This invention relates to a process for preparing 4,6-dιnιtroresorcιnol in one step from resorcinol 4,6-Dιnιtroresorcιnol is used to prepare 4,6-dιamιnoresorcιnol, which is a precursor to polybenzoxazoles (PBOs), polymers which are useful as insulators, solar arrays, and cut-resistant gloves PBOs can be prepared by reacting 4,6-dιamιnoresorcιnol with bisacids, bisacid halides, bisesters, bisamides, or bisnitriles. See Wolfe in Mark et al., The Encyclopedia of Polymer Science and Engineering, Volume 1 1 , pp. 601-635, Wiley-lnterScience Publication, New York, 1988.

Efforts to prepare 4,6-dinιtroresorcinol in one step from resorcinol and nitric acid in yields exceeding 30 percent have been unsuccessful due to the formation of high levels of undesirable by-products, such as 2,4-dιnitroresorcιnol and 2,4,6-trιnιtroresorcιnol (styphnic acid) Improved yields of the desired 4,6-ιsomer have been obtained by introducing bulky protecting groups at the 1- and 3-positιons of resorcinol, thereby inhibiting nitration at the 2-posιtιon. For example, Lysenko et al. (U S. Patent No 4,982,001 ) describes the preparation of 4,6-dιnιtroresorcιnol from resorcinol through a 1 ,3-bιs(methylcarbonato)benzene intermediate. Similarly, Schmitt et al. (J. Orq. Chem 1988, 53, 5568) describes the preparation of 4,6-dinιtroresorcinol in 44 to 66 percent yield from resorcinol diacetate. The steπc hindrance of these intermediates causes nitration to take place mostly at the 4- and 6-posιtιons, so that the desired 4,6-diaminoresorcinol can be prepared upon hydrolysis and hydrogenation. Unfortunately, a significant degree of undesirable nitration still occurs at the 2-posιtιon of the intermediate, and there is the danger of forming the potentially explosive styphnic acid.

It would be desirable to prepare 4,6-dinitroresorcιnol in high yields and at high concentrations in one step from resorcinol without requiring any intermediate protected species.

The present invention is a method of preparing 4,6-dinitroresorcιnol comprising reacting resorcinol with concentrated nitric acid in the substantial absence of suboxides of nitric acid. The present invention addresses the deficiencies in the art by preparing

4,6-dinitroresorcinol in one step from resorcinol in greaterthan 60 percent yields

The present invention is a method of preparing 4,6-dinitroresorcinol comprising reacting resorcinol with concentrated nitric acid in the substantial absence of suboxides of nitric acid. Under these conditions, it has surprisingly been discovered that 4,6-dinιtroresorcιnol can be prepared in one step from resorcinol in higher yields and with higher selectivity than previously known one-step nitrations.

The concentrated nitric acid is an aqueous solution that contains from 80 weight percent, more preferably from 85 weight percent, and most preferably from 89 weight percent,

to 93 weight percent, more preferably 92 weight percent, and most preferably 91 weight percent nitric acid In order for the nitration of resorcinol to be carried out in the substantial absence of suboxides of nitric acid (that is, nitrogen species that have an oxidation state lower than that of nitric acid), it is essential that the concentrated nitric acid be substantially free of suboxides of nitric acid More particularly, the concentrated nitric acid is substantially free of suboxides that are capable of forming nitrosomum (NO + ) ions Examples of suboxides that are substantially absent in the concentrated nitric acid include nιtrogen(IV) species, such as N0 ₂ and N ₂ 0 ₄ , and nιtrogen(lll) species, such as N ₂ 0 ₃ and HIMO2 "Substantially free" means that the reaction mixture, more particularly the concentrated nitric acid, contains less than 2 weight percent, preferably less than 1 weight percent, more preferably less than 0 5 weight percent, and most preferably less than 0 1 weight percent of suboxides of nitric acid based on the weight of nitric acid The levels of suboxides of nitric acid can be determined by titration with, for example, hydrogen peroxide Selectivity and yield of the desired 4,6-dιnιtroresorcιnol is improved at the preferred, more preferred, and most preferred concentrations of nitric acid and suboxides of nitric acid

Concentrated nitric acid that is substantially free of suboxides of nitric acid can be prepared by known methods, including, for example, first combining a less concentrated nitric acid (that is, less than about 80 weight percent aqueous nitric acid) with a dehydrating reagent, such assulfuπc acid, then distilling the nitric acid, preferably at reduced pressure, to form about 100 weight percent nitric acid The desired concentration of nitric acid can then be prepared by adding water, preferably distilled water, to the distilled nitric acid

Alternatively, the concentrated nitric acid that is substantially free of suboxides of nitric acid can be prepared by first adding a sufficient amount of a peroxide, preferably hydrogen peroxide, to greater than about 90 weight percent, more preferably greater than about 95 weight percent nitric acid, to oxidize any suboxides of nitric acid to nitric acid, then adding sufficient water to achieve the desired concentration of nitric acid

In general, less concentrated nitric acid inherently contains lower levels of suboxides of nitric acid than the more concentrated nitric acid This is because the less concentrated nitric acid is more stable to suboxide formation than the more concentrated nitric acid Therefore, the preferred preparation of concentrated nitric acid that is substantially free of suboxides of nitric acid depends on the initial concentration of nitric acid that is being purified Lower initial concentrations tend to require a dehydrating reagent, whereas higher initial concentrations tend to require an oxidizing reagent, or perhaps a combination of an oxidizing reagent and a dehydrating reagent It is most preferable to prepare concentrated nitric acid that is substantially free of suboxides of nitric acid by adding a sufficient amount of hydrogen peroxide to greater than about 95 weight percent nitric acid to oxidize any suboxides of nitric acid to nitric acid, then adding sufficient water to achieve the desired concentration of nitric acid

Surprisingly high initial concentrations of resorcinol can be used in this reaction without negatively affecting the product yields. The initial concentration of resorcinol ranges preferably from 1 weight percent, more preferably from 5 weight percent, and most preferably from 10 weight percent to 18 weight percent, more preferably to 16 weight percent, and most preferably to 14 weight percent, based on the weight of resorcinol and the weight of pure nitric acid in the concentrated nitric acid medium. Higher throughput can be obtained at the most preferred initial concentrations of resorcinol without sacrificing product yield.

The nitration of resorcinol is carried out at subambient temperatures, preferably from 0°C, more preferably from -5°C, and most preferably from -15°C to -50°C, more preferably to -35°C, and most preferably to -25°C. It is also preferred that resorcinol be added to a stoichiometric excess of the nitric acid, more preferably at least about a 5-fold stoichiometric excess (that is, at least about 10 moles of nitric acid per mole of resorcinol), and at such a rate to control the resulting exotherm. Though the resorcinol is preferably added to the concentrated nitric acid in the absence of a solvent, solvent exclusion is not essential. For example, the resorcinol may be first dissolved in preferably a minimal amount of 50 weight percent to 70 weight percent aqueous nitric acid that is substantially free of suboxides of nitric acid, to form a resorcinol/aqueous nitric acid solution that is relatively stable to nitration. This solution can then be added to the concentrated nitric acid under the reaction conditions described above. It is preferred that the reaction be allowed to proceed to substantial completion, as determined, for example, by the substantial absence of 4-nitroresorcinol in the product mixture, more preferably when the product contains less than 1 weight percent 4- nitroresorcinol based on the ideal yield of the desired product. To this end, the reaction is advantageously monitored by HPLC. The selectivity and yield of 4,6-dinitro-resorcinol are improved at the preferred, more preferred, and most preferred temperature ranges, as well as the most preferred mode of addition.

The reaction can be quenched with sufficient waterto inhibit further nitration. The product is a mixture predominantly of 4,6-dinitroresorcinol and 2,4-dinitroresorcinol. The overall selectivity for 4,6- and 2,4-dinitroresorcinol is generally found to exceed 90 weight percent, and the overall selectivity for 4-nitroresorcinol and styphnic acid is generally found to be less than 2 percent when the most preferred conditions are used. Furthermore, the ratio of 4,6-dinitroresorcinol to 2,4-dinitroresorcinol in the final product is generally found to be greater than 2: 1 , and the yield of the 4,6-dinitroresorcinol is generally greater than 60 percent when the most preferred conditions are used.

The desired "4,6-isomer can easily be separated from the 2,4-isomer, due to the marked differences in the solubility of the two isomers in various solvents. For example, the 4,6-isomer can be isolated in about 99 weight percent purity by recrystallization in a suitable solvent, such as acetic acid.

The 4,6-dιnιtroresorcιnol can be reduced to 4,6-dιamιnoresorcιnol by known methods (see Lysenko, supra) The 4,6-dιamιnσresorcιnol can then be reacted with bisacids, bisacid halides, bisesters, bisamides, or bisnitriles to form polybisbenzoxazoles (PBOs), polymers which are useful as insulators, solar arrays, and cut-resistant gloves (See Wolfe, supra ) The invention disclosed herein suitably may be practiced in the absence of any component not specifically disclosed herein

The following example is provided to illustrate the invention, but is not intended to limit the scope thereof All parts and percentages are by weight unless otherwise indicated Example 1 To a solution of 1 1 1 g of 90 percent nitric acid (diluted from 100 percent distilled nitric acid that contained less than 0 1 weight percent suboxides of nitric acid) in a 250 mL 3- necked round-bottom flask was added granular resorcinol (13 0 g, 1 18 mmol, 13 weight percent based on the weight of pure nitric acid) over a 1 3 hour period The solution was stirred vigorously and the reaction temperature was maintained between -21 °C and -15°C After the addition of resorcinol was complete, the reaction was stirred until less than 1 percent of 4-nιtroresorcιnol remained in the reaction mixture (about 15 minutes), as monitored by HPLC The reaction was then quenched with 100 mL of water, while the temperature was maintained at below -5°C for 15 minutes The resultant slurry was filtered and the orange- yellow solid was washed with 40 mL of water The solid was then dried to constant weight in vacuo to afford 19 65 g of a 72:28 mixture of a 4,6-dιnιtroresorcιnol to 2,4-dιnιtroresorcιnol Quantitative assay of the composite reaction mixture (the dried solid and the mother liquor) by an external standard HPLC method was 1 0 percent styphnic acid, 0 78 percent 4-nιtroresorcιnol, 30 8 percent 2,4-dιnιtroresorcιnol, and 60 percent 4,6-dιnιtroresorcιnol This dried material was recrystal zed from 140 g of hot acetic acid to give yellow crystalline 4,6-dιnιtroresorcιnol which assayed at 98 91 percent purity (0 54 percent 2, 4-dιnιtro-resorcιnol) The isolated yield was 54 percent