Nitration of o-xylene

The nitration of o-xylene at around 30 °C and separation of the product isomers by distillation and crystallization yields 3,4-dimethylnitrobenzene, which is transformed into 3,4-xylidine by catalytic reduction. 

4-Xylidine is used as a raw material in the production of riboflavin (vitamin B2), the synthesis of which is based on the condensation of 3,4-dimethylaniline with D-ribose to give the Schiff s base. By hydrogenation with Raney nickel and coupling the amine with benzenediazonium chloride followed by condensation with barbituric acid, riboflavin is obtained. 

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Nitroxylenes are especially important because 4-amino-o-xylene (xylidine), formed from 4-nitro-o-xylene (4-o-NX) upon reduction, is used as a starting material for the production of riboflavin. Nitration of o-xylene by the conventional mixed acid method gives a mixture of 4-o-NX 31-55% and 3-o-NX 45-69 %. 

Recently Milczak et al.[57] have reported the nitration of o-xylene using 100% nitric acid over silica supported metal oxide solid acid catalysts with high yields (up to 90 %) but low selectivity to 4-o-NX (40-57 %). Choudary et a/. 5X 591 performed the nitration of o-xylene and other aromatic hydrocarbons by azeotropic removal of water over modified clay catalysts achieving low yields of 4-o-NX and a selectivity of 52%. Better results were obtained when HBeta zeolite was used as catalyst, performing the reaction in dichloromethane at reflux temperature.[60] Conversions of 40 % and maximum selectivity 68 % of 4-o-NX were obtained. Similar conversions and higher selectivities for 4-o-NX (65-75 %) were reported by Rao et al M 1 using a nanocrystaUine HBeta sample and working at 90 °C in the absence of solvent. 

Nitration of o-xylene with N02 has been performed in the gas phase over several zeolites (HBeta, HY, HZSM-5 and HMordenite), as well as on sulfuric acid supported on silica and sulfated zirconia at temperatures between 50 and 130°C.[72] HBeta was found the most active and selective catalyst for the production of 4-o-NX giving ratios of 4-o-NX 3-o-NX as high as 6 1, whereas no dinitro-o-xylene compounds were detected. 

Regioselective vapour phase nitration of o-xylene to 4-o-NX with dilute nitric acid (30%) using HBeta catalyst at 150 °C has been reported.[80,81] Under these... 

Patil, P. T., Malshe, K. M., Dagade, S. P. and Dongare, M. K. Regioselective nitration of o-xylene to 4-nitro-o-xylene using nitric acid over solid acid catalysts. Catal. Commun., 2003, 4, 429-434. 

Riboflavin or vitamin B, is obtained entirely by synthesis and its usage and role is predominantly as a vitamin rather than as a permitted colourant. The synthesis by Karrer was later improved by Tischler and proceeds from 3,4-xylidine (available through the nitration of o-xylene and reduction of the product), the Schiff s base of which with D-ribose is catalytically reduced. Benzenediazonium chloride is coupled with the hydrogenation product and the resultant azo compound is then condensed with barbituric acid in acetic acid (with loss of aniline) to give the final product. The process is illustrated in Scheme 28. 

Because of the inefficiency of the nitrodeiodination reaction and the faster rate of nitration of o-xylene relative to 4-iodo-o-xylene, I2 cannot be used as a catalyst to alter the substitution pattern in the nitration of o-xylene. ° Mononitration of m-xylene with HNO3 in a liquid mixture of m-benzenedisulphonic acid and phosphoric acid gives excellent yields and a 4-nitro-isomer content higher than that obtained by the usual method. In a typical experiment, the yield was 96% and the 4- to 2-isomer ratio ca. 7.5. A white product obtained during the nitration of toluene has been identified as (46). The catalytic para-nitration of substituted benzenes, especially toluene, has been reviewed, but in Japanese. 

The addition of sulphuric acid increased the rate of nitration of benzene, and under the influence of this additive the rate became proportional to the first powers of the concentrations of aromatic, acetyl nitrate and sulphuric acid. Sulphuric acid markedly catalysed the zeroth-order nitration and acetoxylation of o-xylene without affecting the kinetic form of the reaction. ... 

In the nitration and acetoxylation of o-xylene the addition of acetic acid increased the rate in proportion to its concentration, the presence of 3-0 mol 1" accelerating the rate by a factor of 30. In the presence of a substantial concentration (2-2 mol 1 ) of acetic acid the rate of reaction obeyed the following kinetic expression... 

Acetoxylation has been found to accompany the nitration of o- and m-xylene... 

Pertechnetate in neutral and alkaline media can be extracted into solutions of tetra-alkylammonium iodides in benzene or chloroform. With tetra-n-heptylammo-nium iodide (7.5 x 10 M) in benzene distribution coefficients up to 18 can be obtained . A solution of fV-benzoyl-iV-phenylhydroxylamine (10 M) in chloroform can be used to extract pertechnetate from perchloric acid solution with a distribution coefficient of more than 200, if the concentration of HCIO is higher than 6 M The distribution of TcO between solutions of trilauryl-ammonium nitrate in o-xylene and aqueous solutions of nitrate has been measured. In 1 M (H, Li) NOj and 0.015 M trilaurylammonium nitrate the overall equilibrium constant has been found to be log K = 2.20 at 25 °C. The experiments support an ion exchange reaction . Pertechnetate can also be extracted with rhodamine-B hydrochloride into organic solvents. The extraction coefficient of Tc (VII) between nitrobenzene containing 0.005 %of rhodamine-B hydrochloride and aqueous alcoholic " Tc solution containing 0.0025 % of the hydrochloride, amounts to more than 5x10 at pH 4.7 . 

Major products reported from the photooxidation of o-xylene with nitrogen oxides include formaldehyde, acetaldehyde, peroxyacetyl nitrate, glyoxal, and methylglyoxal (Altshuller, 1983). The rate constant for the reaction of o-xylene and OH radicals at room temperature was 1.53 x 10 " cmVmolecule-sec (Hansen et al, 1975). A rate constant of 8.4 x 10 L/molecule-sec was reported for the reaction of o-xylene with OH radicals in the gas phase (Darnall et al., 1976). Similarly, a room temperature rate constant of 1.34 x 10 " cmVmolecule-sec was reported for the vapor-phase reaction of o-xylene with OH radicals (Atkinson, 1985). At 25 °C, a rate constant of 1.25 X 10 " cmVmolecule-sec was reported for the same reaction (Ohta and Ohyama, 1985). 

Chemical/Physical. Under atmospheric conditions, the gas-phase reaction of o-xylene with OH radicals and nitrogen oxides resulted in the formation of o-tolualdehyde, o-methylbenzyl nitrate, nitro-o-xylenes, 2,3-and 3,4-dimethylphenol (Atkinson, 1990). Kanno et al. (1982) studied the aqueous reaction of o-xylene and other aromatic hydrocarbons (benzene, toluene, w and p-xylene, and naphthalene) with hypochlorous acid in the presence of ammonium ion. They reported that the aromatic ring was not chlorinated as expected but was cleaved by chloramine forming cyanogen chloride. The amount of cyanogen chloride formed increased at lower pHs (Kanno et al., 1982). In the gas phase, o-xylene reacted with nitrate radicals in purified air forming the following products 5-nitro-2-methyltoluene and 6-nitro-2-methyltoluene, o-methylbenzaldehyde, and an aryl nitrate (Chiodini et ah, 1993). 

There are several cases where o-xylene degradation did not occur under nitrate-reducing conditions (see Table 4.5). Possible reasons for these observations were considered above. However, two reports not considered in the previous subsection failed to measure the biodegradation of o-xylene (Morgan et al., 1993 Flyvbjerg et al., 1993) In both cases there was probably very little microbial biomass as only groundwater was used as an inoculum and rates of o-xylene metabolism were likely too slow to measure. 

In m-xylene the two methyl groups agree in activating the same positions, and this is the only one of the three isomeric xylenes which can be nitrated satisfactorily to yield a trinitro derivative. Since the three isomers occur in the same fraction of coal tar and cannot readily be separated by distillation, it is necessary to separate them by chemical means. When the mixed xylenes are treated with about their own weight of 93 per cent sulfuric acid for 5 hours at 50°, the o-xylene (b.p. 144°) and the m-xylene (b.p. 138.8°) are converted into water-soluble sulfonic acids, while the p-xylene (b.p. 138.5°) is unaffected. The aqueous phase is removed, diluted with water to about 52 per cent acidity calculated as sulfuric acid, and then heated in an autoclave at 130° for 4 hours. The m-xylene sulfonic acid is converted to m-xylene, which is removed. The o-xylene sulfonic acid, which remains in solution, may be converted into o-xylene by autoclaving at a higher temperature. The nitration of m-xylene is conveniently carried out in three steps. The effect of the two methyl... 

The effects of added species. The rate of nitration of benzene, according to a rate law kinetically of the first order in the concentration of aromatic, was reduced by sodium nitrate, a concentration of io 3 mol l-1 of the latter retarding nitration by a factor of about 4.llc>28 Lithium nitrate anticatalysed the nitration and acetoxylation of o-xylene in solutions of acetyl nitrate in acetic anhydride. The presence of 6 x io-4 mol 1 1 of nitrate reduced the rate by a factor of 4, and modified the kinetic form of the nitration from a zeroth-order dependence on the concentration of aromatic towards a first-order dependence. However, the ratio of acetoxylation to nitration remained constant.146 Small concentrations of sodium nitrate similarly depressed the rate of nitration of anisole and again modified the reaction away from zeroth to first-order dependence on the concentration of the aromatic.116... 

Ortho- and p- xylene nitrate more readily than m- xylene. They are also more readily oxidized especially during mononitration as well as during vigorous nitration to trinitro derivatives. For this reason the yields obtained in the nitration of o- and p- xylene are lower than those obtained with m- xylene. 

In an investigation of the extraction of trivalent actinides, An(III) from 0.01 M nitric acid solutions of various LiNOj concentrations into o-xylene containing the tertiary amine salt trilaurylmethylammonium nitrate, TLMA HNOj, Van Ooyen [29] found that the amine was monomeric only at very low concentrations (<0.1M in the organic phase) but at higher concentration formed both dimers and trimers. 

Fig. 4.16 Distribution ratio of ions between the trUaurylmethyl ammonium nitrate (TLMA) in o-xylene and aqueous phases of varying LiNOj concentrations, (a) As a function of TLMANO3 concentration at 1-7 M, 2-5 M, 3-3 M LiNOj. (b) Extraction of Eu(in) and tree actinide(III) ions at 0.1 M TLMANO3 in o-xylene and varying aqueous salt concentrations. (From Ref. 29.)...

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