Aromatic amines nitroanilines

It is convenient to include under Aromatic Amines the preparation of m-nitroaniline as an example of the selective reduction of one group in a polynitro compound. When wt-dinitrobenzene is allowed to react with sodium polysulphide (or ammonium sulphide) solution, only one of the nitro groups is reduced and m-nitroanUine results. Some sulphur separates, but the main reaction is represented by ... 

The rate-determining step in the diazotization of aniline in aqueous perchloric acid below concentrations of 0.05 m (pH >0.7) is the formation of N203. The following A-nitrosation step is faster (rate equation of Scheme 3-12). However, with aromatic amines that are weaker nucleophiles than aniline, e.g. 4-nitroaniline, nitrosation is slower than the formation of N203, and the rate is second-order with respect to nitrous acid and first-order in amine (Scheme 3-13, Larkworthy, 1959). 

Resonance effects are also important in aromatic amines. m-Nitroaniline is a weaker base than aniline, a fact that can be accounted for by the —7 effect of the nitro group. But p-nitroaniline is weaker still, though the —I effect should be less because of the greater distance. We can explain this result by taking into account the canonical form A. Because A contributes to the resonance hybrid, " the electron density of the unshared pair is lower in p-nitroaniline than in m-nitroaniline, where a canonical form such as Ais impossible. The basicity is lower in the para compound for two reasons, both... 

The synthesis can be carried out with most aromatic amines and is usually termed the Skraup reaction., The nitrobenzene is frequently replaced by arsenic acid, as in the preparation of 8-nltroquinoIine from o-nitroaniline ... 

A is an anchor compound, one whose ionization ratios are measurable in dilute aqueous add pAbh+ = 100 for p-nitroaniline is used for H0.22 23 Equation (9) is known as the cancellation assumption using it on a series of overlapping weak base indicators of similar type (primary aromatic amines in the case of Ho) leads to Ho values covering a wide addity range according to equation (8), once all the P bh+ values are known. 

Aromatic amines from the (bio)degradation of azo dyes or nitroaromatic explosives must also be monitored, mainly through the sum of their concentrations. However, taking account of the standard solution used for the calibration of the colorimetric reference method (4-nitroaniline), some attempts are proposed for the on-line specific determination of the most important single compounds [44,45]. 

Sparingly soluble salts of primary aromatic amines are diazotised in suspension, with vigorous stirring. Very weak bases such as halo-genated anilines and nitroanilines require a large excess of acid for salt formation they are first dissolved in just sufficient hot concentrated hydrochloric acid, which is then simultaneously cooled in ice and diluted. In this way the salts, which are mostly sparingly soluble, are precipitated in a finely divided condition. Dissolution in concentrated sulphuric acid and direct diazotisation of the sulphate, precipitated as a fine powder by means of ice, is often to be recommended. The free amines, however, should never be diazotised in acid suspension because they react far too slowly. Salt formation should first be ensured. 

The new method of Yoneda s group132 is also a one-pot diazotization-fluoro-de-diazoniation in a liquid-liquid two-phase mixture of pyridine and hydrogen fluoride. Yields for 25 aromatic amines and diamines are 50-100%, except for 2-and 3-fluorobenzoic acid, the three nitroanilines, 3- and 4-diaminobenzene and 4,4 -diaminodiphenyl-oxide (10-50%). In their 1994 paper the authors demonstrate that, in the same system, the photochemical decomposition gives in many cases significantly higher yields than the thermal reaction. The most spectacular increase in yield was found for the fluorination of 2-fluoroaniline where o-difluorobenzene was obtained photochemically in 80.2% yield, but thermally only in 0.6% ... 

Fig. 3.61. HPLC-UV chromatogram at 230 nm for the analysis of the aromatic amines listed. (1) 1,4-Diaminobenzene (2) 2-chloro-l,4-diaminobenzene (3) 2,4-diaminotoluene (4) benzidine (5) 4,4 -oxidianiline (6) aniline and 4-nitroaniline (7) o-toluidine (8) 4,4 -methylenedianiline (9) 3,3 -dimethoxibenzidine (10) 3,3 -dimethylbenzidine (11) 4-chloroaniline and 2-amino-4-nitrotoluene (12) 4,4 -thiodianiline (13) p-cresidine (14) 2,4-dimethylaniline (15) 2-naphty-lamine (16) 4-chloro-o-toluidine (17) 4,4 -methylene-di-o-toluidine (18) 2,4,5-trimethylaniline (19) 4-aminobiphenyl (20) 3,3 -dichlorobenzidine (21) 4,4 -methylenbis (2-chloroaniline) and (22) o-aminoazotoluene. Reprinted with permission from M. C. Garrigos et al. [130].

Dichloronitrobenzene has been prepared by deamination of 3,5-dichloro-4-nitroaniline and of 2,4-dichloro-3-nitroaniline. This procedure is an example of the rather general oxidation of anilines to nitrobenzenes with peroxytrifluoroacetic acid. Use of this reagent is frequently the method of choice for carrying out this transformation, and it is particularly suitable for oxidation of negatively substituted aromatic amines. Conversely, those aromatic amines, such as />-anisidine and j8-naphthylamine, whose aromatic nuclei are unusually sensitive to electrophilic attack give intractable mixtures with this reagent. This is not... 

This effect is increased if there is a suitable electron-withdrawing group in the ortho or para position on the aromatic ring. Thus, p-nitroaniline and o-nitroaniline have pATa 1-0 and —0.3 respectively. These aromatic amines are thus even weaker bases than aniline, a result of improved delocalization in the free base. The increased basicity of the ortho isomer is a result of the very close inductive effect of the nitro group the meta isomer has only the inductive effect, and its is about 2.5. 

With the pK value of o-nitroaniline known. Ho values can be obtained for the region of aqueous sulfuric acid from 24 to 35% [w/w]. The use of even weaker aromatic amines allowed the extension of the Ho scale up to 100% sulfuric acid. 

Further study of the reaction has indicated that nitroanilines, as well as aminobenzoic acids, are not substantially converted into azo compounds, whereas polycyclic aromatic amines give polymeric products. 

Sulfur Bake. The yellow, orange, and brown sulfur dyes belong to this group. The dyes are usually made from aromatic amines, diamines, and their acyl and nuclear alkyl derivatives. These may be used in admixture with nitroanilines and nitrophenols or aminophenols to give the desired shade. The color formed is said to be the result of the formation of the thiazole chromophore, evident in dye structure (1). 

Resonance effects are also important in aromatic amines. m-Nitroaniline is a weaker base than aniline, a fact that can be accounted for by the —I effect of the nitro group, But... 

Monoazo dyes derived from aromatic amines as coupling components and carbo-cyclt aromatic amines as diazo components are the class of disperse azo dyes with the greatest economic importance. Commercial products are most often represented by structure 2, in which 4-nitroaniline [100-01-6] and its substituted derivatives constitute the diazo component. 

Hydrazone Dyes If the diazonium salts of aromatic amines are coupled with the methylene derivatives of N heterocycles, hydrazone dyes (i.e., the monoaza derivatives of the enamine dyes) are obtained. Paper and leather can be dyed in yellow to red shades with these dyes [23], They are moderately lightfast on polyacrylonitrile. If the diazo components contain substituents in the 2-position relative to the amino group that can form a hydrogen bond (e.g., 2-nitroaniline [24] or 1-ami-noanthraquinone [25]), lightfastness is improved considerably (10). 

Chloro-2-nitroaniline. Primary aromatic amines should be protected by acetylation before nitration to avoid the possibility of accidental diazotization.19 Crotonaldehyde. Ignites in concentrated HN03.2... 

N-nitrosation and simultaneous C-nitration of secondary aromatic amines by aqueous solution of N204 has been studied by Stoermer [61] and Ruff and Stein [62], Thus N-methylaniline yielded N-methyl-n-nitroso-p-nitroaniline. 

Reaction of a primary aliphatic amine, n-butylamine, with nitronium tetrafluoroborate in methylene chloride or acetonitrile produced not n-butylnitramine, but n-butyl nitrate in about 20% yield. However, treating an electronegatively substituted primary aromatic amine, picramide, with nitronium tetrafluoroborate did give the primary nitramine, N,2,4,6-tetra-nitroaniline, in 85% yield. Olah (16) had reported previously that aniline was oxidized vigorously by nitronium salts. 

In some instances, diazo compounds and primary aromatic amines undergo an exchange reaction. Thus p-nitrobenzenediazonium chloride and aniline hydrochloride in a weakly acidic solution are converted to p-nitroaniline and benzenediazonium chloride... 

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