Alkanes preparation for the exam in chemistry. Properties of hydrocarbons. Obtaining hydrocarbons. VI. Incomplete hydrogenation of dienes and alkynes
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1.Which hydrocarbon molecule can have a branched carbon chain?
1) methane CH 4 2) ethane C 2 N 6 3) propane C 3 N 8 4) butane C 4 N 10
2.Structural formula of propane C 3 N 8
1)CH 2 -SN 3 -SN 3 2)CH 3 -SN 2 -SN 3 3) CH 3 -CH-CH 4 4) CH 2 -SN 4 -SN 2
3. Main part of natural gas
1) methane 2) ethane 3) propane 4) butane
4.The methane molecule has a carbon-carbon bond
1) simple 2) double 3) triple 4) no such connection
5.Formula of methane homolog 1) C 3 N 6 2) C 4 N 8 3) C 6 N 12 4) C 5 N 12
6.Indicate the correct judgment
A. alkanes are characterized by addition reactions
B. alkanes are characterized by substitution reactions
7.Isomers are different
1) qualitative composition 2) quantitative composition 3) structure 4) homologous difference
8. An isomer of heptane is
1) 2,3-dimethylheptane 2) 2,3-dimethylpentane 3) 2,3-dimethylbutane 4) 2,3-dimethylhexane
9.Nitroethane formula
1) CH 3 -SN 2 - ONO 2 2) CH 3 -SN 2 - NO 2
3) CH 3 -SN 2 - NN 2 4) CH 3 -SN 2 -SN 2 -SN 2 - NO 2
10. What substance is missing from natural gas?
1) methane 2) ethane 3) pentane 4) butane
11. Determine which of the listed compounds has the maximum oxidation state of the carbon atom?
1)CH 2 O 2) CH 3 OH 3)SSl 4 4) C 2 N 4
12.Each carbon atom in an ethane molecule forms
1) two σ-bonds and two π-bonds 2) three σ-bonds and one π bond
3) four σ bonds 4) one σ and three bonds
13.methane is the main component
1) oil 2) natural gas 3) synthesis gas 4) coke oven gas
14.Indicate the correct judgment
A. alkanes are characterized by substitution reactions
B. ethane decolorizes potassium permanganate solution
1) only A is true 2) only B is true 3) both judgments are correct 4) both judgments are incorrect
15.What products can be obtained by reacting bromoethane and bromine propane with sodium metal?
1) butane 2) hexane
3) a mixture of butane and hexane 4) a mixture of hexane, butane, pentane
16.Indicate the name of the radical – CH 3
1) butyl 2) methane 3) ethyl 4) methyl
17. Specify the formula of the ethyl radical
1) -C 2 H 6 2b) -C 3 H 7 3) –C 2 H 5 4d) –C 4 H 9
18. Length of C-C bonds in alkane molecules
1) 0.109 nm 2) 0.154 nm 3) 0.120 nm 4) 0.134 nm
19. As a result of the dehydrogenation reaction of alkanes, the following is split off:
1) water 2) hydrogen 3) carbon 4) oxygen
20.What conditions are necessary for the reaction between methane and chlorine to start?
1) cooling 2) heating 3) increasing pressure 4) lighting
21.According to the state of aggregation of alkanes:
1) gases, liquids 3B) liquids, solids
2) gases, liquids, solids 4) gases, solids
22. A methane molecule has the form:
1) quadrangular pyramid 2) tetrahedron 3) octahedron 4) square
23. The isomer of 2,3-dimethylbutane is:
1) hexane 2) 2,3 – dimethylcyclohexane 3) cyclohexane 4) 2-methylbutane
24. NOT applicable to substitution reaction
1) dehydrogenation 2) bromination 3) nitration 4) chlorination
25. At the second stage of chlorination of methane,
1) carbon tetrachloride 2) trichloromethane 3) dichloromethane 4) 1,2 – dichloroethane
26.Ethane interacts with each of a pair of substances:
1) I 2 and N 2 2 ) HBr and H 2 O 3)Cl 2 and O 2 4) N 2 and NaOH
27.Methane chloride can be obtained as a result of the reaction
A) methane with hydrogen chloride B) methane with hydrochloric acid
B) methane with chlorine under lighting D) methane with a solution of chlorine in water
28. As a result of the thermal decomposition of methane at 1500 °C,
1) C 2 N 2 and N 2 2) CO and H 2 3) C and N 2 4)CO 2 and N 2 ABOUT
29. In the propane bromination reaction, the necessary condition is:
1) illumination by sunlight 3) presence of a catalyst
2) the reaction occurs under normal conditions 4) heating
30.How are alkanes oxidized during the combustion process?
1) hydrogen in the air 2) oxygen in the air 3) potassium permanganate 4) alkanes do not burn
31. The Wurtz reaction is the reaction….
A) nitration of alkanes B) interaction of a monohalogen derivative withNa
B) bromination D) no such reaction
32. The reaction leading to lengthening of the carbon chain is
1) isomerization of alkanes 3) hydrogenation of alkenes
2) decarboxylation of sodium salts of carboxylic acids 4) Wurtz reaction
33.What is not formed during the dehydrogenation of alkanes? 1) arenes 2) alkynes 3) alkenes 4) adcadienes
34.Name the “closest homologues” of propane.
1) C 4 N 10 2) CH 4 3) C 6 N 12 4) C 2 N 6
35Alkanes in the laboratory are obtained:
A) during cracking of oil B) hydrogenation of coal
B) Wurtz reaction D) Kucherov reaction
36. Indicate the formula of an alkane, which is a liquid under normal conditions
1) C 4 N 10 2) C 16 N 34 3) C 7 N 16 4) CH 4
37. With an increase in the number of carbon atoms in hydrocarbon molecules, the boiling point of these hydrocarbons
1) does not change 2) decreases
3) increases 4) first increases, then decreases
38.During the thermal decomposition of methane at a temperature of 1000 0 C are formed
1) soot and hydrogen 2) carbon monoxide and hydrogen
3) carbon dioxide and hydrogen 4) acetylene and hydrogen
39.When potassium acetate and potassium hydroxide are fused, a gaseous product is released
1) hydrogen 2) carbon dioxide 3) methane 4) ethane
40. Hexane does not react with hydrogen chloride because
1) there are no π bonds in its molecule 2) hexane is a hydrocarbon
3) the hexane molecule is non-polar 4) there are no hydrogen bonds between the atoms
41. The reaction product (predominant) of 2-bromopropane with sodium is
1) 2,3-dimethylbutane 2) hexane 3) cyclohexane 4) propene
43. How many different substances are shown in the picture: 1) 7 2) 4 3) 3 4) 2
44. When reforming methylcyclohexane, as a result of isomerization and dehydrogenation reactions, it turns into
1) ethylcyclopentane 2) hexene 3) benzene 4) toluene
45. Alkanes undergo the following reactions: a) substitution; b) accession; c) oxidation; d) polymerization; e) isomerization
1)a,b,c 2)a,c,e 3)a,b,c,d,e 4) b,d,e
46. Ethane interacts with
1) halogens 2) hydrogen3) carboxylic acids 4)hydrogen halides
47. The conversion of butane to butene refers to the reaction
1) polymerization 2) dehydrogenation 3) dehydration 4) isomerization
48. The structural isomer of normal n-hexane is
1) 3-ethylpentane 2) 2-methylpropane 3) 2,2-dimethylpropane 4)2,2-dimethylbutane
49. The interaction of methane with chlorine is a reaction
1) compounds, exothermic 2) substitution, endothermic
3) compounds, endothermic 4) substitution, exothermic
50. Are the following statements about hydrocarbons true?
A. All alkanes are gaseous.
B. Methane discolors an aqueous solution of potassium permanganate.
1) only A is true 2) only B is true 3) both judgments are correct 4) both judgments are incorrect
51. Butane can be obtained by the Wurtz reaction, the scheme of which is
1) C 4 H 8 t°"cat → 2) C 4 H 9 C1 + KOH (alcohol) → 3) C 2 H 5 C1 + Na → 4) 2 C 2 H 4 t°"cat →
Test on the topic “Alkanes” 2016
52. Isobutane reacts with
1) hydrochloric acid 2) hydrogen 3) hydrogen bromide 4) nitric acid
53. The interaction of methane with chlorine is a reaction
1) substitution, irreversible 3) exchange, irreversible
2) substitution, reversible 4) exchange, reversible
54. When heating a mixture of 2-chloropropane and chloroethane with sodium metal, sodium chloride and a mixture are formed
1) 2,3-dimethylbutane, butane, 2-methylbutane 2) hexane, 2-methylbutane, 1,2-dichloroethane
3) 2,3-dimethylbutane, butane, 2-methylbutane 4) 2,3-dimethylbutane, 2-methylbutane, butene
55 Which of the following statements are true?
A. The dehydrocyclization of n-heptane produces benzene.
B. Methane does not undergo addition reactions.
1) only A is true 2) only B is true 3) both statements are true 4) both statements are false
56.Name of alkane CH 3 -CH (CH 3 ) -CH 2 -C (CH 3 ) 2 -CH 3
57. 2-chlorobutane is mainly formed by the reaction
1) butene-1 and chlorine 2) butine-1 and hydrogen chloride
3) butane and chlorine 4) butine-2 and hydrogen chloride
58. Methane reacts
1) with hydrogen chloride 2) with water vapor on a catalyst
3) isomerization 4) with bromine water
59. Contains six carbon atoms
1)2-methylbutane2)2,2-
dimethylbutane 3) 2-methylpropane 4) 3-methylhexane
60. 2-Methylpentane and 2-methylhexane are relative to each other
1) analogues 2) radicals 3) homologs 4) isomers
61. Ethane can be produced
1) dehydration of ethanol 2) electrolysis of potassium acetate solution
3) hydrogenation of ethanol 4) dehydration of ethanoic acid
62. Isomers are
1)3-methylhexane and octane 2)3-ethylpentane and 3-methylpentane
3)2,2-dimethylpentane and 2,2-dimethylhexane 4)2-methylpentane and hexane
64. The number of organic substances that are formed when bromomethane and bromoethane are heated with sodium metal is 1) 1 2)2 3)3 4)4
Tasks of increased difficulty level
66. All alkanes can react with:
1) hydrogen 2) oxygen 3) water
4) chlorine 5) hydrogen chloride 6) nitric acid Answer____________
67 Methane is characterized by:
1) hydrogenation reaction 2) tetrahedral shape of the molecule
3) availability π -bonds in the molecule 5) reactions with hydrogen halides
4) sp 3 -hybridization of the orbitals of the carbon atom in the molecule
6) combustion in air Answer: ________
68. The reaction of propane with chlorine occurs
1) by chain radical mechanism2) with the intermediate formation of a CH particle 3 -SN + -SN 3
3) without catalyst 4) in aqueous solution
5) with the formation of propene 6) with ruptureσ -bonds in the propane moleculeAnswer____________
69 Reaction of propane and bromine
3) leads to the preferential formation of 2-bromopropane
4) leads to the preferential formation of 1-bromopropane
5) usually occurs in the dark
6) is a catalytic process Answer: _______
69.Chlorination of methane
1)consistently leads to the formation of various chlorine-substituted methane
2) begins with the process of breaking the bond in the methane molecule
3) refers to radical reactions
4)carried out in the dark
5) is a typical catalytic process
6) refers to exothermic processes Answer: _________
70.Chlorination of methane
1) proceeds through the ionic mechanism 2) refers to radical reactions
3) begins with the process of breaking the bond in the chlorine molecule 5) refers to endothermic processes
4) proceeds through an intermediate reaction: CH 4 →C + 4H 6) leads to the formation of chloromethane
Answer: __________
71. The mechanism of the methane chlorination reaction includes the following stages:
1)CH 4 → CH 3 + H 2)C1 2 → 2C1
3)C1 + CH 4 → CH 3 C1 + H 4)CH 4 → C + 4H
5) C1 2 + CH 4 → CH 3 C1 + HC1 6) H + Cl → HC1
Answer: __________
72. Reaction of 2-methylpropane and bromine
1) refers to substitution reactions
2) proceeds through a radical mechanism
3) leads to the preferential formation of 1-bromo-2-methylpropane
4) leads to the preferential formation of 2-bromo-2-methylpropane
5) usually occurs in the dark
6) is a catalytic process Answer: __________
73. 2-Methylbutane is characterized by the fact that it
1) used to produce isoprene
2) interacts with chlorine in the presence of A1C1 3
3) upon chlorination it forms predominantly 2-chloro-2-methylbutane
4) is an isomer of dimethylpropane
5) when interacting with copper (II) hydroxide, it forms 2-methylbutanal
6) does not form explosive mixtures with air
Answer:__________
74.Ethan is characterized by the fact that he
1) can be obtained by electrolysis of potassium propionate 2) interacts with bromine in the light
3) during dehydrogenation it is successively converted into ethylene and acetylene
4) undergoes a Wurtz reaction 5) is oxidized by air at ambient conditions.
6) is a homologue of octane Answer: ______________
75. Methane bromination reaction proceeds
1) by radical mechanism 2) in one stage
3) with the formation of various bromo derivatives 4) in the dark and without heating
5) with heat release
6) in accordance with V.V. Markovnikov’s rule Answer: _____.
76. To produce methane you can use the following reactions:
1) heating potassium acetate with potassium hydroxide 2) decomposition of ethane when heated
3) hydrolysis of aluminum carbide 4) chloromethane with sodium
5) reduction of methanal 6) hydrogen with carbon Answer: _________
77. The interaction proceeds according to the radical mechanism
1) propene and bromine water 2) propene and hydrogen bromide
3) propene and chlorine (in aqueous solution) 4) propene and chlorine (at 500°C)
5) ethane and oxygen 6) methane and chlorine
Answer: ________
78. Methane is formed when
1) hydrolysis of calcium carbide CaC 2 2) hydrolysis of aluminum carbide A1 4 WITH 3
3) hydrogenation of ethylene 4) calcination of sodium acetate with sodium hydroxide
5) decomposition of benzene 6) dehydration of ethyl alcohol Answer: ____________.
79. Butane is characterized by:
1) isomerization 4) interaction with sodium
2) hydration 5) hydrogenation
3) interaction with halogens 6) catalytic oxidation
Answer: ____________
80. Select the characteristics characteristic of ethane:
A) gaseous substance B) burns with a pale bluish flame
C) has a pungent odor D) 1.5 times heavier than hydrogen
E) soluble in water E) undergoes addition reactions
Answer: _____________________
Characteristic chemical properties of hydrocarbons: alkanes, alkenes, dienes, alkynes, aromatic hydrocarbons
Alkanes
Alkanes are hydrocarbons in whose molecules the atoms are connected by single bonds and which correspond to the general formula $C_(n)H_(2n+2)$.
Homologous series of methane
As you already know, homologs- these are substances that are similar in structure and properties and differ by one or more $CH_2$ groups.
Saturated hydrocarbons make up the homologous series of methane.
Isomerism and nomenclature
Alkanes are characterized by so-called structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. As you already know, the simplest alkane, which is characterized by structural isomers, is butane:
Let's take a closer look at the basics of IUPAC nomenclature for alkanes:
1. Selecting the main circuit.
The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in the molecule, which is, as it were, its basis.
2.
The atoms of the main chain are assigned numbers. The numbering of the atoms of the main chain begins from the end to which the substituent is closest (structures A, B). If the substituents are located at an equal distance from the end of the chain, then numbering starts from the end at which there are more of them (structure B). If different substituents are located at equal distances from the ends of the chain, then numbering begins from the end to which the senior one is closest (structure D). The seniority of hydrocarbon substituents is determined by the order in which the letter with which their name begins appears in the alphabet: methyl (—$СН_3$), then propyl ($—СН_2—СН_2—СН_3$), ethyl ($—СН_2—СН_3$ ) etc.
Please note that the name of the substituent is formed by replacing the suffix -an to suffix -il in the name of the corresponding alkane.
3. Formation of the name.
At the beginning of the name, numbers are indicated - the numbers of the carbon atoms at which the substituents are located. If there are several substituents at a given atom, then the corresponding number in the name is repeated twice separated by a comma ($2.2-$). After the number, the number of substituents is indicated with a hyphen ( di- two, three- three, tetra- four, penta- five) and the name of the deputy ( methyl, ethyl, propyl). Then, without spaces or hyphens, the name of the main chain. The main chain is called a hydrocarbon - a member of the homologous series of methane ( methane, ethane, propane, etc.).
The names of substances whose structural formulas are given above are as follows:
— structure A: $2$ -methylpropane;
— structure B: $3$ -ethylhexane;
— structure B: $2,2,4$ -trimethylpentane;
— structure G: $2$ -methyl$4$-ethylhexane.
Physical and chemical properties of alkanes
Physical properties. The first four representatives of the homologous series of methane are gases. The simplest of them is methane, a colorless, tasteless, and odorless gas (the smell of gas, upon sensing it, you need to call $104$, is determined by the smell of mercaptans - sulfur-containing compounds specially added to methane used in household and industrial gas appliances so that people , located next to them, could detect the leak by smell).
Hydrocarbons of composition from $С_5Н_(12)$ to $С_(15)Н_(32)$ are liquids; heavier hydrocarbons are solids.
The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents.
Chemical properties.
1. Substitution reactions. The most characteristic reactions for alkanes are free radical substitution reactions, during which a hydrogen atom is replaced by a halogen atom or some group.
Let us present the equations of the most characteristic reactions.
Halogenation:
$CH_4+Cl_2→CH_3Cl+HCl$.
In case of excess halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms with chlorine:
$CH_3Cl+Cl_2→HCl+(CH_2Cl_2)↙(\text"dichloromethane (methylene chloride)")$,
$CH_2Cl_2+Cl_2→HCl+(CHСl_3)↙(\text"trichloromethane(chloroform)")$,
$CHCl_3+Cl_2→HCl+(CCl_4)↙(\text"carbon tetrachloride(carbon tetrachloride)")$.
The resulting substances are widely used as solvents and starting materials in organic syntheses.
2. Dehydrogenation (elimination of hydrogen). When alkanes are passed over a catalyst ($Pt, Ni, Al_2O_3, Cr_2O_3$) at high temperatures ($400-600°C$), a hydrogen molecule is eliminated and an alkene is formed:
$CH_3—CH_3→CH_2=CH_2+H_2$
3. Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons are burning with the formation of carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode. The combustion of saturated hydrocarbons is a free radical exothermic reaction, which is very important when using alkanes as fuel:
$СН_4+2О_2→СО_2+2Н_2O+880 kJ.$
In general, the combustion reaction of alkanes can be written as follows:
$C_(n)H_(2n+2)+((3n+1)/(2))O_2→nCO_2+(n+1)H_2O$
Thermal splitting of hydrocarbons:
$C_(n)H_(2n+2)(→)↖(400-500°C)C_(n-k)H_(2(n-k)+2)+C_(k)H_(2k)$
The process occurs via a free radical mechanism. An increase in temperature leads to homolytic cleavage of the carbon-carbon bond and the formation of free radicals:
$R—CH_2CH_2:CH_2—R→R—CH_2CH_2·+·CH_2—R$.
These radicals interact with each other, exchanging a hydrogen atom, to form an alkane molecule and an alkene molecule:
$R—CH_2CH_2·+·CH_2—R→R—CH=CH_2+CH_3—R$.
Thermal decomposition reactions underlie the industrial process of hydrocarbon cracking. This process is the most important stage of oil refining.
When methane is heated to a temperature of $1000°C$, methane pyrolysis begins - decomposition into simple substances:
$CH_4(→)↖(1000°C)C+2H_2$
When heated to a temperature of $1500°C$, the formation of acetylene is possible:
$2CH_4(→)↖(1500°C)CH=CH+3H_2$
4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton are formed:
5. Aromatization. Alkanes with six or more carbon atoms in the chain cyclize in the presence of a catalyst to form benzene and its derivatives:
What is the reason that alkanes undergo free radical reactions? All carbon atoms in alkane molecules are in a state of $sp^3$ hybridization. The molecules of these substances are built using covalent nonpolar $C-C$ (carbon-carbon) bonds and weakly polar $C-H$ (carbon-hydrogen) bonds. They do not contain areas with increased or decreased electron density, or easily polarizable bonds, i.e. such bonds, the electron density in which can shift under the influence of external factors (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, because bonds in alkane molecules are not broken by the heterolytic mechanism.
Alkenes
Unsaturated include hydrocarbons containing multiple bonds between carbon atoms in their molecules. Unlimited are alkenes, alkadienes (polyenes), alkynes. Cyclic hydrocarbons containing a double bond in the ring (cycloalkenes), as well as cycloalkanes with a small number of carbon atoms in the ring (three or four atoms) also have an unsaturated character. The property of unsaturation is associated with the ability of these substances to enter into addition reactions, primarily hydrogen, with the formation of saturated, or saturated, hydrocarbons - alkanes.
Alkenes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, one double bond between carbon atoms and corresponding to the general formula $C_(n)H_(2n)$.
Its second name is olefins- alkenes were obtained by analogy with unsaturated fatty acids (oleic, linoleic), the remains of which are part of liquid fats - oils (from lat. oleum- oil).
Homologous series of ethene
Unbranched alkenes form the homologous series of ethene (ethylene):
$С_2Н_4$ - ethene, $С_3Н_6$ - propene, $С_4Н_8$ - butene, $С_5Н_(10)$ - pentene, $С_6Н_(12)$ - hexene, etc.
Isomerism and nomenclature
Alkenes, like alkanes, are characterized by structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkene, characterized by structural isomers, is butene:
A special type of structural isomerism is isomerism of the position of the double bond:
$CH_3—(CH_2)↙(butene-1)—CH=CH_2$ $CH_3—(CH=CH)↙(butene-2)—CH_3$
Almost free rotation of carbon atoms is possible around a single carbon-carbon bond, so alkane molecules can take on a wide variety of shapes. Rotation around the double bond is impossible, which leads to the appearance of another type of isomerism in alkenes - geometric, or cis-trans isomerism.
Cis- isomers differ from trance- isomers by the spatial arrangement of molecular fragments (in this case, methyl groups) relative to the plane of the $π$ bond, and, consequently, by their properties.
Alkenes are isomeric to cycloalkanes (interclass isomerism), for example:
The IUPAC nomenclature for alkenes is similar to that for alkanes.
1. Selecting the main circuit.
Naming a hydrocarbon begins with identifying the main chain—the longest chain of carbon atoms in the molecule. In the case of alkenes, the main chain must contain a double bond.
2. Numbering of main chain atoms.
The numbering of the atoms of the main chain begins from the end to which the double bond is closest. For example, the correct connection name is:
$5$-methylhexene-$2$, not $2$-methylhexene-$4$, as one might expect.
If the position of the double bond cannot determine the beginning of the numbering of atoms in the chain, then it is determined by the position of the substituents, just as for saturated hydrocarbons.
3. Formation of the name.
The names of alkenes are formed in the same way as the names of alkanes. At the end of the name, indicate the number of the carbon atom at which the double bond begins, and a suffix indicating that the compound belongs to the class of alkenes - -en.
For example:
Physical and chemical properties of alkenes
Physical properties. The first three representatives of the homologous series of alkenes are gases; substances of the composition $С_5Н_(10)$ - $С_(16)Н_(32)$ - liquids; Higher alkenes are solids.
Boiling and melting points naturally increase with increasing molecular weight of compounds.
Chemical properties.
Addition reactions. Let us recall that a distinctive feature of representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed according to the mechanism
1. Hydrogenation of alkenes. Alkenes are capable of adding hydrogen in the presence of hydrogenation catalysts, metals - platinum, palladium, nickel:
$CH_3—CH_2—CH=CH_2+H_2(→)↖(Pt)CH_3—CH_2—CH_2—CH_3$.
This reaction occurs at atmospheric and elevated pressure and does not require high temperature, because is exothermic. When the temperature rises, the same catalysts can cause a reverse reaction—dehydrogenation.
2. Halogenation (addition of halogens). The interaction of an alkene with bromine water or a solution of bromine in an organic solvent ($CCl_4$) leads to rapid discoloration of these solutions as a result of the addition of a halogen molecule to the alkene and the formation of dihalogen alkanes:
$CH_2=CH_2+Br_2→CH_2Br—CH_2Br$.
3.
$CH_3-(CH)↙(propene)=CH_2+HBr→CH_3-(CHBr)↙(2-bromopropene)-CH_3$
This reaction obeys Markovnikov's rule:
When a hydrogen halide is added to an alkene, the hydrogen is added to the more hydrogenated carbon atom, i.e. the atom at which there are more hydrogen atoms, and the halogen to the less hydrogenated one.
Hydration of alkenes leads to the formation of alcohols. For example, the addition of water to ethene underlies one of the industrial methods for producing ethyl alcohol:
$(CH_2)↙(ethene)=CH_2+H_2O(→)↖(t,H_3PO_4)CH_3-(CH_2OH)↙(ethanol)$
Note that a primary alcohol (with a hydroxo group on the primary carbon) is only formed when ethene is hydrated. When propene or other alkenes are hydrated, secondary alcohols are formed.
This reaction also proceeds in accordance with Markovnikov’s rule - a hydrogen cation attaches to a more hydrogenated carbon atom, and a hydroxo group to a less hydrogenated one.
5. Polymerization. A special case of addition is the polymerization reaction of alkenes:
$nCH_2(=)↙(ethene)CH_2(→)↖(UV light, R)(...(-CH_2-CH_2-)↙(polyethylene)...)_n$
This addition reaction occurs via a free radical mechanism.
6. Oxidation reaction.
Like any organic compounds, alkenes burn in oxygen to form $СО_2$ and $Н_2О$:
$СН_2=СН_2+3О_2→2СО_2+2Н_2О$.
In general:
$C_(n)H_(2n)+(3n)/(2)O_2→nCO_2+nH_2O$
Unlike alkanes, which are resistant to oxidation in solutions, alkenes are easily oxidized by potassium permanganate solutions. In neutral or alkaline solutions, alkenes are oxidized to diols (dihydric alcohols), and hydroxyl groups are added to those atoms between which a double bond existed before oxidation:
Alkadienes (diene hydrocarbons)
Alkadienes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, two double bonds between carbon atoms and corresponding to the general formula $C_(n)H_(2n-2)$.
Depending on the relative arrangement of double bonds, three types of dienes are distinguished:
- alkadienes with cumulated arrangement of double bonds:
- alkadienes with conjugated double bonds;
$CH_2=CH—CH=CH_2$;
- alkadienes with isolated double bonds
$CH_2=CH—CH_2—CH=CH_2$.
These three types of alkadienes differ significantly from each other in structure and properties. The central carbon atom (the atom that forms two double bonds) in alkadienes with cumulated bonds is in a state of $sp$-hybridization. It forms two $σ$-bonds lying on the same line and directed in opposite directions, and two $π$-bonds lying in perpendicular planes. $π$-Bonds are formed due to the unhybridized p-orbitals of each carbon atom. The properties of alkadienes with isolated double bonds are very specific, because conjugate $π$-bonds significantly influence each other.
p-orbitals forming conjugated $π$-bonds constitute practically a single system (it is called a $π$-system), because p-orbitals of neighboring $π$-bonds partially overlap.
Isomerism and nomenclature
Alkadienes are characterized by both structural isomerism and cis-, trans-isomerism.
Structural isomerism.
— carbon skeleton isomerism:
— isomerism of the position of multiple bonds:
$(CH_2=CH—CH=CH_2)↙(butadiene-1,3)$ $(CH_2=C=CH—CH_3)↙(butadiene-1,2)$
Cis-, trans- isomerism (spatial and geometric)
For example:
Alkadienes are isomeric compounds of the classes of alkynes and cycloalkenes.
When forming the name of an alkadiene, the numbers of double bonds are indicated. The main chain must necessarily contain two multiple bonds.
For example:
Physical and chemical properties of alkadienes
Physical properties.
Under normal conditions, propandiene-1,2, butadiene-1,3 are gases, 2-methylbutadiene-1,3 is a volatile liquid. Alkadienes with isolated double bonds (the simplest of them is pentadiene-1,4) are liquids. Higher dienes are solids.
Chemical properties.
The chemical properties of alkadienes with isolated double bonds differ little from the properties of alkenes. Alkadienes with conjugated bonds have some special features.
1. Addition reactions. Alkadienes are capable of adding hydrogen, halogens, and hydrogen halides.
A special feature of the addition to alkadienes with conjugated bonds is the ability to add molecules both in positions 1 and 2, and in positions 1 and 4.
The ratio of products depends on the conditions and method of carrying out the corresponding reactions.
2.Polymerization reaction. The most important property of dienes is the ability to polymerize under the influence of cations or free radicals. The polymerization of these compounds is the basis of synthetic rubbers:
$nCH_2=(CH—CH=CH_2)↙(butadiene-1,3)→((... —CH_2—CH=CH—CH_2— ...)_n)↙(\text"synthetic butadiene rubber")$ .
Polymerization of conjugated dienes proceeds as 1,4-addition.
In this case, the double bond turns out to be central in the unit, and the elementary unit, in turn, can take on both cis-, so trance- configuration
Alkynes
Alkynes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, one triple bond between carbon atoms and corresponding to the general formula $C_(n)H_(2n-2)$.
Homologous series of ethyne
Straight-chain alkynes form the homologous series of ethyn (acetylene):
$С_2Н_2$ - ethine, $С_3Н_4$ - propine, $С_4Н_6$ - butine, $С_5Н_8$ - pentine, $С_6Н_(10)$ - hexine, etc.
Isomerism and nomenclature
Alkynes, like alkenes, are characterized by structural isomerism: isomerism of the carbon skeleton and isomerism of the position of the multiple bond. The simplest alkyne, which is characterized by structural isomers of the multiple bond position of the alkyne class, is butine:
$СН_3—(СН_2)↙(butine-1)—С≡СН$ $СН_3—(С≡С)↙(butine-2)—СН_3$
Isomerism of the carbon skeleton in alkynes is possible, starting with pentine:
Since a triple bond assumes a linear structure of the carbon chain, geometric ( cis-, trans-) isomerism is impossible for alkynes.
The presence of a triple bond in hydrocarbon molecules of this class is reflected by the suffix -in, and its position in the chain is the number of the carbon atom.
For example:
Compounds of some other classes are isomeric to alkynes. Thus, the chemical formula $C_6H_(10)$ has hexine (alkyne), hexadiene (alkadiene) and cyclohexene (cycloalkene):
Physical and chemical properties of alkynes
Physical properties. The boiling and melting points of alkynes, as well as alkenes, naturally increase with increasing molecular weight of the compounds.
Alkynes have a specific odor. They are more soluble in water than alkanes and alkenes.
Chemical properties.
Addition reactions. Alkynes are unsaturated compounds and undergo addition reactions. Mostly reactions electrophilic addition.
1. Halogenation (addition of a halogen molecule). An alkyne is capable of attaching two halogen molecules (chlorine, bromine):
$CH≡CH+Br_2→(CHBr=CHBr)↙(1,2-dibromoethane),$
$CHBr=CHBr+Br_2→(CHBr_2-CHBr_2)↙(1,1,2,2-tetrabromoethane)$
2. Hydrohalogenation (addition of hydrogen halide). The addition reaction of a hydrogen halide, which occurs via an electrophilic mechanism, also occurs in two stages, and at both stages the Markovnikov rule is satisfied:
$CH_3-C≡CH+Br→(CH_3-CBr=CH_2)↙(2-bromopropene),$
$CH_3-CBr=CH_2+HBr→(CH_3-CHBr_2-CH_3)↙(2,2-dibromopropane)$
3. Hydration (addition of water). Of great importance for the industrial synthesis of ketones and aldehydes is the reaction of addition of water (hydration), which is called Kucherov's reaction:
4. Hydrogenation of alkynes. Alkynes add hydrogen in the presence of metal catalysts ($Pt, Pd, Ni$):
$R-C≡C-R+H_2(→)↖(Pt)R-CH=CH-R,$
$R-CH=CH-R+H_2(→)↖(Pt)R-CH_2-CH_2-R$
Since the triple bond contains two reactive $π$ bonds, alkanes add hydrogen in a stepwise manner:
1) trimerization.
When ethyne is passed over activated carbon, a mixture of products is formed, one of which is benzene:
2) dimerization.
In addition to the trimerization of acetylene, its dimerization is possible. Under the influence of monovalent copper salts, vinyl acetylene is formed:
$2HC≡CH→(HC≡C-CH=CH_2)↙(\text"butene-1-in-3(vinylacetylene)")$
This substance is used to produce chloroprene:
$HC≡C-CH=CH_2+HCl(→)↖(CaCl)H_2C=(CCl-CH)↙(chloroprene)=CH_2$
by polymerization of which chloroprene rubber is obtained:
$nH_2C=CCl-CH=CH_2→(...-H_2C-CCl=CH-CH_2-...)_n$
Oxidation of alkynes.
Ethine (acetylene) burns in oxygen, releasing a very large amount of heat:
$2C_2H_2+5O_2→4CO_2+2H_2O+2600kJ$ The action of an oxygen-acetylene torch is based on this reaction, the flame of which has a very high temperature (over $3000°C$), which allows it to be used for cutting and welding metals.
In air, acetylene burns with a smoky flame, because the carbon content in its molecule is higher than in the molecules of ethane and ethene.
Alkynes, like alkenes, discolor acidified solutions of potassium permanganate; In this case, the multiple bond is destroyed.
Reactions characterizing the main methods for producing oxygen-containing compounds
1. Hydrolysis of haloalkanes. You already know that the formation of halokenalkanes when alcohols react with hydrogen halides is a reversible reaction. Therefore, it is clear that alcohols can be obtained by hydrolysis of haloalkanes- reactions of these compounds with water:
$R-Cl+NaOH(→)↖(H_2O)R-OH+NaCl+H_2O$
Polyhydric alcohols can be obtained by hydrolysis of haloalkanes containing more than one halogen atom per molecule. For example:
2. Hydration of alkenes- addition of water via the $π$ bond of an alkene molecule - is already familiar to you, for example:
$(CH_2=CH_2)↙(ethene)+H_2O(→)↖(H^(+))(C_2H_5OH)↙(ethanol)$
Hydration of propene leads, in accordance with Markovnikov’s rule, to the formation of a secondary alcohol - propanol-2:
3. Hydrogenation of aldehydes and ketones. You already know that the oxidation of alcohols under mild conditions leads to the formation of aldehydes or ketones. It is obvious that alcohols can be obtained by hydrogenation (reduction with hydrogen, addition of hydrogen) of aldehydes and ketones:
4. Oxidation of alkenes. Glycols, as already noted, can be obtained by oxidation of alkenes with an aqueous solution of potassium permanganate. For example, ethylene glycol (ethanediol-1,2) is formed by the oxidation of ethylene (ethene):
$CH_2=CH_2+[O]+H_2O(→)↖(KMnO_4)HO-CH_2-CH_2-OH$
5. Specific methods for producing alcohols. Some alcohols are obtained using methods that are unique to them. Thus, methanol is produced industrially by the interaction of hydrogen with carbon monoxide (II) (carbon monoxide) at elevated pressure and high temperature on the surface of a catalyst (zinc oxide):
$CO+2H_2(→)↖(t,p,ZnO)CH_3-OH$
The mixture of carbon monoxide and hydrogen required for this reaction, also called synthesis gas ($CO + nH_2O$), is obtained by passing water vapor over hot coal:
$C+H_2O(→)↖(t)CO+H_2-Q$
6. Fermentation of glucose. This method of producing ethyl (wine) alcohol has been known to man since ancient times:
$(C_6H_(12)O_6)↙(glucose)(→)↖(yeast)2C_2H_5OH+2CO_2$
Methods for producing aldehydes and ketones
Aldehydes and ketones can be produced oxidation or dehydrogenation of alcohols. Let us note once again that the oxidation or dehydrogenation of primary alcohols can produce aldehydes, and of secondary alcohols - ketones:
Kucherov's reaction. As a result of the hydration reaction, acetylene produces acetaldehyde, and ketones are obtained from acetylene homologues:
When heated calcium or barium salts carboxylic acids form a ketone and metal carbonate:
Methods for producing carboxylic acids
Carboxylic acids can be prepared by oxidation of primary aldehyde alcohols:
Aromatic carboxylic acids are formed by the oxidation of benzene homologues:
Hydrolysis of various carboxylic acid derivatives also produces acids. Thus, the hydrolysis of an ester produces an alcohol and a carboxylic acid. As mentioned above, acid-catalyzed esterification and hydrolysis reactions are reversible:
Hydrolysis of the ester under the influence of an aqueous solution of alkali proceeds irreversibly; in this case, not an acid, but its salt is formed from the ester.
Alkenes– these are hydrocarbons whose molecules have ONE double C=C bond.
General formula of alkenes:
CnH2n
Type of hybridization of the carbon atom of a double bond – sp 2 . The remaining carbon atoms in the alkene molecule have sp 3 - hybridization.
The molecule has a flat structure, the angle between σ bonds is 120 0
The length of a double bond is shorter than the length of a single bond.
Alkene nomenclature: a suffix appears in the name -EN.
The first member of the homologous series is C 2 H 4 (ethene).
For the simplest alkenes, historical names are also used:
ethylene (ethene),
propylene (propene),
The following monovalent alkene radicals are often used in nomenclature:
CH 2 -CH=CH 2
Types of isomerism of alkenes:
1. Carbon skeleton isomerism:(starting from C 4 H 8 – butene and 2-methylpropene)
2. Isomerism of multiple bond position:(starting from C 4 H 8): butene-1 and butene-2.
3. Interclass isomerism: With cycloalkanes(starting with propene):
C 4 H 8 - butene and cyclobutane.
4. Spatial isomerism of alkenes:
Due to the fact that free rotation around the double bond is impossible, it becomes possible cis-trans- isomerism .
Alkenes with each of two carbon atoms at a double bond various substituents, can exist in the form of two isomers, differing in the arrangement of substituents relative to the π-bond plane:
Chemical properties of alkenes.
Alkenes are characterized by:
addition reactions to a double bond,
oxidation reactions,
substitution reactions in the “side chain”.
1. Double bond addition reactions: the weaker π bond is broken and a saturated compound is formed.
These are electrophilic addition reactions - A E.
1) Hydrogenation:
CH 3 -CH = CH 2 + H 2 CH 3 -CH 2 -CH 3
2) Halogenation:
CH 3 -CH = CH 2 + Br 2 (solution) CH 3 -CHBr -CH 2 Br
Discoloration of bromine water is a qualitative reaction to a double bond.
3) Hydrohalogenation:
CH 3 -CH = CH 2 + HBr CH 3 -CHBr -CH 3
(MARKOVNIKOV'S RULE: hydrogen attaches to the most hydrogenated carbon atom).
4) Hydration - addition of water:
CH 3 -CH = CH 2 + HOH CH 3 -CH -CH 3
(annexation also occurs according to Markovnikov’s rule)
2. Addition of hydrogen bromide to presence of peroxides (Harash effect) - this is a radical addition - A R
CH 3 -CH = CH 2 + HBr - (H 2 O 2) CH 3 -CH 2 -CH 2 Br
(the reaction with hydrogen bromide in the presence of peroxide occursagainst Markovnikov's rule )
3. Combustion– complete oxidation of alkenes with oxygen to carbon dioxide and water.
C 2 H 4 + 3O 2 = 2CO 2 + 2H 2 O
4. Mild oxidation of alkenes – Wagner reaction : reaction with a cold aqueous solution of potassium permanganate.
3CH 3 - CH=CH 2 + 2KMnO 4 + 4H 2 O 2MnO2 + 2KOH + 3 CH 3 - CH - CH 2
ô ô
OH OH
( diol is formed)
Discoloration of an aqueous solution of potassium permanganate by alkenes is a qualitative reaction to alkenes.
5. Severe oxidation of alkenes– hot neutral or acidic solution of potassium permanganate. Comes with the cleavage of the C=C double bond.
1. When potassium permanganate acts in an acidic environment, depending on the structure of the alkene skeleton, the following is formed:
Fragment of the carbon chain at the double bond
What does it turn into?
=C H 2
C O 2
= CH – R
R– C OOH carboxylic acid
= C–R
ô
R
ketone R – C – R
║
O
CH 3 -WITH -1 N=WITH -2 H 2 +2 KMn +7 O 4 + 3H 2 SO 4
CH 3 -C +3 OOH+ C +4 O 2 + 2Mn +2 SO 4 + K 2 SO 4 + 4H 2 O
2. If the reaction occurs in a neutral environment when heated, then the following results are obtained: potassium salt:
Chain fragment at a double bond
What does it turn into?
=C H 2
K 2 C O 3
= CH – R
R– C OO K - carboxylic acid salt
= C–R
ô
R
ketone R – C – R
║
O
3CH 3 WITH -1 N=WITH-2 N 2 +10 K MnO 4 -t 3CH 3 C +3 OO K + + 3K 2 C +4 O 3 + 10MnO 2 +4H 2 O+ K OH
6. Oxidation oxygen of ethylene in the presence of palladium salts.
CH 2 =CH 2 + O 2 –(kat) C H 3 CHO
(acetic aldehyde)
7. Chlorination and brominationto the side chain: if the reaction with chlorine is carried out in the light or at high temperature, hydrogen is replaced in the side chain.
CH 3 -CH = CH 2 + Cl 2 – (light) CH 2 -CH = CH 2 +HCl
8. Polymerization:
n CH 3 - CH= CH 2 (-CH–CH 2 -)n
propylene polypropylene
CH 3
OBTAINING ALKENES
I . Cracking alkanes:
C 7 H 16 –(t) CH 3 - CH =CH 2 + C 4 H 10
Alkene alkane
II. Dehydrohalogenation of haloalkanes under the action of an alcohol solution of alkali - reaction ELIMINATION.
Zaitsev's rule: The abstraction of a hydrogen atom in elimination reactions occurs predominantly from the least hydrogenated carbon atom.
III . Dehydration of alcohols at elevated temperatures (above 140°C) in the presence of water-removing reagents - aluminum oxide or concentrated sulfuric acid - an elimination reaction.
CH 3 - CH-CH 2 -CH 3 – (H 2 SO 4,t>140 o) H 2 O+CH 3 - CH=CH-CH 3
OH
(also obeys Zaitsev's rule)
IV . Dehalogenation of dihaloalkanes having halogen atoms at neighboring carbon atoms, under the action of active metals.
CH 2 Br-CH Br-CH3+ Mg CH 2 =CH-CH 3 + Mg Br 2
Zinc can also be used.
V . Dehydrogenation of alkanes at 500°C:
VI. Incomplete hydrogenation of dienes and alkynes
C 2 H 2 + H 2 (disadvantage) – (kat) C 2 H 4
Alkanes. Structure of alkanes
Alkanes (paraffins)– aliphatic (non-cyclic) saturated hydrocarbons, in which the carbon atoms are connected to each other by simple (single) bonds in straight or branched chains.
Alkanes have the general formula C n H 2 n +2 , Where n– number of carbon atoms.
Chemical structure. Valkans have two types of chemical bonds:
S–S And S–H.
The C–C bond is covalent nonpolar. The C–H bond is covalent, weakly polar, because carbon and hydrogen are close in electronegativity (2.5 for carbon and 2.1 for hydrogen). The formation of covalent bonds in alkanes due to shared electron pairs of carbon and hydrogen atoms can be shown using electronic formulas:
Electronic and structural formulas reflect chemical structure, but do not give an idea about spatial structure of molecules, which significantly affects the properties of the substance.
Spatial structure , i.e. the relative arrangement of the atoms of a molecule in space depends on the direction of the atomic orbitals (AO) of these atoms. In hydrocarbons, the main role is played by the spatial orientation of the atomic orbitals of carbon, since the spherical 1s-AO of the hydrogen atom lacks a specific orientation.
The spatial arrangement of carbon AO, in turn, depends on the type of its hybridization. The saturated carbon atom in alkanes is bonded to four other atoms. Therefore, its state corresponds to sp 3 hybridization. In this case, each of the four sp 3 -hybrid carbon AOs participates in axial (-) overlap with the s-AO of hydrogen or with the sp 3 -AO of another carbon atom, forming -CH or C-C bonds.
Four -bonds of carbon are directed in space at a tetrahedral angle of 109 o 28". Therefore, the molecule of the simplest representative of alkanes - methane CH 4 - has the shape of a tetrahedron, in the center of which there is a carbon atom, and at the vertices there are hydrogen atoms:
The H-C-H bond angle is 109°28'. The spatial structure of methane can be shown using volumetric (scale) and ball-and-stick models.
For recording, it is convenient to use a spatial (stereochemical) formula.
In the molecule of the next homologue - ethane C 2 H 6 - two tetrahedral sp 3 - carbon atoms form a more complex spatial structure:
Alkane molecules containing more than 2 carbon atoms are characterized by curved shapes.
Nomenclature
According to the IUPAC nomenclature, the names of saturated hydrocarbons are characterized by the suffix –an. The first four hydrocarbons have historically established names, starting with the fifth, the name of the hydrocarbon is based on the Greek name for the corresponding number of carbon atoms.
Hydrocarbons with a normal chain of carbon atoms have the following names:
| CH 4 - methane | CH 3 -(CH 2) 5 - CH 3 - heptane |
The names of branched chain hydrocarbons are constructed as follows:
1. The name of this compound is based on the name of the hydrocarbon corresponding to the number of carbon atoms in the main chain.
The main chain of carbon atoms is considered to be:
a) the longest;
b) the most complex (with the maximum number of branches). If two or more equally long chains can be distinguished in a hydrocarbon, then the one with the largest number of branches is chosen as the main one:
2. After establishing the main chain, it is necessary to renumber the carbon atoms. Numbering begins from the end of the chain to which any of the alkyls is closest. If different alkyls are located at equal distances from both ends of the chain, then numbering begins from the end to which the radical with the smaller number of carbon atoms is closer (methyl, ethyl, propyl, etc.). For example:
If identical radicals that determine the beginning of numbering are located at an equal distance from both ends of the chain, but there are more of them on one side than on the other, then numbering begins from the end where the number of branches is greater: 
2, 2, 4-trimethylpentane
2, 3, 6-trimethylheptane
When naming a compound, first list the substituents in alphabetical order (numerals are not taken into account), and before the name of the radical they put a number corresponding to the number of the carbon atom of the main chain at which this radical is located. After this, the hydrocarbon corresponding to the main chain of carbon atoms is named, separating the word from the numbers with a hyphen.
If a hydrocarbon contains several identical radicals, then their number is denoted by a Greek numeral (di, tri, tetra, etc.) and placed in front of the name of these radicals, and their position is indicated, as usual, by numbers, with the numbers separated by commas, arranged in order their increases and are placed before the name of these radicals, separating them from it with a hyphen.
CYCLOALKANES
The names of cycloalkanes are formed by adding a prefix cyclo- to the name of the corresponding unbranched saturated hydrocarbon with the same number of carbon atoms:
Substituents are numbered according to their position in the cycle in such a way that the sum of the numbers is minimal:
Isomerism
Isomers- these are substances that have the same composition and the same molecular formula and mass, but a different chemical structure, and therefore have different physical and chemical properties.
Structural isomerism
The reason for the manifestation of structural isomerism in the series of alkanes is the ability of carbon atoms to form chains of different structures. This type of structural isomerism is called carbon skeleton isomerism.
Structural isomers have the same composition, but differ in chemical structure, while the chemical properties of the isomers are similar, but the physical properties are different. Alkanes with a branched structure, due to less dense packing of molecules and, accordingly, fewer intermolecular interactions, boil at a lower temperature than their unbranched isomers.
In the molecules of methane CH 4, ethane C 2 H 6 and propane C 3 H 8 there can be only one order of connection of atoms, that is, the first three members of the homologous series of alkanes do not have isomers. For butane C4H10, two structures are possible:
One of these isomers (n-butane) contains a straight carbon chain, and the other, isobutane, contains a branched one (isostructure).
With an increase in the number of carbon atoms in the molecules, the possibilities for chain branching increase, i.e. the number of isomers increases with the number of carbon atoms.
In the series of radicals we also encounter the phenomenon of isomerism. Moreover, the number of isomers in radicals is significantly greater than in their corresponding alkanes. For example, propane, as is known, has no isomers, and the propyl radical has two isomers: n-propyl and iso-propyl:
|
CH 3 -CH 3 -CH 2 - and H 3 C-CH-CH 3
Rotational isomerism of alkanes
Rotation of atoms around the s-bond will not lead to its breaking. As a result of intramolecular rotation along C–C s-bonds, alkane molecules, starting with ethane C 2 H 6, can take on different geometric shapes.
Various spatial forms of a molecule that transform into each other by rotating around C–C s-bonds are called conformations or rotary isomers(conformers).
Rotational isomers of a molecule are its energetically unequal states. Their interconversion occurs quickly and constantly as a result of thermal movement. Therefore, rotary isomers cannot be isolated in individual form, but their existence has been proven by physical methods. Some conformations are more stable (energetically favorable) and the molecule remains in such states for a longer time.
Physical properties
Under normal conditions, the first four members of the homologous series of alkanes are gases, C 5 -C 17 are liquids, and starting from C 18 are solids. The melting and boiling points of alkanes of their density increase with increasing molecular weight. All alkanes are lighter than water and are insoluble in it, but they are soluble in non-polar solvents (for example, benzene) and are themselves good solvents.
The physical properties of some alkanes are presented in the table.
| Name | Formula | tpl°C | tbale°C | d 20 4 |
| Methane | CH 4 | (at -164 °C) |
||
| Ethane | WITH 2 N 6 | (at -100 °C) |
||
| Propane | WITH 3 N 8 | (at -44.5 °C) |
||
| Butane | WITH 4 N 10 | (at 0°C) |
||
| Pentane | C 5 H 12 | |||
| Hexane | WITH 6 N 14 | |||
| Heptane | WITH 7 H 16 | |||
| Octane | C 8 H 18 | |||
| Nonan | WITH 9 N 20 | |||
| Dean | C 10 H 22 | |||
| Pentadecane | C 15 H 32 | |||
| Eikosan | WITH 20 N 42 | (at 37 °C) |
||
| Pentacosan | C 25 H 52 | |||
| Triacontan | WITH 30 N 62 | |||
| *
d 4
20
– relative density, i.e. |
||||
Chemical properties
The trivial (historical) name for alkanes - "paraffins" - means "having no affinity." Alkanes are chemically inactive. The low reactivity of alkanes is due to the very low polarity of the C-C and C-H bonds in their molecules due to the almost identical electronegativity of the carbon and hydrogen atoms. Saturated hydrocarbons under normal conditions do not interact with concentrated acids, alkalis, or even such an active reagent as potassium permanganate.
They are characterized by substitution reactions of hydrogen atoms and splitting.
In these reactions, homolytic cleavage of covalent bonds occurs, i.e. they are carried out by a free radical (chain) mechanism.
Due to the strength of the C–C and C–H bonds, reactions occur either with heating, or in light, or with the use of catalysts.
Let's look at some examples of reactions of this type.
Halogenation. This is one of the characteristic reactions of saturated hydrocarbons. The halogenation of alkanes occurs in stages - no more than one hydrogen atom is replaced in one stage:
CH 4 + Cl 2 → CH 3 Cl + HCl (chloromethane)
CH 3 Cl + Cl 2 → CH 2 Cl 2 + HCl (dichloromethane)
CH 2 Cl 2 + Cl 2 → CHCl 3 + HCl (trichloromethane)
CHCl 3 + Cl 2 → CCl 4 + HCl (carbon tetrachloride).
Nitration. Despite the fact that under normal conditions alkanes do not interact with concentrated nitric acid, when they are heated to 140°C with dilute (10%) nitric acid under pressure, a nitration reaction occurs - the replacement of a hydrogen atom with a nitro group (M.I. Konovalov’s reaction ). All alkanes enter into a similar liquid-phase nitration reaction, but the reaction rate and yields of nitro compounds are low. The best results are observed with alkanes containing tertiary carbon atoms.
Cracking. At high temperatures in the presence of catalysts, saturated hydrocarbons undergo splitting, which is called cracking. During cracking, carbon-carbon bonds are homolytically broken to form saturated and unsaturated hydrocarbons with shorter chains.
CH 3 –CH 2 –CH 2 –CH 3 (butane) –– 400°C CH 3 –CH 3 (ethane)+ CH 2 =CH 2 (ethylene)
An increase in the process temperature leads to deeper decomposition of hydrocarbons and, in particular, to dehydrogenation, i.e. to splitting off
hydrogen. Thus, methane at 1500ºС leads to acetylene.
2CH 4 –– 1500°C H–C =
C–H(acetylene) + 3H 2
Isomerization. Under the influence of catalysts, when heated, hydrocarbons of normal structure undergo isomerization - rearrangement of the carbon skeleton with the formation of branched alkanes.
Oxidation. Under normal conditions, alkanes are resistant to oxygen and oxidizing agents. When ignited in air, alkanes burn, turning into carbon dioxide and water and releasing large amounts of heat.
CH 4 + 2O 2 – flame CO 2 + 2H 2 O
C 5 H 12 + 8O 2 –– flame 5CO 2 + 6H 2 O
Being in nature and receiving
The main sources of alkanes are oil and natural gas. 
Methane makes up the bulk of natural gas; it also contains small amounts of ethane, propane and butane. Methane is found in emissions from swamps and coal seams. Along with light homologues, methane is present in associated petroleum gases. These gases are dissolved in oil under pressure and are also located above it. Alkanes make up a significant portion of petroleum products. Oil also contains cycloalkanes - they are called naphthenes (from the Greek. naphtha- oil). Gas hydrates of alkanes, mainly methane, are also widespread in nature; they occur in sedimentary rocks on continents and at the bottom of the oceans. Their reserves probably exceed the known reserves of natural gas and in the future may become a source of methane and its closest homologues. Alkanes are also obtained by pyrolysis (coking) of coal and its hydrogenation (production of synthetic liquid fuel). Solid alkanes are found in nature in the form of deposits of mountain wax - ozokerite, in the waxy coatings of leaves, flowers and plant seeds, and are part of beeswax. 
In industry, alkanes are obtained by catalytic hydrogenation of carbon oxides CO
Mountain wax
and CO 2 (Fischer–Tropsch method). In the laboratory, methane can be obtained by heating sodium acetate with solid alkali: CH 3 COONa + NaOH → CH 4 + Na 2 CO 3, as well as by hydrolysis of some carbides: Al 4 C 3 + 12H 2 O → 3CH 4 + 4Al(OH) 3. Homologs of methane can be obtained by the Wurtz reaction, for example: 2CH 3 Br + 2Na→CH 3 –CH 3 + 2NaBr. In the case of dihaloalkanes, cycloalkanes are obtained, for example: Br–CH 2 –(CH 2) 4 –CH 2 Br + 2Na→ cyclo-C 6 H 12 + 2NaBr. Alkanes are also formed during decarboxylation of carboxylic acids and during their electrolysis.
Applications of alkanes
Saturated hydrocarbons are widely used in a wide variety of areas of human life and activity.
Gaseous alkanes (methane and propane-butane mixture) are used as valuable fuel. 
Liquid hydrocarbons make up a significant proportion of motor and rocket fuels and are used as solvents.
Vaseline oil (a mixture of liquid hydrocarbons with up to 15 carbon atoms) is a transparent, odorless and tasteless liquid, used in medicine, perfumery and cosmetics. 
Vaseline (a mixture of liquid and solid saturated hydrocarbons with up to 25 carbon atoms) is used to prepare ointments used in medicine.
Paraffin (a mixture of solid alkanes C 19 -C 35) - a white solid mass without odor and taste (mp 50-70 ° C) - used for making candles, impregnating matches and wrapping paper, for thermal procedures in medicine. Serves as a raw material for the production of organic acids and alcohols, detergents and surfactants. 
Normal saturated hydrocarbons of average molecular weight are used as a nutrient substrate in the microbiological synthesis of protein from oil.
Of great importance are the halogen derivatives of alkanes, which are used as solvents, coolants and raw materials for further syntheses. In the modern petrochemical industry, saturated hydrocarbons are the basis for the production of various organic compounds, an important raw material in the processes of obtaining intermediates for the production of plastics, rubbers, synthetic fibers, detergents and many other substances.
Alkanes- saturated (saturated) hydrocarbons. A representative of this class is methane ( CH 4). All subsequent saturated hydrocarbons differ by CH 2- a group that is called a homologous group, and compounds are called homologues.
General formula - WITHnH 2 n +2 .
Structure of alkanes.
Each carbon atom is in sp 3- hybridization, forms 4 σ - communications (1 S-S and 3 S-N). The shape of the molecule is in the form of a tetrahedron with an angle of 109.5°.
The bond is formed through the overlap of hybrid orbitals, with the maximum area of overlap lying in space on the straight line connecting the atomic nuclei. This is the most efficient overlap, so the σ bond is considered the strongest.
Isomerism of alkanes.
For alkanes isomerism of the carbon skeleton is characteristic. Limit connections can take on different geometric shapes while maintaining the angle between the connections. For example,
The different positions of the carbon chain are called conformations. Under normal conditions, the conformations of alkanes freely transform into each other through the rotation of C-C bonds, which is why they are often called rotary isomers. There are 2 main conformations - “inhibited” and “eclipsed”:
Isomerism of the carbon skeleton of alkanes.
The number of isomers increases with increasing carbon chain growth. For example, butane has 2 isomers:
For pentane - 3, for heptane - 9, etc.
If a molecule alkane subtract one proton (hydrogen atom), you get a radical:
Physical properties of alkanes.
Under normal conditions - C 1 -C 4- gases , From 5 to From 17- liquids, and hydrocarbons with more than 18 carbon atoms - solids.
As the chain grows, the boiling and melting points increase. Branched alkanes have lower boiling points than normal ones.
Alkanes insoluble in water, but soluble in non-polar organic solvents. Mix easily with each other.
Preparation of alkanes.
Synthetic methods for producing alkanes:
1. From unsaturated hydrocarbons - the “hydrogenation” reaction occurs under the influence of a catalyst (nickel, platinum) and at a temperature:
2. From halogen derivatives - Wurtz reaction: the interaction of monohaloalkanes with sodium metal, resulting in alkanes with double the number of carbon atoms in the chain:
3. From salts of carboxylic acids. When a salt reacts with an alkali, alkanes are obtained that contain 1 less carbon atom compared to the original carboxylic acid:
4. Production of methane. In an electric arc in a hydrogen atmosphere:
C + 2H 2 = CH 4.
In the laboratory, methane is obtained as follows:
Al 4 C 3 + 12H 2 O = 3CH 4 + 4Al(OH) 3.
Chemical properties of alkanes.
Under normal conditions, alkanes are chemically inert compounds; they do not react with concentrated sulfuric and nitric acid, with concentrated alkali, or with potassium permanganate.
Stability is explained by the strength of the bonds and their non-polarity.
Compounds are not prone to bond breaking reactions (addition reactions); they are characterized by substitution.
1. Halogenation of alkanes. Under the influence of a light quantum, radical substitution (chlorination) of the alkane begins. General scheme:
The reaction follows a chain mechanism, in which there are:
A) Initiating the circuit:
B) Chain growth:
B) Open circuit:
In total it can be presented as:
2. Nitration (Konovalov reaction) of alkanes. The reaction occurs at 140 °C:
The reaction proceeds most easily with the tertiary carbon atom than with the primary and secondary ones.
3. Isomerization of alkanes. Under specific conditions, alkanes of normal structure can transform into branched ones:
4. Cracking alkane. Under the action of high temperatures and catalysts, higher alkanes can break their bonds, forming alkenes and lower alkanes:
5. Oxidation of alkanes. Under different conditions and with different catalysts, alkane oxidation can lead to the formation of alcohol, aldehyde (ketone) and acetic acid. Under conditions of complete oxidation, the reaction proceeds to completion - until water and carbon dioxide are formed:
Application of alkanes.
Alkanes have found wide application in industry, in the synthesis of oil, fuel, etc.
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