2,3-dithiolato)molybdate(IV), [Mo(qdt)3]2, in the presence of protons provides a clear demonstration of the chemical versatility that is possible for a redox-active metal dithiolene center that involves a pyrazine ring linked to the dithiolene group. In an aprotic solvent, two reversible, Nernstian, waves are observed that (formally) correspond to the Mo(V)/Mo(IV) and Mo(IV)/Mo(III) couples. However, on addition of trifluoroacetic acid (Htfa), the Mo(V)/Mo(IV) couple slightly shifts to a higher potential and becomes non-Nernstian and a new three-electron, quasir-eversible, couple occurs some 900 mV less negative than the original Mo(IV)/ Mo(HI) couple. The latter is attributed to the addition of one electron and one... 

Local -deficiency (LOC D) is the largest positive charge on any one carbon atom in the ring and this property decreases in a different order 1,3,5-triazine > pyrimidine > 1,2,4,5-tetrazine > pyridazine pyridine > pyrazine. This order is in closer agreement with a -deficiency scale ( A) based on 13C NMR chemical shifts <19820MR192, CHEC-111(9.02.2)97 >. 

As has already been described in Table 9.1, transport properties are enhanced in CXLs compared with conventional solvents. For example, diffusivities of solutes are enhanced up to 7-fold in carbon dioxide expanded methanol, with little effect being seen on the nature of the solute (benzene pyrazine). Therefore, it is thought that physical rather than chemical interactions are causing this phenomenon, including reduced viscosity and surface tension upon carbon dioxide addition. The solubility of solids, liquids and gases in CXLs will... 

This volume summarizes published pyrazine chemistry with emphasis on syntheses, properties, and reactions of pyrazines and pyrazine iV-oxides (Chapters 1-X). Treatment of theoretical aspects is minimal. Although not strictly relevant. Chapter XI is presented as a summary of earlier reviews and more recent literature of reduced pyrazines (including piperazines). The literature recorded in Beilstein to 1929 and Chemical Abstracts through 1978 (Volume 89) has been covered together with selected references to 1980. Whereas every reasonable effort has been made to incorporate most significant material, no attempt has been made to include all relevant data. Tables have been incorporated in the text to extend the range of examples. 

The pyrido[2,3-/ ]pyrazines, 185-187, have been identified in the defensive secretion of the giant springtail, Tetrodontophora hidanensisP The secretion appears to confuse and strongly disorient potential predators. An unusual quinoline-derived betaine (188) with defensive properties was recently identified from the blood of Photuris versicolor fireflies. In addition, these fireflies chemical defense relies on sequestered cardiotonic steroids, the lucibufagins (see Section 2.04.3.3) (Figure 44). ... 

Since modern structural determination depends heavily upon H and C NMR analysis, information from IR and UV spectroscopies is less abundant than formerly. However, these spectroscopies are still useful to study photophysical properties, chemical behavior, and electronic orientation of the substituents in heteroaromatics. The pH dependence of both electronic and IR spectra of pyrazine has been examined to obtained p. values and information about the solute-solvent interactions, respectively <84JST(i 14)367). Electronic absorption taken in conjunction with fluorescence spectra indicate that 1,4-dihydro-2,3-quinoxalinedione exists in its diketo form irrespective of the nature of the solvent, by comparison with the spectra of the parent quinoxaline and 1,4-dimethyl-2,3-quinoxalinedione <87MI 603-01). New measurements of the singlet-triplet absorption spectra of pyrazine and pyrazine-d 4 at longer pathlengths and higher vapor pressures have been recorded <93aci537). 

The only report of the synthesis and properties of this bicyclic heterocycle adopts a different numbering system to that used by Chemical Abstracts (shown above). The alternative system (18) corresponds to that used for the related pyrido[l,2-a]pyrazine system (19). 

In addition to determining which GC peaks have an odor, GC/0 work can aid in compound identifications. An experienced chemist can provide a great deal of information about a chemical by sniffing it as it exits the GC column. The chemist can typically give molecular weight and functional group or compound class estimates. The chemist may be able to identify a chemical simply based on its aroma and GC retention properties. For example, it is relatively easy to distinguish between an alkyl pyrazine and a methoxy pyrazine by odor. 

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