The most important methods for the synthesis of alkanes:
1.2.1. Alkenes hydrogenation
Alkenes are hydrogenated under hydrogen pressure in the presence of a catalyst (Pt, Pd or Ni).
1.2.2. Hydrogenation or reduction of alkyl halides
During catalytic hydrogenation in the presence of palladium, alkyl halides are converted to alkanes.
Alkanes are also obtained by reduction of alkyl halides with a metal in acid.
Iodalkanes can be easily reduced in a sealed ampoule with hydroiodic acid.
1.2.3. Reaction of haloalkanes with sodium (Wurtz reaction)
The Wurtz reaction makes it possible to obtain a hydrocarbon with a greater number of carbon atoms than in the starting compound.
It is suitable for the synthesis of only symmetric alkanes using primary (!) alkyl halides. In the case of using various haloalkanes as starting compounds, a mixture of three hydrocarbons is obtained as a result of the reaction:
This mixture has to be separated, which is not always possible.
Instead of sodium, other metals can be used in this reaction, for example, magnesium, zinc, lithium.
1.2.4. Kolbe synthesis - electrolysis of sodium or potassium salts of carboxylic acids
The electrolysis of sodium or potassium salts of carboxylic acids also leads to the production of symmetric hydrocarbons.
2. Alkenes
Open-chain hydrocarbons of the composition C n H 2n containing one double bond are called alkenes ... The simplest hydrocarbon of this series is ethylene CH 2 = CH 2. The carbon atom in ethylene is in the sp 2 -hybrid state (trigonal carbon). Due to three hybridized orbitals, each carbon atom forms three -bonds: one - with an adjacent carbon atom, two - with two hydrogen atoms. Side overlap two 2p-orbitals of carbon atoms gives -connection and makes it impossible to rotate around -bonds carbon-carbon. This is due to the phenomenon geometric isomerism.
Geometric isomers (the composition and method of bonding atoms are the same, the arrangement of groups and atoms in space is different). These isomers are named E, Z -nomenclature. In this case, it is possible to use classic cis- and trance- designations to determine the spatial location of identical or similar groups relative to the comparison plane.
The relative precedence of substituents at each carbon atom with a double bond is determined by the atomic number: H (atomic number - 1) - junior, C (atomic number - 6) - senior substituent; if the atoms at carbon with a double bond are the same, then the precedence of the subsequent atoms is considered: - CH 3 (subsequent atoms - H, H, H) - the junior substituent; -СН (СН 3) 2 (subsequent atoms - Н, С, С) - senior deputy.
Electrolysis of aqueous solutions of carboxylic acid salts (anodic synthesis) leads to the formation of alkanes:
The first stage of the process is the anodic oxidation of acid anions to radicals:
Hydrogen and hydroxide of the corresponding metal are formed at the cathode. The Kolbe reaction is applicable to obtain both unbranched and branched alkanes.
Exercise 2. Write down the reaction equations for the production by the Kolbe method: (a) 2,5-dimethylhexane and (b) 3,4-dimethylhexane.
Recovery of alkyl halides
A convenient way to obtain alkanes is the reduction of alkyl halides with zinc in aqueous solutions of acids:
Common reagents such as lithium aluminum hydride, sodium borohydride, sodium or lithium in tert- butyl alcohol , and catalytic reduction with hydrogen. Alkyl iodides can also be reduced by heating with hydroiodic acid.
Decarboxylation of carboxylic acids (Dumas)
When carboxylic acids are heated with alkalis, alkanes are formed with the number of carbon atoms one less than that of the original acid:
This reaction can be used to obtain only lower alkanes, since in the case of using higher carboxylic acids, a large number of by-products are formed.
Alkane reactions
Compared to other classes of organic compounds, alkanes are little reactive. The chemical inertness of alkanes explains their name “paraffins”. The reason for the chemical stability of alkanes is the high strength of the non-polar σ-bonds C-C and C-H. In addition, the C – C and C – H bonds are characterized by very low polarizability.
Because of this, bonds in alkanes do not exhibit a tendency to heterolytic rupture. Alkanes are not affected by concentrated acids and alkalis and they are not oxidized even by strong oxidants. At the same time, nonpolar bonds of alkanes are capable of homolytic decomposition.
Despite the fact that the C-C bond is less strong than the C-H bond (the C-C bond energy is about 88 kcal / mol, and the C-H bond is 98 kcal / mol), the latter breaks more easily, since it is on the surface of the molecule and is more accessible for attack by the reagent.
Chemical transformations of alkanes usually take place as a result of homolytic cleavage of the CH bond with the subsequent replacement of hydrogen atoms by other atoms. Thus, substitution reactions are characteristic of alkanes.
Halogenation
Methane, ethane and other alkanes react with fluorine, chlorine and bromine, but practically do not react with iodine. The reaction between an alkane and a halogen is called halogenation.
A. Chlorination of methane
Chlorination of methane is of practical importance. The reaction is carried out under the influence of illumination or when heated to 300 o C.
Let us consider the mechanism of this reaction using the example of the formation of methyl chloride. The mechanism means a detailed description of the process of converting reagents into products. It was found that the chlorination of methane proceeds according to the radical chain mechanism S R.
When exposed to light or heat, the chlorine molecule breaks down into two chlorine atoms - two free radicals.
The chlorine radical, interacting with a methane molecule, removes a hydrogen atom from the latter to form an HCl molecule and a free methyl radical:
CH 4 + Cl. ® CH 3. + HCl chain extension
CH 3. + Cl-Cl® CH 3 -Cl + Cl. chain continuation
The chlorine atom will further react with the methane molecule, etc. Theoretically, a single chlorine atom can cause the chlorination of an infinite number of methane molecules, and therefore the process is called a chain process. Chains can be broken when radicals interact with each other:
CH 3. + Cl. ® CH 3 -Cl
CH 3. + CH 3. ® CH 3 -CH 3 Open circuit
Cl. + Cl. ® Cl-Cl
or with the vessel wall
Formally, a free methyl radical has a tetrahedral structure:
However, due to the small value inversion barrier(transition of one form of a molecule to another), its statistically most probable state is flat.
As a result of the methane chlorination reaction, a mixture of all four possible products of the replacement of hydrogen atoms with chlorine atoms is formed:
The ratio between the various chlorination products depends on the ratio of methane to chlorine. If it is necessary to obtain methyl chloride, an excess of methane should be taken, and an excess of carbon tetrachloride - chlorine.
Carboxylation of phenolates according to the Kolbe - Schmidt reaction makes it possible to obtain ortho-hydroxyaromatic carboxylic acids from sodium phenolates. The Kolbe - Schmidt reaction occurs with the participation of carbon dioxide $ CO_2 $:
Picture 1.
Features of the Kolbe - Schmidt reaction
An original technique for introducing carboxyl groups into the aromatic system was discovered by G. Kolbe in 1860. When dry alkaline phenolate is heated with carbon dioxide at temperatures above 150 $ ^ \ circ $ C and a pressure of about 5 atm, an alkaline salt of salicylic acid is formed:
Figure 2.
With the participation of phenolates of potassium, rubidium and cesium, a similar reaction proceeds with the formation of predominantly para-substituted hydroxyaromatic acids.
Figure 3.
It is not phenols that are introduced into the reaction, but phenolates that are active for electrophilic substitution, because carbon dioxide is a very weak electrophile. This is due to the formation of an intermediate complex of sodium phenolate and carbon dioxide, in which the sodium atom is coordinated with two oxygen atoms, one of which is included in the $ CO_2 $ molecule. Due to a certain polarization, the carbon atom acquires a greater positive charge and a convenient location for attacking the opto position of the phenolic ring.
Figure 4.
Application of the Kolbe - Schmidt reaction
Rearrangement of monosalicylates and alkaline salts of 2-naphthol
Anhydrous potassium and rubidium monosalicylates when heated above 200-220 $ ^ \ circ $ С give dipotassium and dirubidium salts pair-hydroxybenzoic acid and phenol.
Figure 7.
Alkaline salts of potassium and cesium of 2-hydroxybenzoic (salicylic) acid are rearranged to form di-alkali salts 4 -hydroxybenzoic acid:
Figure 8.
Alkaline sodium and lithium salts pair-hydroxybenzoic acid, on the contrary, when heated, are rearranged into the di-alkali salt of salicylic acid:
Figure 9.
It follows from this that the carboxylation of alkali phenolates is a reversible reaction and their direction depends only on the nature of the cation. Similar patterns are observed during the corboxylation of alkaline salts of 2-naphthol:
Figure 10.
In contrast to monohydric phenols, diatomic and triatomic phenols are carboxylated under milder conditions. Thus, resorcinol is carboxylated when $ CO_2 $ is passed into an aqueous solution of its dipotassium salt at 50 $ ^ \ circ $ C with the formation of 2,4-dihydroxybenzoic acid.
Figure 11.
Reimer - Timan reaction
Phenols and some heterocyclic compounds such as pyrrole and indole can be formalized with chloroform under basic conditions (Reimer - Timan reaction). The entry of the aldehyde group is oriented to the ortho-position, and only in the case when both of them are occupied, para-substituted derivatives are formed.
Figure 12.
It is known that chloroform in the presence of strong bases forms dichlorocarbene $: CCl_2 $, which is a real electrophilic particle.
Figure 13.
This is confirmed by the formation of ring expansion products characteristic of the action of $: CCl_2 $, namely, pyridine in the reaction with pyrrole, and the isolation of products of addition of dichlorocarbene to aromatic rings in the ipso position, as this is observed in the formylation reaction of para-cresol. In the latter case, methyl groups cannot be cleaved off, like a proton, under the action of an electrophile, and stabilization occurs through the migration of a proton to the dichloromethyl group.
Figure 14.
Or Kolbe process(named after Adolf Wilhelm Hermann Kolbe and Rudolf Schmitt) is a chemical reaction of sodium phenolate carboxylation by the action of carbon dioxide under severe conditions (pressure 100 atm., temperature 125 ° C) followed by treatment of the product with acid. In industry, this reaction is used to synthesize salicylic acid, which is a precursor of aspirin, as well as β-hydroxy naphthoic and other acids. A review article was devoted to the Kolbe - Schmitt reaction and its application.
Reaction mechanism
The key stage in the Kolbe - Schmitt reaction mechanism is the nucleophilic addition of the phenolate ion to carbon dioxide, which leads to the formation of the corresponding salicylate.
The direction of the reaction depends on which phenolate is used as the starting compound. When sodium phenolate is introduced into the reaction, ortho-substituted product. This is because the sodium ion is able to stabilize the six-membered transition state, from which the electrophilic attack of the aromatic ring of phenol occurs. When potassium phenolate is used, the formation of a six-membered transition complex is less favorable, and therefore pair-substituted product.
The reaction is facilitated by the presence of electron donor substituents, for example, polyatomic phenols (phloroglucinol, resorcinol, pyrocatechol) are carboxylated in an aqueous solution of potassium carbonate.
An industrial version of the Kolbe-Schmitt reaction, used for the synthesis of salicylic acid and its derivatives (p-amino, 5-chlorosalicylic acid, etc.) is the Marassé modification - carboxylation of a mixture of phenol and potassium carbonate with carbon dioxide at 170 ° C and a pressure of 9-13 MPa.
Organic synthesisMechanisms of chemical processes
Name reactions
Kolbe synthesisWürz reaction
Kucherov's reaction
Lebedev's reaction
Konovalov's reaction
Zaitsev's rule
Markovnikov's rule
Wöhler reaction
Dumas reaction
Wagner reaction
Berthelot reaction
Diels - Alder reaction
Zelinsky - Kazansky reaction
Wöhler reaction
Friedrich Wöhler,1800 - 1882
Oxalic acid synthesis
in the hydrolysis of cyanogen in
acidic environment, 1824
Synthesis of urea from
carbon dioxide and ammonia
at high temperatures and
pressure, 1828
Obtaining acetylene at
hydrolysis of calcium carbide
(obtained by fusion
coke and lime), 1829
Wöhler reactions
Hydrolysis of cyanogen to form oxalic acidacid, 1824
Wöhler reactions
Synthesis of urea from carbon dioxide and ammonia,1828 g.
“I can’t be silent any longer,” Wöhler writes to his
teacher, J. Ya. Berzelius, - and must inform
You that I can get urea without the help of the kidneys
dogs, humans and generally without the participation of any
living creature ... "
T0
CO2 + 2NH3 → H2O +
Wöhler reactions
Obtaining acetylene by hydrolysis of carbidecalcium, 1862
In 1892 Moissan (France) and Wilson (Canada)
proposed the design of an electric arc furnace,
suitable for industrial use:
obtaining calcium carbide by fusion
burnt lime and coal
Or CaCO3 → CaO + CO2; CaO + 3C → CaC2 + CO
Dumas reaction
Fusion of carboxylic acid saltswith alkalis:
0
CaO, T
Н3С-СООNa + NaOH → CH4 + Na2CO3
Decarboxylation of carboxylic acid salts (- CO2)
French chemist.
Member of the French Academy of Sciences
(1832)
Member of the Paris Academy
medicine (1843)
President of the Academy of Sciences (1843)
He was also involved in state
activities. In 1850-1851 the Minister of Agriculture and
trade in government
Jean Baptiste André Dumas,
Napoleon Bonoparte.
1800 - 1884
Wagner reaction
Mild oxidation of alkenesaqueous solution
potassium permanganate with
formation of a diatomic
alcohol
Egor Egorovich Wagner,
1849 - 1903
Konovalov's reaction
Mikhail IvanovichKonovalov,
1858 - 1906
Nitration of hydrocarbons
diluted НNО3 at
increased or
normal pressure (by
free radical
mechanism).
Doctoral dissertation
"Nitrating action
weak nitrous acid on
saturated hydrocarbons
character "(1893)
10. Berthelot reaction
Ethanol synthesis by ethylene hydration:French chemist.
Member of the Paris Academy of Sciences
(1873)
Corresponding member
Petersburg Academy of Sciences (since
1876)
In 1895-1896. Berthelot was
foreign minister
France.
Marcelin Berthelot,
1827-1907
11. Rules of A.M. Zaitsev (1875), V.V. Markovnikov (1869)
AlexanderMikhailovich Zaitsev,
1841-1910
Vladimir Vasilievich
Markovnikov,
1837-1904
12. Rules of A.M. Zaitsev (1875), V.V. Markovnikov (1869)
When protic acids or water are attached toasymmetric unsaturated hydrocarbons
the hydrogen proton joins the most
hydrogenated carbon atom
(product formation proceeds through the most
stable carbocation) - rule
Markovnikov. Repeat. excl. from the rule.
When split off, a hydrogen proton is split off
from the least hydrogenated atom
carbon - Zaitsev's rule.
13. Exercises according to the rules of Zaitsev and Markovnikov
Of which halogenated hydrocarbonsunder the action of an alcohol solution
potassium hydroxide can be obtained:
1) 2-methylpentene-1
2) 3-methylpentene-2
3) 4-methyl-3-ethylpentene-2
4) 3-ethylhexene-2?
14. Würz reaction, 1865
Synthesis of symmetric alkanesfrom alkyl halides to
reactions with sodium (even easier
with potassium)
Charles Adolph Würz,
1817- 1884
President of the Paris
academies of sciences
15. Kolbe synthesis, 1849
Electrolysis of aqueous solutionspotassium and sodium salts
carboxylic acids.
Adolph Wilhelm
Herman Kolbe,
1818-1884, Germany
16. Grignard's reagent, 1912
Organomagnesium chemicalconnections like
magnesium methyl iodide CH3MgI
magnesium benzene bromide C6H5MgBr.
Victor Grignard,
1871-1935, France
Nobel laureate
chemistry awards
17. Diels - Alder reaction
Diene synthesis - reaction, cycloadditiondienophiles and conjugated dienes to form
six-membered cycle:
18. The Diels - Alder reaction
Kurt Albert, Germany1902 - 1958
Otto Paul Hermann Diels,
Germany, 1876 - 1954
In 1950 they were awarded for diene synthesis
Nobel Prize in Chemistry
19. Reaction of Zelinsky - Kazansky
ɳ = 70%20. Reaction of Zelinsky - Kazansky
Graduated from NovorossiyskUniversity in Odessa (1884)
Professor of Moscow
university (1911-1917)
Organized the institute
organic chemistry of the USSR Academy of Sciences
(1935), since 1953 the institute has worn it
name
Created the first coal
gas mask (1915) taken on
Nikolay Dmitrievich
armament during the First
Zelinsky,
world war in Russian and
Russian empire,
allied armies.
1861 - 1953
21. Coal gas masks
Soldiers of the Czech Legion of the Russian Army ingas masks Zelinsky-Kummant
22. Reaction of Zelinsky - Kazansky
Boris AlexandrovichKazansky,
1891 - 1973
Graduated from Moscow University
(1919)
Worked in Moscow
university led by
N. D. Zelinsky
Taught at Moscow
university workshop on
general chemistry, quality and
quantitative analysis, and
later in organic chemistry,
petroleum chemistry, organic
catalysis
Academician of the USSR Academy of Sciences
23. Kucherov's reaction
Hydration of alkynes inpresence of Hg2 + salts in
acidic environment.
Mikhail Grigorievich
Kucherov,
1850 - 1911
24. Lebedev's reaction
Lebedev proposed a one-step methodobtaining butadiene from ethyl alcohol
(catalysts: ZnO, Al2O3; T 400-5000C)
2CH3CH2OH
Sergei
Vasilevich
Lebedev,
1874-1934.
2H2O + CH2 = CH-CH = CH2 + H2
Thanks to the works of Lebedev
industrial production
synthetic rubber started in the Soviet
Union in 1932 - for the first time in the world.
25. Reagents
Grignard reagentTollens' reagent OH
Ammonia solution of copper chloride (I)
[Cu (NH3) 2] Cl
26. Catalysts
Na catalyst in liquid ammoniaLindlar catalyst
Na to NH3
Pd // Pb2 +
An acidic solution of copper (I) chloride in ammonium chloride
NH4Cl, CuCl
Ziegler - Natta
See which reactions are used for (workbook)
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