Nitro compounds structure. Qualitative reactions of nitro compounds
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 medium, the equilibrium shifts towards the aci-nitro form. Thus, 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,
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 added 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 the districts 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:
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.
Chloroacetic acid Sodium chloroacetate
Nitroacetic acid
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:
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,
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.
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:
(a) 
(b) 
According to the systematic nomenclature, amines are named by adding the prefix amine to the name of the hydrocarbon. According to the rational nomenclature, they are considered as alkyl or arylamines.
Methanamine Ethanamine N-Methylethanamine N-Ethylethaneamine
(methylamine) (ethylamine) (methylethylamine) (diethylamine)
N,N-Diethylethanamine 2-Aminoethanol 3-Aminopropane
triethylamine) (ethanolamine) acid
Cyclohexanamine Benzolamine N-Methylbenzenamine 2-Methylbenzenamine
(cyclohexylamine) (aniline) (N-methylaniline) (o-toluidine)
Heterocyclic amines are named after the corresponding hydrocarbon, inserting the prefix aza-, diaza- or triaza- to indicate the number of nitrogen atoms.
1-Azacyclopeta- 1,2-Diazacyclopeta- 1,3-Diazacyclopeta-
2,4 diene 2,4 diene 2,4 diene
3. Nitro compounds. Methods of obtaining and the most important properties.
Nitro compounds- organic substances containing the nitro group -N0 2 .
The general formula is R-NO 2 .
Depending on the radical R, aliphatic (limiting and unsaturated), acyclic, aromatic and heterocyclic nitro compounds are distinguished. According to the nature of the carbon atom to which the nitro group is attached, nitro compounds are divided into primary, secondary and tertiary.
Methods for the preparation of aliphatic nitro compounds
Direct nitration of alkanes in the liquid or gas phase under the action of 50-70% aqueous nitric acid at 500-700 ° C or nitrogen tetroxide at 300-500 ° C is of industrial importance only for obtaining the simplest nitroalkanes, since nitration under these conditions is always accompanied by cracking of hydrocarbons and leads to a complex mixture of a wide variety of nitro compounds. This reaction was not widely used for this reason.
The most common laboratory method for obtaining nitroalkanes is still the alkylation reaction of the nitrite ion, discovered by V. Meyer as early as 1872. In the classical method of W. Meyer, silver nitrite reacts with primary or secondary alkyl bromides and alkyl iodides in ether, petroleum ether or without solvent at 0-20 o C to form a mixture of nitroalkane and alkyl nitrite.
The nitrite ion is one of the degenerate ambident anions with two independent nucleophilic centers (nitrogen and oxygen) that are not linked into a single mesomeric system.
The reactivity of an ambident nitrite ion with two independent nucleophilic centers (nitrogen and oxygen) differs sharply from the reactivity of enolate ions with two nucleophilic centers bound into a single mesomeric system.
The ratio of N- and O-alkylation products (nitroalkane/alkyl nitrite) in the Meyer reaction of alkyl bromides and iodides with silver nitrite depends crucially on the nature of the alkyl group in the alkyl halide. Yields of primary nitroalkanes reach 75–85%, but they sharply decrease to 15–18% for secondary nitroalkanes and to 5% for tertiary nitroalkanes.
Thus, neither tertiary nor secondary alkyl halides are suitable for the synthesis of nitroalkanes by reaction with silver nitrite. The Meyer reaction seems to be the best way to prepare primary nitroalkanes, arylnitromethanes, and -nitroesters of carboxylic acids.
To obtain nitroalkanes, only alkyl bromides and alkyl iodides should be used, since alkyl chlorides, alkyl sulfonates and dialkyl sulfates do not interact with silver nitrite. From -dibromoalkanes, -dinitroalkanes are easily obtained.
N. Kornblum (1955) proposed a modified general method for the preparation of primary and secondary nitroalkanes, as well as dinitroalkanes and nitro-substituted ketones.
This method is based on the alkylation of alkali metal nitrites with primary or secondary alkyl halides in the dipolar aprotic solvent DMF. In order to prevent subsequent nitrosation of the nitroalkane by the alkyl nitrite formed in parallel, it is necessary to introduce urea or polyhydric phenols, resorcinol or phloroglucinol, into the reaction mixture. The yield of primary nitroalkanes by this method does not exceed 60%; lower than with the alkylation of silver nitrite (75-80%). However, secondary nitroalkanes can be obtained in good yield by alkylation of sodium nitrite in DMF.
Tertiary alkyl halides undergo elimination under the action of the nitrite ion and do not form nitro compounds. Esters of -chloro- or -bromo-substituted acids are smoothly converted into esters of -nitro-substituted acids with a yield of 60-80% when interacting with sodium nitrite in DMSO or DMF.
Another common method for the synthesis of nitroalkanes is the oxidation of ketone oximes with trifluoroperacetic acid in acetonitrile.
In addition to oximes, primary amines can also be oxidized with peracetic acid or m-chloroperbenzoic acid:
More than a hundred years ago, G. Kolbe described a method for producing nitromethane by reacting sodium chloroacetate and sodium nitrite in an aqueous solution at 80-85 o C:
The intermediate nitroacetic acid anion is decarboxylated to nitromethane. For the preparation of nitromethane homologues, the Kolbe method is of no value due to the low yield of nitroalkanes. The idea of this method was ingeniously used in the development of a modern general method for the preparation of nitroalkanes. Dianions of carboxylic acids are nitrated by the action of alkyl nitrate with simultaneous decarboxylation of the α-nitro-substituted carboxylic acid.
Nitration of carbanions with alkyl nitrates is also widely used to obtain - dinitroalkanes. For this purpose, enolate ions of cyclic ketones are treated with two equivalents of alkyl nitrate. Opening of the ring followed by decarboxylation leads to the -nitroalkane.
Methods for the preparation of aromatic nitro compounds
Aromatic nitro compounds are most often obtained by nitration of arenes, which was considered in detail in the study of electrophilic aromatic substitution. Another common method for preparing nitroarenes is the oxidation of primary aromatic amines with trifluoroperacetic acid in methylene chloride. Trifluoroperacetic acid is obtained directly in the reaction mixture by reacting trifluoroacetic acid anhydride and 90% hydrogen peroxide. The oxidation of the amino group to the nitro group with trifluoroperacetic acid is important for the synthesis of nitro compounds containing other electron-withdrawing groups in the ortho and para positions, for example, for the production of ortho and para dinitrobenzene, 1,2,4 trinitrobenzene, and 2,6 dichloronitrobenzene and etc..
Reactions of aliphatic nitro compounds:
Primary and secondary nitroalkanes are in tautomeric equilibrium with the aci form of the nitro compound, otherwise called nitronic acid.
Of the two tautomeric forms, the nitro form is much more stable and dominates in equilibrium. For nitromethane at 20 o the concentration of the aci-form does not exceed 110 -7 of the fraction of nitroalkane, for 2-nitropropane it increases to 310 -3. The amount of aci-form increases for phenylnitromethane. The isomerization of the aci-nitro compound to the nitro compound is slow. This makes it possible to determine the concentration of the aci-form by titration with bromine with a very high degree of accuracy.
The low rate of interconversion of two tautomeric forms allowed A. Ganch back in 1896 to isolate both tautomeric forms of phenylnitromethane individually. Phenylnitromethane is completely soluble in cold aqueous sodium hydroxide solution. When treated with aqueous acetic acid at 0°, a colorless solid is formed, which is the aci form of phenylnitromethane. It instantly turns red when treated with iron(III) chloride and titrated quantitatively with bromine.
On standing, the solid aci form slowly isomerizes to the more stable liquid form of phenylnitromethane. For simple nitroalkanes, for example, nitromethane, nitroethane, and 2-nitropropane, the aci form cannot be isolated individually, since it isomerizes into the nitro form quite easily at 0 o and the content of the aci form can only be judged from titrimetric bromination data.
The concentration of the two tautomeric forms for any compound is always inversely proportional to the acidity of the tautomeric forms, the aci form of nitroalkanes being in all cases a stronger acid than the nitro form. For nitromethane in water, pKa ~ 10.2, while for its aci-form CH 2 \u003d N (OH) -O, pKa ~ 3.2. For 2-nitropropane, this difference is much smaller, pKa (CH 3) 2 CHNO 2 is 7.68, and for (CH 3) 2 C=N(OH)-O pKa is 5.11.
The difference in pKa values for the two forms is not unexpected since the aci form is an O-H acid, while the nitro form is a C-H acid. Recall that a similar pattern is observed for the keto- and enol forms of carbonyl and 1,3-dicarbonyl compounds, where enol is a stronger O-H acid compared to the C-H acidity of the keto form.
Aci-nitro compounds are fairly strong acids that form salts even when reacting with sodium carbonate, in contrast to the nitro form of nitroalkanes, which does not react with the carbonate ion. Tautomeric transformations of both forms of nitroalkanes are catalyzed by both acids and bases, similarly to the enolization of aldehydes and ketones.
Reactions of ambident anions of nitroalkanes.
Under the action of a base on both the nitro form and the aci form of the nitro compound, a mesomeric ambident anion common to both of them is formed, in which the charge is delocalized between the oxygen and carbon atoms.
The ambident anions of nitroalkanes are in all respects close analogues of the enolate ions of carbonyl compounds and are characterized by the same substitution reactions as for the enolate ions.
The most typical and important reactions involving nitroalkane anions are: halogenation, alkylation, acylation, condensations with carbonyl compounds, Mannich and Michael reactions - all those that are typical for enolate ions. Depending on the nature of the electrophilic agent and, to some extent, on the structure of the nitroalkane, substitution can occur with the participation of either oxygen or carbon, or both centers of the ambident nitroalkane anion.
Halogenation of alkaline salts of nitro compounds is carried out only at the carbon atom, the reaction can be stopped at the stage of introduction of one halogen atom.
Nitrosation of primary nitroalkanes is also carried out only at the carbon atom and leads to the formation of so-called nitrolic acids.
Secondary nitroalkanes give pseudonitrols under the same conditions.
Nitrolic acids are colorless and, when shaken with sodium hydroxide solution, form red salts.
In contrast, pseudonitrols have a blue color in a neutral medium. These compounds can be used to identify primary and secondary nitroalkanes. Tertiary nitroalkanes do not react at 0° or below with nitrous acid.
Alkylation of ambident anions of nitroalkanes proceeds, in contrast to halogenation and nitrosation, predominantly at the oxygen atom with the formation of aci-form esters as intermediates, which are called nitrone esters. Esters of the aci-form of nitroalkanes can be isolated individually by alkylation of nitroalkane salts with trialkyloxonium tetrafluoroborates in methylene chloride at -20 o.
Nitron ethers are thermally unstable and above 0-20° undergo redox decomposition into oxime and carbonyl compound.
The oxime is always formed as the end product of the reduction of the nitroalkane, while the aldehyde is the end product of the oxidation of the alkylating agent. This reaction has found wide application in the synthesis of aromatic aldehydes.
When alkali salts of 2-nitropropane react with substituted benzyl halides, the end products are acetone oxime and an aromatic aldehyde.
An even more important role is played by the alkylation of ambident anions of nitroalkanes under the action of allyl halides to obtain ,-unsaturated aldehydes.
As follows from the above examples, in contrast to enolate ions, nitroalkane anions undergo regioselective O-alkylation. Such a sharp difference in the behavior of two related classes of ambident anions is due to the high degree of charge localization on the oxygen atom of the nitroalkane anion.
In the presence of one or more strong electron-withdrawing groups in the benzyl halide, such as NO 2 , NR 3 , SO 2 CF 3 , etc., the reaction mechanism and its regioselectivity change. In this case, C-alkylation of the nitroalkane anion is observed by a mechanism involving radical anions, which is essentially similar to the S RN 1 mechanism of aromatic nucleophilic substitution.
The discovery of the anion-radical mechanism of C-alkylation of nitroalkanes and other ambident anions allowed N. Kornblum in 1970-1975 to develop an extremely effective method for the alkylation of ambident anions with the help of -nitro-substituted esters, nitriles, etc., which contribute to the implementation of the anion-radical chain process.
It should be noted that in these reactions substitution occurs even at the tertiary carbon atom.
C-alkylation can be made practically the only direction of the reaction in the case of alkylation of nitroalkane dianions. Nitroalkane dianions are formed by treating primary nitroalkanes with two equivalents of n-butyllithium in THF at -100 o.
These dianions also undergo regioselective C-acylation upon interaction with acyl halides or anhydrides of carboxylic acids.
Condensation of nitroalkane anions with carbonyl compounds(Henri's reaction).
Condensation of anions of primary and secondary nitroalkanes with aldehydes and ketones leads to the formation of -hydroxynitroalkanes or their dehydration products - ,-unsaturated nitro compounds.
This reaction was discovered by L. Henri in 1895 and can be considered as a kind of aldol-crotonic condensation of carbonyl compounds.
The anion of the nitroalkane, and not the carbonyl compound, takes part in the condensation, since the acidity of nitroalkanes (pKa ~ 10) is ten orders of magnitude higher than the acidity of carbonyl compounds (pKa ~ 20).
Effective catalysts for the Henri reaction are hydroxides, alkoxides and carbonates of alkali and alkaline earth metals.
The alkalinity of the medium must be carefully controlled to avoid aldol condensation of carbonyl compounds or the Canizzaro reaction for aromatic aldehydes. Primary nitroalkanes can also react with two moles of a carbonyl compound, so the ratio of reactants must be observed very carefully. During the condensation of aromatic aldehydes, only -nitroalkenes are usually formed, and it is very difficult to stop the reaction at the stage of formation of -hydroxynitroalkane.
The addition of nitroalkane anions to an activated double bond according to Michael andMannich reaction involving nitroalkanes.
Anions of primary and secondary nitroalkanes add via a multiple bond
,-unsaturated carbonyl compounds, esters and cyanides, in the same way as it happens when enolate ions are attached to an activated double bond.
For primary nitroalkanes, the reaction can go further with the participation of the second mole of CH 2 =CHX. The nitroalkane anions in the Michael addition reaction are prepared in the usual manner using sodium ethoxide or diethylamine as the base.
α-Nitroalkenes can also be used as Michael acceptors in addition reactions of conjugated carbanions. Addition of nitroalkane anions to - nitroalkenam is one of the simplest and most convenient methods for the synthesis of aliphatic dinitro compounds.
This type of addition can also occur under Henri reaction conditions as a result of dehydration of the condensation product of an aldehyde or ketone with a nitroalkane and subsequent addition of the nitroalkane.
Primary and secondary aliphatic amines enter into the Mannich reaction with primary and secondary nitroalkanes and formaldehyde.
In terms of its mechanism and scope, this reaction is no different from the classical version of the Mannich reaction involving carbonyl compounds instead of nitroalkanes.
Reactions of aromatic nitro compounds:
The nitro group is highly stable with respect to electrophilic reagents and various oxidizing agents. Most nucleophilic agents, with the exception of organolithium and magnesium compounds, as well as lithium aluminum hydride, do not act on the nitro group. The nitro group is among the excellent nucleophilic groups in activated aromatic nucleophilic substitution (S N A r) processes. For example, the nitro group in 1,2,4-trinitrobenzene is easily replaced by hydroxide, alkoxide ions or amines.
The most important reaction of aromatic nitro compounds is the reduction of their pre-primary amines.
This reaction was discovered in 1842 by N.N. Zinin, who was the first to reduce nitrobenzene to aniline by the action of ammonium sulfide. Currently, catalytic hydrogenation is used to reduce the nitro group in arenes to the amino group under industrial conditions. Copper is used as a catalyst on silica gel as a carrier. The catalyst is prepared by applying copper carbonate from a suspension in sodium silicate solution and then reducing with hydrogen while heating. The yield of aniline over this catalyst is 98%.
Sometimes in the industrial hydrogenation of nitrobenzene to aniline, nickel is used as a catalyst in combination with vanadium and aluminum oxides. Such a catalyst is effective in the range of 250-300 about and is easily regenerated by air oxidation. The yield of aniline and other amines is 97-98%. The reduction of nitro compounds to amines can be accompanied by hydrogenation of the benzene ring. For this reason, the use of platinum as a catalyst is avoided in the production of aromatic amines. palladium or Raney nickel.
Another method for the reduction of nitro compounds is metal reduction in an acidic or alkaline medium.
The reduction of the nitro group to the amino group occurs in several stages, the sequence of which differs greatly in acidic and alkaline media. Let us consider successively the processes that occur during the reduction of nitro compounds in acidic and alkaline media.
When reducing in an acidic medium, iron, tin, zinc and hydrochloric acid are used as a reducing agent. An effective reducing agent for the nitro group is tin(II) chloride in hydrochloric acid. This reagent is especially effective in cases where the aromatic nitro compound contains other functional groups: CHO, COR, COOR, etc., which are sensitive to the action of other reducing agents.
The reduction of nitro compounds to primary amines in an acidic medium proceeds stepwise and includes three stages with the transfer of two electrons at each stage.
In an acidic environment, each of the intermediate products is rapidly reduced to the final product of aniline, and they cannot be isolated individually. However, in aprotic solvents in a neutral medium, intermediate reduction products can be detected.
In the reduction of nitrobenzene with sodium or potassium in THF, the radical anion of nitrobenzene is first formed due to the transfer of one electron from the alkali metal.
The alkali metal cation is bound into a contact ion pair with the oxygen atom of the nitro group of the radical anion. Upon further reduction, the radical anion is converted into a dianion, which, after protonation, gives nitrosobenzene.
Nitrozobenzene, like other aromatic nitroso compounds, has a high oxidizing potential and is very rapidly reduced to N-phenylhydroxylamine. Therefore, nitrosobenzene cannot be isolated as a reduction intermediate, although electrochemical reduction data unambiguously indicate its formation.
Further reduction of nitroso compounds to N-arylhydroxylamine includes two similar stages of one-electron reduction to the radical anion and then to the dianion of the nitroso compound, which is converted to N-arylhydroxylamine upon protonation.
The last step in the reduction of arylhydroxylamine to a primary amine is accompanied by heterolytic cleavage of the nitrogen-oxygen bond after protonation of the substrate.
In a neutral aqueous solution, phenylhydroxylamine can be obtained as a product of the reduction of nitrobenzene. Phenylhydroxylamine is obtained by reducing nitrobenzene with zinc in an aqueous solution of ammonium chloride.
Arylhydroxylamines are easily reduced to amines by treatment with iron or zinc and hydrochloric acid.
Since phenylhydroxylamine is a reduction intermediate, it can not only be reduced to aniline but also oxidized to nitrosobenzene.
This is probably one of the best methods for obtaining aromatic nitroso compounds, which cannot otherwise be isolated as an intermediate in the reduction of nitro compounds.
Aromatic nitroso compounds readily dimerize in the solid state, and their dimers are colorless. In the liquid and gaseous state, they are monomeric and colored green.
The reduction of nitro compounds with metals in an alkaline medium differs from the reduction in an acidic medium. In an alkaline environment, nitrosobenzene reacts rapidly with the second reduction intermediate, phenylhydroxylamine, to form azoxybenzene. This reaction is essentially similar to the addition of nitrogenous bases to the carbonyl group of aldehydes and ketones.
Under laboratory conditions, azoxybenzene is obtained in good yield by reducing nitro compounds with sodium borohydride in DMSO, sodium methoxide in methyl alcohol, or in the old way using As 2 O 3 or glucose as a reducing agent.
Under the action of zinc in an alcoholic solution of alkali, azoxybenzene is first reduced to azobenzene, and under the action of excess zinc, further to hydrazobenzene.
In synthetic practice, azoxybenzene derivatives can be reduced to azobenzene by the action of a trialkyl phosphite as a reducing agent. On the other hand, azobenzene is easily oxidized to azoxybenzene by peracids.
Azobenzene exists as cis and trans isomers. The reduction of azoxybenzene results in a more stable trans isomer, which is converted to the cis isomer upon irradiation with UV light.
Asymmetric azobenzene derivatives are obtained by the condensation of nitroso compounds and primary aromatic amines.
When aromatic nitro compounds are reduced with lithium aluminum hydride in ether, azo compounds are also formed with a yield close to quantitative.
Azobenzene is reduced by zinc dust and alcohol alkali to hydrazobenzene. Hydrazobenzene is thus the end product of metal reduction of nitrobenzene in an alkaline medium. In air, colorless hydrazobenzene readily oxidizes to orange-red azobenzene. At the same time, hydrazobenzene, as well as azobenzene and azoxybenzene, is reduced to aniline by the action of sodium dithionite in water or tin (II) chloride in hydrochloric acid.
The overall process of reduction of aromatic nitro compounds with metals in acidic and alkaline media can be represented as the following sequence of transformations.
In an acidic environment:
In an alkaline environment:
In industry, aniline is obtained by catalytic reduction of nitrobenzene on a copper or nickel catalyst, which replaced the old method of reducing nitrobenzene with cast iron shavings in an aqueous solution of ferric chloride and hydrochloric acid.
The reduction of the nitro group to the amino group with sodium sulfide and sodium hydrosulfide is currently only relevant for the partial reduction of one of the two nitro groups, for example, in m-dinitrobenzene or 2,4-dinitroaniline.
With the stepwise reduction of polynitro compounds using sodium sulfide, this inorganic reagent is converted into sodium tetrasulfide, which is accompanied by the formation of alkali.
The high alkalinity of the medium leads to the formation of azoxy and azo compounds as by-products. In order to avoid this, sodium hydrosulfide should be used as a reducing agent, where no alkali is formed.
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NITRO COMPOUNDS, contain in the molecule one or several. nitro groups directly attached to the carbon atom. N- and O-nitro compounds are also known. 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.
Nitro 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:
Ethers of nitrone to-t exist in the form of cis- 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.
NITRO COMPOUNDS OF THE ALIPHATIC SERIES
Nitroalkanes have the general formula C n H 2n+1 NO 2 or R-NO 2 . They are isomeric alkyl nitrites (esters of nitric acid) with the general formula R-ONO. The isomerism of nitroalkanes is related to the isomerism of the carbon skeleton. Distinguish primary RCH 2 NO 2 , secondary R 2 CHNO 2 and tertiary R 3 CNO 2 nitroalkanes, for example:
Nomenclature
The name of the nitroalkanes is based on the name of the hydrocarbon with the prefix nitro(nitromethane, nitroethane, etc.). According to the systematic nomenclature, the position of the nitro group is indicated by a number:
^ Methods for obtaining nitroalkanes
1. Nitration of alkanes with nitric acid (Konovalov, Hess)
Concentrated nitric acid or a mixture of nitric and sulfuric acids oxidize alkanes. Nitration proceeds only under the action of dilute nitric acid (sp. weight 1.036) in the liquid phase at a temperature of 120-130 ° C in sealed tubes (M.I. Konovalov, 1893):
^ R-H + HO-NO 2 → R-NO 2 + H 2 O
For nitration Konovalov M.I. first used nonaphthene
It was found that the ease of replacing a hydrogen atom with a nitro group increases in the series:
The main factors affecting the rate of the nitration reaction and the yield of nitro compounds are the concentration of the acid, the temperature, and the duration of the process. So, for example, the nitration of hexane is carried out with nitric acid (d 1.075) at a temperature of 140 ° C:
The reaction is accompanied by the formation of polynitro compounds and oxidation products.
The method of vapor-phase nitration of alkanes has gained practical importance (Hess, 1936). Nitration is carried out at a temperature of 420°C and a short stay of the hydrocarbon in the reaction zone (0.22-2.9 sec). Nitration of alkanes according to Hass leads to the formation of a mixture of nitroparaffins:
The formation of nitromethane and ethane occurs as a result of the cracking of the hydrocarbon chain.
The nitration reaction of alkanes proceeds according to the free radical mechanism, and nitric acid is not a nitrating agent, but serves as a source of nitrogen oxides NO 2:
2. Meyer reaction (1872)
The interaction of halide alkyls with silver nitrite leads to the production of nitroalkanes:
A method for producing nitroalkanes from alkyl halides and sodium nitrite in DMF (dimethylformamide) was proposed by Kornblum. The reaction proceeds according to the mechanism S N 2.
Along with nitro compounds, nitrites are formed in the reaction, this is due to the ambivalence of the nitrite anion:
^ Structure of nitroalkanes
Nitroalkanes can be represented by the Lewis octet formula or by resonance structures:
One of the bonds of the nitrogen atom with oxygen is called donor-acceptor or semipolar.
^
Chemical properties
Chemical transformations of nitroalkanes are associated with reactions at the a-hydrogen carbon atom and the nitro group.
Reactions at the a-hydrogen atom include reactions with alkalis, with nitrous acid, aldehydes and ketones.
1. Formation of salts
Nitro compounds are pseudoacids - they are neutral and do not conduct electric current, however, they interact with aqueous solutions of alkalis to form salts, upon acidification of which the aci-form of the nitro compound is formed, which then spontaneously isomerizes into a true nitro compound:
The ability of a compound to exist in two forms is called tautomerism. Nitroalkane anions are ambident anions with dual reactivity. Their structure can be represented by the following forms:
2. Reactions with nitrous acid
Primary nitro compounds react with nitrous acid (HONO) to form nitrolic acids:
Nitrolic acids, when treated with alkalis, form a blood-red salt:
Secondary nitroalkanes form pseudonitrols (heme-nitronitroso-alkanes) of blue or greenish color:
Tertiary nitro compounds do not react with nitrous acid. These reactions are used for the qualitative determination of primary, secondary and tertiary nitro compounds.
3. Synthesis of nitroalcohols
Primary and secondary nitro compounds interact with aldehydes and ketones in the presence of alkalis to form nitro alcohols:
Nitromethane with formaldehyde gives trioxymethylnitromethane NO 2 C(CH 2 OH) 3 . When the latter is reduced, an amino alcohol NH 2 C (CH 2 OH) 3 is formed - the starting material for the production of detergents and emulsifiers. Tri(oxymethyl)nitromethane trinitrate, NO 2 C(CH 2 ONO 2) 3 , is a valuable explosive.
Nitroform (trinitromethane) when interacting with formaldehyde forms trinitroethyl alcohol:
4. Recovery of nitro compounds
The complete reduction of nitro compounds to the corresponding amines can be carried out by many methods, for example, by the action of hydrogen sulfide, iron in hydrochloric acid, zinc and alkali, lithium aluminum hydride:
Methods of incomplete reduction are also known, as a result of which oximes of the corresponding aldehydes or ketones are formed:
5. Interaction of nitro compounds with acids
Of practical value are the reactions of nitro compounds with acids. Primary nitro compounds, when heated with 85% sulfuric acid, are converted into carboxylic acids. It is assumed that the 1st stage of the process is the interaction of nitro compounds with mineral acids with the formation of the aci-form:
Salts of aci-forms of primary and secondary nitro compounds in the cold in aqueous solutions of mineral acids form aldehydes or ketones (Nef reaction):
. Aromatic nitro compounds. Chemical properties
Chemical properties. Recovery of nitro compounds in acidic, neutral and alkaline media. The practical significance of these reactions. Activating effect of the nitro group on nucleophilic substitution reactions. Aromatic polynitro compounds.
