Density oleum

The density of oleum at 20°C (76) and at 25°C (39) has been reported. The boiling points of oleum are presented in Figure 15 (86). Freezing points are shown in Figure 16 (75,87). An excellent discussion on the crystallisation points of oleum is available (69). The solubiUty of sulfur dioxide in oleum has been reported (68,69). Viscosity of oleum is summarized in Figure 17 (55) additional viscosity data are available (76). 

Electrophilic substitution reactions of unsubstituted quinoxaline or phenazine are unusual however, in view of the increased resonance possibilities in the transition states leading to the products one would predict that electrophilic substitution should be more facile than with pyrazine itself (c/. the relationship between pyridine and quinoline). In the case of quinoxaline, electron localization calculations (57JCS2521) indicate the highest electron density at positions 5 and 8 and substitution would be expected to occur at these positions. Nitration is only effected under forcing conditions, e.g. with concentrated nitric acid and oleum at 90 °C for 24 hours a 1.5% yield of 5-nitroquinoxaline (19) is obtained. The major product is 5,6-dinitroquinoxaline (20), formed in 24% yield. 

Halogen substituents withdraw electron density from the aromatic nucleus but direct olp-through resonance effects. The result is that halobenzenes undergo nitration with more difficulty relative to benzene. The nitration of chlorobenzene with strong mixed acid gives a mixture of 2,4- and 2,6-isomeric dinitrochlorobenzenes in which the former predominates." The nitration of 2,4-dinitrochlorobenzene to 2,4,6-trinitrochlorobenzene (picryl chloride) requires an excess of fuming nitric acid in oleum at elevated temperature. Both are useful for the synthesis of other polynitroarylene explosives but only 2,4-dinitrochlorobenzene finds industrial importance (Sections 4.8.1.2 and 4.8.1.3). 

Fig. A.6. Absolute viscosity of sulfuric acid versus mass% H2S04 in sulfuric acid. For viscosity in kg m 1 s 1, multiply cP by 0.001. Source Bright, N.F., Hutchinson, H. and Smith, D., (1946) The viscosity and density of sulfuric acid and oleum, J. Soc. Chem. Ind. 65, 385 388.

Fig. 4.8 Etch pits on the a-b surface plane of a tetracene crystal, after etching with oleum (H2S2O7). The edges of the pits correspond to the crystallographic directions 110 and 110. The number density is ca. 10 cm . From ). Niemax, Dissertation,...

Prolonged reaction with pyridine-sulfiir trioxide complex can yield the 2,5-disulfonic aci4 and if both the a-positions are blocked sulfonation can occur in the p- or 3-position.In pyrrole, electron donation from the heteroatom favours electrophilic attack in the 2- and 5-positions since these are the sites of maximum jr-electron density. It is found that pyrroles containing electron-withdrawing substituents are stabilized relative to the parent heterocycle and may consequently be sulfonated by chlorosulfonic acid or oleum. ... 

Isothiazole is also sulfonated by oleum to yield the 4-sulfonic acid derivative in both thiazole and isothiazole the orientation of sulfonation is in agreement with the calculated r-electron density patterns of the parent heterocycles. ... 

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