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 N-O connections: for primary nitro compounds 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 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 positive. 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 N. 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 rupture C-N connections, by N=O, O=N O, C=N -> O bonds and reactions 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 N. during heating. with miner. 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 intermediate nitroso compounds. s and . Carrying out the process in 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 ethers 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 such as 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:
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 . Characteristic qualitative reaction primary and secondary nitro compounds are reacted with nitrous acid.
For tertiary aliphatic nitro compounds specific reactions no detection available.
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 turns pink; 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 - ester or amide
Qualitative reactions of thiols (thioalcohols, thioethers)
The most important properties of 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.
1. Nitro compounds
1.2. Reactions of nitro compounds
1. NITRO COMPOUNDS
Nitro compounds are hydrocarbon derivatives 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 causes it to have a strong -I effect. Along with the strong -I effect, the nitro group has a strong -M effect.
Ex. 1. Consider the structure of the nitro group and its effect on the direction and rate of the electrophilic substitution reaction in aromatic core.
1.1. Methods for obtaining nitro compounds
Almost all methods for producing nitro compounds have already been discussed in previous chapters. Aromatic nitro compounds are usually obtained by direct nitration of arenes and aromatic heterocyclic compounds. Nitrocyclohexane is produced industrially by nitration of cyclohexane:
(1)
Nitromethane is also obtained in the same way, but in laboratory conditions it is obtained from chloroacetic acid as a result of reactions (2-5). The key stage of these is reaction (3), which occurs 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, the a-hydrogen atoms have increased mobility and therefore primary and secondary nitro compounds are CH-acids. So, nitromethane is quite strong acid(pKa 10.2) and in an alkaline environment 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 aqueous solution NaOH.
1.2.2. Condensation of aliphatic nitro compounds with aldehydes and ketones
A nitro group can be introduced into aliphatic compounds by an aldol reaction between a nitroalkane anion and an aldehyde or ketone. In nitroalkanes, the 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 occur, and with aromatic aldehydes, only condensation reactions occur.
Thus, nitromethane adds to cyclohexanone,
(7)
1-Nitromethylcyclohexanol
but condenses with benzaldehyde,
The addition reaction with formaldehyde involves all three hydrogen atoms of nitromethane to form 2-hydroxymethyl-2-nitro-1,3-dinitropropane or trimethylolnitromethane.
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. Reduction of nitro compounds
The nitro group is reduced to an amino group by various reducing agents (11.3.3). Aniline is produced 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 to release hydrogen.
(12)
7-nitro-1,3,5-triazaadamantane 7-amino-1,3,5-triazaadamantane
Nitro compounds are reduced with metals in an acidic environment followed by alkalization
(13) (11 33)
Depending on the pH of the medium and the reducing agent used, different products can be obtained. In a neutral and alkaline environment, the activity of conventional reducing agents towards nitro compounds is less than in an acidic environment. A typical example is the reduction of nitrobenzene with zinc. In excess 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 rearrangement:
(15)
p-Aminophenol is used as a developer in photography. Phenylhydroxylamine can be further oxidized to nitrosobenzene:
(16)
Nitrosobenzene
By reducing nitrobenzene with tin(II) chloride, azobenzene is obtained, and with zinc in an alkaline medium, hydrazobenzene is obtained.
(17)
(18)
By treating nitrobenzene with a solution of alkali in methanol, azoxybenzene is obtained, while the methanol is oxidized to formic acid.
(19)
Methods for incomplete reduction of nitroalkanes are known. One of the industrial methods for producing nylon 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 - nylon:
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 reaction of (a) m-dinitrobenzene with tin in hydrochloric acid, (b) m-dinitrobenzene with hydrogen sulfide, (c) p-nitrotoluene with zinc in a buffer solution of ammonium chloride.
Exercise 5. Complete the reactions:
(A) 
(b) 
By systematic nomenclature amines are named by adding the prefix amine to the name of the hydrocarbon. By rational nomenclature they are considered alkyl or arylamines.
Methaneamine Ethanamine N-Methylethanamine N-Ethylethanamine
(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 by inserting the prefix aza-, diaza- or triaza- to denote the number of nitrogen atoms.
1-Azacyclopeta- 1,2-Diazacyclopeta- 1,3-Diazacyclopeta-
2,4-diene 2,4-diene 2,4-diene
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. The nitro group has a structure intermediate between two limiting resonance 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°. 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-form called aci-nitro compound or nitronic compound:
Esters of nitronic compounds exist in the form of cis- and trans-isomers. There are cyclical ethers, e.g. Isoxazoline N-oxides.
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.
NITRO COMPOUNDS OF ALIPHATIC SERIES
Nitroalkanes have the general formula C n H 2n+1 NO 2 or R-NO 2 . They are isomers of alkyl nitrites (esters of nitric acid) with general formula R-ONO. The isomerism of nitroalkanes is associated with 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 names of nitroalkanes are based on the name of the hydrocarbon with the prefix nitro(nitromethane, nitroethane, etc.). According to 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 oxidizes alkanes. Nitration occurs only under the influence of dilute nitric acid (specific 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 time using nonaphthene
It was found that the ease of replacing a hydrogen atom with a nitro group increases in the series:
The main factors influencing the rate of the nitration reaction and the yield of nitro compounds are the acid concentration, temperature and process duration. For example, 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.
Practical significance obtained the method of vapor-phase nitration of alkanes (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 Hess leads to the formation of a mixture of nitroparaffins:
The formation of nitromethane and ethane occurs as a result of cracking of the hydrocarbon chain.
The nitration reaction of alkanes proceeds by a free radical mechanism, and nitric acid is not a nitrating agent, but serves as a source of nitrogen oxides NO2:
2. Meyer's reaction (1872)
The interaction of alkyl halides 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 ambidentity of the nitrite anion:
^ Structure of nitroalkanes
Nitroalkanes can be represented by Lewis octet formula or resonance structures:
One of the bonds of a 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 involving the a-hydrogen atom include reactions with alkalis, 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 in 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 pseudonitroles (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 nitro alcohols
Primary and secondary nitro compounds react 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, amino alcohol NH 2 C (CH 2 OH) 3 is formed - the starting material for the production of detergents and emulsifiers. Tri(hydroxymethyl)nitromethane trinitrate, NO 2 C(CH 2 ONO 2) 3, is a valuable explosive.
Nitroform (trinitromethane) reacts with formaldehyde to form trinitroethyl alcohol:
4. Reduction of nitro compounds
Complete reduction of nitro compounds into the corresponding amines can be achieved by many methods, for example, 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
The reactions of nitro compounds with acids are of practical value. Primary nitro compounds when heated with 85% sulfuric acid are converted into carboxylic acids. It is assumed that stage 1 of the process is the interaction of nitro compounds with mineral acids to form the aci form:
Aci salts of primary and secondary nitro compounds form aldehydes or ketones in the cold in aqueous solutions of mineral acids (Nef reaction):
. Aromatic nitro compounds. Chemical properties
Chemical properties. Reduction of nitro compounds in acidic, neutral and alkaline media. The practical significance of these reactions. The activating effect of the nitro group on nucleophilic substitution reactions. Polynitro compounds of the aromatic series.
1. Nitro compounds
1.2. Reactions of nitro compounds
1. NITRO COMPOUNDS
Nitro compounds are hydrocarbon derivatives 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 causes it to have a strong -I effect. Along with the strong -I effect, the nitro group has a strong -M effect.
Ex. 1. Consider the structure of the nitro group and its effect on the direction and rate of the electrophilic substitution reaction in the aromatic ring.
1.1. Methods for obtaining nitro compounds
Almost all methods for producing nitro compounds have already been discussed in previous chapters. Aromatic nitro compounds are usually obtained by direct nitration of arenes and aromatic heterocyclic compounds. Nitrocyclohexane is produced industrially by nitration of cyclohexane:
(1)
Nitromethane is also obtained in the same way, but in laboratory conditions it is obtained from chloroacetic acid as a result of reactions (2-5). The key stage of these is reaction (3), which occurs via the SN2 mechanism.
(2)Chloroacetic acid Sodium chloroacetate
(3) (4)Nitroacetic acid
(5)Nitromethane
1.2. Reactions of nitro compounds
1.2.1. Tautomerism of aliphatic nitro compounds
Due to the strong electron-withdrawing properties of the nitro group, the a-hydrogen atoms have increased mobility and therefore primary and secondary nitro compounds are CH-acids. Thus, nitromethane is a rather strong acid (pKa 10.2) and in an alkaline environment it easily turns into a resonance-stabilized anion:
(6)Nitromethane pKa 10.2 Resonance stabilized anion
Exercise 2. Write the reactions of (a) nitromethane and (b) nitrocyclohexane with an aqueous solution of NaOH.
1.2.2. Condensation of aliphatic nitro compounds with aldehydes and ketones
A nitro group can be introduced into aliphatic compounds by an aldol reaction between a nitroalkane anion and an aldehyde or ketone. In nitroalkanes, the 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 occur, and with aromatic aldehydes, only condensation reactions occur.
Thus, nitromethane adds to cyclohexanone,
(7)
1-Nitromethylcyclohexanol
but condenses with benzaldehyde,
(8)The addition reaction with formaldehyde involves all three hydrogen atoms of nitromethane to form 2-hydroxymethyl-2-nitro-1,3-dinitropropane or trimethylolnitromethane.
(9)By condensation of nitromethane with hexamethylenetetramine we obtained 7-nitro-1,3,5-triazaadamantane:
(10)
Ex. 3. Write the reactions of formaldehyde (a) with nitromethane and (b) with nitrocyclohexane in an alkaline medium.
1.2.3. Reduction of nitro compounds
The nitro group is reduced to an amino group by various reducing agents (11.3.3). Aniline is produced 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 to release hydrogen.
(12)
7-nitro-1,3,5-triazaadamantane 7-amino-1,3,5-triazaadamantane
Nitro compounds are reduced with metals in an acidic environment followed by alkalization
(13) (11 33)
Depending on the pH of the medium and the reducing agent used, different products can be obtained. In a neutral and alkaline environment, the activity of conventional reducing agents towards nitro compounds is less than in an acidic environment. A typical example is the reduction of nitrobenzene with zinc. In excess 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 rearrangement:
(15)
p-Aminophenol is used as a developer in photography. Phenylhydroxylamine can be further oxidized to nitrosobenzene:
(16)
Nitrosobenzene
By reducing nitrobenzene with tin(II) chloride, azobenzene is obtained, and with zinc in an alkaline medium, hydrazobenzene is obtained.
(17)
(18)
By treating nitrobenzene with a solution of alkali in methanol, azoxybenzene is obtained, while the methanol is oxidized to formic acid.
(19)
Methods for incomplete reduction of nitroalkanes are known. One of the industrial methods for producing nylon 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 - nylon:
Reduction of the nitro group of aldol addition products (7) is a convenient way to obtain b-amino alcohols.
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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
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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 buffer solution of ammonium chloride.
Exercise 5. Complete the reactions:
(b) 
