Nitro compounds with nitrous acid. Qualitative reactions of nitro compounds. Aliphatic nitro compounds

NITRO COMPOUNDS

(C-nitro compounds), contain one or several in the 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; atoms N and O have, sp 2 - hybridization, NCHO bonds are equivalent and almost one and a half; bond lengths, ex. for CH 3 NO 2, 0.122 nm (NHO), 0.147 nm (CCHN), ONO angle 127 The СЧNO 2 system is flat with a low barrier of rotation around the СЧN coupling.

N. having at least one a-H-atom can exist in two tautomeric forms with a common mesomeric anion. The O-form is called. aci-H. or nitrone to-that:

Known decomp. derivatives of nitrone to-t: f-ly RR "C \u003d N (O) O - M + (salts of N.), ethers (nitrone ethers), etc. Esters of nitrone to-t exist in the form jis- and trance -isomers. There is a cycle. ethers, for example. N-oxides of isoxazolines.

Name N. is produced by adding the prefix "nitro" to the name. base connections, if necessary adding a digital index, eg. 2-nitropropane. Name N. salts are produced from the name. either C-form or aci -form, or nitronic to-you.

Physical properties. The simplest nitroalkanes are colorless. liquids. Phys. sv-va nek-ryh aliphatic N. are given in the table. Aromatic N.-colorless. or light yellow high-boiling liquids or low-melting solids with a characteristic odor, poor sol. in water, as a rule, they are distilled with steam.

PHYSICAL PROPERTIES OF SOME ALIPHATIC NITROCOMPOUNDS

* 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 NCHO bond: for primary N., 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 \u003d 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 in the region of 1570-1540, and electron donor - in the region 1510-1490 cm -1); for H. salts 1610-1440 and 1285-1135 cm -1; nitrone ethers have an intense band at 1630-1570 cm, the MCHN bond is a weak band at 1100-800 cm -1.

In UV spectra aliphatic H. l max 200-210 nm (strong band) and 270-280 nm (weak band); for salts and esters of nitrone to-t acc. 220-230 and 310-320 nm; for heme - dinitroed. 320-380 nm; for aromatic N. 250-300 nm (the intensity of the band sharply decreases with violation of coplanarity).

In the PMR spectrum of chem. shifts a-H-atom, depending on the structure of 4-6 ppm. In the NMR spectrum 14 N and 15 N chem. shift 5 from -50 to + 20 ppm

In the mass spectra of aliphatic N. (with the exception of CH 3 NO 2) the peak mol. the ion is absent or very small; main the fragmentation process is the elimination of NO 2 or two oxygen atoms with the formation of a fragment equivalent to nitrile. Aromatic N. is characterized by the presence of a pier peak. and she; main 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 Naib. strong electron-withdrawing groups and is able to effectively delocalize negative. charge. In aromatic. conn. as a result of induction and especially mesomeric effects, it affects the distribution of electron density: the nucleus acquires a partial posit. charge, to-ry localized hl. arr. in ortho- and couple - positions; 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 for the district with electrophilic. reagents. This determines N.'s widespread use in org. synthesis: the NO 2 group is introduced into the desired position of the org molecule. conn., carry out decomp. p-tion, associated, as a rule, with a change in the carbon skeleton, and then transformed into another f-tion or removed. In aromatic. In this series, a shorter scheme is often used: nitration-transformation of the NO 2 group.

Mn. aliphatic N.'s transformations take place with pre. isomerization to nitronic 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 share aci - forms for nitromethane 1X10 -7, for nitropropane 3. 10 -3. Nitron to-you in free. form, as a rule, unstable; they are obtained by careful acidification of N.'s salts. Unlike N., they conduct current in solutions and give red coloration with FeCl 3. Aci- H.- stronger CH-acids (p To a~ 3-5) than the corresponding N. (p K a\u003e~ 8-10); N.'s acidity increases with the introduction of electron-withdrawing substituents in the a-position to the NO 2 group.

Formation of nitronic to - t in a series of aromatic N. is associated with isomerization of the benzene ring into the quinoid form; for example, forms with conc. H 2 SO 4 colored salt-like product f-ly I, o-nitrotoluene as a result of intramol. proton transfer with the formation of a bright blue O-derivative:

Under the action of bases on primary and secondary N., salts of N. are formed; ambident salts in p-tions with electrophiles are capable of giving both O- and C-derivatives. Thus, the alkylation of hydrogen salts with alkyl halides, trialkylchlorosilanes, or R 3 O + BF - 4 gives O-alkylation products. Last m. B. obtained also by the action of diazomethane or N, O- bis - (trimethylsilyl) acetamide to nitroalkanes with p To a< 3> or nitrone to-you, e.g .:

Acyclic. alkyl ethers of nitronic to - t are thermally unstable and disintegrate on intramol. mechanism:

p-tion 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 p-tions with a break in the SCHN bond, along the bonds N \u003d O, O \u003d N O, C \u003d N -\u003e O and p-tions with preservation of the NO 2 group.

R-ts and s r a z r s v o m with v y z and chN. Primary and secondary N. at loading. with a miner. to-tami in the presence. alcohol or water solution of alkali form carbonyl compounds. (cm. Nefa reaction). P-tion passes through the interim. formation of nitronic to-t:

As a starting point silyl nitrone ethers can be used. The action of strong to - t on aliphatic N. can lead to hydroxamic to - there, for example:

The method is used in the industry for the synthesis of CH 3 COOH and hydroxylamine from nitroethane. Aromatic N. are inert to the action of strong to-t.

Under the action of reducing agents (for example, TiCl 3 -H 2 O, VCl 2 -H 2 O-DMF) on N. or oxidants (KMnO 4 -MgSO 4, O 3) on N. salts, aldehydes are 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 flows similarly. decomposition of nitroalkanes at t-ts above 450 °. Vicinal dinitrosoed. when treating with Ca amalgam in hexamstanol, both groups of NO 2 are cleaved 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, under the action of thiolate ions on tertiary nitroalkanes in aprotic p-solvents, the NO 2 group is replaced by a hydrogen atom. P-tion proceeds by 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. nucleophile. substitution of the NO 2 group depends on its position with respect to other substituents: the NO 2 group located in meta -position with respect to electron-withdrawing substituents and in ortho- and couple- positions to electron donor, has a low reaction. ability; reaction. ability of the NO 2 group located in ortho- and couple -position to electron-withdrawing substituents increases markedly. In some cases, the deputy enters into ortho - position to the leaving group NO 2 (for example, when heating aromatic N. with alcohol solution KCN, Richter's district):

R-ts and p about with I z and N \u003d O. One of the most important p-tions-recovery, leading in the general case to a set of products:

Azoxy (II), azo (III) and hydrazoed. (IV) are formed in an alkaline environment as a result of condensation of intermediate nitrosoat. with amines and hydroxylamines. Carrying out the process in an acidic environment excludes the formation of these substances. Nitrosoeater. recover faster than the corresponding N., and select them from reactions. the mixture usually fails. Aliphatic N. are reduced in azoxy or under the action of Na alcoholates, aromatic ones under the action of NaBH 4; treatment of the latter with LiAlH 4 leads to azo compounds. Electrochem. aromatic N., under certain conditions, makes it possible to obtain any of the derivatives presented (with the exception of nitrosoed.); by the same method it is convenient to obtain hydroxylamines from mononitroalkanes and amidoximes from salts heme -dinitroalkanes:

There are many methods of restoration of N. to. Iron filings, Sn and Zn are widely used in the presence. to-t; at catalytic. hydrogenation as catalysts using Ni-Raney, Pd / C or Pd / PbCO 3, and others. Aliphatic N. is easily reduced to amines LiAlH 4 and NaBH 4 in the presence. Pd, Na and Al amalgams, at heat. with hydrazine over Pd / C; for aromatic N., TlCl 3, CrCl 2 and SnCl 2 are sometimes used, aromatic. poly-H. selectively reduced to nitramines with Na hydrosulfide in CH 3 OH. There are ways to elect. restoration of the NO 2 group in polyfunctional N. without affecting other f-tions.

Under the action of P (III) on aromatic N., a sequential process occurs. deoxygenation of the NO 2 group with the formation of highly reactive nitrenes. P-tion is used for the synthesis of condenser. heterocycles, e.g .:

Under the same conditions, silyl ethers of nitronic acid are transformed into silyl derivatives of oximes. Treatment of primary nitroalkanes with PCl 3 in pyridine or NaBH 2 S leads to nitriles. Aromatic N., containing in ortho - the position of a substituent with a double bond or a cyclopropyl substituent, in an acidic medium rearranges into o-nitroso ketones, for example:

N. and nitrone esters react with an excess of Grignard reagent to give hydroxylamine derivatives:

P-tion on bonds O \u003d N O and C \u003d N O. N. enter into p-tions of 1,3-dipolar cycloaddition, for example:

Naib. easily this p-tion proceeds between nitrone ethers and olefins or acetylenes. In the products of cycloaddition (mono- and bicyclic. Dialkoxyamines) under the action of nucleoph. and electroph. reagents of the connection N CH O are readily cleaved, which leads to degradation. aliphatic and hetero-cyclic. conn .:

For preparative purposes, stable nitrone silyl ethers are used in the district.

R-ts and with preservation of m group NO 2. Aliphatic nitrogen containing an a-H-atom are easily alkylated and acylated with the formation, as a rule, of O-derivatives. However, inter-mod. dilithium salts of primary N. with alkyl halides, anhydrides, or halogenated carbonic acid halides leads to the products of C-alkylation or C-acylation, for example:

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 district); in the district you can use the previously obtained methylol derivatives of N. or aminosoed .:

Nitromethane and nitroethane can condense with two methylolamine molecules, while higher nitroalkanes can condense with only one. At certain ratios of reagents, the p-tion can lead to heterocyclic. conn., for example: with interaction. primary nitroalkane with two equivalents of primary amine and an excess of formaldehyde are formed comp. f-ly V, if the reagents are taken in a ratio of 1: 1: 3-conne. f-ly VI.

Aromatic N. easily enter the nucleophile district. substitution and much more difficult in the district of electrophysiology. substitution; in this case, the nucleophile is sent to ortho- and position-position, and electrophile-in meta- position to group NO 2. Electrophysical rate constant nitrobenzene nitration is 5-7 orders of magnitude less than benzene; this produces m-dinitrobenzene.

The activating effect of the NO 2 group on the nucleoph. substitution (especially for ortho -position) are widely used in org. synthesis and industry. P-tion proceeds according to the scheme of addition-elimination with interm. the formation of an s-complex (Meisenheimer's complex). According to this scheme, halogen atoms are easily replaced by nucleophiles:

Examples of substitution by the radical anion mechanism with aromatic electron capture are known. compound and release of a halide ion or other groups, for example. alkoxy, amino, sulfate, NO - 2. In the latter case, the p-tion passes the easier, the greater the deviation of the NO 2 group from coplanarity, for example: in 2,3-dinitrotoluene is substituted in the basic. group NO 2 in position 2. The H atom in aromatic N. is also capable of nucleophilization. substitution-nitrobenzene at heating. with NaOH forms o-nitrophenol.

The nitro group facilitates the rearrangement of the aromatic. conn. by the mechanism of intramol. nucleophile. substitution or through the stage of carbanion formation (see. Smiles regrouping).

The introduction of the second group NO 2 accelerates the nucleophilus. substitution. N. in the presence. bases bind to aldehydes and ketones, giving nitro alcohols (see. Henri reaction), primary and secondary N. - to the connection, containing aktivir. double bond (Michael's district), e.g .:

Primary N. can enter into Michael's district with a second molecule of unsaturated compound; this district with the last. transformation of the NO 2 group is used for the synthesis of poly-functional. aliphatic connections. The combination of the Henri and Michael districts leads to 1,3-dinitro compounds, for example:

K inactive double bond only Hg-derivatives are attached gem- di- or trinitro compounds, as well as IC (NO 2) 3 and C (NO 2) 4, while C- or O-alkylation products are formed; the latter can enter into a p-cyclo-addition with the second olefin molecule:

Easily enter into the district of addition of nitroolefins: with water in a weakly acidic or weakly alkaline medium, followed by. by Henri's retroreaction they form carbonyl compounds. and nitroalkanes; with N. containing a-H-atom, -poly-N; add other CH-acids, such as esters of acetoacetic and malonic acid, Grignard reagents, as well as nucleophiles of the OR -, NR - 2 type, etc., for example:

Nitroolefins can act as dienophiles or dipolarophiles in solutions of diene synthesis and cycloaddition, and 1,4-dinitrodienes can act as diene components, for example:

The nitrosation of primary N. leads to nitrolic acids RC (\u003d NOH) NO 2, secondary N. form pseudo-nitroles RR "C (NO) NO 2, tertiary N. do not enter the district.

Nitroalkanes are easily halogenated in the presence. bases with sequential. substitution of H atoms at a-C-atom:

With fotdheim. chlorination replaces more distant H atoms:

When carboxylation of primary nitroalkanes by the action of CH 3 OMgOCOOCH 3, a-nitrocarboxylic to-you or their esters are formed.

When processing salts with mono-H. C (NO 2) 4., Ag or alkali metal nitrites or under the action of nitrites on a-halogennitroalkanes in an alkaline medium (Ter Meer district) are formed heme -dinitro compounds. The electrolysis of a-halogen-nitroalkanes in aprotic p-solvents, as well as the treatment of H. Cl 2 in an alkaline medium or the electrooxidation of N. salts lead to vic - dinitroconnections:

The nitro group does not render creatures. influence on free-radical or aromatic arylation. conn .; p-tion leads to the main. to ortho- and couple -substituted products.

To restore N. without affecting the NO 2 group, use NaBH 4, LiAlH 4 at low t-pax or solution of dibora-na in THF, for example:

Aromatic. di- and tri-H., in particular 1,3,5-trinitrobenzene, form stable brightly colored crystalline. pier complexes with aromatic. connection-donors of electrons (amines, phenols, etc.). Complexes with picrinic acid are used for the isolation and purification of aromatic substances. hydrocarbons. Reciprocity. di- and trinitrobenzenes with strong bases (HO -, RO -, N - 3, RSO - 2, CN -, aliphatic amines) leads to the formation of Meisen-Heimer complexes, which are isolated in the form of colored alkali metal salts.

Receiving. In the industry, lower nitroalkanes are obtained by liquid-phase (Konovalov district) or vapor-phase (Hess method) nitration of a mixture of ethane, propane and butane, isolated from natural gas or obtained by oil refining (see. Nitration). Higher N. is obtained by this method, for example. nitrocyclohexane is an intermediate product in the production of caprolactam.

In the laboratory, to obtain nitroalkanes, nitrogen to-that compound is used. with activator methylene group; a convenient method for the synthesis of primary nitroalkanes is nitration of 1,3-indandione followed by. alkaline hydrolysis of a-nitroketone:

Aliphatic N. also receive interaction. AgNO 2 with alkyl halides or NaNO 2 with esters of a-halogencarboxylic acids (see. Meyer's reaction). Aliphatic N. are formed during the oxidation of amines and oximes; oximes - a method of obtaining heme -di- and heme -trinitro compounds, e.g .:

Nitroalkanes m. B. obtained by heating acyl nitrates to 200 ° C.

Mn. Methods of synthesis of N. are based on the nitration of olefins with nitrogen oxides, HNO 3, nitronium salts, NO 2 Cl, org. nitrates, etc. As a rule, this gives a mixture vic -dinitrocompounds, nitronitrates, nitronitrites, unsaturated N., as well as products of conjugated 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 afterbirth. hydrolysis of salts of a, a "-dinitro-ketones:

Poly-N. synthesized by destructive nitration dec. org. conn .; for example, tri- and are obtained by the action of HNO 3 on acetylene in the presence. Hg (II) ions.

Main method of obtaining aromatic N.-electroph. nitration. The active nitrating group is the nitronium ion NO 2 generated from HNO 3 under the action of strong protonic or aprotic to-t. For nitration under mild conditions, nitronium salts (NO 2 BF 4, NO 2 ClO 4, etc.) are used, as well as N 2 O 5 in inert p-solvents.

In the industry for nitration aromatic. conn. use, as a rule, nitrating mixtures (H 2 SO 4 + HNO 3). In the laboratory, to increase the concentration of the nitronium ion, instead of H 2 SO 4, they use AlCl 3, SiCl 4, BF 3, etc., often nitration is carried out in inert p-solvents (CH 3 COOH, nitromethane, etc.). Easily replaced by the NO 2 sulfo- and diazo groups. For the introduction of the second group of NO 2 into nitrobenzene in ortho- and couple -position first get the corresponding diazo derivative, and then replace the diazo group at the Sandmeyer p-tions. Aromatic nitrogen is also obtained by oxidation of nitroso, diazo, and amino groups.

Application. Poly-H, especially aromatic, are used as explosives and to a lesser extent as components of propellants. Aliphatic N. are used as p-agents in the paint and varnish industry and in the production of polymers, in particular cellulose ethers; for cleaning miner. oils; oil dewaxing, etc.

A number of N. are used as biologically active in... So, ethers of phosphoric to - you, containing a nitroaryl fragment, - insecticides; derivatives of 2-nitro-1,3-propanediol and 2-nitrostyrene -; derivatives of 2,4-dinitrophenol -; a-nitrofurans are the most important antibacterial drugs, based on which drugs have been created that have a wide spectrum of action (furazolidine, etc.). Certain aromatic N.-fragrant substances.

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 conn. in theoretical. org. chemistry.

Nitroparaffins have a strong local irritant effect and are relatively toxic in you. They belong to cellular poisons of general action, especially dangerous for the liver. LD 50 0.25-1.0 g / kg (with oral administration). Chlorinated and unsaturated nitrogen is 5-10 times more toxic. Aromatic N. depress the nervous and especially the circulatory system, disrupting the supply of oxygen to the body. Signs of poisoning - hyperemia, higher. mucus discharge, watery eyes, cough, dizziness, headache. Wed-va first aid-quinine and. N.'s metabolism is associated with oxidation-reduction. p-tions and, in particular, with oxidizes. phosphorylation. For example, 2,4-dinitrophenol is one of the Naib. 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 volume of production of the most important aliphatic N. is tens of thousands of tons, aromatic-hundreds of thousands of tons; For example, the USA produces 50 thousand tons / year of C 1 -C 3 nitroalkanes and 250 thousand tons of nitrobenzene / year.

see also m-Dinitrobenzene, Nitroanisoles, Nitrobenzene, Nitrometap, Nitrotoluenes and etc.

Lit .: Chemistry of nitro and nitroso groups, ed. G. Foyer, trans. from English, t. 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 VA, "Izv. AN SSSR. Ser. Chem.", 1984, No. 1, p. 165-73.

V.A.Tartakovsky.

Chemical encyclopedia. - M .: Soviet encyclopedia. Ed. I. L. Knunyants. 1988 .

Limit nitro compounds with an open chain (non-cyclic) have the general formula C n H 2n + 1 NO 2. They are isomeric to alkyl nitrites (nitrous acid esters) with the general formula R-ONO. The differences are as follows:

Alkyl nitrites have more low temperatures boiling

Nitroelines are strongly polar and have a large dipole moment

Alkyl nitrites are easily saponified with alkalis and mineral acids to form the corresponding alcohols and nitrous acid or its salt.

Reduction of nitro compounds leads to amines, alkyl nitrites to alcohols and hydroxylamine.

Receiving

According to the Konovalov reaction - by nitration of paraffins with dilute nitric acid when heated. All hydrocarbons enter the liquid-phase nitration reaction, but the reaction rate is low and the yields are low. The reaction is accompanied by oxidation and the formation of poly (nano) compounds. Best results are obtained with hydrocarbons containing a tertiary carbon atom. Vapor-phase nitration takes place at 250-500 ° C with nitric acid vapors. The reaction is accompanied by cracking of hydrocarbons, as a result, all kinds of nitro derivatives are obtained, and oxidation, as a result of which alcohols, aldehydes, ketones, acids are formed. Unsaturated hydrocarbons are also formed. Nitric acid can be replaced with nitrogen oxides. Nitration proceeds according to the S R mechanism.

Interaction of halogenated derivatives of saturated hydrocarbons with silver nitrite upon heating. The attacking particle is NO 2 - ion exhibiting dual reactivity (ambiguity), i.e. add a radical at nitrogen (S N 2) to form a nitro compound R-NO 2 or oxygen to form an ester of nitrous acid R-O-N \u003d O. (S N 1). The reaction mechanism and its direction strongly depend on the nature of the solvent. Solvating solvents (water, alcohols) favor the formation of ether.

Chemical properties

When nitro compounds are reduced, primary amines are formed:

Primary and secondary nitro compounds are soluble in alkalis to form salts. Hydrogen atoms at the carbon bonded to the nitro group are activated, as a result, in an alkaline medium, the nyro compounds are rearranged into the aci-nitro form:

When an alkaline solution of a nitro compound is treated with a mineral acid, a strongly acidic aci-form is formed, which quickly isomerized to the usual neutral form:

Nitro compounds are referred to as pseudoacids. Pseudo acids are neutral and not electrically conductive, but nevertheless form neutral alkali metal salts. Neutralization of nitro compounds with alkalis occurs slowly, and true acids - instantly.

Primary and secondary nitro compounds react with nitrous acid, tertiary ones do not react:

Alkaline salts of nitrolic acids in solution are red, pseudonitrols are blue or greenish-blue.

Primary and secondary nyro compounds are condensed in the presence of alkalis with aldehydes, forming nitro alcohols (nucleophilic addition):

Aci-forms of primary and secondary nitro compounds in aqueous solutions under the action of mineral acids form aldehydes or ketones:

When heated with 85% sulfuric acid, primary nitro compounds are converted into carboxylic acids with the elimination of hydroxylamine. This occurs as a result of hydrolysis of the resulting aci-form.

Nitro compounds 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 the 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 complete positive charge on the nitrogen atom determines the presence of a strong -I-effect in it. Along with the strong -I-effect, the nitro group has a strong -M-effect.

Control. 1. Consider the structure of the nitro group and its effect on the direction and rate of the electrophilic substitution reaction in the aromatic nucleus.

Methods for obtaining nitro compounds

Almost all methods of obtaining nitro compounds have already been discussed in the previous chapters. Aromatic nitro compounds are obtained, as a rule, by direct nitration of arenes and aromatic heterocyclic compounds. Nitrocyclohexane is industrially obtained by nitration of cyclohexane:

Nitromethane is also obtained in the same way, but under laboratory conditions it is obtained from chloroacetic acid as a result of reactions (2-5). The key stage of these is reaction (3), which proceeds via the SN2 mechanism.

Chloroacetic acid Sodium chloroacetate

Nitroacetic acid

Nitromethane

Reactions of nitro compounds

Tautomerism of aliphatic nitro compounds

Due to the strong electron-withdrawing properties of the nitro group, -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 medium easily transforms 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.

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, hydrogen atoms are even more mobile than in aldehydes and ketones, and therefore they can enter with aldehydes and ketones in the addition and condensation reactions, providing their hydrogen atoms. Addition reactions usually take place with aliphatic aldehydes, and only condensation with aromatic aldehydes.

So, nitromethane is attached to cyclohexanone,

1-Nitromethylcyclohexanol

but condenses with benzaldehyde,

All three hydrogen atoms of nitromethane are involved in the addition reaction with formaldehyde, and 2-hydroxymethyl-2-nitro-1,3-dinitropropane or trimethylolnitromethane is formed.

By condensing nitromethane with hexamethylenetetramine, we obtained 7-nitro-1,3,5-triazaadamantane:

Control. 3. Write the reactions of formaldehyde (a) with nitromethane and (b) with nitrocyclohexane in an alkaline medium.

Recovery of nitro compounds

The nitro group is reduced to the amino group with various reducing agents (11.3.3). Aniline is obtained by hydrogenation of nitrobenzene under pressure in the presence of Raney nickel under industrial conditions

In laboratory conditions, instead of hydrogen, you can use hydrazine, which decomposes in the presence of Raney nickel with the release of hydrogen.

7-nitro-1,3,5-triazaadamantane 7-amino-1,3,5-triazaadamantane

Nitro compounds are reduced by metals in an acidic medium, followed by alkalinization

Various products can be obtained depending on the pH of the medium and the reducing agent used. In a neutral and alkaline medium, the activity of conventional reducing agents with respect to nitro compounds is less than in an acidic medium. 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 to phenylhydroxylamine:

In an acidic medium, arylhydroxylamines undergo rearrangement:

p-Aminophenol is used as a developer in photography. Phenylhydroxylamine can be further oxidized to nitrosobenzene:

Nitrosobenzene

Reduction of nitrobenzene with tin (II) chloride gives azobenzene, and zinc in an alkaline medium - hydrazobenzene.

By treating nitrobenzene with an alkali solution in methanol, azoxybenzene is obtained, while the methanol is oxidized to formic acid.

Known methods for incomplete reduction and nitroalkanes. 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 - starting material for the preparation of fiber - nylon:

The reduction of the nitro group of the products of aldol addition (7) is a convenient way to obtain α-amino alcohols.

1-Nitromethylcyclohexanol 1-Aminomethylcyclohexanol

The use of hydrogen sulphide as a reducing agent allows the reduction of one of the nitro groups in dinitroarenes:

m-Dinitrobenzene m-Nitroaniline

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 reactions:

Lecture number 40

NITRO COMPOUNDS

Nitro compounds are hydrocarbon derivatives in which one or more hydrogen atoms are replaced by a nitro group - NO 2.

Nitroalkanes are alkane derivatives in which one or more hydrogen atoms are replaced by a nitro group.

The general formula of mononitroalkanes is C n H 2n + 1 NO 2.

When forming the names of nitroalkanes, the longest hydrocarbon chain is selected, the numbering of which starts from the end to which the nitro group is located closer. The latter is indicated with the prefix "nitro". For instance:

Synthesis methods

1. Nitration of alkanes

Nitromethane is obtained from methane; nitration of methane homologues results in a mixture of nitroalkanes:

2. Alkylation of nitrites

R-Br + AgNO 2 ® R-NO 2 + AgBr

R-Br + NaNO 2 ® R-NO 2 + NaBr

Since nitrite anions are ambident in nature, aprotic non-polar solvents and moderate temperatures are used to obtain a high yield of nitroalkane.

Physical properties and structure

Nitroalkanes are colorless or yellowish liquids or crystalline substances with a faint odor.

Mononitroalkanes are characterized by large dipole moments. The reason for the significant polarity of nitroalkanes lies in the electronic structure of the nitro group, which contains a semipolar bond

The alignment of the N-O bonds is confirmed by X-ray structural analysis: the N-O bond in the nitro group is shorter communication n-o in hydroxylamine, but longer than the bond in the nitroso group –N \u003d O.

The high electronegativity of the N and O atoms, the multiplicity of the N \u003d O bond and its semipolar character determine the significant electron-withdrawing properties of the nitro group (-I and -M effects).

Nitroalkanes are characterized by weak absorption in the UV region of 270-280 nm. This is due to electronic transitions of the n ® p * type of the lone electron pair of the oxygen atom on the LUMO.

In the IR spectra, absorption maxima are observed associated with symmetric and antisymmetric vibrations of N \u003d O bonds in the regions of 1370 cm -1 and 1550 cm -1.

Chemical properties of nitroalkanes

Acidity and tautomeric transformations of nitroalkanes

Primary and secondary nitroalkanes are CH-acids .

The acidity is due to the stabilization of the carbanion formed due to the electron-withdrawing properties of the nitro group.

The acidity of mononitroalkanes in aqueous solutions is comparable to the acidity of phenols. If one carbon atom has two or three nitro groups, the acidity rises sharply.

The nitroalacane anion is ambident like the enolate anion. For example, when it is protonated, another tautomeric form, in addition to nitroalkane, can be formed.

The tautomeric form of nitroalkane is called aciform or nitronic acid, which has not been obtained in pure form. Nitronic acid is an OH-acid of medium strength (pKa \u003d 3.2).

Thus, nitro compounds should be considered as tautomers reacting in nitro and aci forms.

Under normal conditions, the concentration of the aci-form is negligible (10 -5 -10 -7%). Equilibrium shifts to the right in an alkaline environment due to the formation of salts.

Crystalline salts of alkali and alkaline earth metals are stable and readily soluble in water. They are sometimes called nitronic acid salts. Upon acidification of solutions, nitronic acid itself (aciform) is first formed, which is then isomerized to nitroalkane.

Nitro compounds belong to pseudoacids, which are characterized by the fact that they themselves are neutral, do not have electrical conductivity, but nevertheless form neutral salts of alkali and alkaline earth metals.

"Neutralization" of nitro compounds with bases occurs slowly, and true acids - instantly.

Among other reactions of nitroalkanes, we note the following.

Hydrolysis in an acidic medium with cleavage of C-N bonds.

This reaction is used in the art for the synthesis of hydroxylamine and its sulfate.

Substitution of H-atoms ata- C to halogens, nitrous acid residues, aldehydes, ketones, etc.

The reaction with HNO 2 is qualitative for nitroalkanes. Tertiary nitroalkanes do not react, secondary R 2 CH-NO 2 form nitrosonitroalkanes

Primary form with HNO 2 nitrooximes (nitrolic acids)

These colorless compounds form blood-red salts of nitrolic acids with alkalis.

Aromatic nitro compounds

1. Methods of obtaining

1. Arenas nitration

This is the main method for producing nitroarenes; considered in detail in the study of electrophilic aromatic substitution (see lek. No. 18).

1. Oxidation of arylamines

The method consists in the oxidation of primary aromatic amines with peroxy compounds. The most effective oxidation reagent is trifluoroperoxyacetic acid in methylene chloride. Trifluoroperoxyacetic acid is obtained directly in the reaction mixture by the interaction of trifluoroacetic acid anhydride and 90% hydrogen peroxide. This method is important for the synthesis of nitro compounds containing in ortho- and couple- positions to the nitro group, other electron-withdrawing groups, for example:

2. Physical properties and structure

Nitroarenes are yellow substances with a peculiar odor. Nitrobenzene is a liquid with a bitter almond smell. Di- and polynitroarenes are crystalline substances.

The nitro group is a strong electron acceptor; therefore, nitroarenes have large dipole moments directed towards the nitro group.

Polynitroarene molecules are strong electron acceptors. For example, the electron affinity of 1,3-dinitrobenzene is 1.35 eV, and that of 1,3,5-trinitrobenzene is 1.75 eV.

3. Chemical properties

Recovery of nitro group

The amino group is the product of the exhaustive reduction of the nitro group in nitroarenes. At present, catalytic hydrogenation is used to reduce nitroarenes to arylamines under industrial conditions. Copper on silica gel is used as a catalyst as a carrier. The yield of aniline over this catalyst is 98%.

Under laboratory conditions, metals in an acidic or alkaline environment are used to reduce the nitro group. Recovery occurs in several stages, the sequence of which is very different in acidic and alkaline media.

When recovering in an acidic environment the process proceeds stepwise and includes the following stages.

In an acidic medium, each of the intermediate products is rapidly reduced to the final product of aniline, and they cannot be isolated individually. Iron, tin or zinc and hydrochloric acid are used as reducing agents. An effective reducing agent for the nitro group is tin (II) chloride in hydrochloric acid. The end product of acid reduction is an amine, for example:

C 6 H 5 NO 2 + 3Zn + 7HCl® C 6 H 5 NH 2 HCl + 3ZnCl 2 + 2H 2 O

In a neutral solution, For example, in the reduction of nitroarenes with zinc in an aqueous solution of ammonium chloride, the reduction process slows down and stops at the stage of formation of arylhydroxylamine.

When recovering in an alkaline environment in excess of reducing agent end product reduction of nitroarene is hydrazoarene (diarylhydrazine)

The process can be represented as the following sequence of transformations.

azoxyarene

azoarene g

hydrazoarene

In an alkaline medium, the reduction of nitrosoarene and hydroxylamine slows down so much that the main process becomes their condensation with the formation of azoxyarene. This reaction is essentially similar to the addition of nitrogenous bases to the carbonyl group of aldehydes and ketones.

Azoxybenzene under the action of zinc in an alcoholic alkali solution is reduced first to azobenzene, and under the action of excess zinc further to hydrazobenzene.

Azoxybenzene itself can be obtained by reducing nitrobenzene with sodium methoxide in methanol.

Sulfides of alkali metals and ammonium are also used as reducing agents for nitroarenes.

4ArNO 2 + 6Na 2 S + 7H 2 O® 4ArNH 2 + 3Na 2 S 2 O 3 + 6NaOH

As follows from the stoichiometric equation, in the process of reduction with sulfide, the alkalinity of the medium increases, which leads to the formation of azoxy and azo compounds as by-products. In order to avoid this, hydrosulfides and polysulfides should be used as reducing agents, since in this case no alkali is formed.

ArNO 2 + Na 2 S 2 + H 2 O® ArNH 2 + Na 2 S 2 O 3

The rate of the reduction of the nitro group with sulfides strongly depends on the electronic effects of substituents in the aromatic ring. Thus, m-dinitrobenzene is reduced by sodium disulfide 1000 times faster than m-nitroaniline. This is used for partial recovery nitro groups in polynitro compounds.

Products of incomplete reduction of the nitro group

Nitrosoarenes

Nitrosoarenes are easily reduced; therefore, they are difficult to obtain by reduction of nitroarenes. The best method for the preparation of nitrosoarenes is the oxidation of arylhydrazines.

It is possible to directly introduce the nitroso group into the aromatic ring by the action of nitrous acid on phenols and tertiary arylamines (see lectures # 29 and 42)

In the crystalline state, aromatic nitroso compounds exist in the form of colorless dimers. In liquid and gaseous state there is an equilibrium between dimer and monomer. Monomers are colored green.

Nitroso compounds, like carbonyl compounds, react with nucleophiles. For example, upon condensation with arylhydroxylamines, azoxy compounds are formed (see above), and with arylamines, azo compounds are formed.

Arylhydroxylamines

In addition to the method described above, by reduction of nitroarenes in a neutral medium, arylhydroxylamines are synthesized by nucleophilic substitution in activated arenes.

As intermediate products of the reduction of nitroarenes, arylhydroxylamines can be oxidized to nitroso compounds (see above) and reduced to amines by catalytic hydrogenation or the action of a metal in an acidic medium.

ArNHOH + Zn + 3HCl ® ArNH 2 . HCl + ZnCl 2 + H 2 O

In an acidic medium, arylhydroxylamines rearrange aminophenols, which is used to obtain the latter, for example:

Azoxyarenes

In addition to the methods described above - condensation of nitroso compounds with arylhydroxylamines and reduction of nitroarenes with sodium methoxide, azoxyarenes can be obtained by oxidation of azoarenes with peroxy compounds.

In an alkaline medium, azoxyarenes are reduced to azo- and then hydrazoarenes (see above).

Azoarenes

Formed upon reduction of nitroarenes, arylhydrazines and azoxyarenes in an alkaline medium, for example:

Unsymmetrical azo compounds are obtained by condensation of nitroso compounds with amines (see above). An important method for the synthesis of azo compounds - the azo coupling reaction will be discussed in detail below (see lek. No. 43)

Azoarenes exist in the form cis- and trance- isomers. More stable under irradiation trance-isomer is converted to cis-isomer. The reverse transformation occurs when heated.

Azo compounds are colored; many of them are used as dyes.

Hydrazoarenes

These are the end products of nitroarene reduction in an alkaline environment. Hydrazoarenes are colorless crystalline substances; they oxidize in air to colored azo compounds. For preparative purposes, oxidation is carried out by the action of bromine water

Ar-NHN-HAr + Br 2 + 2NaOH ® Ar-N \u003d N-Ar + 2NaBr + 2H 2 O

Upon reduction under severe conditions, hydrazoarenes give arylamines.

An important property of hydrazo compounds is their rearrangement into 4,4 / -diaminobiphenyls. This transformation was named benzidine rearrangement. Currently, this term is used to unite a whole group of related rearrangements leading to the formation of a mixture ortho- and couple-isomeric derivatives of diaminobiphenyl.

When hydrazobenzene itself is rearranged, a mixture of diamines is obtained containing 70% benzidine and 30% 2,4 / -diaminobiphenyl.

If couple- the position in one of the benzene nuclei of hydrazobenzene is occupied by some substituent, the product of the rearrangement is the diphenylamine derivative (the so-called semidine rearrangement).

When studying the mechanism of benzidine and related rearrangements, it was found that they occur intramolecularly. If two different hydrazobenzenes are rearranged together, there are no cross rearrangement products. For the rearrangement of hydrazobenzene itself, it was found that the reaction rate is proportional to the concentration of hydrazobenzene and the square of the proton concentration. This means that the diprotonated form of hydrazobenzene undergoes rearrangement. It was also shown that the monoprotonated form of hydrazobenzene is converted entirely to benzidine only upon repeated acid treatment. These data are consistent with the following mechanism of benzidine rearrangement.

It is assumed that the transition state is formed from such a conformation of hydrazobenzene, in which two corresponding carbon atoms of both benzene rings are very close to each other. The formation of a new carbon-carbon bond and the breaking of the old bond of two nitrogen atoms occur strictly synchronously. According to modern terminology, the benzidine rearrangement belongs to the group of sigmatropic rearrangements.

Nitration of aromatic compounds is the main way to obtain nitro compounds. The nitration process as a special case of electrophilic substitution in the aromatic series has already been considered earlier. Therefore, it seems appropriate to focus on the synthetic possibilities of this reaction.

Benzene itself is nitrated quite easily and with good results

Under more severe conditions, nitrobenzene is also capable of nitrating to form m-dinitrobenzene

Due to the deactivating effect of two nitro groups, introduce a third nitro group into m-dinitrobenzene is possible only with great difficulty. 1,3,5-Trinitrobenzene was obtained in 45% yield as a result of nitration m-dinitrobenzene at 100-110 about C and the reaction time of 5 days.

Difficulties in obtaining trinitrobenzene by direct nitration of benzene led to the development of indirect methods. According to one of them, trinitrotoluene, more accessible than trinitrobenzene, is oxidized to 2,4,6-trinitrobenzoic acid, which is easily decarboxylated when heated in water

In the same way, one has to resort to indirect methods, if necessary, to obtain 1,2-dinitrobenzene. In this case, the ability of the amino group to oxidize to the nitro group in about-nitroaniline

Even in those cases when the preparation of nitro compounds by nitration should not have encountered special difficulties, one has to turn to indirect methods. So, it is not possible to obtain picric acid by nitration of phenol, because phenol is not nitrated with nitric acid, but oxidized. Therefore, the following scheme is usually used

The subtleties of this scheme are that due to the deactivation of the ring with chlorine and two already existing nitro groups, it is not possible to introduce a third nitro group into it. Therefore, chlorine in dinitrochlorobenzene is preliminarily replaced by hydroxyl, which nitro groups contribute to (bimolecular substitution). The resulting dinitrophenol easily accepts one more nitro group without being oxidized to an appreciable degree. The available nitro groups protect the benzene ring from oxidation.

Another unobvious way to obtain picric acid is sulfonation of phenol to 2,4-phenol disulfonic acid, followed by nitration of the resulting compound. In this case, simultaneously with nitration, the sulfo groups are replaced by nitro groups

One of the most important aromatic nitro derivatives, trinitrotoluene, is obtained in technology by nitration of toluene, which proceeds according to the following scheme

Chemical properties

Aromatic nitro compounds can react with the participation of both the benzene ring and the nitro group. These structural elements affect the reactivity of each other. So, under the influence of the nitro group, nitrobenzene enters into the reaction of electrophilic substitution reluctantly and the new substituent takes m-position. The nitro group affects not only the reactivity of the benzene ring, but also the behavior of adjacent functional groups in chemical reactions.

Let us consider the reactions of aromatic nitro compounds due to the nitro group.

16.2.1. Recovery.One of the most important reactions of nitro compounds is their reduction to aromatic amines, which are widely used in the production of dyes, drugs, and photochemicals.

The possibility of converting a nitro group into an amino group by reducing nitro compounds was first shown by Zinin in 1842 using the example of the reaction of nitrobenzene with ammonium sulfide

Subsequently, the reduction of aromatic nitro compounds was the subject of deep study. It was found that, in the general case, the reduction is complex and proceeds through a number of stages with the formation of intermediate products. Amines are only the end product of the reaction. The recovery result is determined by the strength of the reducing agent and the pH environment. In electrochemical reduction, the composition of the products depends on the value of the potential at the electrodes. By varying these factors, it is possible to delay the recovery process at intermediate stages. In neutral and acidic media, the reduction of nitrobenzene proceeds sequentially through the formation of nitrosobenzene and phenylhydroxylamine

When the reduction is carried out in an alkaline medium, the formed nitrosobenzene and phenylhydroxylamine are able to condense with each other to form azoxybenzene, in which the nitrogen and oxygen atoms are linked by a semipolar bond

The putative condensation mechanism resembles the aldol condensation mechanism

Reduction of azoxybenzene to aniline goes through azo- and hydrazobenzenes

All of the above intermediate products for the reduction of nitrobenzene to aniline can be obtained either directly from nitrobenzene or starting from each other. Here are some examples

16.2.2. Influence of the nitro group on the reactivity of other functional groups.In the study of aromatic halogen derivatives, we have already encountered a case when a suitably located nitro group (nitro groups) significantly affected the nucleophilic substitution of a halogen (bimolecular substitution of an aromatically bound halogen). For example about- and p-dinitrobenzenes, it was found that the nitro group can contribute to the nucleophilic substitution of not only halogen, but even another nitro group

The mechanism of bimolecular substitution of a nitro group for a hydroxyl group can be represented as the following two-stage process

The carbanion formed in the first stage of the reaction under consideration is resonantly stabilized due to the contribution of the limiting structure 1, in which the nitro group draws electrons from the very carbon of the benzene ring, which has an excess of them.

A feature of the nucleophilic substitution of one nitro group under the influence of another nitro group is that the reaction is very sensitive to the arrangement of nitro groups relative to each other. It is known that m-dinitrobenzene does not react with an alcoholic ammonia solution even at 250 o C.

Other examples of facilitating the substitution of a nitro group, in this case hydroxyl, are picric acid conversions

16.2.3. Complexation with aromatic hydrocarbons.A characteristic property of aromatic nitro compounds is their tendency to form complexes with aromatic hydrocarbons. The bonds in such complexes are electrostatic in nature and arise between electron-donor and electron-acceptor particles. The complexes under consideration are called π -complexes or charge transfer complexes.

π –Complexes in most cases are crystalline substances with characteristic melting points. If necessary π - the complex can be destroyed with the release of hydrocarbon. Due to the combination of these properties π -complexes are used for the isolation, purification and identification of aromatic hydrocarbons. Picric acid is especially often used for complexation, the complexes of which are incorrectly called picrates.

Chapter 17

Amines

Primary, secondary and tertiary amines are distinguished according to the degree of substitution of hydrogen atoms in ammonia for alkyl and aryl substituents. Depending on the nature of the substituents, amines can be fatty aromatic or purely aromatic.

Aromatic amines are named by adding the ending "amine" to the names of groups associated with nitrogen. In difficult cases, an amino group with a smaller substituent is designated with the prefix "amino" (N-methylamino, N, N-dimethylamino), which is added to the name of a more complex substituent. Below are the most common amines and their names

Receiving methods

We have already encountered many of the methods for obtaining amines in the study of aliphatic amines. When these methods are applied to the synthesis of aromatic amines, some peculiarities are encountered; therefore, without fear of repetitions, we will consider them.

17.1.1. Recovery of nitro compounds.The reduction of nitro compounds is the main method of both laboratory and industrial production of amines, which can be carried out in several ways. These include catalytic hydrogenation, atomic hydrogen reduction, and chemical reduction.

Catalytic reduction is carried out with molecular hydrogen in the presence of finely ground nickel or platinum, copper complex compounds on supports. When choosing a catalyst and reduction conditions, it should be borne in mind that other functional groups can also be reduced. In addition, the catalytic reduction of nitro compounds must be carried out with some caution due to the extreme exothermicity of the reaction.

When ammonium sulfide is used as a chemical reducing agent, it becomes possible to restore only one of several nitro groups

17.1.2. Amination of halogenated derivatives.Difficulties are known that arise during the amination of aromatic halogen derivatives by the "elimination - addition" mechanism. However, as already mentioned more than once, the electron-withdrawing substituents in the benzene ring, arranged in the right order, greatly facilitate the substitution of halogen in aryl halides, directing the process according to the bimolecular mechanism. For comparison, below are the conditions for the amination of chlorobenzene and dinitrochlorobenzene

17.1.3. Splitting according to Hoffmann.The Hoffmann cleavage of acid amides allows one to obtain primary amines, which contain one carbon less than the starting amides

The reaction proceeds with the migration of phenyl from the carbonyl carbon to the nitrogen atom (1,2-phenyl shift) according to the following proposed mechanism

17.1.4. Alkylation and arylation of amines.Alkylation of primary and secondary aromatic amines with haloalkyl or alcohols allows obtaining secondary and tertiary fatty aromatic amines

Unfortunately, when the primary amines participate in the reaction, a mixture is obtained. This can be avoided if the starting amine is pre-acylated and only then alkylated

This method of protecting the amino group allows one to obtain pure secondary aromatic amines, as well as tertiary amines with various substituent radicals.

Arylation of amines makes it possible to obtain pure secondary and tertiary aromatic amines

Chemical properties

Aromatic amines react with both the amino group and the benzene ring. Moreover, each functional group is influenced by another group.

Amino group reactions

Due to the presence of an amino group, aromatic amines undergo numerous reactions. Some of them have already been considered: alkylation, acylation, reaction with aldehydes to form azomethines. The other reactions to which attention will be paid are easily predictable, but they have certain peculiarities.

Basicity

The presence of a lone pair of electrons at the nitrogen atom, which can be presented for the formation of a bond with a proton, provides aromatic amines with basic properties

It is of interest to compare the basicity of aliphatic and aromatic amines. As already shown in the study of aliphatic amines, it is convenient to judge the basicity of amines by the basicity constant To in

Let's compare the basicity of aniline, methylamine and ammonia

Ammonia 1.7. 10 -5

Methylamine 4.4. 10 -4

Aniline 7.1. 10 -10

It can be seen from these data that the appearance of an electron-donating methyl group increases the electron density at the nitrogen atom and leads to an increase in the basicity of methylamine as compared to ammonia. At the same time, the phenyl group weakens the basicity of aniline by more than 10 5 times as compared to ammonia.

The decrease in the basicity of aniline in comparison with aliphatic amines and ammonia can be explained by the conjugation of the lone pair of nitrogen electrons with the sextet of electrons of the benzene ring

This reduces the ability of a lone pair of electrons to attach a proton. This tendency is even more pronounced for aromatic amines, which contain electron-withdrawing substituents in the benzene ring.

So, m-nitroaniline as a base is 90 times weaker than aniline.

As might be expected, electron-donating substituents on the benzene ring enhance the basicity of aromatic amines

Fatty aromatic amines under the influence of the alkyl group exhibit greater basicity than aniline and amines with electron-withdrawing groups in the ring.