Nitroanilines isomer separations

The photometric determination of mixtures of aniline, p-nitroaniline and o-nitroaniline was described. Distribution coefficients and separation efficiency of these compounds by LLE in various solvents were compared517. Substituted nitroanilines such as 2-chloro-4-nitroaniline and 2,4-dinitroaniline are intermediates in the manufacture of the dye D C Red No. 36 and were identified as impurities by RP-LC518. A spectrophotometric method was developed for the determination of aniline and m-nitroaniline in a mixture of aniline and nitroaniline isomers by derivatization with 5,7-dichloro-4,6-dinitrobenzofuroxan (244). The relative error of the determination is <5%519. See also Section IV.D.3.b for similar derivatives. 

Example 2 Chromatography of nitroaniline isomers. The elution order of the nitroaniline isomers was ortho, meta, and para in normal-phase liquid chromatography using H-butanol-w-hexane mixtures as the eluent, when the stationary phase material was either silica gel, alumina, an ion-exchanger, polystyrene gel, or octadecyl-bonded silica gel. The results indicate that the separation of these compounds can be performed on a range of different types of stationary phase materials if the correct eluent is selected. The best separation will be achieved by the right combination of stationary phase material and eluent.68... 

Separation of the stereoisomers cis- and iraws-stilbene, or p-nitroaniline (p-NA) and o-NA was studied using 0.02—0.7 M 3-CD aqueous solution as a LM. Celgard X-10 microporous hollow fibers were used for permeators design. The characteristics of the HFCLMP module see in Ref. [93]. For nitroandine isomers separation, an equimolar 0.005 M solution of o-NA and p-NA in 80% 1-octanol and 20% heptane was used as a feed. For stilbene isomers separation, an equimolar 0.01 M solution of cis- and trans-stilbene in pure heptane was used as a feed. 

FIGURE 16 LSC separation of nitroaniline isomers on 10-/im alumina, 15 cm x 2.4 mm column, 40% CH2CI2 in hexane mobile phase, flow rate 1.7 ml/mln, 1 fig of each isomer. [From Majors, R. E. (1973). Anal. Chen). 45, 757. Reprinted with permission by the American Chemical Society.]... 

The impurities present in aromatic nitro compounds depend on the aromatic portion of the molecule. Thus, benzene, phenols or anilines are probable impurities in nitrobenzene, nitrophenols and nitroanilines, respectively. Purification should be carried out accordingly. Isomeric compounds are likely to remain as impurities after the preliminary purifications to remove basic and acidic contaminants. For example, o-nitrophenol may be found in samples ofp-nitrophenol. Usually, the ri-nitro compounds are more steam volatile than the p-nitro isomers, and can be separated in this way. Polynitro impurities in mononitro compounds can be readily removed because of their relatively lower solubilities in solvents. With acidic or basic nitro compounds which cannot be separated in the above manner, advantage may be taken of their differences in pK values (see Chapter 1). The compounds can thus be purified by preliminary extractions with several sets of aqueous buffers... 

The same methodology was also used starting from the ethyl 6-amino-7-chloro-l-ethyl-4-oxo-l,4-dihydroquinoline-3-carboxylate, prepared by reduction of the nitro derivative. The requisite nitro derivative was prepared by nitration of ethyl 7-chloro-l-ethyl-4-oxo-l,4-dihydroquinoline-3-carboxylate. A second isomer was prepared from 4-chloro-3-nitroaniline by reaction with diethyl ethoxymethylene-malonate, subsequent thermal cyclization, and further ethylation because of low solubility of the formed quinolone. After separation and reduction, the ethyl 7-amino-6-chloro-l-ethyl-4-oxo-l,4-dihydroquinoline-3-carboxylate 32 was obtained. The ort/io-chloroaminoquinolones 32,33 were cyclized to the corresponding 2-substituted thiazoloquinolines 34 and 35, and the latter were derivatized (Scheme 19) (74JAP(K)4, 79CPB1). 

In the recent past separation of isomers has been attempted using aqueous liquid membranes based on p-cyclodextrin. Thus, separation of a mixture of o- and p-nitroaniline (in 80% i-octanol, 20% -heptane) has been studied, with the p-isomer showing a selectivity of 5 at 0.7 molar p-cyclodextrin. Even stereoisomers of stilbene cis and trans) were separated using a 0.02 to 0.2 M cyclodextrin solution, but the selectivity was less than 2 (Mandal et al, 1998). 

Chromatography on silica and alumina is unique among the liquid chromatographic methods in providing maximum selectivity for tfie separation of isomers. It is no problem to separated m- and p-dibromobenzene (a s 1.8 in pentane) (2) or the three nitroanilines (79) on silica or alumina stationary phases with dichloromethane as eluent. 

Structural Isomers. Chromatograms illustrating the separation of ortho, meta and para isomers of cresol (22) and and xylene ( O)on RP columns are shown in Figures 4 and 5. They enable a comparison of the chromatographic properties and selectivities due to <. - and -CD complexation between positional isomers of the above compounds.Similar behaviour was observed for ortho,meta and para isomers of fluoronitrobenzene, chloronitrobenzene, iodoni-trobenzene, nitrophenol, nitroaniline, dinitrobenzene (22), nitrocinnamic acid (22) some mandelic acid derivatives (19,21,34) and ethyltoluene (28). Both [Pg.225]

Nitrochlorobenzene-4-sulfonic acid can be prepared ako by sulfonation of o-nitrochlorobenzene (see Table III).This method,however,is inherently more costly and less reliable than the one given above (nitration of chlorobenzene-4-sulfonic acid), because it requires the isolation of o-nifrochlorobenzene and its separation from the para isomer. Nevertheless, the second method may be preferable technically under those circumstances where the demand for p-nitrochlorobenzene is so large that a use for the by-product ortho isomer is need. At the present time, the situation is reversed. The manufacture of o-nitroanisole uses so much o-nitrochloroben-zene that the accumulated para compound is partly used in making p-nitroaniline. 

Nitration of acetanilide yields a mixture of ortho and para substitution products. The para isomer is separated, then subjected to hydrolysis to give p-nitroaniline. 

Figure 21.19. LSC separation of nitro-aniline isomers on 10-fim alumina, (1) o-nitroaniline (2) m-nitroaniline (3) p-nitroaniline. Column Micropak Al-10. Packing lO-fim alumina, type T. Dimensions IScm x.lAmm. Mobile phase 40% CH2CI2 in hexane. Flow rate 100 mljhr. Sample size 1 y,g of each isomer. Detector 254-nm ultraviolet absorption. From R. E. Majors, Anal. Chem., 45, 757 (1973), by permission of the publisher. Copyright 1973 by the American Chemical Society. 

CD-containing mobile phases in HPLC have been successfully used for the separation of various isomers such as structural isomers, diastereomers and enantiomers. For example, ortho, meta, para isomers of cresol, xylene and aU six isomers of nitrocinnamic acid were separated on the Lichrosorb RP-C18 column with jS-CD solution as mobile phase [41]. Similar results were also obtained for ortho, meta and para isomers of nitrophenol, nitroaniline, fluoronitrobenzene. 

Figure 9 serves to demonstrate this equalizing of the stationary phases in the presence of buffers even for non-ionic analytes. In Fig. 9a, the separation of the isomers of nitroaniline on four rather different stationary phases with the help of an alkaline acetonitrile buffer is shown. Apart from small differences in the retention time, the separation of the three peaks looks rather similar on each of the four columns. Fig. 9b shows the separation of the nitroanilines on Symmetry Shield and on Zorbax Bonus in a methanol/water mixture. The chromatograms look absolutely different even an inversion of the elution order is observed. This means that to exploit the individual properties of the stationary phases in the realm of ultimate selectivity, one should dispense with buffers, which is not easy to realize in routine work, where reproducible retention times are required. Nevertheless, one should remember this in the case of orthogonal tests see below. These phenomena are observed even with simple, polar, non-ionizable analytes such as ketones (see Fig. 10). 

Mandal et al. (in preparation) incorporated p-cyclodextrin in an aqueous solution (0.7 M P-cyclodextrin, 7.5% NaOH, 37.5% urea, pH = 12) and employed it as a CLM in a permeator containing 300 symmetric hydrophobic feed fibers and 300 similar strip fibers. The feed was an equimolar (0.005 M) mixture of structural isomers o-nitroaniline and /7-nitroaniline in 80% octanol-20% heptane with the same solvent mixture as the strip liquid (Figure 3a). They obtained a selectivity of almost 5 for /7-nitroaniline over the ortho isomer. The operation was stable. The longest run continued for a period of almost two days with the feed solution and the strip solvent in coxmtercurrent flow. This successful separation indicated that employing a reflux arrangement at the end of the extraction cascade, it would be possible to continuously separate structural isomers into high purity (99%). ... 

In this example, the isomers had significant water solubility. Further, water has a small selectivity for p- over o-nitroaniline. Mandal et al (in preparation) also studied HFCLM separation of stereoisomers cis- and /raTis-stilbene which have very low water solubility. The solubility and the observed selectivity in the P-cyclodextrin liquid was significant (selectivity of almost two for the cis- over the /rans-isomer). Further, the separation was stable and continuous which opens a way for continuous separation of isomers as opposed to chromatographic separations. 

MOFs can also be used as the stationary phase for both NP- and RP-HPLC. Fu et alP explored MIL-lOO(Fe) as a novel stationary phase for both NP and RP HPLC. Two groups of analytes (benzene, toluene, ethylbenzene, naphthalene, and 1-chloronaphthalene aniline, acetanilide, 2-nitroaniline, and 1-naphthylamine) were used to test the performance of MIL-lOO(Fe) in RP mode, whereas chloroaniline and toluidine isomers were employed to evaluate its performance in NP mode. Baseline separation of all the tested analytes was achieved on MIL-100(Fe)-packed colunm with good precision. The hydrophobic property stemmed from the aromatic ring walls of the pores in MIL-100(Fe) frameworks dominates the selective separation of neutral analytes and basic anilines in RP mode, whereas the interactions between the nitrogen atoms in the analyte and the Fe active sites in MIL-1 OO(Fe) govern the separation of chloroaniline and toluidine isomers in NP mode. The mesoporous cages, accessible windows, excellent chemical and solvent stability, metal active sites, and aromatic pore walls... 

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