Nitro- compounds

Nitro groups adjacent to alkyl or other substituents containing abstractable hydrogen atoms can undergo intramolecular reactions leading to diradicals that can cyclize or follow other reaction paths. An example is the photoreaction of 2-(2-nitrophenyl)-ethanol to an indoline derivative (Bakke, 1970). 

Nitro compounds. Aliphatic nitro compounds are acidic. They are freed from alcohols or alkyl halides by standing for a day with concentrated sulphuric acid, then washed with water, dried with magnesium sulphate followed by calcium sulphate and distilled. The principal impurities are isomeric or homologous nitro compounds. In cases where the nitro compound was originally prepared by vapour phase nitration of the aliphatic hydrocarbon, fractional distillation should separate the nitro compound from the corresponding hydrocarbon. Fractional crystallisation is more effective than fractional distillation if the melting point of the compound is not too low. 

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 of p-nitrophenol. Usually, the o-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 pKg values. The compounds can thus be purified by preliminary extractions with several sets of aqueous buffers of known pH (see for example Table 19, p. 43) from a solution of the substance in a suitable solvent such as ethyl ether. This method is more satisfactory and less laborious the larger the difference between the pK value of the impurity and the desired compound. Heterocyclic nitro compounds require similar treatment to the nitroanilines. Neutral nitro compounds can be steam distilled. 

Phenols. Because phenols are weak acids, they can be freed from neutral impurities by dissolution in aqueous N 

Salts (organic), (a) With metal ions Water-soluble salts are best purified by preparing a concentrated aqueous solution to which, after decolorising with charcoal and filtering, ethanol or acetone is added so that the salts crystallise. They are collected, washed with aqueous ethanol or aqueous acetone, and dried. In some cases, water-soluble salts can be recystallised satisfactorily from alcohols. Water-insoluble salts are purified by Soxhlet extraction, first with organic solvents and then with water, to remove soluble contaminants. The purified salt is recovered from the thimble. 

Their most common impurities are sulphonyl chlorides (neutral) or the sulphinic acid or disulphide from which they are 

Phenols. Because phenols are weak acids, they can be freed from neutral impurities by dissolution in aqueous N sodium hydroxide and extraction with a solvent such as ethyl ether, or by steam distillation to remove the non-acidic 

Sulphur compounds, (a) Disulphides can be purified by extracting acidic and basic impurities with aqueous base or acid, respectively. However, they are somewhat sensitive to strong alkali which slowly cleaves the disulphide bond. The lower-melting members can be fractionally distilled under vacuum. The high members can be recrystallised from alcohol, toluene or glacial acetic acid. 

Nitro Compounds Recent Advances in Synthesis and Chemistry, Eds. H. Feuer and 

Nitro Compounds. - Catalytic hydrogenation of nitro compounds to amines takes place readily at room temperature and atmospheric pressure. The mechanism proceeds via the nitroso and hydroxylamine stages and azo, azoxy, and hydrazo compounds may also be produced.  

Aliphatic nitro compounds are reduced at Ni and Pt cathodes to yield hydroxylamines, whilst at cathodes of high H2 overvoltage amines are the major product.  

The electrochemical reduction of aromatic nitro compounds has been studied in much greater detail than have aliphatic nitro compounds. The products of reduction are highly dependent on the pH of the electrolyte. In acid solutions the process proceeds via the nitroso and phenylhydroxyl-amine compounds to the amine - 

In alkaline and neutral solutions condensation reactions between the nitroso and hydroxylamine compounds and between the hydroxylamine and nitro compounds occur. The result is the formation of azoxy compounds which may undergo further reduction to the azo and hydrazo compounds- 

The extent of advancement of reduction may be controlled by choice of cathode material, the higher the hydrogen overvoltage, the further advanced 

Nitro compounds show two strong bands in the infrared spectrum. One appears near 1550 cm and the other near 1350 cm . Although these two bands may partially overlap the aromatic ring region, 1600-1450 cm , it is usually easy to see the NO2 peaks. 

O Aliphatic nitro compounds asymmetric stretch (strong), 1600-1530 cm symmetric stretch (medium), 1390-1300 cm .  

O Aromatic nitro compounds (conjugated) asymmetric stretch (strong), 

FIGURE 2.65 The infrared spectrum of 1-nitrohexane (neat hquid, KBr plates). 

Polynitro compounds such as 2,4,6-trinitrotoluene (TNT 2-methyl-1,3,5-trinitrobenzene) are shock sensitive and used as explosives. 

The reduction of w-nitrochlorobenzene to the corresponding amine by ethanolic stannous chloride follows simple second order kinetics . The reaction is retarded by addition of acid but no effort was made to preserve constant ionic strength. 

Because phenols are weak acids, they can be freed from neutral impurities by dissolution in aqueous N sodium hydroxide and extraction with a solvent such as diethyl ether, or by steam distillation to remove the non-acidic material. The phenol is recovered by acidification of the aqueous phase with 2N sulfuric acid, and either extracted with ether or steam distilled. In the second case the phenol is extracted from the steam distillate after saturating it with sodium chloride (salting out). A solvent is necessary when large quantities of liquid phenols are purified. The phenol is fractionated by distillation under reduced pressure, preferably in an atmosphere of nitrogen to minimise oxidation. Solid phenols can be crystallised from toluene, petroleum ether or a mixture of these solvents, and can be sublimed under vacuum. Purification can also be effected by fractional crystallisation or zone refining. For further purification of phenols via their acetyl or benzoyl derivatives vide supra). 

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. 

Early work on simple alkyl nitro-compounds [3, 4] indicated that whilst nitromethane absorbed at 1580 cm and 1375 cm higher homologues showed the asymmetric absorption at somewhat lower frequencies. This has been fully substantiated by later studies. Haszeldine [24] repeated the earlier work of Smith [4] et al and added some further compounds. The overall ranges for simple alkyl derivatives were 1567—1550 cm (as.) and 1379—1368 cm (s.). These values probably relate to the liquid state, but changes on solution are relatively small in this series. Brown [25], and Komblum Ungnade and Smiley [26] have each examined about thirty-five alkyl nitro-compounds with essentially similar results. More recently, Lunn [59] has studied the effects of substitution at the a-carbon atom, and Geiseler et al. [60,61] have reviewed the infra-red and Raman data. Additional Raman data have been provided by Popov et al [62] and Slovetskii et al [63]. 

In most cases there are sufficient differences between the frequencies of primary, secondary and tertiary nitro compounds to enable them to be differentiated. Slovetskii et al [63] suggest the following ranges, based on Raman data. 

This is generally in accord with the results of other authors but some caution is needed in the use of the lower frequency symmetric mode as this is much more prone to coupling than is the 

Conjugation of the nitro-group by attachment to an ethylenic double bond leads to a fall in both frequencies similar to that shown by the carbonyl absorption. Shechter and Sheppard [27], for example, find that 2-methyl-l-nitropropene absorbs at 1515 cm and 1350cm , whereas 2-methyl-3-nitropropene absorbs at 1555 cm and 1366 cm . This has been confirmed also by Brown [25], who finds the ranges 1524 4cm and 1353 6 cm for monoalkyl nitroethylenes and slightly lower values of 1515 4 cm and 1346 9 cm for di- and tri-alkylnitroethylenes. 

1332 cm 5 cm . Although fewer data are available on nitrocompounds with other a-electronegative groups, it would seem likely that similar effects will occur. 

These syntheses are limited to aromatic nitro compounds. 

Organic compounds that are oxidizing agents will oxidize ferrous hydroxide (blue) to ferric hydroxide (brown). The most common organic compounds that function in this way are aliphatic and aromatic nitro compounds, which are reduced to amines by the reaction (Eq. 25.60). Other less common types of compounds that give the same test are nitroso compounds, hydroxylamines, alkyl nitrates, alkyl nitrites, and quinones. 

The sulphonic acids, it will be recalled, are strong acids, their acid character being due to the remaining acid hydroxyl left in the compound. Sulphuric acid is di-basic and only one of the two hydroxyls is eliminated by the substitution in the ring. Nitric acid, however, is mono-basic and possesses only one acid hydroxyl. 

Not Acids.—When, therefore, this hydroxyl is removed by the reaction of nitration the residue contains no remaining acid hydroxyl and the compound can not be acid. Nitro benzene and the other nitro compounds of this series are unlike the sulphonic acids then in that they are neutral compounds. 

Not Esters, Non-hydrolyzable.—The nitro compounds resemble the sulphonic acids, however, in that they are non-hydrolyzable, and, therefore, are not esters. In them the benzene ring is linked directly to the nitrogen as in the sulphonic acids the ring is linked directly to the sulphur. 

Nitro benzene can be heated with water for a long time at 200° without decomposition. 

Reduction.—Another reaction of the nitro compounds which proves that the nitrogen is directly linked to the ring is their reduction to ammonia derivatives. As will be explained more fully when we take up the ammonia derivatives, just as nitric acid by complete reduction yields ammonia, so nitro benzene and other nitro compounds are 34 

The decomposition reactions of RDX and HMX are stoichiometrically balanced-when it is assumed that CO, rather than COj, is formed as a combustion product  

Though the adiabatic flame temperatures are 3300 K for RDX and 3290 K for HMX at 10 MPa, no excess oxidizer fragments are produced. Thus, RDX and HMX are not used as oxidizers in propellants. 

NO2 and N2O act as oxidizers and CH2O acts as the fuel component. Since nitrogen dioxide reacts quite rapidly with formaldehyde, the gas-phase reaction 

Nitroguanidine (NQ) is a nitramine compound containing one N-NOj group in its molecular structure. In contrast to cyclic nitramines such as HMX and RDX, its density is low and its heat of explosion is also comparatively low. However, the Mg of its combustion products is low because of the high mass fraction of hydrogen contained within the molecule. Incorporating NQ particles into a double-base propellant forms a composite propellant termed a triple-base propellant, as used in guns. 

Ammonium dinitramide (ADN) is a crystalline oxidizer with the formula NH4N(N02)2, that is, it is composed ofionicaUy bonded ammonium cations, NH4, and dinitramide anions, N(N02)2- Though ADN is crystalline and has a high oxygen content, similar to AP and KP, it has no halogen or metal atoms within its structure. ADN is used as an oxidizer in smokeless composite propellants, similar to AN and HNF. It melts at about 364 K, accompanied by the latent heat of fusion. 

Alkyl Halides. Alkyl halides are reduced by lithium aluminum hydride to give a hydrocarbon in what is known as a hydrogenolysis reaction (R3C—X R3C—H sec. 4.8.E).6l Reduction of 1-bromobutane 

For directions and examples see Cheronis, p 625, Linstead, p 69, Shriner, p 262, Vogel, p 529, Wild, p 247 

From catalytic hydrogenation (Raney nickel, platinum oxide and palladium on charcoal) of the nitro compound in ethanol, methanol or dioxane 

From the nitro compound and lithium aluminum hydride in ethers 

For partial reduction of polymtro compounds with sodium or ammonium polysulhde see Linstead, p 71, Vogel, p 551 

NOTE For directions and examples for the preparation of the derivatives of the amine formed on reduction of the nitro compounds see explanations and references to Table XVIII, p 291,292, 293, 294 

March s Advanced Organic Chemistry Reactions, Mechanisms, and Structure, 5th ed., John Wiley Sons, New York, 2001. 

Butler, A. R., Chem. Rev. 1975, 75, 241, Patai, S., The Chemistry of Diazonium and Diazo Groups, Wiley, New York, 1978. 

Williams, D. L. H., Nitrosation, Cambridge University Press, Cambridge, 1988. 

Stradins, J., Turovska, B., Glezer, V., Markava, E., Gustina, D., J. Elec-troanal. Chem. Interfac. Electrochem. 1991, 317, 243. 

Seyden-Penne, J., Reductions by the Alumino- and Borohydrides in Organic Synthesis, VCH, New York, 1991. 

Nilroglycrln., CHJVOjCHNO,CHJVO Is, aie yellow, viscous liquid. It is Slishtly soluble ui water, wuh a speciac gravity f i yj, i,iei, u tovier Ilian water. 

When the structure of picric acid is compared with the structure of TNT, the only difference is the fuel that the nitro functional groups were placed on the number of nitro groups is exactly the same. The explosive power of picric acid is similar to that of TNT. There are other Class 1.1 to 1.3 materials that are nitro compounds and some that are made up of other chemicals. Black powder is a low-order explosive made up of a mixture of potassium or sodium nitrate, charcoal, and sulfur in 75, 15, and 10% proportions, respectively. It has an appearance of a fine powder to dense pellets, which may be black or have a grayish-black color. It is a dangerous fire and explosion risk, is sensitive to heat, and will deflagrate rapidly. 

The ammonium radical has a different bonding configuration, which accounts for the four hydrogen atoms hooked to the nitrogen atom. This is one of those street chemistry concepts that it is better to just accept rather than trying to understand 

The lack of a workable system to warn everyone within the danger zone of an explosion 

The failnre to notify emergency service personnel promptly and accurately of the hazards 

Molecular ion Odd nominal mass number for odd number of N atoms Ahphatic weak or absent Aromatic strong 

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C. A typical aromatic amine. Best prepared by the prolonged action of concentrated ammonia solution at a high temperature upon anthraquinone-l-sulphonic acid in the presence of BaClj and by reduction of the corresponding nitro compound or by amination of the chloroanthraquinone. 

Azo-compounds can be obtained by reduction of nitro-compounds, or by oxidation of hydrazo-compounds. They are usually prepared, however, by reacting a phenol or amine with a diazonium salt. The coupling usually takes place in the position para to the hydroxyl or amino group, but if this position is occupied it goes to the ortho position, e.g. 

NH3) are of great commercial importance practically all modem explosives contain high proportions of ammonium nitrate or organic nitro-compounds. Nitrates are also employed as fertilizers. 

Nitro-compounds are prepared by the direct action of nitric acid. The reaction is greatly facilitated if a mixture of nitric and sulphuric acid is used. 

On reduction, the nitro-compounds give rise to a series of products, e.g. 

N-phenylhydroxylamine, PhNHOH and further reduction can give azoxybenzene, azobenzene, hydrazobenzene and aniline. The most important outlet commercially for the nitro-compounds is the complete reduction to the amines for conversion to dyestufTs. This is usually done in one stage with iron and a small amount of hydrochloric acid. 

Some nitro-compounds are themselves coloured and can be used as dyestuffs, e.g. picric acid. In this case the nitro-group can be considered to be the chromophore. For aliphatic nitro-compounds see nitroparaffins. 

It must be kept under an atmosphere of nitrogen or carbon dioxide it reduces, for example, Fe(III) to Fe(II) and nitro-organic compounds RNO2 to amines RNH2 (it may be used quantitatively to estimate nitro-compounds). In neutral solution, hydrolysis occurs to give species such as [Ti(0H)(H20)s], and with alkali an insoluble substance formulated as Ti203 aq is produced this is rapidly oxidised in air. 

Problems still exist with AM 1, freatment of phosphorus-oxygen bonds is inaccurate, nitro compounds are still too positive in energy, and the peroxide bond, for example, is still too short. In many cases, l M3 is an improvement over AM 1,... 

Both aliphatic and aromatic nitro-compounds can be readily reduced in acid solution to the corresponding primary amine. Thus when a mixture of nitrobenzene and tin is treated with hydrochloric acid, the tin dissolves to give stannous chloride, SnCh, which in these circumstances then reacts with more acid to give stannic chloride, SnCl, and the nascent hydrogen produced from... 

The alkaline reduction of an aromatic nitro-compound to give the azoxy... 

Place about 1 g. of the nitro-hydrocarbon in a boiling-tube and add 5 ml. of cone. HCl and several pieces of granulated tin. Warm the mixture and shake continuously to break up the oily drops of the nitro-compound. When all the oil has disappeared (about 3 minutes heating) pour off the liquid from any undissolved tin into a 100 ml. conical flask. Cool and add cautiously 30% aqueous NaOH solution until the precipitate formed redissolves to give a dark-coloured solution. Cool the latter thoroughly and shake well with about 15 ml. of ether. Separate the ethereal layer in a separating-funnel, wash with water and evaporate the ether in a basin on a previously heated water-bath in a fume-cupboard atoay from all flames. The residue is either... 

Yellow. Quinones, m- and p-nitroaniline, o-nitrophenol, and many other nitrO"Compounds. [Note that some nitro-compounds often appear yellow (e.g. m-dinitrobenzene and 3, 5 -dinitro-benzoic acid), but are colourless when absolutely pure.] Iodoform. 

Monohydric alcohols, aldehydes (including chloral hydrate), ketones, cinnamic acid, amines (2-naphthylaminc is odourless), nitrophenols (resemble both phenol and nitro-compound),... 

It should be noted that only representative substances are indicated in the above list. Substituted derivatives of the compounds in most classes may be encountered, e.g., nitrobenzoic acid in the aromatic carboxylic acids (p. 347). This acid will contain CH(0)N, but the salient properties are still those of a carboxylic acid, CH(0), Section 14, although the properties of an aromatic nitro-compound (e.g.y reduction to an amino-compound) will also be evident. 

Aliphatic nitro compounds. These are isomeric with the alkyl nitrites and may be prepared from the alkyl halide and silver nitrite, for example C,H,aBr + AgNOj — C Hj NO + AgBr... 

For a discussion of the Reactions and Characterisation of Aliphatic Nitro Compounds, see Section IV,16B.)... 

Nitro compounds, when hquid, have characteristic odours, are insoluble in water, highly refractive and with a density greater than unity. Many are crystalline sohds. Most nitro compounds are slightly coloured, generally yellow the intensity of the colour increases with the number of nitro groups. The following reactions will assist in their detection. 

Dissolve 0-5 g. of the substance in 10 ml. of 50 per cent, alcohol, add 0-5 g. of solid ammonium chloride and about 0 -5 g. of zinc powder. Heat the mixture to boiling, and allow the ensuing chemical reaction to proceed for 5 minutes. Filter from the excess of zinc powder, and teat the filtrate with Tollen s reagent Section 111,70, (i). An immediate black or grey precipitate or a silver mirror indicates the presence of a hydroxyl-amine formed by reduction of the nitro compound. Alternatively, the filtrate may be warmed with Fehling s solution, when cuprous oxide will be precipitated if a hydroxylamine is present. Make certain that the original compound does not aflfect the reagent used. 

Oxidation of side chains. Aromatic nitro compounds that contain a side chain (e.g., nitro derivatives of alkyl benzenes) may be oxidised to the corresponding acids either by alkahne potassium permanganate (Section IV,9, 6) or, preferably, with a sodium dichromate - sulphuric acid mixture in which medium the nitro compound is more soluble. 

Mix 1 g. of the nitro compound with 4 g, of sodium dichromate and 10 ml. of water in a 50 ml. flask, then attach a reflux condenser to the flask. Add slowly and with shaking 7 ml. of concentrated sulphuric acid. The reaction usually starts at once if it does not, heat the flask gently to initiate the reaction. When the heat of reaction subsides, boil the mixture, cautiously at first, under reflux for 20-30 minutes. Allow to cool, dilute with 30 ml. of water, and filter oflF the precipitated acid. Purify the crude acid by extraction with sodium carbonate solution, precipitation with dUute mineral acid, and recrystaUisation from hot water, benzene, etc. 

A number of selected aromatic nitro compounds are collected in Table IV,16A, It will be noted that a few nitro aromatic esters have been included in the Table. These are given here because the nitro group may be the first functional group to be identified aromatic nitro esters should be treated as other esters and hydrolysed for final identification. 

The following reactions will assist in the detection of aliphatic nitro compounds. 

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