Limit open-chain nitro compounds (non-cyclic) have the general formula C n H 2n+1 NO 2 . They are isomeric to alkyl nitrites (esters of nitrous acid) with the general formula R-ONO. The differences are:
Alkyl nitrites have lower boiling points
Nitro compounds are highly polar and have a large dipole moment
Alkyl nitrites are easily saponified by alkalis and mineral acids to form the corresponding alcohols and nitrous acid or its salt.
Reduction of nitro compounds leads to amines, alkyl nitrites to alcohols and hydroxylamine.
Receipt
According to the Konovalov reaction - by nitration of paraffins with dilute nitric acid when heated. All hydrocarbons enter into the liquid-phase nitration reaction, but the reaction rate is low and the yields are low. The reaction is accompanied by oxidation and the formation of polynyrocompounds. The best results are obtained with hydrocarbons containing a tertiary carbon atom. Vapor-phase nitration proceeds at 250-500 o C with nitric acid vapor. The reaction is accompanied by cracking of hydrocarbons, resulting in all kinds of nitro derivatives, and oxidation, which results in the formation of alcohols, aldehydes, ketones, acids. Unsaturated hydrocarbons are also formed. Nitric acid can be replaced by nitrogen oxides. Nitration proceeds by the S R mechanism.
Interaction of halogen derivatives of saturated hydrocarbons with silver nitrite when heated. The attacking particle is the NO 2 - ion, which exhibits dual reactivity (ambivalence), i.e. add a radical on nitrogen (S N 2) to form a nitro compound R-NO 2 or oxygen to form a nitrous acid ester R-O-N=O.(S N 1). The mechanism of the reaction and its direction strongly depend on the nature of the solvent. Solvating solvents (water, alcohols) favor the formation of ether.
Chemical properties
When reducing nitro compounds, primary amines are formed:
Primary and secondary nitro compounds are soluble in alkalis with the formation of salts. Hydrogen atoms at the carbon bound to the nitro group are activated, as a result, in an alkaline environment, the niro compounds are rearranged into the aci-nitro form:
When an alkaline solution of a nitro compound is treated with a mineral acid, a strongly acidic aci form is formed, which quickly isomerizes into the usual neutral form:
Nitro compounds are referred to as pseudoacids. Pseudoacids are neutral and non-conductive, but nevertheless form neutral alkali metal salts. Neutralization of nitro compounds with alkalis occurs slowly, and true acids - instantly.
Primary and secondary nitro compounds react with nitrous acid, tertiary ones do not react:
Alkaline salts of nitrolic acids in solution are red, pseudonitrols are blue or greenish-blue.
Primary and secondary niro compounds condense in the presence of alkalis with aldehydes, forming nitro alcohols (nucleophilic addition):
Aci-forms of primary and secondary nitro compounds in aqueous solutions under the action of mineral acids form aldehydes or ketones:
Primary nitro compounds, when heated with 85% sulfuric acid, transform into carboxylic acids with the elimination of hydroxylamine. This occurs as a result of hydrolysis of the resulting aci-form.
NITRO COMPOUNDS
(C-nitro compounds), contain one or several in the molecule. nitro groups directly attached to the carbon atom. N- and O-nitro compounds are also known (see Nitramines And organic nitrates).
The nitro group has a structure intermediate between the two limiting resonance structures:
The group is planar; the N and O atoms have, sp 2 - hybridization, NChO bonds are equivalent and almost one and a half; bond lengths, eg. for CH 3 NO 2 , 0.122 nm (NChO), 0.147 nm (CHN), ONO angle 127°. The MFNO 2 system is flat with a low barrier of rotation around the SCN connection.
N., having at least one a-H-atom, can exist in two tautomeric forms with a common mesomeric anion. O-shape aci-H. or nitrone to-that:
Known diff. derivatives of nitronic acids: f-ly RR "C \u003d N (O) O - M + (salts of H.), ethers (nitronic esters), etc. Ethers of nitronic acids exist in the form iis- And trance-isomers. There are cyclic ethers, for example. N-oxides of isoxazolines.
Name N. is produced by adding the prefix "nitro" to the name. base connections, if necessary adding a digital indicator, e.g. 2-nitropropane. Name N. salts are produced from the names. either C-form or aci-forms, or nitrone to-you.
physical properties. The simplest nitroalkanes are colorless. liquids. Phys. Holy Islands of certain aliphatic N. are shown in the table. Aromatic N.-bestsv. or light yellow high-boiling liquids or low-melting solids with a characteristic odor, poorly sol. in water, as a rule, are distilled with steam.
PHYSICAL PROPERTIES OF SOME ALIPHATIC NITRO COMPOUNDS
* At 25°C. ** At 24°C. *** At 14°C.
In N.'s IK spectra there are two characteristic. bands corresponding to antisymmetric and symmetric stretching vibrations of the NChO bond: for primary N. resp. 1560-1548 and 1388-1376 cm -1 , for secondary 1553-1547 and 1364-1356 cm -1 , for tertiary 1544-1534 and 1354-1344 cm -1 ; for nitroolefins RCH=CHNO 2 1529-1511 and 1351-1337 cm -1 ; for dinitroalkanes RCH(NO 2) 2 1585-1575 and 1400-1300 cm -1 ; for trinitroalkanes RC(NO 2) 3 1610-1590 and 1305-1295 cm -1; for aromatic H. 1550-1520 and 1350-1330 cm -1 (electron-withdrawing substituents shift the high-frequency band to the region 1570 -1540, and electron-donor - to the region 1510-1490 cm -1); for salts H. 1610-1440 and 1285-1135 cm -1; nitrone esters have an intense band at 1630-1570 cm, the CCHN bond has a weak band at 1100-800 cm -1 .
In UV spectra, aliphatic H. l max 200-210 nm (intense band) and 270-280 nm (weak band); for salts and esters of nitrone to-t resp. 220-230 and 310-320 nm; For gem-dinitrocomponent. 320-380 nm; for aromatic H. 250-300 nm (the intensity of the band sharply decreases when the coplanarity is violated).
In the PMR spectrum, chem. shifts a-H-atom depending on the structure of 4-6 ppm In the NMR spectrum 14 N and 15 N chem. shift 5 from - 50 to + 20 ppm
In the mass spectra of aliphatic N. (with the exception of CH 3 NO 2) peak mol. ion is absent or very small; main the fragmentation process is the elimination of NO 2 or two oxygen atoms to form a fragment equivalent to a nitrile. Aromatic N. is characterized by the presence of a peak they say. and she; main the peak in the spectrum corresponds to the ion produced by elimination of NO 2 .
Chemical properties. The nitro group is one of the most strong electron-withdrawing groups and is able to effectively delocalize negative. charge. In the aromatic conn. as a result of induction and especially mesomeric effects, it affects the distribution of electron density: the nucleus acquires a partial positive. charge, to-ry localized Ch. arr. V ortho- And pair-provisions; Hammett constants for the NO 2 s group m 0.71s n 0.778,s+ n 0.740, s - n 1.25. So arr., the introduction of the NO 2 group dramatically increases the reaction. ability org. conn. in relation to the nucleoph. reagents and makes it difficult to R-tion with elektrof. reagents. This determines the widespread use of N. in org. synthesis: the NO 2 group is introduced into the desired position of the org molecule. Comm., carry out decomp. p-tion associated, as a rule, with a change in the carbon skeleton, and then transformed into another function or removed. In the aromatic In a row, a shorter scheme is often used: nitration-transformation of the NO 2 group.
Mn. aliphatic N.'s transformations pass with preliminary. isomerization to nitrone to-you or the formation of the corresponding anion. In solutions, the balance is usually almost completely shifted towards the C-form; at 20 °C share aci- forms for nitromethane 1X10 -7, for nitropropane 3. 10 -3 . Nitronovye to-you in svob. the form is usually unstable; they are obtained by careful acidification of H salts. Unlike H., they conduct current in solutions and give a red color with FeCl 3 . Aci- N.-stronger CH-acids (p K a~ 3-5) than the corresponding N. (p K a >~ 8-10); N.'s acidity increases with the introduction of electron-withdrawing substituents in the a-position to the NO 2 group.
The formation of nitrone to - t in a number of aromatic N. is associated with the isomerization of the benzene ring into the quinoid form; for example, forms with conc. H 2 SO 4 colored salt product f-ly I, o-nitrotoluene shows as a result vnutrimol. proton transfer to form a bright blue O-derivative:
Under the action of bases on primary and secondary N., N. salts are formed; ambident salts in p-tions with electrophiles are able to give both O- and C-derivatives. So, during the alkylation of H. salts with alkyl halides, trialkylchlorosilanes, or R 3 O + BF - 4, O-alkylation products are formed. The last m. b. also obtained by the action of diazomethane or N,O- bis-(trimethylsilyl)acetamide to nitroalkanes with p K a< 3>
or nitrone to-you, for example:
Acyclic alkyl esters of nitrone to-t are thermally unstable and decompose according to vnutrimol. mechanism:
p-tion can be used to obtain carbonyl compounds. Silyl ethers are more stable. See below for the formation of C-alkylation products.
N. is characterized by p-tions with a break in the bond SChN, by bonds N=O, O=N O, C=N -> O, and p-tions with the preservation of the NO 2 group.
R-ts and and with r and r y v o m s s vyaz i sChN. Primary and secondary N. at loading. with a miner. to-tami in the presence. alcohol or aqueous solution of alkali form carbonyl Comm. (cm. Nefa reaction). R-tion passes through the interval. the formation of nitrone to-t:
As a source Comm. silyl nitrone ethers can be used. The action of strong to-t on aliphatic N. can lead to hydroxamic to-there, for example:
The method is used in the industry for the synthesis of CH 3 COOH and hydroxylamine from nitroethane. Aromatic N. are inert to the action of strong to-t.
Under the action of reducing agents (eg, TiCl 3 -H 2 O, VCl 2 -H 2 O-DMF) on H. or oxidizing agents (KMnO 4 -MgSO 4 , O 3) on H. salts, aldehydes are also formed.
Aliphatic H., containing mobile H in the b-position to the NO 2 group, under the action of bases, easily eliminate it in the form of HNO 2 with the formation of olefins. Thermal flows in the same way. decomposition of nitroalkanes at temperatures above 450 °. Vicinal dinitrocomponents. when treated with Ca amalgam in hexamstanol, both NO 2 groups, Ag-salts of unsaturated H. are cleaved off. With the loss of NO 2 groups, they can dimerize:
Nucleof. substitution of the NO 2 group is not typical for nitroalkanes, however, when thiolate ions act on tertiary nitroalkanes in aprotic p-solvents, the NO 2 group is replaced by a hydrogen atom. P-tion proceeds by an anion-radical mechanism. In the aliphatic and heterocyclic. conn. the NO 2 group with a multiple bond is relatively easily replaced by a nucleophile, for example:
In the aromatic conn. nucleoph. substitution of the NO 2 group depends on its position in relation to other substituents: the NO 2 group located in meta- position with respect to electron-withdrawing substituents and in ortho- And pair- positions to electron donor, has a low reaction. ability; reaction the ability of the NO 2 group located in ortho- And pair- positions to electron-withdrawing substituents, increases markedly. In some cases, the deputy enters into ortho-position to the leaving group NO 2 (for example, when aromatic N. is heated with an alcohol solution of KCN, Richter's solution):
R-ts and and about with I z and N \u003d O. One of the most important p-tsy-restoration, leading in the general case to a set of products:
Azoxy-(II), azo-(III) and hydrazo compounds. (IV) are formed in an alkaline environment as a result of the condensation of intermediate nitroso compounds. with amines and hydroxylamines. Carrying out the process in an acidic environment excludes the formation of these substances. Nitroso-compound. recover faster than the corresponding N., and select them from the reactions. mixtures usually fail. Aliphatic N. are restored in azoxy- or under the action of Na alcoholates, aromatic - under the action of NaBH 4, the treatment of the latter with LiAlH 4 leads to azo compounds. Electrochem. aromatic N. under certain conditions allows you to get any of the presented derivatives (with the exception of nitrosocompound.); by the same method it is convenient to obtain hydroxylamines from mononitroalkanes and amidoximes from salts gem-dinitroalkanes:
There are many methods of recovering N. to. Widely used iron filings, Sn and Zn in the presence. to-t; with catalytic hydrogenation, Ni-Raney, Pd / C or Pd / PbCO 3, etc. are used as catalysts. Aliphatic N. are easily reduced to amines LiAlH 4 and NaBH 4 in the presence. Pd, Na and Al amalgams, when heated. with hydrazine over Pd/C; for aromatic N., TlCl 3, CrCl 2 and SnCl 2 are sometimes used, aromatic. poly-N. are selectively reduced to nitramines with Na hydrosulfide in CH 3 OH. There are ways to choose. recovery of the NO 2 group in polyfunctional N. without affecting other f-tions.
Under the action of P(III) on aromatic N., a succession occurs. deoxygenation of the NO 2 group with the formation of highly reactive nitrenes. R-tion is used for the synthesis of condenser. heterocycles, for example:
Under the same conditions, silyl esters of nitrone acids are transformed into silyl derivatives of oximes. Treatment of primary nitroalkanes with PCl 3 in pyridine or NaBH 2 S leads to nitriles. Aromatic N., containing in ortho- position substituent with a double bond or cyclopropyl substituent, in an acidic environment rearrange to o-nitrosoketones, for example:
N. and nitrone ethers react with an excess of the Grignard reagent, giving hydroxylamine derivatives:
R-tions on the bonds O = N O and C = N O. N. enter the p-tions of 1,3-dipolar cycloaddition, for example:
Naib. this p-tion easily flows between nitrone ethers and olefins or acetylenes. In cycloaddition products (mono- and bicyclic dialkoxyamines) under the action of nucleoph. and elektrof. N C O bond reagents are easily cleaved, which leads to decomp. aliphatic and hetero-cyclic. conn.:
For preparative purposes, stable silyl nitrone esters are used in the district.
R-ts and with the preservation of the NO 2 group. Aliphatic N., containing an a-H-atom, are easily alkylated and acylated with the formation, as a rule, of O-derivatives. However, mutually mod. dilithium salts of primary N. with alkyl halides, anhydrides or carboxylic acid halides to - t leads to products of C-alkylation or C-acylation, for example:
Known examples vnutrimol. C-alkylations, e.g.:
Primary and secondary N. react with aliphatic. amines and CH 2 O with the formation of p-amino derivatives (p-tion Mannich); in the district, you can use previously obtained methylol derivatives of N. or amino compounds:
Nitromethane and nitroethane can condense with two molecules of methylolamine, and higher nitroalkanes with only one. At certain ratios of reagents p-tion can lead to heterocyclic. connection, for example: with interaction. primary nitroalkane with two equivalents of primary amine and an excess of formaldehyde are formed Comm. f-ly V, if the reagents are taken in a ratio of 1:1:3-comm. forms VI.
Aromatic N. easily enter into the district of the nucleoph. substitution and much more difficult, in the district of the electroph. substitution; in this case, the nucleophile is directed to ortho- and pore-positions, and the electrophile-in meta- position to the NO 2 group. Velocity constant nitration of nitrobenzene is 5-7 orders of magnitude less than that of benzene; this produces m-dinitrobenzene.
The activating effect of the NO 2 group on the nucleoph. substitution (especially ortho-position) is widely used in org. synthesis and industry. R-tion proceeds according to the scheme of accession-cleavage from the intermediate. the formation of an s-complex (Meisenheimer complex). According to this scheme, halogen atoms are easily replaced by nucleophiles:
Known examples of substitution by the anion-radical mechanism with electron capture aromatic. connection and emission of a halide ion or other groups, for example. alkoxy, amino, sulfate, NO - 2. In the latter case, the district passes the easier, the greater the deviation of the NO 2 group from coplanarity, for example: in 2,3-dinitrotoluene it is replaced in the main. the NO 2 group in position 2. The H atom in aromatic H. is also capable of nucleophage. substitution-nitrobenzene at heating. with NaOH forms o-nitrophenol.
The nitro group facilitates aromatic rearrangements. conn. according to the intramol mechanism. nucleoph. substitution or through the stage of formation of carbanions (see. Smiles regrouping).
The introduction of the second NO 2 group accelerates the nucleophane. substitution. N. in the presence. bases are added to aldehydes and ketones, giving nitroalcohols (see. Henri reaction), primary and secondary N. - to Comm., containing aktivir. double bond (Michael region), for example:
Primary N. can enter into Michael's p-tion with the second molecule of an unsaturated compound; this p-tion with the last. transformation of the NO 2 group is used for the synthesis of poly-function. aliphatic connections. The combination of Henri and Michael p-tions leads to 1,3-dinitro compounds, for example:
To inactivated double bond, only Hg-derivatives are added gem- di-or trinitro compounds, as well as IC(NO 2) 3 and C(NO 2) 4, with the formation of C- or O-alkylation products; the latter can enter into a cyclo-addition p-tion with the second olefin molecule:
Easily enter into p-tion accession nitroolefins: with water in a slightly acidic or slightly alkaline medium with the latter. Henri retroreaction they form carbonyl Comm. and nitroalkanes; with N., containing a-H-atom, poly-N.; add other CH-acids, such as acetoacetic and malonic acid esters, Grignard reagents, as well as nucleophiles such as OR -, NR - 2, etc., for example:
Nitroolefins can act as dienophiles or dipolarophiles in p-tions of diene synthesis and cycloaddition, and 1,4-dinitrodienes can act as diene components, for example:
Nitrosation of primary N. leads to nitrolic to-there RC (=NOH) NO 2, secondary N. form pseudo-nitrols RR "C (NO) NO 2, tertiary N. do not enter into the district.
Nitroalkanes are easily halogenated in the presence. bases with succession. substitution of H atoms at a-C-atom:
With photodhym. chlorination, more distant H atoms are replaced:
When carboxylation of primary nitroalkanes by the action of CH 3 OMgOCOOCH 3 a-nitrocarboxylic to-you or their esters are formed.
When processing salts mono-N. C (NO 2) 4 ., nitrites of Ag or alkali metals or under the action of nitrites on a-halo-nitroalkanes in an alkaline medium (Ter Meer district) are formed gem-dinitro compounds. The electrolysis of a-halo-nitroalkanes in aprotic p-solvents, as well as the treatment of H. Cl 2 in an alkaline medium or the electrooxidation of H. salts, lead to vic- dinitro compounds:
The nitro group does not render beings. influence on free-radical or aromatic arylation. conn.; p-tion leads to the main. To ortho- And pair- substituted products.
To restore N. without affecting the NO 2 group, NaBH 4, LiAlH 4 are used at low temperatures or diborane solution in THF, for example:
Aromatic di- and tri-N., in particular 1,3,5-trinitrobenzene, form stable brightly colored crystals. they say complexes with aromatic Comm.-donors of electrons (amines, phenols, etc.). Complexes with picric to-one is used to isolate and purify aromatic. hydrocarbons. Intermod. di- and trinitrobenzenes with strong bases (HO - , RO - , N - 3 , RSO - 2 , CN - , aliphatic amines) leads to the formation of Meisen-heimer complexes, which are isolated as colored alkali metal salts.
Receipt. In industry, lower nitroalkanes are obtained by liquid-phase (Konovalov district) or vapor-phase (Hess method) nitration of a mixture of ethane, propane and butane, isolated from natural gas or obtained by oil refining (see. Nitration). Higher N., for example, are also obtained by this method. nitrocyclohexane is an intermediate in the production of caprolactam.
In the laboratory, to obtain nitroalkanes, nitric acid is used. with activated a methylene group; a convenient method for the synthesis of primary nitroalkanes is the nitration of 1,3-indanedione with the last. alkaline hydrolysis of a-nitroketone:
Aliphatic N. also receive interaction. AgNO 2 with alkyl halides or NaNO 2 with esters of a-halocarboxylic-new to-t (see. Meyer reaction). Aliphatic N. are formed during the oxidation of amines and oximes; oximes - a method of obtaining gem-di-and gem- trinitro compounds, e.g.:
Nitroalkanes m. b. obtained by heating acyl nitrates to 200 °C.
Mn. N. synthesis methods are based on the nitration of olefins with nitrogen oxides, HNO 3 , nitronium salts, NO 2 Cl, org. nitrates, etc. As a rule, this results in a mixture vic-dinitro compounds, nitronitrates, nitronitrites, unsaturated N., as well as products of conjugated addition of the NO 2 group and a p-solvent molecule or their hydrolysis products, for example:
a,w-Dinitroalkanes are obtained by the action of alkyl nitrates on cyclic. ketones with last. hydrolysis of salts a, a "-dinitro-ketones:
Poly-N. synthesized by destructive nitration decomp. org. conn.; eg, three - and get by the action of HNO 3 on acetylene in the presence. Hg(II) ions.
Main method of obtaining aromatic N. - electrophor. nitration. The active nitrating group is the nitronium ion NO 2 generated from HNO 3 under the action of strong protic or aprotic acids. For nitration under mild conditions, nitronium salts are used (NO 2 BF 4, NO 2 ClO 4, etc.), as well as N 2 O 5 in inert p-solvents.
In the industry for nitration aromatic. conn. as a rule, nitrating mixtures are used (H 2 SO 4 + HNO 3). In the laboratory, instead of H 2 SO 4, AlCl 3, SiCl 4, BF 3, etc. are used to increase the concentration of the nitronium ion, nitration is often carried out in inert p-solvents (CH 3 COOH, nitromethane, etc.). Easily replaced by the NO 2 group of sulfo and diazo groups. To introduce the second NO 2 group into nitrobenzene in ortho- And pair-positions first receive the corresponding diazo derivative, and then they replace the diazo group according to the Sandmeyer p-tion. Aromatic N. are also obtained by the oxidation of nitroso, diazo, and amino groups.
Application. Poly-N., especially aromatic ones, are used as explosives and to a lesser extent as components of rocket fuels. Aliphatic N. are used as solvents in the paint and varnish industry and in the production of polymers, in particular cellulose ethers; for cleaning the miner. oils; oil dewaxing, etc.
A number of N. find application as biologically active in-in. So, esters of phosphoric acid, containing a nitroaryl fragment, are insecticides; derivatives of 2-nitro-1,3-propanediol and 2-nitrostyrene -; derivatives of 2,4-dinitrophenol -; a-nitrofurans are the most important antibacterial drugs, based on them, drugs with a wide spectrum of action (furazolidin, etc.) have been created. Some aromatic N.-fragrant in-va.
N. - intermediate products in the production of synthetic. dyes, polymers, detergents and corrosion inhibitors; wetting, emulsifying, dispersing and flotation agents. agents; plasticizers and modifiers of polymers, pigments, etc. They are widely used in org. synthesis and as a model Comm. in the theoretical org. chemistry.
Nitroparaffins have a strong local irritant effect and are relatively toxic substances. They belong to cellular poisons of general action, especially dangerous for the liver. LD 50 0.25-1.0 g / kg (with oral administration). Chlorinated and unsaturated N. are 5-10 times more toxic. Aromatic N. depress the nervous and especially the circulatory system, disrupting the supply of oxygen to the body. Signs of poisoning - hyperemia, elevated. mucus secretion, lacrimation, cough, dizziness, headache. Wed first aid-quinine and. N.'s metabolism is connected with okislit. - restore. p-tions and, in particular, with oxidizing. phosphorylation. For example, 2,4-dinitrophenol is one of the largest. powerful reagents that uncouple the processes of oxidation and phosphorylation, which prevents the formation of ATP in the cell.
The world produces several hundred different N. The volume of production of the most important aliphatic N. is tens of thousands of tons, aromatics is hundreds of thousands of tons; for example, in the USA 50 thousand tons/year of C 1 -C 3 nitroalkanes and 250 thousand tons/year of nitrobenzene are produced.
see also m-Dinitrobenzene, Nitroanisols, Nitrobenzene, Nitromethap, Nitrotoluenes and etc.
Lit.: Chemistry of nitro- and nitrosogroups, ed. G. Feuer, trans. from English, vol. 1-2, M., 1972-73; Chemistry of aliphatic and alicyclic nitro compounds, M., 1974; General Organic, trans. from English, vol. 3, M., 1982, p. 399-439; Tartakovsky V. A., "Izv. AN SSSR. Ser. chem.", 1984, No. 1, p. 165-73.
V. A. Tartakovsky.
Chemical encyclopedia. - M.: Soviet Encyclopedia. Ed. I. L. Knunyants. 1988 .
Aromatic nitro compounds are divided into two groups: compounds containing a nitro group bonded to the carbon atom of the aromatic nucleus, and compounds containing a nitro group in the side chain:
Depending on which (primary, secondary, tertiary) carbon atom has a nitro group, nitro compounds are primary, secondary or tertiary.
The names of nitro compounds are formed by adding the prefix nitro- to the name of the corresponding hydrocarbon, indicating the position of the nitro group:
Nitroarenes containing a nitro group in the side chain are considered as derivatives of saturated hydrocarbons containing an aromatic radical and a nitro group as substituents:
How to get
1. Nitration of alkanes (Konovalov reaction). The saturated hydrocarbon is treated with dilute nitric acid (10–25%) at elevated temperature and pressure.
2. Nitration of arenes. Nitrocompounds containing a nitro group linked to an aromatic radical are obtained by nitration of arenes with a mixture of concentrated nitric and sulfuric acids, called a "nitrating mixture". The reaction proceeds by the mechanism of electrophilic substitution (SE),
A maximum of three nitro groups can be introduced into the benzene core. The nitro group deactivates the benzene core so much that more stringent conditions are required for the introduction of the second nitro group, and the third is introduced with great difficulty,
3. The action of salts of nitrous acid on halogen derivatives of alkanes:
It is advisable to carry out this reaction in an aprotic solvent medium to reduce the formation of by-products - esters of nitrous acid,
3. Oxidation of tert-alkylamines. This method is used only to obtain tertiary nitro compounds:
According to the physical properties of the nitro compounds of the series, these are liquid or crystalline, colorless or yellow-colored substances. The reason for staining is the presence of a chromophore - the -NO 2 group. Nitro compounds have a pleasant odor and are poisonous. Slightly soluble in water, soluble in most organic solvents.
Chemical properties
Nitro compounds are characterized by two series of reactions: reactions involving the nitro group and reactions involving mobile hydrogen atoms at the α-carbon atom.
1. Tautomerism and salt formation. Due to the presence of mobile hydrogen atoms at the α-carbon atom, primary and secondary nitro compounds are tautomeric substances.
In solution, a dynamic equilibrium is established between these forms. This type of tautomerism is called aci-nitro-taut. series. In a neutral medium, the equilibrium is almost completely shifted towards the nitro form. In an alkaline environment, the equilibrium shifts towards the aci-nitro form. So, primary and secondary nitroalkanes dissolve in an aqueous solution of alkali, forming salts of nitronic acids.
Salts of nitronic acids are easily destroyed by mineral acids with the formation of initial nitroalkanes.
Tertiary nitro compounds, due to the absence of mobile hydrogen atoms at the α-carbon atom, are not capable of tautomerism, and therefore do not interact with alkalis.
2. Reaction with nitrous acid. Primary, secondary and tertiary nitro compounds react differently to the action of nitrous acid. Only those nitro compounds that have mobile hydrogen atoms at the α-carbon atom react with HNO 2.
Primary nitro derivatives form alkyl nitro acids:
Nitrolic acids dissolve in alkalis, forming red salts.
Secondary nitro compounds with nitrous acid form pseudonitrols (nitroso-nitro compounds):
Pseudonitrols are colorless substances that are associated compounds in the crystalline state, but in solution or in the melt, the associates are destroyed and a blue color appears.
Tertiary nitro compounds do not react with nitrous acid.
The reaction with nitrous acid is used to distinguish primary, secondary and tertiary nitro compounds from each other.
3. Condensation reaction with aldehydes and ketones. Due to mobile hydrogen atoms in the α-position, nitro compounds are able to enter into condensation reactions with aldehyde in a weakly alkaline medium to form nitroalcohols (nitroalkanols):
Nitroalcohols are easily dehydrated to form unsaturated nitrocompounds.
4. Recovery reaction. When nitroalkanes are reduced, alkylamines are formed:
When aromatic nitro compounds are reduced, aromatic amines are formed (Zinin reaction). Depending on the pH of the reaction medium, the reduction process can proceed in two directions, differing in the formation of different intermediate products.
In a neutral and acidic environment (pH< 7) в качестве промежуточных соединений образуются ароматические нитрозосоединения и арилгидроксиламины:
In an alkaline environment (pH>7), the nitroso compounds formed during the reaction are condensed with sarylhydroxylamine and azoxy compounds are formed. The latter add hydrogen and turn into hydrazo compounds, which, in turn, easily turn into arylamines:
The reduction reaction of nitroarenes in an alkaline environment (pH>7) can be stopped at any of the above steps. It serves as the main method for obtaining azo- and hydrazo compounds. The reaction was discovered in 1842 by the Russian scientist N.N. Zinin,
1. Nitro compounds
1.2. Reactions of nitro compounds
1. NITRO COMPOUNDS
Nitrocompounds are derivatives of hydrocarbons in which one or more hydrogen atoms are replaced by a nitro group -NO 2 . Depending on the hydrocarbon radical to which the nitro group is attached, nitro compounds are divided into aromatic and aliphatic. Aliphatic compounds are distinguished as primary 1o, secondary 2o and tertiary 3o, depending on whether a nitro group is attached to the 1o, 2o or 3o carbon atom.
The nitro group -NO2 should not be confused with the nitrite group -ONO. The nitro group has the following structure:
The presence of a total positive charge on the nitrogen atom determines the presence of a strong -I-effect. Along with a strong -I-effect, the nitro group has a strong -M-effect.
Ex. 1. Consider the structure of the nitro group and its influence on the direction and rate of the electrophilic substitution reaction in the aromatic nucleus.
1.1. Methods for obtaining nitro compounds
Almost all methods for obtaining nitro compounds have already been considered in previous chapters. Aromatic nitro compounds are obtained, as a rule, by direct nitration of arenes and aromatic heterocyclic compounds. Nitrocyclohexane under industrial conditions is obtained by nitration of cyclohexane:
(1)
Nitromethane is also obtained in the same way, however, under laboratory conditions, it is obtained from chloroacetic acid as a result of reactions (2-5). The key step among them is reaction (3) proceeding via the SN2 mechanism.
(2)Chloroacetic acid Sodium chloroacetate
(3) (4)Nitroacetic acid
(5)Nitromethane
1.2. Reactions of nitro compounds
1.2.1. Tautomerism of aliphatic nitro compounds
Due to the strong electron-withdrawing properties of the nitro group, a-hydrogen atoms have increased mobility and therefore primary and secondary nitro compounds are CH-acids. So, nitromethane is a rather strong acid (pKa 10.2) and in an alkaline medium it easily turns into a resonance-stabilized anion:
(6)Nitromethane pKa 10.2 Resonance stabilized anion
Exercise 2. Write the reactions of (a) nitromethane and (b) nitrocyclohexane with an aqueous solution of NaOH.
1.2.2. Condensation of aliphatic nitro compounds with aldehydes and ketones
The nitro group can be introduced into aliphatic compounds by an aldol reaction between the nitroalkane anion and an aldehyde or ketone. In nitroalkanes, a-hydrogen atoms are even more mobile than in aldehydes and ketones, and therefore they can enter into addition and condensation reactions with aldehydes and ketones, providing their a-hydrogen atoms. With aliphatic aldehydes, addition reactions usually take place, and with aromatic ones, only condensations.
So, nitromethane is added to cyclohexanone,
(7)
1-nitromethylcyclohexanol
but condenses with benzaldehyde,
(8)All three hydrogen atoms of nitromethane participate in the addition reaction with formaldehyde and 2-hydroxymethyl-2-nitro-1,3-dinitropropane or trimethylolnitromethane is formed.
(9)By condensation of nitromethane with hexamethylenetetramine, we obtained 7-nitro-1,3,5-triazaadamantane:
(10)
Ex. 3. Write the reactions of formaldehyde (a) with nitromethane and (b) with nitrocyclohexane in an alkaline medium.
1.2.3. Recovery of nitro compounds
The nitro group is reduced to the amino group by various reducing agents (11.3.3). Aniline is obtained by hydrogenation of nitrobenzene under pressure in the presence of Raney nickel under industrial conditions.
(11) (11 32)
In laboratory conditions, instead of hydrogen, hydrazine can be used, which decomposes in the presence of Raney nickel with the release of hydrogen.
(12)
7-nitro-1,3,5-triazaadamantane 7-amino-1,3,5-triazaadamantane
Nitro compounds are reduced with metals in an acid medium, followed by alkalization
(13) (11 33)
Depending on the pH of the medium and the reducing agent used, various products can be obtained. In a neutral and alkaline environment, the activity of conventional reducing agents with respect to nitro compounds is less than in an acidic environment. A typical example is the reduction of nitrobenzene with zinc. In an excess of hydrochloric acid, zinc reduces nitrobenzene to aniline, while in a buffer solution of ammonium chloride it reduces to phenylhydroxylamine:
(14)
In an acidic environment, arylhydroxylamines undergo a rearrangement:
(15)
p-Aminophenol is used as a developer in photography. Phenylhydroxylamine can be further oxidized to nitrosobenzene:
(16)
Nitrosobenzene
The reduction of nitrobenzene with tin (II) chloride produces azobenzene, and with zinc in an alkaline medium, hydrazobenzene is obtained.
(17)
(18)
Treatment of nitrobenzene with a solution of alkali in methanol gives azoxybenzene, while the methanol is oxidized to formic acid.
(19)
Known methods of incomplete recovery and nitroalkanes. One of the industrial methods for producing capron is based on this. By nitration of cyclohexane, nitrocyclohexane is obtained, which is converted by reduction into cyclohexanone oxime and then, using the Beckmann rearrangement, into caprolactam and polyamide - the starting material for the preparation of fiber - capron:
Reduction of the nitro group of aldol addition products (7) is a convenient way to obtain b-amino alcohols.
(20)
1-Nitromethylcyclohexanol 1-Aminomethylcyclohexanol
The use of hydrogen sulfide as a reducing agent makes it possible to reduce one of the nitro groups in dinitroarenes:
(11 34)
m-Dinitrobenzene m-Nitroaniline
(21)
2,4-Dinitroaniline 4-Nitro-1,2-diaminobenzene
Exercise 4. Write the reduction reactions of (a) m-dinitrobenzene with tin in hydrochloric acid, (b) m-dinitrobenzene with hydrogen sulfide, (c) p-nitrotoluene with zinc in a buffered ammonium chloride solution.
Exercise 5. Complete reactions:
(b) 
The nitro group has a structure intermediate between the two limiting resonance structures:
The group is planar; the N and O atoms have sp 2 hybridization, the N-O bonds are equivalent and practically one and a half; bond lengths, eg. for CH 3 NO 2, 0.122 nm (N-O), 0.147 nm (C-N), ONO angle 127°. The C-NO 2 system is planar with a low barrier to rotation around the C-N bond.
H Itro compounds having at least one a-H-atom can exist in two tautomeric forms with a common mesomeric anion. O-shape aci-nitro compound or nitrone to-that:
Known diff. derivatives of nitronic acids: salts of the f-ly RR "C \u003d N (O) O - M + (salts of nitro compounds), ethers (nitronic esters), etc. Ethers of nitronic acids exist in the form of iis- and trans- isomers There are cyclic ethers, for example N-oxides of isoxazolines.
Name nitro compounds are produced by adding the prefix "nitro" to the name. base connections, if necessary adding a digital indicator, e.g. 2-nitropropane. Name salts of nitro compounds are produced from the names. either C-form, or aci-form, or nitrone to-you.
physical properties. The simplest nitroalkanes are colorless. liquids. Phys. Holy Islands of certain aliphatic nitro compounds are given in the table. Aromatic nitro compounds-bestsv. or light yellow, high-boiling liquids or low-melting solids, with a characteristic odor, poorly sol. in water tends to be distilled with steam.
PHYSICAL PROPERTIES OF SOME ALIPHATIC NITRO COMPOUNDS
* At 25°C. ** At 24°C. *** At 14°C.
In the UV spectra of aliphatic nitro compounds l max 200-210 nm (intense band) and 270-280 nm (weak band); for salts and esters of nitrone to-t resp. 220-230 and 310-320 nm; for gem-dinitrocomponent. 320-380 nm; for aromatic nitro compounds, 250–300 nm (the intensity of the band sharply decreases when the coplanarity is violated).
In the PMR spectrum, chem. shifts of a-H-atom depending on the structure 4-6 ppm In the NMR spectrum 14 N and 15 N chem. shift 5 from - 50 to + 20 ppm
In the mass spectra of aliphatic nitro compounds (with the exception of CH 3 NO 2), the peak mol. ion is absent or very small; main fragmentation process - elimination of NO 2 or two oxygen atoms to form a fragment equivalent to nitrile. Aromatic nitro compounds are characterized by the presence of a peak mol. and she ; main the peak in the spectrum corresponds to the ion produced by elimination of NO 2 .
Chemical properties. The nitro group is one of the most strong electron-withdrawing groups and is able to effectively delocalize negative. charge. In the aromatic conn. as a result of induction and especially mesomeric effects, it affects the electron density distribution: the nucleus acquires a partial positive. charge, to-ry localized Ch. arr. in ortho and para positions; Hammett constants for the NO 2 group s m 0.71, s n 0.778, s + n 0.740, s - n 1.25. So arr., the introduction of the NO 2 group dramatically increases the reaction. ability org. conn. in relation to the nucleoph. reagents and makes it difficult to R-tion with elektrof. reagents. This determines the widespread use of nitro compounds in org. synthesis: the NO 2 group is introduced into the desired position of the org molecule. Comm., carry out decomp. p-tion associated, as a rule, with a change in the carbon skeleton, and then transformed into another function or removed. In the aromatic In a row, a shorter scheme is often used: nitration-transformation of the NO 2 group.
Mn. transformations of aliphatic nitro compounds take place with a preliminary. isomerization to nitrone to-you or the formation of the corresponding anion. In solutions, the balance is usually almost completely shifted towards the C-form; at 20 °С, the proportion of the aci-form for nitromethane is 1 10 -7, for nitropropane 3. 10 -3 . Nitronovye to-you in svob. the form is usually unstable; they are obtained by careful acidification of salts of nitro compounds. Unlike nitro compounds, they conduct current in solutions and give a red color with FeCl 3 . Aci-nitro compounds are stronger CH-acids (pK a ~ 3-5) than the corresponding nitro compounds (pK a ~ 8-10); the acidity of nitro compounds increases with the introduction of electron-withdrawing substituents in the a-position to the NO 2 group.
The formation of nitrone to-t in a series of aromatic nitro compounds is associated with the isomerization of the benzene ring into the quinoid form; for example, nitrobenzene forms with conc. H 2 SO 4 colored salt product f-ly I, o-nitrotoluene exhibits photochromism as a result vnutrimol. proton transfer to form a bright blue O-derivative:
Under the action of bases on primary and secondary nitro compounds, salts of nitro compounds are formed; ambident anions of salts in p-tions with electrophiles are able to give both O- and C-derivatives. So, during the alkylation of salts of nitro compounds with alkyl halides, trialkylchlorosilanes or R 3 O + BF - 4, O-alkylation products are formed. Recent m.b. also obtained by the action of diazomethane or N,O-bis-(trimethylsilyl)acetamide on nitroalkanes with pK a< 3 или нитроновые к-ты, напр.:
Acyclic alkyl esters of nitrone to-t are thermally unstable and decompose according to intramol. mechanism:
; this
p-tion can be used to obtain carbonyl compounds. Silyl ethers are more stable. See below for the formation of C-alkylation products.
For nitro compounds, p-tions with a break in the C-N bond, along the bonds N \u003d O, O \u003d N O, C \u003d N -\u003e O and p-tions with the preservation of the NO 2 group are characteristic.
R-ts and and with r and ry v o m s vyaz z and C-N. Primary and secondary nitro compounds at loading. with a miner. to-tami in the presence. alcohol or aqueous solution of alkali form carbonyl Comm. (see Neph reaction). R-tion passes through the interval. the formation of nitrone to-t:
As a source Comm. silyl nitrone ethers can be used. The action of strong to-t on aliphatic nitro compounds can lead to hydroxamic to-there, for example:
The method is used in the industry for the synthesis of CH 3 COOH and hydroxylamine from nitroethane. Aromatic nitro compounds are inert to the action of strong to-t.
Under the action of reducing agents (eg, TiCl 3 -H 2 O, VCl 2 -H 2 O-DMF) on nitro compounds or oxidizing agents (KMnO 4 -MgSO 4, O 3) on salts of nitro compounds, ketones and aldehydes are formed.
Aliphatic nitro compounds containing a mobile H atom in the b-position to the NO 2 group, under the action of bases, easily eliminate it in the form of HNO 2 with the formation of olefins. Thermal flows in the same way. decomposition of nitroalkanes at temperatures above 450 °. Vicinal dinitrocomponents. when treated with Ca amalgam in hexamstanol, both NO 2 groups are cleaved off, Ag-salts of unsaturated nitro compounds can dimerize upon loss of NO 2 groups:
Nucleof. substitution of the NO 2 group is not typical for nitroalkanes, however, when thiolate ions act on tertiary nitroalkanes in aprotic p-solvents, the NO 2 group is replaced by a hydrogen atom. P-tion proceeds by an anion-radical mechanism. In the aliphatic and heterocyclic. conn.the NO 2 group with a multiple bond is relatively easily replaced by a nucleophile, for example:
In the aromatic conn. nucleoph. the substitution of the NO 2 group depends on its position with respect to other substituents: the NO 2 group, which is in the meta position with respect to the electron-withdrawing substituents and in the ortho and para positions to the electron donor, has a low reaction. ability; reaction the ability of the NO 2 group, located in the ortho- and para-positions to electron-withdrawing substituents, increases markedly. In some cases, the substituent enters the ortho position to the leaving NO 2 group (for example, when aromatic nitro compounds are loaded with an alcohol solution of KCN, Richter's solution):
R-ts and and about with I z and N \u003d O. One of the most important p-tsy-restoration, leading in the general case to a set of products:
Azoxy-(II), azo-(III) and hydrazo compounds. (IV) are formed in an alkaline environment as a result of the condensation of intermediate nitroso compounds. with amines and hydroxylamines. Carrying out the process in an acidic environment excludes the formation of these substances. Nitroso-compound. recover faster than the corresponding nitro compounds, and select them from the reaction. mixtures usually fail. Aliphatic nitro compounds are reduced to azoxy or azo compounds by the action of Na alcoholates, aromatic ones by the action of NaBH 4, the treatment of the latter with LiAlH 4 leads to azo compounds. Electrochem. the reduction of aromatic nitro compounds under certain conditions allows you to get any of the presented derivatives (with the exception of nitroso compounds); it is convenient to obtain hydroxylamines from mononitroalkanes and amidoximes from salts of gem-dinitroalkanes by the same method:
Many methods are known for the reduction of nitro compounds to amines. Widely used iron filings, Sn and Zn in the presence. to-t; with catalytic hydrogenation as catalysts use Ni-Raney, Pd / C or Pd / PbCO 3, etc. Aliphatic nitro compounds are easily reduced to amines LiAlH 4 and NaBH 4 in the presence. Pd, Na and Al amalgams, when heated. with hydrazine over Pd/C; for aromatic nitro compounds, TlCl 3, CrCl 2 and SnCl 2 are sometimes used, aromatic. polynitro compounds are selectively reduced to nitramines with Na hydrosulfide in CH 3 OH. There are ways to choose. recovery of the NO 2 group in polyfunctional nitro compounds without affecting other f-tions.
Under the action of P(III) on aromatic nitro compounds, a succession occurs. deoxygenation of the NO 2 group with the formation of highly reactive nitrenes. R-tion is used for the synthesis of condenser. heterocycles, for example:
Under the same conditions, silyl esters of nitrone acids are transformed into silyl derivatives of oximes. Treatment of primary nitroalkanes with PCl 3 in pyridine or NaBH 2 S leads to nitriles. Aromatic nitro compounds containing a double bond substituent or a cyclopropyl substituent in the ortho position are rearranged in an acidic medium into o-nitrosoketones, for example:
H itro compounds and nitrone ethers react with an excess of Grignard's reagent to give hydroxylamine derivatives:
R-tions for bonds O \u003d N O and C \u003d N O. Nitro compounds enter into p-tions of 1,3-dipolar cycloaddition, for example:
Naib. this p-tion easily flows between nitrone esters and olefins or acetylenes. In cycloaddition products (mono- and bicyclic dialkoxyamines) under the action of nucleoph. and elektrof. N - O bond reagents are easily cleaved, which leads to decomp. aliphatic and hetero-cyclic. conn.:
For preparative purposes, stable silyl nitrone esters are used in the district.
R-ts and with the preservation of the NO 2 group. Aliphatic nitro compounds containing an a-H-atom are easily alkylated and acylated to form, as a rule, O-derivatives. However, mutually mod. dilithium salts of primary nitro compounds with alkyl halides, anhydrides or carboxylic acid halides leads to products of C-alkylation or C-acylation, for example:
Known examples vnutrimol. C-alkylations, e.g.:
Primary and secondary nitro compounds react with aliphatic. amines and CH 2 O with the formation of p-amino derivatives (p-tion Mannich); in the district, you can use pre-obtained methylol derivatives of nitro compounds or amino compounds:
The activating effect of the NO 2 group on the nucleoph. substitution (especially in the ortho position) is widely used in org. synthesis and industry. P-tion proceeds according to the scheme of accession-cleavage from the intermediate. the formation of an s-complex (Meisenheimer complex). According to this scheme, halogen atoms are easily replaced by nucleophiles:
Known examples of substitution by the anion-radical mechanism with electron capture aromatic. connection and emission of a halide ion or other groups, for example. alkoxy, amino, sulfate, NO - 2. In the latter case, the district passes the easier, the greater the deviation of the NO 2 group from coplanarity, for example: in 2,3-dinitrotoluene it is replaced in the main. the NO 2 group in position 2. The H atom in aromatic nitro compounds is also capable of nucleophage. substitution-nitrobenzene at heating. with NaOH forms o-nitrophenol.
The nitro group facilitates aromatic rearrangements. conn. according to the intramol mechanism. nucleoph. substitution or through the stage of formation of carbanions (see Smiles rearrangement).
The introduction of the second NO 2 group accelerates the nucleophane. substitution. H introconnections in the presence. bases are added to aldehydes and ketones, giving nitroalcohols (see Henri reactions), primary and secondary nitro compounds, to Comm., containing activir. double bond (Michael region), for example:
Primary nitro compounds can enter into the Michael p-tion with the second molecule of the unsaturated compound; this p-tion with the last. trancethe formation of the NO 2 group is used for the synthesis of poly-function. aliphatic connections. The combination of Henri and Michael p-tions leads to 1,3-dinitro compounds, for example:
To inactivated only Hg-derivatives of gem-di- or trinitro compounds, as well as IC (NO 2) 3 and C (NO 2) 4, are attached to the double bond, while products of C- or O-alkylation are formed; the latter can enter into a cyclo-addition p-tion with the second olefin molecule:
Easily enter into p-tion accession nitroolefins: with water in a slightly acidic or slightly alkaline medium with the latter. Henri retroreaction they form carbonyl Comm. and nitroalkanes; with nitro compounds containing a-H-atom, poly-nitro compounds; add other CH-acids, such as acetylacetone, acetoacetic and malonic esters, Grignard reagents, as well as nucleophiles such as OR -, NR - 2, etc., for example:
Nitroolefins can act as dienophiles or dipolarophiles in p-tions of diene synthesis and cycloaddition, and 1,4-dinitrodienes can act as diene components, for example:
Receipt. In the industry, lower nitroalkanes are obtained by liquid-phase (Konovalov's district) or vapor-phase (Hess method) nitration of a mixture of ethane, propane and butane, isolated from natural gas or obtained by oil refining (see Nitration). Higher nitro compounds are also obtained in this way, for example. nitrocyclohexane is an intermediate in the production of caprolactam.
In the laboratory, nitration of nitric acid is used to obtain nitroalkanes. with activated a methylene group; a convenient method for the synthesis of primary nitroalkanes is the nitration of 1,3-indanedione with the last. alkaline hydrolysis of a-nitroketone:
Aliphatic nitro compounds also receive interaction. AgNO 2 with alkyl halides or NaNO 2 with esters of a-halocarboxylic-new to-t (see Meyer reaction). Aliphatic nitro compounds are formed from the oxidation of amines and oximes; oxidation of oximes - a method for obtaining gem-di- and gem-trinitro compounds, for example:
