NITROBENZENE(mirban oil) C 6 H 5 MO 2, molecular weight 123.11, colorless. or greenish-yellow oily liquid with the smell of bitter almonds; melting point 5.85 °C, boiling point 211.03 °C, 108.2°/30 mmHg; d 4 20 1.2037; n D 20 1.1562, h 2.165 mPa. s (15°C), 1.634 mPa. s (30°C); g 4.335. 10 -6 N/m; C 0 p 0.1774 kJ/(kg K); DH 0 arr -17.165 kJ/mol, DH 0 burn (for liquid) -3094.88 kJ/mol, DH 0 exp 46.05 kJ/mol, DH 0 ex 58.19 kJ/mol. At - 30 °C - crystals of the monoclinic system (a = 0.386 nm, b = 1.165 nm, c = 1.324 nm, b = 95.58 °, z = 4). Poorly soluble in water (0.19% by weight at 20 °C, 0.8% at 80 °C); miscible in all respects with diethyl ether, benzene; well sol. in other organic solvents, distilled with steam.
In terms of chemical properties, it is a typical aromatic nitro compound. Electroph. substitution (chlorination, nitration, sulfonation) is more difficult than for benzene, which is due to the strong electron-withdrawing effect of the NO 2 group. Substitution occurs predominantly in the meta position. Nitration of NITROBENZENE proceeds according to scheme 1 or 2:
Nucleof. substitution is easy; the second substituent enters the ortho or para position, for example, fusion with KOH at 100°C leads to o-nitrophenol. NITROBENZENE does not undergo the Friedel-Crafts reaction. The reduction of NITROBENZENE depends on the nature of the reducing agent and the reaction conditions. When reduced by metals (Fe, Zn or Sn) in an acidic environment, metal sulfides, H 2 in the presence of metallic. catalysts or SnCl 2 in CH 3 COOH NITROBENZENE are converted into aniline; at
the action of Zn in an alkaline medium or LiAlH 4 forms a mixture of azo- and azoxybenzenes, which are then converted into hydrazo-benzene, and when interacting with Ka 3 AsO 3 - azoxybenzene. Treatment of NITROBENZENE with Zn in a neutral environment leads to N-phenyl-hydroxylamine, a mixture of Na 2 S 2 O 3 with Na 3 PO 4 -K phenylsulfamate Na. During electrochemical reduction in a sulfuric acid environment, NITROBENZENE is converted into n-aminophenol.
In industry, to obtain NITROBENZENE, a continuous process of nitration of benzene with a mixture of conc. HNO 3 and H 2 SO 4; yield 96-99%. In the laboratory, NITROBENZENE is obtained by adding benzene to a mixture of HNO 3 (density 1.4 g/cm 3) and H 2 SO 4 (1.84 g/cm 3) in a 1:1 ratio at 55-60 ° C; holding time 45 min; yield 81%.
NITROBENZENE is determined polarographically or by reduction with TlCl 2 or TiSO 4 in an acidic medium, followed by titration of excess Ti 2+, as well as converting NITROBENZENE to m-dinitrobenzene with the determination of the latter colorimetrically. NITROBENZENE turns a 10% KOH solution pink.
NITROBENZENE is toxic and absorbed through the skin; has a strong effect on the central nervous system, disrupts metabolism, causes liver diseases, oxidizes hemoglobin into meth-hemoglobin; MPC 3 mg/m3. Flash point 88 °C, ignition temperature 482 °C.
NITROBENZENE is a feedstock in the production of aniline, an aromatic nitrogen-containing compound (eg, benzidine, quinoline, azobenzene); cellulose ether solvent; component of polishing compounds for metals.
More than 250 thousand tons of NITROBENZENE are produced annually in the USA
Literature: Orlova E.Yu., Chemistry and technology of high explosives and substances, 3rd ed., L., 1981; Kirk-Othmer encyclopedia, 3 ed., v. 15, N.Y., 1981.
V. I. Erashko.
m-NITROBENZENESULPHONIC ACID(3-nitrobenzene sulfonic acid), formula I, molecular weight 203.17; colorless crystals; melting point 70°C; hygroscopic; soluble in water and ethanol. With alkaline and alkaline-earth. metals form water-soluble salts; solubility at 25 °C for K-salt is 3.04%, for Pb-salt 6.09%.
NITROBENZENE has the properties of aromatic nitro compounds and benzene sulfonic acids. With SOCl 2 at 70 ° C it forms the corresponding anhydride (melting point 130-140 ° C) with an admixture of sulfonyl chloride (melting point 142 ° C), at 180-200 ° C they turn into m-dichlorobenzene. K-salt NITROBENZENE with PCl 5 at 100 ° C is converted into 3-nitrobenzenesulfonyl chloride (melting point 63-64 ° C), Na-salt with a mixture of PBr 3 and PBr 5 into bis-(3-nitrophenyl) disulfide. NITROBENZENE is easily reduced under the influence of various agents. Thus, the action of Fe in H 2 SO 4, hydrazine in aqueous-alcoholic alkali and catalytic hydrogenation in the presence of Pd or Ni leads to 3-aminobenzenesulfonic acid (methanilic acid); reduction of Al into dilute H 2 SO 4 at 100°C to 5-amino-2-hydroxybenzenesulfonic acid. With hydrogen sulfide in a solution of NaHS or Zn in an alkali solution, NITROBENZENE is reduced first to azobenzene-3,3"-disulfonic acid (formula II), and then to hydrazobenzene-3,3"-disulfonic acid (III); the latter under the influence of conc. HCl at 20 °C rearranges into benzidine-2,2"-disulfonic acid (IV):
NITROBENZENE oxidizing agent; oxidation-reduction potential in water at 24 °C is: 0.06 V (pH 0.76); 0.03 V (pH 3.76); -0.06 V (pH 6.77); -0.18 V (pH 9.6); -0.31 V (pH 11.8).
NITROBENZENE is obtained in the form of Na-salt (ludigol) by sulfonation of nitrobenzene with 25% oleum at 70 °C followed by salting out. Nitrobenzene-3-sulfonic chloride is synthesized by chlorinating nitrobenzene with chlorosulfonic acid.
NITROBENZENE is an intermediate in the production of methanilic acid, benzidine-2,2"-disulfonic acid, and some drugs. drugs; oxidizing agent in the production of dyes, photographs. T. self-igniting ludigola 380 °C; MPC 109 g/m 3 NITROBENZENE sulfonyl chloride is used in the production of 3-aminobenzenesulfamide.
For literature, see Art. Benzenesulfonic acids. NITROBENZENE B. Karpova.
Chemical encyclopedia. Volume 3 >>
Nitrobenzene
With 6 H 5 NO 2 - in technology and trade it is also called mirban essence, mirban oil And artificial chorcomandal oil. Being very similar to the latter in smell, it is used in significant quantities in perfumery, especially in the production of fragrant varieties of soap (toilet, etc.), but the bulk of it is used for the production of aniline and then from it fuchsin and other coal pigments (see .Organic artificial paints). The use of N. in explosives technology is very limited, because by itself it is capable of exploding only under certain particularly favorable conditions. N. was first obtained by Mitscherlich in the city by dissolving benzene in fuming nitric acid and precipitating the solution with water. In technology, benzene is nitrated with a mixture of 10 parts by weight of nitric acid. weight 1.40 - 1.42 and 14 - 15 parts of oil of vitriol, and this mixture is taken to taste 3 parts to 1 part benzene. Currently, they usually strive to obtain N. as pure as possible, which is equally important both for the purposes of perfumery [Pure N., not containing nitrotoluene impurities, has a much more pleasant odor] and for the production of aniline dyes (see Organic artificial dyes ); Therefore, to prepare it, take benzene at temp. kip. 80 - 81°, obtained by rectification in column apparatuses of Savall, Heckmann, etc., similar to ordinary alcohol rectifiers, and sometimes even further purified by crystallization in the cold, or even free from thiophene. The nitration operation is carried out in closed cast iron vessels, usually cylindrical in shape with a rounded or cone-shaped bottom, equipped inside with a stirrer operating from a mechanical drive, a thermometer, at the top (in the lid) a hole for pouring benzene and pouring in an acid mixture and a lead pipe for draining acidic substances developing during the reaction. vapors and gases, and below (in the bottom) with a drain valve. Up to 100 kg, and in England up to 400 and even up to 800 kg of benzene are immediately poured into such a vessel of the appropriate size, and gradually, with continuous stirring and cooling of the vessel from the outside with a stream of water using certain devices, the above amount of pre-cooled acid mixture is poured [The reaction occurs faster and more completely if you do the opposite, that is, add benzene to the acid mixture (cf. Nitroglycerin); but since with this method of nitration, due to a more energetic course of the reaction and strong heating of the mixture (according to Berthelot, the replacement of 1 atom of N-benzene with a group NO 2 is accompanied by the release of 36 b. cal.), benzene partly volatilizes, partly nitrates further, turning into dinitrobenzene , and in addition there is also a danger of its ignition, then in factories they always prefer to carry out the operation as indicated above]. Its influx occurs within 8 - 12 hours. At the end, its cooling is stopped, the temperature is allowed to rise to 80 - 90° and, having finished pouring the acid, stirring is continued for several more hours. Having then allowed the liquid to completely cool and settle, the spent acid is drained, followed by the oily layer of N that has floated to its surface. The resulting raw N. is washed in large clay vessels or oak vats with hand mixers, first with water, then with a weak solution of soda and again several times. times with clean water [Wash waters, which carry with them quite significant amounts of N., are subjected to settling before being lowered anywhere, and the settling, not yet completely washed N. is added to subsequent portions of the raw nitro product], is freed by blowing with steam from unreacted hydrocarbon, which is collected and put back into use during the next treatment, and is sometimes further purified by steam distillation from a boiler with a steam jacket or a blind coil inside for heating. The yield of N. is usually 150 - 152 parts per 100 parts of pure benzene (theoretical yield = 157 hours). Pure N. is a colorless [Not completely pure N. is usually more or less yellowish in color] liquid of specific gravity 1.2002 (at 0°) and 1, (at 14.5°) with a pleasant bitter-almond odor, solidifying in the cold at a mass of needle-shaped crystals, melting at + 3° (Mitscherlich), boiling at 210°, quite easily volatile with water vapor and very poisonous. It is slightly soluble in water, but very easily in wine and wood alcohol, ether, benzene, etc. Halides (Cl, Br) at ordinary temperatures have no effect on nitrogen, but in the presence of iodine, antimony chloride, and ferric chloride they give halogen-substituted nitrogen. , mainly metachloro- or bromine-H. (C 6 H 4 ClNO 2, C 6 H 4 BrNO 2). When a mixture of N. is heated with aniline, glycerin, and strong sulfuric acid, quinoline is formed (synthesis of Scroup's quinoline, see Quinoline). When heated with a mixture of oil of vitriol and fuming nitric acid, N. is converted into dinitrobenzene C 6 H 4 (NO 2) 2, main. way metadi-N. The most important reaction is its reduction to aniline (see Organic artificial paints and Nitro compounds) according to the equation:
C 6 H 5 NO 2 + 3H 2 = C 6 H 5 NH 2 + 2H 2 O.
This reaction is easily accomplished by heating N. to 104° with hydroiodic acid under the action of tin chloride SnCl 2 (quantitatively), ammonium sulfide, iron in the presence of a small amount of hydrochloric acid. The last reaction serves for the factory production of aniline. Reduction of N. is also accomplished partly by alcohol when the alcohol solution of N. is left in direct sunlight, which results in the formation of a small amount of aniline with the simultaneous formation of acetaldehyde. About the effect of N. on aniline oil using the nitrobenzene method for producing fuchsin, see Organic artificial paints. To test the purity of N. it is heated with a solution of caustic potassium. Pure N. should not color the alkaline solution yellow or brown, and only such N. can be used for making the highest grades of toilet soaps without fear that the latter will turn yellow over time during storage. If N. contains at least traces of dinitrothiophene, then its alcohol solution turns into a beautiful red color by adding one drop of caustic potassium solution (W. Meyer). In addition to the above-described pure N., used for perfumery, for the preparation of quinoline and pure aniline (aniline for blue) and for the oxidation of aniline using the nitrobenzene method, nitration of various types of crude benzene (90%, 50%, etc., see Coal tar) are prepared, however, now rarely; other commercial varieties are also prepared, and namely, mainly two varieties of N.: the so-called, in contrast to pure, or “light”, N., “heavy” and “very heavy” N. [The names “light” and “heavy” are not entirely successful, since they are in direct contradiction with the beat. the weight of the N varieties they designate; here they mean to indicate only the height of the boiling temperature of the corresponding varieties of N.]. The first is a mixture of N. with ortho- and paranitrotoluene, has a specific gravity of 1.19 (at 0°), boils at 210° - 220° and is used for the preparation of “aniline oil for red paints” (see Organic artificial paints); the second consists mainly of ortho- and paranitrotoluene with a slight admixture of N. and partly nitroxylenes, has a specific gravity of 1.167 (at 0°) and boils at 222 - 235°; it is used in technology to obtain a mixture of ortho- and paratoluidine (see Toluidine).
Dinitrobenzenes C 6 H 4 (NO 2) 2 (ortho-, para- and meta-) are obtained by nitration of benzene or N. with a mixture of strong sulfuric and fuming nitric acids when heated, and the main product is metadinitrobenzene (ordinary dinitrobenzene). Di-N. are crystalline substances, poisonous, soluble in wine and wood alcohol, ether, chloroform, etc., as well as in hot water (ortho-N. - little), upon reduction they give the corresponding phenylenediamines C 6 H 4 (NH 2) 2 (see .). Ortodi-N. represents needles or single-clinomer tablets, melts at 118°, when boiled with caustic soda it gives orthonitrophenol C 6 H 4 (OH)NO 2, with alcoholic ammonia - orthonitroaniline C 6 H 4 (NH 2)NO 2, does not change with an alcohol solution of potassium synoxide even when boiling. Metadi-N.- thin rhombic tablets, melts at 91°, boils without decomposition at 297°, with an alcohol solution of potassium bluehydride gives nitrile C 6 H 3 (OCH 5) (NO 2) CN, ammonium sulphide is reduced to metanitroaniline and nitialin C 12 H 16 N 4 S 4 O, with bromine in the presence of ferric chloride at 180 - 230° gives C 6 Br 6, C 6 Br 5 Cl, C 6 Br 4 Cl 2, and when heated with ferric chloride alone, it completely decomposes. Paradis-N.- single-clinomer needles with temp. melted 171 - 172°, easily sublimes, with naphthalene it forms a compound that is very difficult to dissolve in alcohol; it changes with an alcohol solution of potassium synoxide only when boiled. Di-N. poisonous, like N., and just like the latter, it explodes on its own only if certain conditions are met, for example. with instant heating to a very high temp. (Berthelot), but with nitrate and other oxygen-rich substances it is capable of forming well-detonating mixtures, such as, for example, bellite, roburite, helgophyte etc. [They will be considered together with other similar mixtures in Art. Picric acid, Sprengel explosives, Panclastite], and therefore finds quite frequent use in explosives technology. In large form in di-N factories. is prepared either by directly nitrating benzene with a more concentrated acid mixture, or by first obtaining N. as described above and, after the reaction is completed, adding the same amount of acid mixture, converting N. into di-N., while the mass is not cooled and the temperature is allowed to rise to 100°. At the end of nitration, the mass is allowed to cool and settle and the acid is drained. The resulting crude di-N. washed first with cold, then warm water, separated from the water using filters, melted and cast into tin molds. A good commercial di-N., always consisting of a mixture of all three isomers with a predominance of the meta-compound, is a yellow solid crystalline mass, melting at 85 - 87° and soluble in warm water, alcohol and other solvents. It should be odorless and should not contain N impurities at all.
Trinitrobenzenes C6H3(NO2)3. Of the three isomers possible according to theory, only two are known so far: a symmetric one, obtained from metadinitrobenzene, and one of the asymmetric ones (1: 2: 4), obtained from paradinitrobenzene. To prepare them, the corresponding dinitro compounds are treated by heating with a mixture of fuming sulfuric and nitric acids. They represent the highest degree of nitro substitution of benzene obtained and are crystalline substances, more or less easily soluble in benzene, ether, wine and wood alcohol, chloroform, acetone and sparingly soluble in carbon sulphide and water. Simm. C 6 H 3 (NO 2) 3 (1:3:5) crystallizes in leaves or rhombus. tablets, melts at 121 - 122°, sublimates with careful heating, directly combines with hydrocarbons and aniline, with bromine in the presence of bromine iron gives perbromobenzene C 6 Br 6, upon oxidation with red salt and soda - trinitrophenol (picric acid) [Dinitrobenzene in those conditions gives dinitrophenol, but it is much more difficult to oxidize, while H. does not oxidize at all], it is vigorously reduced with potassium synoxide, and when heated with caustic soda or barite, it decomposes to form nitrous acid salts. Not simmm. C 6 H 3 (NO 3) 3 (1:2:4) presents light yellow crystals, with alcoholic ammonia it easily turns into dinitroaniline C 6 H 3 (NH 2)(NO 2) 2, when boiled with a weak solution of sodium hydroxide gives dinitrophenol C 6 H 3 (OH)(NO 2) 2, and with methyl alcohol at 150° - methyl ester of dinitrophenol C 6 H 3 (СOH 3)(NO 2) 2. Trinitrobenzenes are already substances with more clearly expressed explosive properties, but still have almost no practical significance. See also Art. Nitro compounds and aromatic hydrocarbons (nitro derivatives).
P. P. Rubtsov.Δ.
Nitrobenzene(nitrobenzoin) - in medicine proposed for external use exclusively against scabies, but, due to its extreme toxicity, N. did not enter into medical practice. Symptoms of poisoning are expressed by general weakness, nausea, then loss of consciousness, a state of pronounced depression of the central nervous system, alternating with attacks of convulsions, cyanosis, dilated pupils, and difficulty breathing may occur. Death occurs due to respiratory failure; A spectral blood test detects a hematinic band. When giving benefits to poisoned people, they try to remove the poison by prescribing emetics (if the poisoning occurred from the introduction of N. into the stomach), in addition, by using energetic irritants inside, under the skin, cold douches and artificial respiration.
Under the action of reducing agents, the nitro group transforms into a primary amino group:
R-NO 2 a R-NH 2
This reaction occurs with compounds of both the fatty and aromatic series. This method is of especially great practical importance in the aromatic series, since the aromatic nitro derivatives themselves are obtained very easily and are therefore quite accessible products.
In most cases, metals are used as reducing agents: tin, zinc or iron in the presence of hydrochloric acid.
The fastest and most complete recovery occurs under the action of tin:
2R-NO 2 + 3Sn + 14HCl and 2Cl + 3SnCl 4 + 4H 2 O
In technology for the reduction of nitro compounds, iron is used, which is much cheaper than tin:
R-NO 2 + 3Fe + 7HCl a Cl + 3FeCl 2 + 2H 2 O
It was experimentally established that to carry out this reaction it is possible to take 40 times less acid than required by the above equation, which is explained by the oxidation of divalent iron into ferric iron.
Ammonium sulfide or sodium sulfide can also be used as reducing agents:
R - NO 2 + 3(NH 4) 2 S and R - NH 2 + 6NH 3 + 3S + 2H 2 O
It was in this way that N. N. Zinin first obtained aniline from nitrobenzene in 1841 and proved the generality of this reaction for nitro compounds of the benzene and naphthalene series, as well as for polynitro compounds.
Using ammonium sulphide, it is possible to carry out partial reduction of polynitro compounds, for example, to obtain nitroaniline from dinitrobenzene.
The reduction of nitro compounds can also be carried out in the gas phase with hydrogen in the presence of metallic copper as a catalyst.
Aniline is obtained by the reduction of nitrobenzene with tin or iron in the presence of hydrochloric acid.
Aniline is a base (albeit a weak one) and therefore forms salts with acids. After the reduction reaction is completed, these salts are decomposed by adding alkali:
Cl + NaOH a C 6 H 5 -NH 2 + NaCl + H 2 O
and the released aniline is distilled off with steam.
a) Reduction of nitrobenzene with tin
Reagents:
Nitrobenzene...................18.5 g (0.15 mol)
Tin granulated.........36 g (0.35 mol)
Hydrochloric acid conc......80 ml (about 1 mol)
Sodium hydroxide; calcium chloride; ether; caustic potassium
Tin and nitrobenzene are added to a round-bottomed half-liter flask, 10 ml of hydrochloric acid is added and the flask is closed with a stopper with a wide glass tube inserted into it, which serves as a reflux condenser. The contents of the flask are mixed well. After some time, a vigorous reaction begins, accompanied by strong heating of the mixture. If the reaction goes too violently, you should immerse the flask in cold water for a while.
Gradually add the remaining amount of hydrochloric acid into the flask, maintaining the vigorous course of the reaction all the time. After all the acid has been added, the flask is heated for 1 hour in a water bath.
Add 30 ml of water to the still warm solution and gradually add (until a strongly alkaline reaction) a solution of 45 g of sodium hydroxide in 60 ml of water. Aniline and water vapor are distilled from the hot liquid (the device is shown in Fig. 18). An aqueous aniline emulsion is collected in the receiver, which gradually separates. After a completely transparent distillate begins to drain from the refrigerator, about 100 ml of liquid is distilled off.
Aniline is noticeably soluble in water (100 g of water dissolves 3 g of aniline) and therefore, to more completely isolate it from an aqueous solution, the resulting product is saturated with sodium chloride, in concentrated solutions of which aniline is practically insoluble. For every 100 ml of shoulder strap, add 20-25 g of well-ground sodium chloride. The salt is dissolved with stirring and the aniline is extracted with ether, shaking the solution in a separating funnel successively with 50, 30 and again with 30 ml of ether.
The combined ethereal extracts are dried with several pieces of solid potassium hydroxide, the ether is distilled off in a water bath and the aniline is distilled from a small distillation flask using an air cooler.
Yield about 12 g.
Pace. kip. 184.4°; beat weight 1.022.
b) Reduction of nitrobenzene with iron
Reagents:
Nitrobenzene...................18.5 g (0.15 mol)
Iron (fine sawdust).......30 g (0.55 grams)
Hydrochloric acid conc......90 ml (1.1 mol)
Sodium hydroxide; sodium chloride; ether; caustic potassium
The work should be carried out under traction.
Nitrobenzene and iron filings are added to a half-liter flask and then hydrochloric acid is added in small portions (about 1-2 ml). After each addition of acid, the flask is closed with a rubber stopper into which a glass tube 25-30 cm long is inserted, and the contents of the flask are mixed well. When 30 ml of hydrochloric acid is added, the remaining amount can be added in larger portions, 10-20 ml each. If the reaction proceeds too violently, the flask should be cooled with water. After adding all the acid, the flask is heated for another half hour in a boiling water bath. The absence of nitrobenzene odor indicates the end of the reaction. Next, an alkali solution is added until the reaction is strongly alkaline, the aniline is distilled off with steam and extracted with ether, as described in the previous work.
m-Nitroaniline is obtained as a result of partial reduction m-dinitrobenzene sodium sulfide:
Reagents:
Dinitrobenzene......10 g (0.06 mol)
Sodium sulphide crystal.........25 g (0.1 mol)
The work is being carried out under traction.
In a 250 ml flask, heat 50 ml of water to 85° and pour in finely ground dinitrobenzene. Stirring vigorously, a thin suspension of dinitrobenzene is obtained, after which a solution of sodium sulfide is gradually added to 20 ml of water.
The end of the reaction is determined by placing a drop of the solution on a filter paper moistened with a solution of copper sulfate. If the resulting black spot of copper sulfide does not disappear within 20 seconds, the reaction can be considered complete.
The solution is cooled and left to stand overnight. The next day, the resulting nitroaniline crystals are sucked off and crystallized from water.
Lecture No. 40
NITRO COMPOUNDS
Nitro compounds are derivatives of hydrocarbons in which one or more hydrogen atoms are replaced by a nitro group - NO 2.
Nitroalkanes are derivatives of alkanes in which one or more hydrogen atoms are replaced by a nitro group.
The general formula of mononitroalkanes is C n H 2n+1 NO 2.
When naming nitroalkanes, the longest hydrocarbon chain is selected, the numbering of which begins from the end to which the nitro group is located closest. The latter is indicated using the prefix “nitro”. For example:
Synthesis methods
1. Nitration of alkanes
Nitromethane is obtained from methane; the nitration of methane homologues produces a mixture of nitroalkanes:
2. Alkylation of nitrites
R-Br + AgNO 2 ® R-NO 2 + AgBr
R-Br + NaNO 2 ® R-NO 2 + NaBr
Since nitrite anions are ambident in nature, aprotic nonpolar solvents and moderate temperatures are used to obtain a high yield of nitroalkane.
Physical properties and structure
Nitroalkanes are colorless or yellowish liquids or crystalline substances with a faint odor.
Mononitroalkanes are characterized by large dipole moments. The reason for the significant polarity of nitroalkanes lies in the electronic structure of the nitro group containing a semipolar bond
The alignment of N-O bonds is confirmed by X-ray diffraction analysis: the N-O bond in the nitro group is shorter than the N-O bond in hydroxylamine, but longer than the bond in the nitroso group –N=O.
The high electronegativity of the N and O atoms, the multiplicity of the N=O bond and its semipolar nature determine the significant electron-withdrawing properties of the nitro group (-I and –M effects).
Nitroalkanes are characterized by weak absorption in the UV region of 270-280 nm. This is due to electronic transitions of the n ® p* type of the lone electron pair of the oxygen atom on the LUMO.
In the IR spectra, absorption maxima are observed associated with symmetric and antisymmetric vibrations of N=O bonds in the regions of 1370 cm -1 and 1550 cm -1 .
Chemical properties of nitroalkanes
Acidity and tautomeric transformations of nitroalkanes
Primary and secondary nitroalkanes are CH-acids .
The acidity is due to the stabilization of the resulting carbanion due to the electron-withdrawing properties of the nitro group.
The acidity of mononitroalkanes in aqueous solutions is comparable to the acidity of phenols. If one carbon atom has two or three nitro groups, the acidity increases sharply.
The nitroalakane anion is ambident like the enolate anion. For example, when it is protonated, in addition to the nitroalkane, another tautomeric form can be formed.
The tautomeric form of a nitroalkane is called aciform or nitronic acid, which has not been obtained in its pure form. Nitronic acid is an OH acid of medium strength (pKa = 3.2).
Thus, nitro compounds should be considered as tautomers, reacting in nitro and aci forms.
Under normal conditions, the concentration of the aci form is negligible (10-5-10-7%). The equilibrium shifts to the right in an alkaline environment due to the formation of salts.
Crystalline salts of alkali and alkaline earth metals are stable and highly soluble in water. They are sometimes called nitronic acid salts. When solutions are acidified, nitronic acid itself (aciform) is first formed, which then isomerizes into a nitroalkane.
Nitro compounds belong to pseudoacids, which are characterized by the fact that they themselves are neutral, do not have electrical conductivity, but nevertheless form neutral salts of alkali and alkaline earth metals.
“Neutralization” of nitro compounds by bases occurs slowly, but of true acids - instantly.
Among other reactions of nitroalkanes, we note the following.
Hydrolysis in an acidic environment with cleavage of C-N bonds.
This reaction is used in technology for the synthesis of hydroxylamine and its sulfate.
Substitution of H-atoms ata- C for halogens, nitrous acid residues, aldehydes, ketones, etc.
The reaction with HNO 2 is qualitative for nitroalkanes. Tertiary nitroalkanes do not react, secondary R 2 CH-NO 2 form nitrosonitroalkanes
Primary ones form nitrooximes (nitrolic acids) with HNO 2
These colorless compounds form blood-red salts of nitrolic acids with alkalis.
Aromatic nitro compounds
1. Methods of obtaining
- Nitration of arenes
This is the main method for preparing nitroarenes; discussed in detail in the study of electrophilic aromatic substitution (see lecture No. 18).
- Oxidation of arylamines
The method involves the oxidation of primary aromatic amines with peroxy compounds. The most effective oxidation reagent is trifluoroperoxyacetic acid in methylene chloride. Trifluoroperoxyacetic acid is obtained directly in the reaction mixture by reacting trifluoroacetic acid anhydride and 90% hydrogen peroxide. This method is important for the synthesis of nitro compounds containing ortho- And pair-positions to the nitro group are other electron-withdrawing groups, for example:
2. Physical properties and structure
Nitroarenes are yellow substances with a peculiar odor. Nitrobenzene is a liquid with the smell of bitter almonds. Di- and polynitroarenes are crystalline substances.
The nitro group is a strong electron acceptor, so nitroarenes have large dipole moments directed towards the nitro group.
Polynitroarene molecules are strong electron acceptors. For example, the electron affinity of 1,3-dinitrobenzene is 1.35 eV, and that of 1,3,5-trinitrobenzene is 1.75 eV.
3. Chemical properties
Reduction of the nitro group
The product of exhaustive reduction of the nitro group in nitroarenes is the amino group. Currently, catalytic hydrogenation is used to reduce nitroarenes to arylamines under industrial conditions. The catalyst uses copper on silica gel as a carrier. The yield of aniline over this catalyst is 98%.
In laboratory conditions, metals are used to reduce the nitro group in an acidic or alkaline environment. Reduction occurs in several stages, the sequence of which varies greatly in acidic and alkaline environments.
During recovery in an acidic environment The process proceeds stepwise and includes the following stages.
In an acidic environment, each of the intermediate products is quickly reduced to the final product aniline and they cannot be isolated individually. Iron, tin or zinc and hydrochloric acid are used as reducing agents. An effective reducing agent for the nitro group is tin(II) chloride in hydrochloric acid. The end product of reduction in an acidic environment is an amine, for example:
C6H5NO2 + 3Zn + 7HCl® C 6 H 5 NH 2HCl + 3ZnCl 2 + 2H 2 O
In a neutral solution, for example, when reducing nitroarenes with zinc in an aqueous solution of ammonium chloride, the reduction process slows down and stops at the stage of formation of arylhydroxylamine.
Upon recovery in an alkaline environment in excess of the reducing agent, the final product of nitroarene reduction is hydrazoarene (diarylhydrazine)
The process can be represented as the following sequence of transformations.
azoxyarene |
azoarene g |
hydrazoarene |
In an alkaline environment, the reduction processes of nitrosoarene and hydroxylamine slow down so much that the main process becomes their condensation with the formation of azoxyarene. This reaction is essentially similar to the addition of nitrogenous bases to the carbonyl group of aldehydes and ketones.
Azoxybenzene, when exposed to zinc in an alcoholic alkali solution, is reduced first to azobenzene, and when exposed to excess zinc, further to hydrazobenzene.
Azoxybenzene itself can be prepared by reducing nitrobenzene with sodium methoxide in methyl alcohol.
Alkali metal and ammonium sulfides are also used as reducing agents for nitroarenes.
4ArNO2 + 6Na2S + 7H2O® 4ArNH 2 + 3Na 2 S 2 O 3 + 6NaOH
As follows from the stoichiometric equation, during the process of reduction with sulfide, the alkalinity of the medium increases, which leads to the formation of azoxy and azo compounds as by-products. In order to avoid this, hydrosulfides and polysulfides should be used as reducing agents, since in this case alkali is not formed.
ArNO 2 + Na 2 S 2 + H 2 O® ArNH 2 + Na 2 S 2 O 3
The rate of reduction of the nitro group by sulfides strongly depends on the electronic effects of the substituents on the aromatic ring. Thus, m-dinitrobenzene is reduced by sodium disulfide 1000 times faster than m-nitroaniline. This is used for partial recovery nitro groups in polynitro compounds.
Products of incomplete reduction of the nitro group
Nitrosoarenes
Nitrosoarenes are easily reduced, so they are difficult to obtain by reducing nitroarenes. The best method for preparing nitrosoarenes is the oxidation of arylhydrazines.
It is possible to directly introduce a nitroso group into the aromatic ring by the action of nitrous acid on phenols and tertiary arylamines (see lectures No. 29 and 42)
In the crystalline state, aromatic nitroso compounds exist in the form of colorless dimers. In liquid and gaseous states, there is an equilibrium between dimer and monomer. Monomers are colored green.
Nitroso compounds, like carbonyl compounds, react with nucleophiles. For example, upon condensation with arylhydroxylamines, azoxy compounds are formed (see above), and with arylamines, azo compounds are formed.
Arylhydroxylamines
In addition to the method described above for the preparation of nitroarenes by reduction in a neutral environment, arylhydroxylamines are synthesized by nucleophilic substitution in activated arenes.
As intermediates in the reduction of nitroarenes, arylhydroxylamines can be oxidized to nitroso compounds (see above) and reduced to amines by catalytic hydrogenation or by the action of a metal in an acidic environment.
ArNHOH + Zn + 3HCl ® ArNH 2 . HCl + ZnCl 2 + H 2 O
In an acidic environment, arylhydroxylamines rearrange aminophenols, which is used to obtain the latter, for example:
Azoxyarenes
In addition to the methods described above - condensation of nitroso compounds with arylhydroxylamines and reduction of nitroarenes with sodium methoxide, azoxyarenes can be obtained by oxidation of azoarenes with peroxy compounds.
In an alkaline environment, azoxyarenes are reduced to azo- and then hydrazoarenes (see above).
Azo arenas
They are formed during the reduction of nitroarenes, arylhydrazines and azoxyarenes in an alkaline medium, for example:
Unsymmetrical azo compounds are obtained by condensation of nitroso compounds with amines (see above). An important method for the synthesis of azo compounds, the azo coupling reaction, will be discussed in detail below (see lecture No. 43)
Azoarenes exist as cis- And trance- isomers. More stable when irradiated trance-isomer turns into cis-isomer. The reverse transformation occurs upon heating.
Azo compounds are colored, many of them are used as dyes.
Hydrazoarenes
These are the end products of the reduction of nitroarenes in an alkaline environment. Hydrazoarenes are colorless crystalline substances that oxidize in air to colored azo compounds. For preparative purposes, oxidation is carried out using bromine water.
Ar-NHN-HAr + Br 2 + 2NaOH ® Ar-N=N-Ar + 2NaBr + 2H 2 O
When reduced under harsh conditions, hydrazoarenes give arylamines.
An important property of hydrazo compounds is rearrangement into 4,4/-diaminobiphenyls. This transformation was called benzidine rearrangement. Currently, this term combines a whole group of related rearrangements leading to the formation of a mixture ortho- And pair-isomeric derivatives of diaminobiphenyl.
Rearrangement of hydrazobenzene itself produces a mixture of diamines containing 70% benzidine and 30% 2,4/-diaminobiphenyl.
If pair-position in one of the benzene rings of hydrazobenzene is occupied by some substituent, the product of the rearrangement is a diphenylamine derivative (the so-called semidine rearrangement).
When studying the mechanism of benzidine and related rearrangements, it was found that they occur intramolecularly. If two different hydrazobenzenes are rearranged together, there are no cross-rearrangement products. For the rearrangement of hydrazobenzene itself, the reaction rate was found to be proportional to the concentration of hydrazobenzene and the square of the proton concentration. This means that the diprotonated form of hydrazobenzene undergoes rearrangement. It has also been shown that the monoprotonated form of hydrazobenzene is converted entirely to benzidine only upon repeated treatment with acid. These data are consistent with the following mechanism of benzidine rearrangement.
It is assumed that the transition state is formed from a conformation of hydrazobenzene in which the two corresponding carbon atoms of both benzene rings are very close to each other. The formation of a new carbon-carbon bond and the breaking of the old bond of two nitrogen atoms occurs strictly synchronously. According to modern terminology, the benzidine rearrangement is one of the sigmatropic rearrangements.
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