The nitro group has a structure intermediate between two limiting resonant structures:
The group is planar; the N and O atoms have sp 2 hybridization, the N-O bonds are equivalent and almost one-and-a-half; bond lengths, e.g. for CH 3 NO 2, 0.122 nm (N-O), 0.147 nm (C-N), ONO angle 127°. C-NO system 2 flat with low barrier of rotation around C-N connections.
N itro compounds having at least one a-H atom can exist in two tautomeric forms with a common mesomeric anion. O-form called aci-nitro compound or nitronic compound:
Various known derivatives of nitronic compounds: salts of the form RR"C=N(O)O - M + (salts of nitro compounds), ethers (nitronic ethers), etc. Esters of nitronic compounds exist in the form of is- 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, adding a digital indicator if necessary, e.g. 2-nitropropane. Name salts of nitro compounds are produced from the name. either the C-form, or the aci-form, or the nitronic acid.
Physical properties. The simplest nitroalkanes are colorless. liquids Phys. The properties of certain aliphatic nitro compounds are given in the table. Aromatic nitro compounds are colorless. or light yellow high-boiling liquids or low-melting solids with a characteristic odor, poorly soluble. in water, as a rule, they are 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 ethers of nitronic acid, resp. 220-230 and 310-320 nm; for heme-dinitro-containing 320-380 nm; for aromatic nitro compounds 250-300 nm (the intensity of the band decreases sharply when coplanarity is violated).
In the PMR spectrum of chem. shifts of the a-H atom, depending on the structure, 4-6 ppm. In the NMR spectrum 14 N and 15 N chemical. 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; basic fragmentation process - the 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 ; basic the peak in the spectrum corresponds to the ion produced by the 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 aromatic conn. as a result of inductive and especially mesomeric effects, it affects the distribution of electron density: the nucleus becomes partially positive. charge, which is 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. Thus, the introduction of the NO 2 group sharply increases the reaction. ability to org. conn. in relation to nucleoph. reagents and makes it difficult to deal with electroph. 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. connection, carry out decomposition. ptions associated, as a rule, with a change in the carbon skeleton, and then transformed into another function or removed. In aromatic In some cases, a shorter scheme is often used: nitration-transformation of the NO 2 group.
Mn. transformations of aliphatic nitro compounds take place with pre-treatment. isomerization into nitronic compounds or the formation of the corresponding anion. In solutions, the equilibrium is usually almost completely shifted towards the C-form; at 20 °C the proportion of the aci form for nitromethane is 1 10 -7, for nitropropane 3. 10 -3. Nitron compounds are free. 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 nitron compounds 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-like product of type I, o-nitrotoluene exhibits photochromism as a result of intramol. proton transfer to form a bright blue O derivative:
When bases act on primary and secondary nitro compounds, salts of nitro compounds are formed; ambident anions of salts in solutions with electrophiles are capable of producing both O- and C-derivatives. Thus, when alkylation of salts of nitro compounds with alkyl halides, trialkylchlorosilanes or R 3 O + BF - 4, O-alkylation products are formed. Latest 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 nitronic acids are thermally unstable and disintegrate intramol. mechanism:
; this
The solution can be used to obtain carbonyl compounds. Silyl ethers are more stable. For the formation of C-alkylation products, see below.
Nitro compounds are characterized by reactions with the rupture of the C-N bond, along the bonds N=O, O=N O, C=N -> O, and solutions with preservation of the NO 2 group.
R-ts and s r a r s in about m with connections and S-N. Primary and secondary nitro compounds when heated. with mineral K-tami is present. alcohol or water solution alkalis form carbonyl compounds. (see Nave reaction). R-tion passes through the gaps. formation of nitron compounds:
As initial conn. Silyl nitrone ethers can be used. Action strong qualities on aliphatic nitro compounds can lead to hydroxamic compounds, for example:
The method is used in industry for the synthesis of CH 3 COOH and hydroxylamine from nitroethane. Aromatic nitro compounds are inert to the action of strong compounds.
When reducing agents (for example, TiCl 3 -H 2 O, VCl 2 -H 2 O-DMF) act on nitro compounds or oxidizing agents (KMnO 4 -MgSO 4, O 3) on the 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, when exposed to bases, easily eliminate it in the form of HNO 2 with the formation of olefins. Thermal flow proceeds similarly. decomposition of nitroalkanes at temperatures above 450°. Vicinal dinitrosoids. when treated with Ca amalgam in hexamstanol, both NO 2 groups are split off; Ag salts of unsaturated nitro compounds are able to dimerize when NO 2 groups are lost:
Nucleof. substitution of the NO 2 group is not typical for nitroalkanes, however, when thiolate ions act on tertiary nitroalkanes in aprotic solutions, the NO 2 group is replaced by a hydrogen atom. The flow flows through anion-radical mechanism. In aliphatic and heterocyclic conn.the NO 2 group at a multiple bond is relatively easily replaced by a nucleophile, for example:
In aromatic conn. nucleoph. the substitution of the NO 2 group depends on its position in relation to other substituents: the NO 2 group, located in the meta position with respect to the electron-withdrawing substituents and in the ortho- and para-positions with respect to the electron-donating ones, has a low reactivity. ability; reaction the ability of the NO 2 group located in the ortho- and para-positions to accept electron-withdrawing substituents increases markedly. In some cases, the substituent enters the ortho position to the NO 2 leaving group (for example, when heating aromatic nitro compounds with an alcohol solution KCN, Richter solution):
R-ts and i about connections and N = O. One of the most important r-tions is restoration, leading to general case to the product set:
Azoxy-(II), azo-(III) and hydrazo-containing. (IV) are formed in an alkaline environment as a result of condensation of intermediate nitroso compounds. with amines and hydroxylamines. Carrying out the process in an acidic environment eliminates the formation of these substances. Nitroso-containing are reduced faster than the corresponding nitro compounds, and isolate them from the reaction. the mixture usually fails. Aliphatic nitro compounds are reduced to azoxy or azo compounds under the action of Na alcoholates, aromatic compounds under the action of NaBH 4, treatment of the latter with LiAlH 4 leads to azo compounds. Electrochem. the reduction of aromatic nitro compounds under certain conditions makes it possible to obtain any of the presented derivatives (with the exception of nitroso compounds); Using the same method, it is convenient to obtain hydroxylamines from mononitroalkanes and amidoximes from salts of gem-dinitroalkanes:
There are many known methods for the reduction of nitro compounds to amines. Iron filings, Sn and Zn are widely used. kit; with catalytic in hydrogenation, Ni-Raney, Pd/C or Pd/PbCO 3 and others are used as catalysts. Aliphatic nitro compounds are easily reduced to the amines LiAlH 4 and NaBH 4 in the presence. Pd, amalgams of Na and Al, with heating. with hydrazine over Pd/C; for aromatic nitro compounds, TlCl 3, CrCl 2 and SnCl 2 are sometimes used, aromatic. poly-nitro compounds are selectively reduced to nitramines by Na hydrosulfide in CH 3 OH. There are ways to choose. reduction of the NO 2 group in polyfunctional nitro compounds without affecting other functions.
When P(III) acts on aromatic nitro compounds, a sequence occurs. deoxygenation of the NO 2 group with the formation of highly reactive nitrenes. The solution is used for the synthesis of condenser. heterocycles, for example:
Under the same conditions, silyl ethers of nitronic compounds are transformed into silyl derivatives of oximes. Treatment of primary nitroalkanes 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 environment to form o-nitrosoketones, for example:
N itro compounds and nitrone esters react with excess Grignard reagent to give hydroxylamine derivatives:
Rations for the O = N O and C = N O bonds. Nitro compounds enter into 1,3-dipolar cycloaddition relationships, for example:
Naib. This process easily occurs between nitron esters and olefins or acetylenes. In cycloaddition products (mono- and bicyclic dialkoxyamines) under the influence of nucleophiles. and electroph. N - O bond reagents are easily broken down, which leads to decomposition. aliphatic and hetero-cyclic. conn.:
For preparative purposes, stable silyl nitrone esters are used in the region.
R-ts and preservation of the NO 2 group. Aliphatic nitro compounds containing an a-H atom are easily alkylated and acylated, usually forming O-derivatives. However, mutual mod. dilithium salts of primary nitro compounds with alkyl halides, anhydrides or carboxylic acid halides leads to C-alkylation or C-acylation products, for example:
There are known examples of intramol. C-alkylation, e.g.:
Primary and secondary nitro compounds react with aliphatic compounds. amines and CH 2 O with the formation of p-amino derivatives (Mannich solution); in the solution you can use previously prepared methylol derivatives of nitro compounds or amino compounds:
The activating effect of the NO 2 group on the nucleophile. Substitution (especially at the ortho position) is widely used in org. synthesis and industry. The reaction proceeds according to the addition-elimination scheme with intermediate. formation of the s-complex (Meisenheimer complex). According to this scheme, halogen atoms are easily replaced by nucleophiles:
There are known examples of substitution by the radical anion mechanism with electron capture in aromatic compounds. connection and release of halide ion or other groups, e.g. alkoxy, amino, sulfate, NO - 2. In the latter case, the reaction is easier, the greater the deviation of the NO 2 group from coplanarity, for example: in 2,3-dinitrotoluene it is replaced in the base. NO 2 group in position 2. The H atom in aromatic nitro compounds is also capable of nucleophiling. substitution-nitrobenzene when heated. 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 a second NO 2 group accelerates the nucleoph. substitution N itro compounds present. bases are added to aldehydes and ketones, giving nitro alcohols (see Henri reactions), primary and secondary nitro compounds - to compounds containing activator. double bond (Michael's r-tion), for example:
Primary nitro compounds can enter into a Michael reaction with a second molecule of an unsaturated compound; this district with the last one. tranceformation of the NO 2 group is used for the synthesis of poly-functional. aliphatic connections. The combination of Henri and Michael solutions leads to 1,3-dinitro compounds, for example:
K inactivated Only Hg derivatives of gem-di- or trinitro compounds, as well as IC(NO 2) 3 and C(NO 2) 4, are added to the double bond, resulting in the formation of C- or O-alkylation products; the latter can enter into a cyclo-addition reaction with a second olefin molecule:
Nitroolefins easily enter into addition solutions: with water in a slightly acidic or slightly alkaline environment with the last. by Henri's retroreaction they form carbonyl compounds. 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 the processes of diene synthesis and cycloaddition, and 1,4-dinitrodienes as diene components, for example:
Receipt. In industry, lower nitroalkanes are obtained by liquid-phase (Konovalov's method) 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 using this method, for example. nitrocyclohexane is an intermediate in the production of caprolactam.
In the laboratory, nitration of nitrogen compounds is used to obtain nitroalkanes. with activated methylene group; a convenient method for the synthesis of primary nitroalkanes is the nitration of 1,3-indanedione with the last one. alkaline hydrolysis of a-nitroketone:
Aliphatic nitro compounds also receive interaction. AgNO 2 with alkyl halides or NaNO 2 with esters of a-halocarboxylic acids (see Meyer reaction). Aliphatic nitro compounds are formed by the oxidation of amines and oximes; oxime oxidation - a method for producing heme-di- and heme-trinitro compounds, for example:
N- and O-nitro compounds are also known (see and Organic Nitrates).
The nitro group has a structure intermediate between two limiting resonance structures:
PHYSICAL PROPERTIES OF SOME ALIPHATIC NITRO COMPOUNDS
*At 25°C. **At 24°C. ***At 14°C.
The IR spectra of nitro compounds contain two characteristics. bands corresponding to antisymmetric and symmetric stretching vibrations of the N-O bond: for primary nitro compounds, respectively. 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 N. 1550-1520 and 1350-1330 cm -1 (electron-withdrawing substituents shift the high-frequency band to the region of 1570-1540, and electron-donating substituents to the region of 1510-1490 cm -1); for N. 1610-1440 and 1285-1135 cm -1 ; nitrone ethers have an intense band at 1630-1570 cm, the C-N bond has a weak band at 1100-800 cm -1.
In the UV spectra of aliphatic nitro compounds, l max 200-210 nm (intense band) and 270-280 nm (weak band); for and esters of nitronic acids, respectively. 220-230 and 310-320 nm; for heme-dinitro-containing 320-380 nm; for aromatic N. 250-300 nm (the intensity of the band decreases sharply when coplanarity is violated).
In the PMR spectrum of chem. shifts of the a-H atom, depending on the structure, 4-6 ppm. In the NMR spectrum 14 N and 15 N chemical. 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. absent or very small; basic fragmentation process - the elimination of NO 2 or two to form a fragment equivalent to . Aromatic nitro compounds are characterized by the presence of a peak mol. ; basic the peak in the spectrum corresponds to that obtained during the 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 aromatic conn. as a result of induction and especially it affects the distribution: the nucleus acquires a partial positivity. a charge that is localized primarily in the 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. Thus, the introduction of the NO 2 group sharply increases the reaction. ability to org. conn. in relation to nucleoph. reagents and complicates reactions with electroph. 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. connection, carry out decomposition. reactions associated, as a rule, with a change in the carbon skeleton, and then transformed into another function or removed. In aromatic In some cases, a shorter scheme is often used: nitration-transformation of the NO 2 group.
Mn. transformations of aliphatic nitro compounds take place with pre-treatment. into nitronic acids or the formation of the corresponding . In solutions, the equilibrium is usually almost completely shifted towards the C-form; at 20 °C the proportion of the aci form for 1 is 10 -7, for nitropropane 3. 10 -3. Nitronic acids in free. the form is usually unstable; they are obtained by careful acidification of N. Unlike N., they conduct current in solutions and give a red color with FeCl 3. Aci-N. 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 nitronic acids in the aromatic series is associated with the benzene ring into the quinoid form; for example, forms with conc. H 2 SO 4 colored salt-like product of type I, o-nitrotoluene results in intramol. transfer to form a bright blue O derivative:
When bases act on primary and secondary nitrogen, nitro compounds are formed; ambident in reactions with electrophiles are capable of producing both O- and C-derivatives. Thus, when H. is alkylated with alkyl halides, trialkylchlorosilanes or R 3 O + BF - 4, O-alkylation products are formed. Latest m.b. also obtained by the action of diazomethane or N,O-bis-(trimethylsilyl)acetamide on nitroalkanes with pK a
Acyclic alkyl esters of nitronic acids are thermally unstable and decompose intramol. mechanism:
r-tion can be used to obtain. Silyl ethers are more stable. For the formation of C-alkylation products, see below.
Nitro compounds are characterized by reactions with the cleavage of the C-N bond, along the N=O, O=N O, C=N -> O bonds, and reactions with the preservation of the NO 2 group.
R-ts and s r a r s in about m with connections and S-N. Primary and secondary N. during heating. with mineral acids in the presence of an alcohol or aqueous solution form carbonyl compounds. (see Nave reaction). R-tion passes through the gaps. formation of nitronic acids:
As initial conn. Silyl nitrone ethers can be used. The action of strong acids on aliphatic nitro compounds can lead to hydroxamic acids, for example:
The method is used in industry for the synthesis of CH 3 COOH and from nitroethane. Aromatic nitro compounds are inert to the action of strong acids.
Aliphatic nitro compounds 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. Thermal flow proceeds similarly. decomposition of nitroalkanes at temperatures above 450°. Vicinal dinitrosoids. when Ca is processed in hexamstanol, both NO 2 groups are eliminated; Ag salts of unsaturated nitro compounds are able to dimerize when NO 2 groups are lost:
Nucleof. substitution of the NO 2 group is not typical for nitroalkanes, however, when thiolate ions act on tertiary nitroalkanes in aprotic solvents, the NO 2 group is replaced by . The reaction proceeds through the anion-radical mechanism. In aliphatic and heterocyclic conn. the NO 2 group at is relatively easily replaced by a nucleophile, for example:
In aromatic conn. nucleoph. the substitution of the NO 2 group depends on its position in relation to other substituents: the NO 2 group, located in the meta position with respect to the electron-withdrawing substituents and in the ortho- and para-positions with respect to the electron-donating ones, has a low reactivity. ability; reaction the ability of the NO 2 group located in the ortho- and para-positions to accept electron-withdrawing substituents increases markedly. In some cases, the substituent enters the ortho position to the NO 2 leaving group (for example, when aromatic N. is heated with an alcohol solution of KCN, Richter reaction):
R-ts and about the connection N = O. One of the most important reactions is reduction, which generally leads to a set of products:
Azoxy-(II), azo-(III) and hydrazo-containing. (IV) are formed in an alkaline environment as a result of intermediately occurring nitroso compounds. s and . Carrying out the process in an acidic environment eliminates the formation of these substances. Nitroso-containing are reduced faster than the corresponding nitro compounds, and isolate them from the reaction. the mixture usually fails. Aliphatic N. are reduced in azoxy-or under the action of Na, aromatic - under the action of NaBH 4, treatment of the latter with LiAlH 4 leads to. Electrochem. aromatic N., under certain conditions, allows you to obtain any of the presented derivatives (with the exception of nitroso compounds); Using the same method, it is convenient to obtain from mononitroalkanes and amidoximes from heme-dinitroalkanes:
Rations for the bonds O = N O and C = N O. Nitro compounds enter into 1,3-dipolar reactions, for example:
Naib. This reaction easily occurs between nitron ethers and or. In products (mono- and bicyclic dialkoxyamines) under the influence of nucleophiles. and electroph. N - O bond reagents are easily broken down, which leads to decomposition. aliphatic and hetero-cyclic. conn.:
For preparative purposes, stable silyl nitrone esters are used in the reaction.
R-ts and preservation of the NO 2 group. Aliphatic Ns containing an a-H atom are easily alkylated and acylated, usually forming O-derivatives. However, mutual mod. dilithium primary N. with alkyl halides, anhydrides or acid halides of carboxylic acids leads to C-alkylation or C-acylation products, for example:
There are known examples of intramol. C-alkylation, for example:
Primary and secondary nitro compounds react with aliphatic compounds. and CH 2 O with the formation of p-amino derivatives (Mannich solution); in the reaction you can use previously prepared methylol derivatives of nitro compounds or amino compounds:
Nitroolefins easily enter into addition reactions: in a slightly acidic or slightly alkaline environment with the last. by Henri's retroreaction they form carbonyl compounds. and nitroalkanes; with nitro compounds containing a-H-atom, -poly-nitro compounds; add other CH acids, such as and malonic acids, Grignard reagents, as well as nucleophiles like OR -, NR - 2, etc., for example:
Nitroolefins can act as dienophiles or dipolarophiles in cycloaddition reactions, and 1,4-dinitrodienes can act as diene components, for example:
Receipt. In industry, lower nitroalkanes are obtained by liquid-phase (Konovalov's method) or vapor-phase (Hess method) mixtures of , and , isolated from natural or obtained by processing (see Nitration). This method is also used to obtain higher nitrates, for example, nitrocyclohexane, an intermediate in the production of caprolactam.
In the laboratory, to obtain nitroalkanes, nitric acid is used. with activated methylene group; a convenient method for the synthesis of primary nitroalkanes is the nitration of 1,3-indanedione with the last one. alkaline a-nitroketone:
Aliphatic nitro compounds also receive interaction. AgNO 2 with alkyl halides or NaNO 2 with esters of a-halocarboxylic acids (see Meyer reaction). Aliphatic N. are formed when and; -method for producing heme-di- and heme-trinitro compounds, for example:
Nitroalkanes m.b. obtained by heating acyl nitrates to 200 °C.
Mn. methods for the synthesis of nitro compounds are based on olefins, HNO 3, nitronium, NO 2 Cl, org. nitrates, etc. As a rule, this produces a mixture of vic-dinitro compounds, nitronitrates, nitronitrites, unsaturated nitro compounds, as well as products of the conjugate addition of the NO 2 group and the solvent or their products, for example:
Nitro compounds
Nitro compounds - organic compounds containing one or more nitro groups -NO2. Nitro compounds usually mean C-nitro compounds in which the nitro group is bonded to a carbon atom (nitroalkanes, nitroalkenes, nitro arenes). O-nitro compounds and N-nitro compounds are divided into separate classes - nitroesters (organic nitrates) and nitramines.
Depending on the radical R, aliphatic (saturated and unsaturated), acyclic, aromatic and heterocyclic nitro compounds are distinguished. Based on the nature of the carbon atom to which the nitro group is bonded, nitro compounds are divided into primary, secondary and tertiary.
Nitro compounds are isomeric to nitrous acid esters HNO2 (R-ONO)
In the presence of α-hydrogen atoms (in the case of primary and secondary aliphatic nitro compounds), tautomerism is possible between nitro compounds and nitronic acids (aci forms of nitro compounds):
From halogen derivatives:
Nitration
Nitration is the reaction of introducing the nitro group -NO2 into the molecules of organic compounds.
The nitration reaction can proceed by an electrophilic, nucleophilic or radical mechanism, with the active species in these reactions being, respectively, the nitronium cation NO2+, the nitrite ion NO2- or the NO2 radical. The process consists of replacing the hydrogen atom at the C, N, O atoms or adding a nitro group to a multiple bond.
Electrophilic nitration[edit | edit source text]
In electrophilic nitration, the main nitrating agent is nitric acid. Anhydrous nitric acid undergoes autoprotolysis according to the reaction:
Water shifts the equilibrium to the left, so in 93-95% nitric acid the nitronium cation is no longer detected. In this regard, nitric acid is used in a mixture with water-binding concentrated sulfuric acid or oleum: in a 10% solution of nitric acid in anhydrous sulfuric acid, the equilibrium is almost completely shifted to the right.
In addition to a mixture of sulfuric and nitric acids, various combinations of nitrogen oxides and organic nitrates with Lewis acids (AlCl3, ZnCl2, BF3) are used. A mixture of nitric acid with acetic anhydride, in which a mixture of acetyl nitrate and nitrogen oxide (V) is formed, as well as a mixture of nitric acid with sulfur oxide (VI) or nitrogen oxide (V) has strong nitrating properties.
The process is carried out either by direct interaction of the nitrating mixture with a pure substance, or in a solution of the latter in a polar solvent (nitromethane, sulfolane, acetic acid). A polar solvent, in addition to dissolving reactants, solvates the + ion and promotes its dissociation.
In laboratory conditions, nitronium nitrates and salts are most often used, the nitrating activity of which increases in the following series:
Mechanism of benzene nitration:
In addition to replacing the hydrogen atom with a nitro group, substitutive nitration is also used, when a nitro group is introduced instead of sulfo-, diazo- and other groups.
The nitration of alkenes under the action of aprotic nitrating agents occurs in several directions, which depends on the reaction conditions and the structure of the starting reagents. In particular, reactions of proton abstraction and addition of functional groups of solvent molecules and counterions can occur:
Nitration of amines leads to N-nitroamines. This process is reversible:
Nitration of amines is carried out with concentrated nitric acid, as well as its mixtures with sulfuric acid, acetic acid or acetic anhydride. The product yield increases when moving from strongly basic to weakly basic amines. Nitration of tertiary amines occurs with the cleavage of the C-N bond (nitrolysis reaction); this reaction is used to produce explosives - hexogen and octogen - from methenamine.
Substitutive nitration of acetamides, sulfonamides, urethanes, imides and their salts proceeds according to the following scheme:
The reaction is carried out in aprotic solvents using aprotic nitrating agents.
Alcohols are nitrated by any nitrating agents; the reaction is reversible:
Nucleophilic nitration[edit | edit source text]
This reaction is used to synthesize alkyl nitrites. The nitrating agents in this type of reaction are alkali metal nitrite salts in aprotic dipolar solvents (sometimes in the presence of crown ethers). The substrates are alkyl chlorides and alkyl iodides, α-halocarboxylic acids and their salts, alkyl sulfates. By-products of the reaction are organic nitrites.
Radical nitration[edit | edit source text]
Radical nitration is used to obtain nitroalkanes and nitroalkenes. Nitrating agents are nitric acid or nitrogen oxides:
In parallel, the oxidation reaction of alkanes occurs due to the interaction of the NO2 radical with the alkyl radical at the oxygen atom rather than nitrogen. The reactivity of alkanes increases when moving from primary to tertiary. The reaction is carried out both in the liquid phase (with nitric acid at normal pressure or nitrogen oxides, at 2-4.5 MPa and 150-220°C), and in gas (nitric acid vapor, 0.7-1.0 MPa, 400-500°C)
Nitration of alkenes by a radical mechanism is carried out with 70-80% nitric acid, sometimes with dilute nitric acid in the presence of nitrogen oxides. Cycloalkenes, dialkyl- and diarylacetylenes are nitrated with N2O4 oxide, resulting in the formation of cis- and trans-nitro compounds, by-products are formed due to the oxidation and destruction of the original substrates.
The anion-radical nitration mechanism is observed in the interaction of tetranitromethane salts of mono-nitro compounds.
Konovalov reaction (for aliphatic hydrocarbons)
Konovalov's reaction is the nitration of aliphatic, alicyclic and fatty-aromatic compounds with dilute HNO3 at elevated or normal pressure (free radical mechanism). The reaction with alkanes was first carried out by M.I. Konovalov in 1888 (according to other sources, in 1899) with 10-25% acid in sealed ampoules at a temperature of 140-150°C.
Typically a mixture of primary, secondary and tertiary nitro compounds is formed. Fatty aromatic compounds are easily nitrated at the α-position of the side chain. Side reactions include the formation of nitrates, nitrites, nitroso and polynitro compounds.
In industry, the reaction is carried out in the vapor phase. This process was developed by H. Hess (1930). The alkane and nitric acid vapors are heated to 420-480°C for 0.2-2 seconds, followed by rapid cooling. Methane gives nitromethane, and its homologues also undergo dissociation C--C bonds, so that a mixture of nitroalkanes is obtained. It is separated by distillation.
The active radical in this reaction is O2NO·, a product of the thermal decomposition of nitric acid. The reaction mechanism is given below.
2HNO3 -t°→ O2NO· + ·NO2 + H2O
R-H + ONO2 → R + HONO2
R· + ·NO2 → R-NO2
Nitration of aromatic hydrocarbons.
Chemical properties[edit | edit source text]
In terms of their chemical behavior, nitro compounds show a certain similarity to nitric acid. This similarity is manifested in redox reactions.
Reduction of nitro compounds (Zinin reaction):
Condensation reactions
Tautomerism of nitro compounds.
Tautomerism (from the Greek ταύτίς - the same and μέρος - measure) is the phenomenon of reversible isomerism, in which two or more isomers easily transform into each other. In this case, tautomeric equilibrium is established, and the substance simultaneously contains molecules of all isomers (tautomers) in a certain ratio.
Most often, tautomerization involves the movement of hydrogen atoms from one atom in a molecule to another and back again in the same compound. A classic example is acetoacetic ester, which is an equilibrium mixture of ethyl ester of acetoacetic (I) and hydroxycrotonic acids (II).
Tautomerism is strongly manifested for a whole range of substances derivatives of hydrogen cyanide. So hydrocyanic acid itself exists in two tautomeric forms:
At room temperature the equilibrium of the conversion of hydrogen cyanide to hydrogen isocyanide is shifted to the left. The less stable hydrogen isocyanide has been shown to be more toxic.
Tautomeric forms of phosphorous acid
A similar transformation is known for cyanic acid, which is known in three isomeric forms, but tautomeric equilibrium binds only two of them: cyanic and isocyanic acids:
For both tautomeric forms, esters are known, that is, products of the substitution of hydrocarbon radicals for hydrogen in cyanic acid. Unlike these tautomers, the third isomer, fulminate (fulmic) acid, is not capable of spontaneous transformation into other forms.
Many chemical and technological processes are associated with the phenomenon of tautomerism, especially in the field of synthesis of medicinal substances and dyes (production of vitamin C - ascorbic acid, etc.). The role of tautomerism in processes occurring in living organisms is very important.
The amide-iminol tautomerism of lactams is called lactam-lactim tautomerism. It plays an important role in the chemistry of heterocyclic compounds. The equilibrium in most cases is shifted towards the lactam form.
The list of organic pollutants is especially large. Their diversity and large numbers make it almost impossible to control the content of each of them. Therefore, they highlight priority pollutants(about 180 compounds, grouped into 13 groups): aromatic hydrocarbons, polynuclear aromatic hydrocarbons (PAHs), pesticides (4 groups), volatile and low-volatile organochlorine compounds, chlorophenols, chloroanilines and chloronitroaromatic compounds, polychlorinated and polybrominated biphenyls, organometallic compounds and others. The sources of these substances are precipitation, surface runoff and industrial and municipal SV.
Related information.
NITRO COMPOUNDS
(C-nitro compounds) contain one or more in a molecule. nitro groups directly bonded 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 two limiting resonance structures:
The group is planar; N and O atoms have, sp 2 -hybridization, NPO bonds are equivalent and almost one-and-a-half; bond lengths, e.g. for CH 3 NO 2, 0.122 nm (NCHO), 0.147 nm (SCN), ONO angle 127°. The SChNO 2 system is flat with a low barrier to rotation around the SChN bond.
N., having at least one a-H atom, can exist in two tautomeric forms with a common mesomeric anion. O-form called atsi-H. or nitronic one:
Various known derivatives of nitronic compounds: RR"C=N(O)O - M + (N salts), ethers (nitronic esters), etc. Esters of nitronic compounds exist in the form iis- And trance-isomers. There are cyclical ethers, e.g. Isoxazoline N-oxides.
Name N. is produced by adding the prefix “nitro” to the name. base connections, adding a digital indicator if necessary, e.g. 2-nitropropane. Name N. salts are produced from the name. either C-form or atsi-form, or nitronic acid.
Physical properties. The simplest nitroalkanes are colorless. liquids. Phys. The properties of certain aliphatic N. are given in the table. Aromatic N.-colorless. or light yellow high-boiling liquids or low-melting solids with a characteristic odor, poorly soluble. in water, as a rule, they are distilled with steam.
PHYSICAL PROPERTIES OF SOME ALIPHATIC NITRO COMPOUNDS
*At 25°C. **At 24°C. ***At 14°C.
In the IR spectra of N. there are two characteristics. bands corresponding to antisymmetric and symmetric stretching vibrations of the NPO 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 N. 1550-1520 and 1350-1330 cm -1 (electron-withdrawing substituents shift the high-frequency band to the region of 1570-1540, and electron-donating substituents to the region of 1510-1490 cm -1); for salts N. 1610-1440 and 1285-1135 cm -1; nitrone ethers have an intense band at 1630-1570 cm, the SCN bond has a weak band at 1100-800 cm -1.
In the UV spectra, aliphatic N. l max 200-210 nm (intense band) and 270-280 nm (weak band); for salts and ethers of nitronic acid, resp. 220-230 and 310-320 nm; For heme-dinitro-containing 320-380 nm; for aromatic N. 250-300 nm (the intensity of the band decreases sharply when coplanarity is violated).
In the PMR spectrum of chem. shifts of the a-H atom depending on the structure are 4-6 ppm. In the NMR spectrum 14 N and 15 N chemical. shift 5 from - 50 to + 20 ppm.
In the mass spectra of aliphatic N. (with the exception of CH 3 NO 2), the peak mol. ion is absent or very small; basic fragmentation process - the elimination of NO 2 or two oxygen atoms to form a fragment equivalent to nitrile. Aromatic N. is characterized by the presence of a peak mol. and she; basic the peak in the spectrum corresponds to the ion produced by the 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 aromatic conn. as a result of inductive and especially mesomeric effects, it affects the distribution of electron density: the nucleus becomes partially positive. charge, which is localized Ch. arr. V ortho- And pair- provisions; Hammett constants for the NO 2 s group m 0.71, s n 0.778, s+ n 0.740, s - n 1.25. Thus, the introduction of the NO 2 group sharply increases the reaction. ability to org. conn. in relation to nucleoph. reagents and makes it difficult to deal with electroph. reagents. This determines the widespread use of N. in the organization. synthesis: the NO 2 group is introduced into the desired position of the org molecule. connection, carry out decomposition. ptions associated, as a rule, with a change in the carbon skeleton, and then transformed into another function or removed. In aromatic In some cases, a shorter scheme is often used: nitration-transformation of the NO 2 group.
Mn. transformations of aliphatic N. take place with preliminary. isomerization into nitronic compounds or the formation of the corresponding anion. In solutions, the equilibrium is usually almost completely shifted towards the C-form; at 20 °C fraction atsi-forms for nitromethane 1X10 -7, for nitropropane 3. 10 -3. Nitron compounds are free. the form is usually unstable; they are obtained by careful acidification of N salts. Unlike N., 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 nitronic compounds in the aromatic N. series is associated with the isomerization of the benzene ring into the quinoid form; for example, forms with conc. H 2 SO 4 colored salt-like product of type I, o-nitrotoluene results in intramol. proton transfer to form a bright blue O derivative:
When bases act on primary and secondary N., N. salts are formed; ambident salts in solutions with electrophiles are capable of producing both O- and C-derivatives. Thus, when 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 into nitroalkanes with p K a< 3>
or nitron compounds, for example:
Acyclic alkyl esters of nitronic acids are thermally unstable and disintegrate intramol. mechanism:
The solution can be used to obtain carbonyl compounds. Silyl ethers are more stable. For the formation of C-alkylation products, see below.
N. is characterized by r-tions with a break in the SChN bond, along the bonds N=O, O=N O, C=N -> O and r-tions with preservation of the NO 2 group.
R-ts and s r a r s in about m s communication with SCHN. Primary and secondary N. during heating. with mineral K-tami is present. alcohol or aqueous solution of alkali form carbonyl compounds. (cm. Nave reaction). R-tion passes through the gaps. formation of nitron compounds:
As initial conn. Silyl nitrone ethers can be used. The effect of strong drugs on aliphatic N. can lead to hydroxamic drugs, for example:
The method is used in industry for the synthesis of CH 3 COOH and hydroxylamine from nitroethane. Aromatic N. are inert to the action of strong drugs.
When reducing agents (for example, TiCl 3 -H 2 O, VCl 2 -H 2 O-DMF) act on N. or oxidizing agents (KMnO 4 -MgSO 4, O 3) on N. salts, aldehydes are also formed.
Aliphatic N., 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 flow proceeds similarly. decomposition of nitroalkanes at temperatures above 450°. Vicinal dinitrosoids. when treated with Ca amalgam in hexamstanol, both NO 2 groups are split off, Ag salts of unsaturated N. with the loss of NO 2 groups are able to dimerize:
Nucleof. substitution of the NO 2 group is not typical for nitroalkanes, however, when thiolate ions act on tertiary nitroalkanes in aprotic solutions, the NO 2 group is replaced by a hydrogen atom. The reaction proceeds through the anion-radical mechanism. In aliphatic and heterocyclic conn. the NO 2 group at a multiple bond is relatively easily replaced by a nucleophile, for example:
In aromatic conn. nucleoph. substitution of the NO 2 group depends on its position relative to other substituents: the NO 2 group located in meta-position relative to electron-withdrawing substituents and in ortho- And pair- positions to electron donors, has low reactivity. ability; reaction the ability of the NO 2 group located in ortho- And pair-positions to electron-withdrawing substituents increases noticeably. In some cases, the deputy enters into ortho-position to the NO 2 leaving group (for example, when heating aromatic N. with alcohol solution KCN, Richter solution):
R-ts and i about connections and N = O. One of the most important r-tions is restoration, which generally leads to a set of products:
Azoxy-(II), azo-(III) and hydrazo-containing. (IV) are formed in an alkaline environment as a result of condensation of intermediate nitroso compounds. with amines and hydroxylamines. Carrying out the process in an acidic environment eliminates the formation of these substances. Nitroso-containing are restored faster than the corresponding N., and isolate them from the reaction. the mixture usually fails. Aliphatic N. are reduced in azoxy-or under the action of Na alcoholates, aromatic - under the action of NaBH 4, treatment of the latter with LiAlH 4 leads to azo compounds. Electrochem. aromatic N., under certain conditions, allows you to obtain any of the presented derivatives (with the exception of nitroso compounds); The same method is convenient for obtaining hydroxylamines from mononitroalkanes and amidoximes from salts heme-dinitroalkanes:
There are many known methods for restoring N. to. Iron filings, Sn and Zn are widely used. kit; with catalytic in hydrogenation, Ni-Raney, Pd/C or Pd/PbCO 3 and others are used as catalysts. Aliphatic N. are easily reduced to the amines LiAlH 4 and NaBH 4 in the presence. Pd, amalgams of Na and Al, with heating. 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 by Na hydrosulfide in CH 3 OH. There are ways to choose. restoration of the NO 2 group in multifunctional N. without affecting other functions.
When P(III) acts on aromatic N., a sequence occurs. deoxygenation of the NO 2 group with the formation of highly reactive nitrenes. The solution is used for the synthesis of condenser. heterocycles, for example:
Under the same conditions, silyl ethers of nitronic acids are transformed into silyl derivatives of oximes. Treatment of primary nitroalkanes PCl 3 in pyridine or NaBH 2 S leads to nitriles. Aromatic N. containing ortho-position, a substituent with a double bond or a cyclopropyl substituent, in an acidic environment, rearranges into o-nitrosoketones, for example:
N. and nitron esters react with an excess of Grignard reagent, giving hydroxylamine derivatives:
The relationships between the O = N O and C = N O bonds enter into the relationships of 1,3-dipolar cycloaddition, for example:
Naib. This process easily occurs between nitron esters and olefins or acetylenes. In cycloaddition products (mono- and bicyclic dialkoxyamines) under the influence of nucleophiles. and electroph. N H O bond reagents are easily broken down, which leads to decomposition. aliphatic and hetero-cyclic. conn.:
For preparative purposes, stable silyl nitrone esters are used in the region.
R-ts and preservation of the NO 2 group. Aliphatic Ns containing an a-H atom are easily alkylated and acylated, usually forming O-derivatives. However, mutual mod. dilithium salts of primary N. with alkyl halides, anhydrides or carbonic acid halides leads to C-alkylation or C-acylation products, for example:
There are known examples of intramol. C-alkylation, e.g.:
Primary and secondary N. react with aliphatic. amines and CH 2 O with the formation of p-amino derivatives (Mannich solution); in the district you can use previously prepared 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, the solution can lead to heterocyclic. connection, for example: when interacting primary nitroalkane with two equivalents of primary amine and excess formaldehyde are formed. Forms V, if the reagents are taken in the ratio 1:1:3-comm. Forms VI.
Aromatic N. easily enter into nucleophilic solutions. substitution and much more difficult - in the district of electroph. substitution; in this case the nucleophile is directed to ortho- and time-position, and the electrophile meta- position to the NO 2 group. Electrof speed constant nitration of nitrobenzene is 5-7 orders of magnitude less than benzene; this produces m-dinitrobenzene.
The activating effect of the NO 2 group on the nucleophile. substitution (especially ortho-position) are widely used in org. synthesis and industry. The reaction proceeds according to the addition-elimination scheme with intermediate. formation of the s-complex (Meisenheimer complex). According to this scheme, halogen atoms are easily replaced by nucleophiles:
There are known examples of substitution by the radical anion mechanism with electron capture in aromatic compounds. connection and release of halide ion or other groups, e.g. alkoxy, amino, sulfate, NO - 2. In the latter case, the reaction is easier, the greater the deviation of the NO 2 group from coplanarity, for example: in 2,3-dinitrotoluene it is replaced in the base. NO 2 group in position 2. The H atom in aromatic N. is also capable of nucleophiling. substitution-nitrobenzene when heated. 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 a second NO 2 group accelerates the nucleoph. substitution N. is present. bases add to aldehydes and ketones, giving nitro alcohols (see. Henri reactions), primary and secondary N.-to compounds containing activ. double bond (Michael's r-tion), for example:
Primary N. can enter into a Michael reaction with a second molecule of an unsaturated compound; this district with the last one. transformation of the NO 2 group is used for the synthesis of poly-functional. aliphatic connections. The combination of Henri and Michael solutions leads to 1,3-dinitro compounds, for example:
K inactivated only Hg derivatives are added to the double bond heme- di-or trinitro compounds, as well as IC(NO 2) 3 and C(NO 2) 4, in which C- or O-alkylation products are formed; the latter can enter into a cyclo-addition reaction with the second olefin molecule:
Nitroolefins easily enter into addition solutions: with water in a slightly acidic or slightly alkaline environment with the last. by Henri's retroreaction they form carbonyl compounds. and nitroalkanes; with N. containing a-H-atom, -poly-N.; add other CH acids, such as 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 the processes of diene synthesis and cycloaddition, and 1,4-dinitrodienes as diene components, for example:
Nitrosation of primary N. leads to nitrolic acids RC(=NOH)NO 2, secondary N. form pseudo-nitroles RR"C(NO)NO 2, tertiary N. do not enter the solution.
Nitroalkanes are easily halogenated in the presence. grounds with subsequent substitution of H atoms at the a-C atom:
When fotdheim. chlorination, more distant H atoms are replaced:
When carboxylation of primary nitroalkanes by the action of CH 3 OMgOCOOCH 3 a-nitrocarbon compounds or their esters are formed.
When processing mono-N salts. C(NO 2) 4., nitrites of Ag or alkali metals, or by the action of nitrites on a-halo-nitroalkanes in an alkaline environment (Ter Meer region) are formed heme-dinitro compounds. The electrolysis of a-halogen-nitroalkanes in aprotic solutions, as well as the treatment of N. Cl 2 in an alkaline medium or the electrooxidation of N. salts lead to vits-dinitro compounds:
The nitro group does not render creatures. influence on free radical or aromatic arylation. conn.; r-tion leads to the basis. To ortho- And pair- substituted products.
To restore N. without affecting the NO 2 group, use NaBH 4, LiAlH 4 with low t-rah or solution of diboron in THF, for example:
Aromatic di- and tri-N., in particular 1,3,5-trinitroben-zol, form stable, brightly colored crystalline particles. they say complexes with aromatic electron donor compounds (amines, phenols, etc.). Complexes with picric acid are used for the isolation and purification of aromatic compounds. hydrocarbons. Interaction di- and trinitrobenzenes with strong reasons(HO - , RO - , N - 3 , RSO - 2 , CN - , aliphatic amines) leads to the formation of Meisen-Haimer complexes, which are isolated in the form of colored alkali metal salts.
Receipt. In industry, lower nitroalkanes are obtained by liquid-phase (Konovalov's method) 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. is also obtained using this method, for example. nitrocyclohexane is an intermediate in the production of caprolactam.
In the laboratory, to obtain nitroalkanes, a nitrogen compound is used. with activated methylene group; a convenient method for the synthesis of primary nitroalkanes is the nitration of 1,3-indanedione with the last one. alkaline hydrolysis of a-nitroketone:
Aliphatic N. also receive interaction. AgNO 2 with alkyl halides or NaNO 2 with a-halocarboxylic esters (see. Meyer reaction). Aliphatic N. are formed during the oxidation of amines and oximes; oximes - method of preparation heme-di- and heme-trinitro compounds, for example:
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 vits-dinitro compounds, nitronitrates, nitronitrites, unsaturated N., as well as products of the conjugate addition of the NO 2 group and the p-solvent molecule or the products of their hydrolysis, for example:
a,w-Dinitroalkanes are obtained by the action of alkyl nitrates on cyclic. ketones with last hydrolysis of a,a"-dinitro-ketone salts:
Poly-N. synthesized by destructive nitration of decomp. org. conn.; for example, tri- and are obtained by the action of HNO 3 on acetylene in the presence. Hg(II) ions.
Basic method of obtaining aromatic N.-electr. nitration. The active nitrating group is the nitronium ion NO 2, generated from HNO 3 under the action of strong protic or aprotic compounds. For nitration in mild conditions use nitronium salts (NO 2 BF 4, NO 2 ClO 4, etc.), as well as N 2 O 5 in inert solutions.
In the industry for nitration of aromatic. conn. As a rule, nitrating mixtures (H 2 SO 4 + HNO 3) are used. In the laboratory, to increase the concentration of nitronium ion, AlCl 3, SiCl 4, BF 3, etc. are used instead of H 2 SO 4; nitration is often carried out in inert solutions (CH 3 COOH, nitromethane, etc.). The sulfo and diazo groups are easily replaced by the NO 2 group. To introduce the second group of NO 2 into nitrobenzene ortho- And pair-position, the corresponding diazo derivative is first obtained, and then the diazo group is replaced according to the Sandmeyer solution. 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 minerals. oils; oil dewaxing, etc.
A number of N. are used as biological active ingredients. Thus, phosphoric esters 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; drugs with a wide spectrum of action (furazolidine, etc.) have been created on their basis. Certain aromatic N.-fragrant herbs.
N. - intermediate products in synthetic production. dyes, polymers, detergents and corrosion inhibitors; wetting, emulsifying, dispersing and flotation. agents; plasticizers and modifiers of polymers, pigments, etc. They are widely used in org. synthesis and as model compounds. in theoretical org. chemistry.
Nitroparaffins have a strong local irritant effect and are relatively toxic. They are classified as cellular poisons of general action, especially dangerous for the liver. LD 50 0.25-1.0 g/kg (for oral administration). Chlorinated and unsaturated N. are 5-10 times more toxic. Aromatic N. depress the nervous and especially the circulatory system, disrupting the body's oxygen supply. Signs of poisoning - hyperemia, higher. mucus secretion, lacrimation, cough, dizziness, headache. First aid agents are quinine and. N.'s metabolism is associated with oxidation-reduction. p-tions and, in particular, with oxidation. phosphorylation. For example, 2,4-dinitrophenol is one of the most powerful reagents that uncouple the processes of oxidation and phosphorylation, which prevents the formation of ATP in the cell.
Several hundred different N are produced in the world. The production volume of the most important aliphatic N is tens of thousands of tons, aromatic N is hundreds of thousands of tons; for example, in the USA, 50 thousand tons/year of nitroalkanes C 1 -C 3 and 250 thousand tons/year of nitrobenzene are produced.
see also m-Dinitrobenzene, Nitroanisoles, Nitrobenzene, Nitrometape, Nitrotoluenes and etc.
Lit.: Chemistry of nitro- and nitroso groups, 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., "Izvestia AN SSSR. Ser. khim.", 1984, No. 1, p. 165-73.
V. A. Tartakovsky.
Chemical encyclopedia. - M.: Soviet Encyclopedia. Ed. I. L. Knunyants. 1988 .
Reduction of nitro compounds . All nitro compounds are reduced to primary amines. If the resulting amine is volatile, it can be detected by a change in the color of the indicator paper:
Reaction with nitrous acid. A characteristic qualitative reaction to primary and secondary nitro compounds is the reaction with nitrous acid.
For tertiary aliphatic nitro compounds There are no specific detection reactions.
Detection of aromatic nitro compounds. Aromatic nitro compounds are usually pale yellow in color. In the presence of other substituents, the intensity and depth of color often increases. To detect aromatic nitro compounds, they are reduced to primary amines, the latter are diazotized and combined with β-naphthol:
| ArNO 2 → ArNH 2 → ArN 2 Cl → ArN=N |
| OH |
This reaction, however, is not specific, since amines are formed during the reduction of not only nitro compounds, but also nitroso, azooxy, and hydrazo compounds. In order to make a final conclusion about the presence of a nitro group in a compound, it is necessary to carry out a quantitative determination.
Qualitative reactions of N-nitroso compounds
Reaction with HI. C-Nitroso compounds can be distinguished from N-nitroso compounds by their relation to an acidified solution of potassium iodide: C-nitroso compounds oxidize hydroiodic acid, N-nitroso compounds do not react with hydroiodic acid.
Reaction with primary aromatic amines. C-Nitroso compounds condense with primary aromatic amines, forming colored azo compounds:
| ArN = O + H 2 N – Ar → Ar – N = N – Ar + H 2 O |
Hydrolysis of N-nitroso compounds. Pure aromatic and fatty aromatic N-nitroso compounds (nitrosamines) are easily hydrolyzed by alcohol solutions of HCl, forming a secondary amine and nitrous acid. If hydrolysis is carried out in the presence of a-naphthylamine, then the latter is diazotized by the resulting nitrous acid, and the diazo compound enters into an azo coupling reaction with excess a-naphthylamine. An azo dye is formed:
The reaction mixture is colored pink color; Gradually the color becomes purple.
Qualitative reactions of nitriles
In the analysis of nitriles RC≡N, ArC≡N, their ability to hydrolyze and be reduced is used. To detect the C≡N group, hydrolysis is carried out:
| RC ≡ N + H 2 O → R – CONH 2 |
Nitriles are most conveniently characterized by the acids that are obtained by their hydrolysis. The acid is isolated from the hydrolyzate by steam distillation or extraction and converted into one of the derivatives - an ester or an amide
Qualitative reactions of thiols (thioalcohols, thioesters)
Most important properties The thiols used in the analysis are the ability to substitute a hydrogen atom in the -SH group and the ability to oxidize. Substances containing the -SH group have a strong unpleasant odor, which weakens with increasing number of carbon atoms in the molecule.
Reaction with HNO 2. Substances containing the SH group give a color reaction when exposed to nitrous acid:
In addition to thiols, thioacids RCOSH also give this reaction. If R is a primary or secondary alkyl, a red color appears; if R is a tertiary alkyl or aryl, the color is first green and then red.
Mercaptide formation. A characteristic qualitative reaction of thiols is also the formation of precipitation of heavy metal mercaptides (Pb, Cu, Hg). For example,
| 2RSH + PbO → (RS)2Pb + H2O |
Lead and copper mercaptides are colored.