Alkanes all formulas. Structural isomerism of alkanes. Examples of problem solving
Acyclic hydrocarbons are called alkanes. There are 390 alkanes in total. Nonacontatrictan (C 390 H 782) has the longest structure. Halogens can attach to carbon atoms to form haloalkanes.
Structure and nomenclature
By definition, alkanes are saturated or saturated hydrocarbons having a linear or branched structure. Also called paraffins. Alkanes contain only single covalent bonds between carbon atoms. General formula -
To name a substance, you must follow the rules. According to the international nomenclature, names are formed using the suffix -an. The names of the first four alkanes have developed historically. Starting from the fifth representative, the names are made up of a prefix indicating the number of carbon atoms, and the suffix -an. For example, octa (eight) makes octane.
For branched chains, the names add up:
- from the numbers indicating the numbers of carbon atoms around which the radicals stand;
- from the name of the radicals;
- from the name of the main chain.
Example: 4-methylpropane - the fourth carbon atom in the propane chain has a radical (methyl).
Rice. 1. Structural formulas with the names of alkanes.
Every tenth alkane names the next nine alkanes. After decane come undecane, dodecane, and so on; after eicosan, geneicosan, docosan, tricosan, etc.
homologous series
The first representative is methane, therefore alkanes are also called the homologous series of methane. The table of alkanes shows the first 20 representatives.
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Name |
Formula |
Name |
Formula |
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Tridecan |
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Tetradecane |
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Pentadecan |
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Hexadecane |
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Heptadecane |
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Octadecan |
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Nanadekan |
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Starting with butane, all alkanes have structural isomers. The prefix iso- is added to the name: isobutane, isopentane, isohexane.
Rice. 2. Examples of isomers.
Physical properties
The aggregate state of substances changes in the list of homologues from top to bottom. The more carbon atoms are contained and, accordingly, the greater the molecular weight of the compounds, the higher the boiling point and the harder the substance.
The rest of the substances containing more than 15 carbon atoms are in the solid state.
Gaseous alkanes burn with a blue or colorless flame.
Receipt
Alkanes, like other classes of hydrocarbons, are obtained from oil, gas, and coal. For this, laboratory and industrial methods are used:
- solid fuel gasification:
C + 2H 2 → CH 4;
- hydrogenation of carbon monoxide (II):
CO + 3H 2 → CH 4 + H 2 O;
- aluminum carbide hydrolysis:
Al 4 C 3 + 12H 2 O → 4Al (OH) 3 + 3CH 4;
- reaction of aluminum carbide with strong acids:
Al 4 C 3 + H 2 Cl → CH 4 + AlCl 3;
- reduction of haloalkanes (substitution reaction):
2CH 3 Cl + 2Na → CH 3 -CH 3 + 2NaCl;
- hydrogenation of haloalkanes:
CH 3 Cl + H 2 → CH 4 + HCl;
- fusion of salts of acetic acid with alkalis (Dumas reaction):
CH 3 COONa + NaOH → Na 2 CO 3 + CH 4.
Alkanes can be obtained by hydrogenation of alkenes and alkynes in the presence of a catalyst - platinum, nickel, palladium.
Chemical properties
Alkanes react with inorganic substances:
- combustion:
CH 4 + 2O 2 → CO 2 + 2H 2 O;
- halogenation:
CH 4 + Cl 2 → CH 3 Cl + HCl;
- nitration (Konovalov reaction):
CH 4 + HNO 3 → CH 3 NO 2 + H 2 O;
- connection:
One of the first types of chemical compounds studied in school curriculum according to organic chemistry, are alkanes. They belong to the group of saturated (otherwise - aliphatic) hydrocarbons. Their molecules contain only single bonds. Carbon atoms are characterized by sp³ hybridization.
Homologs are called chemical substances who have general properties and chemical structure, but differing by one or more CH2 groups.
In the case of methane CH4, the general formula for alkanes can be given: CnH (2n+2), where n is the number of carbon atoms in the compound.
Here is a table of alkanes, in which n is in the range from 1 to 10.
Isomerism of alkanes
Isomers are those substances whose molecular formula is the same, but the structure or structure is different.
The class of alkanes is characterized by 2 types of isomerism: carbon skeleton and optical isomerism.
Let us give an example of a structural isomer (i.e., a substance that differs only in the structure of the carbon skeleton) for butane C4H10.
Optical isomers are called such 2 substances, the molecules of which have a similar structure, but cannot be combined in space. The phenomenon of optical or mirror isomerism occurs in alkanes, starting with heptane C7H16.
To give the alkane the correct name, use the IUPAC nomenclature. To do this, use the following sequence of actions:
According to the above plan, let's try to give a name to the next alkane.
Under normal conditions, unbranched alkanes from CH4 to C4H10 are gaseous substances, from C5H12 to C13H28 they are liquid and have a specific odor, all subsequent ones are solid. It turns out that as the length of the carbon chain increases, the boiling and melting points increase. The more branched the structure of an alkane, the lower the temperature at which it boils and melts.
Gaseous alkanes are colorless. And also all representatives of this class cannot be dissolved in water.
Alkanes that have state of aggregation gas, may burn, while the flame will either be colorless or have a pale blue tint.
Chemical properties
Under normal conditions, alkanes are rather inactive. This is explained by the strength of σ-bonds between atoms C-C and C-H. Therefore, it is necessary to provide special conditions (for example, a fairly high temperature or light) in order to conduct chemical reaction became possible.
Substitution reactions
Reactions of this type include halogenation and nitration. Halogenation (reaction with Cl2 or Br2) occurs when heated or under the influence of light. During the reaction proceeding sequentially, haloalkanes are formed.
For example, you can write the reaction of chlorination of ethane.
Bromination will proceed in a similar manner.
Nitration is a reaction with a weak (10%) solution of HNO3 or with nitric oxide (IV) NO2. Conditions for carrying out reactions - temperature 140 °C and pressure.
C3H8 + HNO3 = C3H7NO2 + H2O.
As a result, two products are formed - water and an amino acid.
Decomposition reactions
Decomposition reactions always require a high temperature. This is necessary to break bonds between carbon and hydrogen atoms.
So, when cracking temperature required between 700 and 1000 °C. During the reaction, -C-C- bonds are destroyed, a new alkane and alkene are formed:
C8H18 = C4H10 + C4H8
An exception is the cracking of methane and ethane. As a result of these reactions, hydrogen is released and alkyne acetylene is formed. Prerequisite is heating up to 1500 °C.
C2H4 = C2H2 + H2
If you exceed the temperature of 1000 ° C, you can achieve pyrolysis with a complete rupture of bonds in the compound:
During the pyrolysis of propyl, carbon C was obtained, and hydrogen H2 was also released.
Dehydrogenation reactions
Dehydrogenation (hydrogen elimination) occurs differently for different alkanes. The reaction conditions are a temperature in the range from 400 to 600 ° C, as well as the presence of a catalyst, which can be nickel or platinum.
From a compound with 2 or 3 C atoms in the carbon skeleton, an alkene is formed:
C2H6 = C2H4 + H2.
If there are 4-5 carbon atoms in the chain of the molecule, then after dehydrogenation, alkadiene and hydrogen will be obtained.
C5H12 = C4H8 + 2H2.
Starting with hexane, during the reaction, benzene or its derivatives are formed.
C6H14 = C6H6 + 4H2
We should also mention the conversion reaction carried out for methane at a temperature of 800 °C and in the presence of nickel:
CH4 + H2O = CO + 3H2
For other alkanes, the conversion is uncharacteristic.
Oxidation and combustion
If an alkane heated to a temperature of not more than 200 ° C interacts with oxygen in the presence of a catalyst, then the products obtained will differ depending on other reaction conditions: these may be representatives of the classes of aldehydes, carboxylic acids, alcohols or ketones.
When complete oxidation alkane burns to end products - water and CO2:
C9H20 + 14O2 = 9CO2 + 10H2O
If during the oxidation the amount of oxygen was insufficient, the final product instead of carbon dioxide will become coal or CO.
Carrying out isomerization
If a temperature of about 100-200 degrees is provided, a rearrangement reaction becomes possible for unbranched alkanes. The second mandatory condition for isomerization is the presence of an AlCl3 catalyst. In this case, the structure of the molecules of the substance changes and its isomer is formed.
Significant the share of alkanes is obtained by separating them from natural raw materials. Most often, natural gas is processed, the main component of which is methane, or oil is subjected to cracking and rectification.
You should also remember about the chemical properties of alkenes. In grade 10, one of the first laboratory methods studied in chemistry lessons is the hydrogenation of unsaturated hydrocarbons.
C3H6 + H2 = C3H8
For example, as a result of the addition of hydrogen to propylene, a single product is obtained - propane.
Using the Wurtz reaction, alkanes are obtained from monohaloalkanes, in the structural chain of which the number of carbon atoms is doubled:
2CH4H9Br + 2Na = C8H18 + 2NaBr.
Another way to obtain is the interaction of a salt of a carboxylic acid with an alkali when heated:
C2H5COONa + NaOH = Na2CO3 + C2H6.
In addition, methane is sometimes produced in an electric arc (C + 2H2 = CH4) or by reacting aluminum carbide with water:
Al4C3 + 12H2O = 3CH4 + 4Al(OH)3.
Alkanes are widely used in industry as a low cost fuel. And they are also used as raw materials for the synthesis of other organic matter. For this purpose, methane is usually used, which is necessary for and synthesis gas. Some other saturated hydrocarbons are used to obtain synthetic fats, and also as a base for lubricants.
For the best understanding of the topic "Alkanes", more than one video tutorial has been created, in which topics such as the structure of matter, isomers and nomenclature are discussed in detail, as well as the mechanisms of chemical reactions are shown.
Alkanes are saturated hydrocarbons, in the molecules of which all carbon atoms are occupied by hydrogen atoms through simple bonds. Therefore, the structural isomerism of alkanes is characteristic of the homologues of the methane series.
Isomerism of the carbon skeleton
Homologs with four or more carbon atoms are characterized by structural isomerism in terms of changes in the carbon skeleton. Methyl groups -CH 2 can attach to any carbon of the chain, forming new substances. The more carbon atoms in the chain, the more isomers homologues can form. The theoretical number of homologs is calculated mathematically.
Rice. 1. Approximate number of isomers of methane homologues.
In addition to methyl groups, long carbon chains can be attached to carbon atoms, forming complex branched substances.
Examples of isomerism of alkanes:
- normal butane or n-butane (CH 3 -CH 2 -CH 2 -CH 3) and 2-methylpropane (CH 3 -CH(CH 3) -CH 3);
- n-pentane (CH 3 -CH 2 -CH 2 -CH 2 -CH 3), 2-methylbutane (CH 3 -CH 2 -CH (CH 3) -CH 3), 2,2-dimethylpropane (CH 3 -C (CH 3) 2 -CH 3);
- n-hexane (CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3), 2-methylpentane (CH 3 -CH (CH 3) -CH 2 -CH 2 -CH 3), 3-methylpentane ( CH 3 -CH 2 -CH (CH 3) -CH 2 -CH 3), 2,3-dimethylbutane (CH 3 -CH (CH 3) -CH (CH 3) -CH 3), 2,2-dimethylbutane ( CH 3 -C(CH 3) 2 -CH 2 -CH 3).
Rice. 2. Examples of structural isomers.
Branched isomers differ from linear molecules in their physical properties. Branched alkanes melt and boil at lower temperatures than their linear counterparts.
Nomenclature
The international IUPAC nomenclature has established rules for naming branched chains. To name a structural isomer, one should:
- find the longest chain and name it;
- number the carbon atoms, starting from the end, where there are the most substituents;
- indicate the number of identical substituents with numerical prefixes;
- name substitutes.
The name consists of four parts, going one after another:
- numbers denoting chain atoms that have substituents;
- numerical prefixes;
- the name of the substitute;
- the name of the main circuit.
For example, in a CH 3 -CH (CH 3) -CH 2 -C (CH 3) 2 -CH 3 molecule, the main chain has five carbon atoms. So it's pentane. The right end has more branches, so the numbering of atoms starts from here. In this case, the second atom has two identical substituents, which is also reflected in the name. It turns out that this substance has the name 2,2,4-trimethylpentane.
Various substituents (methyl, ethyl, propyl) are listed alphabetically in the name: 4,4-dimethyl-3-ethylheptane, 3-methyl-3-ethyloctane.
Usually, numerical prefixes from two to four are used: di- (two), tri- (three), tetra- (four).
What have we learned?
Alkanes are characterized by structural isomerism. Structural isomers are common to all homologues, starting with butane. In structural isomerism, substituents are attached to carbon atoms in the carbon chain, forming complex branched chains. The name of the isomer consists of the names of the main chain, substituents, the verbal designation of the number of substituents, the digital designation of the carbon atoms to which the substituents are attached.
The simplest organic compounds are hydrocarbons composed of carbon and hydrogen. Depending on the nature of the chemical bonds in hydrocarbons and the ratio between carbon and hydrogen, they are divided into saturated and unsaturated (alkenes, alkynes, etc.)
limiting Hydrocarbons (alkanes, hydrocarbons of the methane series) are compounds of carbon with hydrogen, in the molecules of which each carbon atom spends no more than one valency on the connection with any other neighboring atom, and all valences not spent on the connection with carbon are saturated with hydrogen. All carbon atoms in alkanes are in the sp 3 state. Limit hydrocarbons form a homologous series characterized by the general formula WITH n H 2n+2. The ancestor of this series is methane.
Isomerism. Nomenclature.
Alkanes with n=1,2,3 can only exist as one isomer
Starting from n=4, the phenomenon of structural isomerism appears.
The number of structural isomers of alkanes increases rapidly with an increase in the number of carbon atoms, for example, pentane has 3 isomers, heptane has 9, etc.
The number of alkane isomers also increases due to possible stereoisomers. Starting from C 7 H 16, the existence of chiral molecules is possible, which form two enantiomers.
Alkanes nomenclature.
The dominant nomenclature is the IUPAC nomenclature. At the same time, it contains elements of trivial names. Thus, the first four members of the homologous series of alkanes have trivial names.
CH 4 - methane
C 2 H 6 - ethane
C 3 H 8 - propane
C 4 H 10 - butane.
The names of the remaining homologues are derived from Greek Latin numerals. So, for the following members of a series of normal (unbranched) structure, the names are used:
C 5 H 12 - pentane, C 6 H 14 - hexane, C 7 H 18 - heptane,
C 14 H 30 - tetradecane, C 15 H 32 - pentadecane, etc.
Basic IUPAC rules for branched alkanes
a) choose the longest unbranched chain, the name of which is the basis (root). The suffix "an" is added to this stem.
b) number this chain according to the principle of least locants,
c) the substitute is indicated in the form of prefixes in alphabetical order, indicating the location. If there are several identical substituents in the parent structure, then their number is indicated by Greek numerals.
Depending on the number of other carbon atoms with which the considered carbon atom is directly connected, there are distinguished: primary, secondary, tertiary and quaternary carbon atoms.
As substituents in branched alkanes, alkyl groups or alkyl radicals appear, which are considered as the result of the elimination of one hydrogen atom from the alkane molecule.
The name of the alkyl groups is formed from the name of the corresponding alkanes by replacing the last suffix "an" with the suffix "il".
CH 3 - methyl
CH 3 CH 2 - ethyl
CH 3 CH 2 CH 2 - propyl
For the name of branched alkyl groups, chain numbering is also used:
Starting from ethane, alkanes are able to form conformers, which correspond to the hindered conformation. The possibility of transition from one hindered conformation to another through the eclipsed conformation is determined by the rotation barrier. Determining the structure, composition of conformers, and barriers to rotation are the tasks of conformational analysis. Methods for obtaining alkanes.
1. Fractional distillation natural gas or gasoline fraction of oil. In this way, individual alkanes up to 11 carbon atoms can be isolated.
2. Hydrogenation of coal. The process is carried out in the presence of catalysts (oxides and sulfides of molybdenum, tungsten, nickel) at 450-470 about C and pressures up to 30 MPa. Coal and catalyst are ground into powder and hydrogenated in suspension by bubbling hydrogen through the suspension. The resulting mixtures of alkanes and cycloalkanes are used as motor fuels.
3. Hydrogenation of CO and CO 2 .
CO + H 2 alkanes
CO 2 + H 2 alkanes
Co, Fe, etc. are used as catalysts for these reactions. d - elements.
4.Hydrogenation of alkenes and alkynes.
5.organometallic synthesis.
A). Wurtz synthesis.
2RHal + 2Na R R + 2NaHal
This synthesis is of little use if two different haloalkanes are used as organic reagents.
b). Protolysis of Grignard reagents.
R Hal + Mg RMgHal
RMgHal + HOH RH + Mg(OH)Hal
V). Interaction of lithium dialkylcuprates (LiR 2 Cu) with alkyl halides
LiR 2 Cu + R X R R + RCu + LiX
Lithium dialkylcuprates themselves are obtained in a two-stage method
2R Li + CuI LiR 2 Cu + LiI
6. Electrolysis of salts of carboxylic acids (Kolbe synthesis).
2RCOONa + 2H 2 O R R + 2CO 2 + 2NaOH + H 2
7. Fusion of salts of carboxylic acids with alkalis.
The reaction is used to synthesize lower alkanes.
8.Hydrogenolysis of carbonyl compounds and haloalkanes.
A). carbonyl compounds. Synthesis of Clemmens.
b). Halogenalkanes. catalytic hydrogenolysis.
Ni, Pt, Pd are used as catalysts.
c) Halogenalkanes. Reactive recovery.
RHal + 2HI RH + HHal + I 2
Chemical properties of alkanes.
All bonds in alkanes are of low polarity; therefore, they are characterized by radical reactions. The absence of pi bonds makes addition reactions impossible. Alkanes are characterized by substitution, elimination, and combustion reactions.
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Type and name of the reaction | |
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1. Substitution reactions | |
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A) with halogens(With chlorineCl 2 – in the light, Br 2 - when heated) the reaction obeys Markovnik's rule (Markovnikov's rules) - first of all, the halogen replaces the hydrogen at the least hydrogenated carbon atom. The reaction takes place in stages - no more than one hydrogen atom is replaced in one stage. Iodine reacts most difficultly, and moreover, the reaction does not go to the end, since, for example, when methane reacts with iodine, hydrogen iodide is formed, which reacts with methyl iodide to form methane and iodine (reversible reaction): |
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 (tetrachloromethane). |
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B) Nitration (Konovalov's reaction) Alkanes react with a 10% solution of nitric acid or nitrogen oxide N 2 O 4 in the gas phase at a temperature of 140 ° and low pressure to form nitro derivatives. The reaction also obeys Markovnikov's rule. One of the hydrogen atoms is replaced by a NO 2 residue (nitro group) and water is released |
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2. Elimination reactions | |
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A) dehydrogenation- removal of hydrogen. Reaction conditions catalyst-platinum and temperature. |
CH 3 - CH 3 → CH 2 \u003d CH 2 + H 2 |
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B) cracking the process of thermal decomposition of hydrocarbons, which is based on the reactions of splitting the carbon chain of large molecules with the formation of compounds with a shorter chain. At a temperature of 450–700 o C, alkanes decompose due to the breaking of C–C bonds (stronger C–H bonds are retained at this temperature) and alkanes and alkenes with a smaller number of carbon atoms are formed |
C 6 H 14 C 2 H 6 +C 4 H 8 |
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B) complete thermal decomposition |
CH 4 C + 2H 2 |
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3. Oxidation reactions | |
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A) combustion reaction When ignited (t = 600 o C), alkanes react with oxygen, while they are oxidized to carbon dioxide and water. |
С n Н 2n+2 + O 2 ––> CO 2 + H 2 O + Q CH 4 + 2O 2 ––> CO 2 + 2H 2 O + Q |
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B) Catalytic oxidation- at a relatively low temperature and with the use of catalysts, it is accompanied by the breaking of only a part of the C–C bonds, approximately in the middle of the molecule and C–H, and is used to obtain valuable products: carboxylic acids, ketones, aldehydes, alcohols. |
For example, with incomplete oxidation of butane (breaking the C 2 -C 3 bond), acetic acid is obtained |
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4. Isomerization reactions not typical for all alkanes. Attention is drawn to the possibility of converting some isomers into others, the presence of catalysts. |
C 4 H 10 C 4 H 10 |
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5.. Alkanes with 6 or more carbon backbones also react dehydrocyclization, but always form a 6-membered cycle (cyclohexane and its derivatives). Under the reaction conditions, this cycle undergoes further dehydrogenation and turns into an energetically more stable benzene cycle of an aromatic hydrocarbon (arene). |
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Mechanism of halogenation reaction:
Halogenation
Halogenation of alkanes proceeds by a radical mechanism. To initiate the reaction, a mixture of alkane and halogen must be irradiated with UV light or heated. Chlorination of methane does not stop at the stage of obtaining methyl chloride (if equimolar amounts of chlorine and methane are taken), but leads to the formation of all possible substitution products, from methyl chloride to carbon tetrachloride. Chlorination of other alkanes leads to a mixture of hydrogen substitution products at different atoms carbon. The ratio of chlorination products depends on temperature. The rate of chlorination of primary, secondary, and tertiary atoms depends on temperature; at low temperatures, the rate decreases in the series: tertiary, secondary, primary. As the temperature rises, the difference between the speeds decreases until it becomes the same. In addition to the kinetic factor, the distribution of chlorination products is influenced by a statistical factor: the probability of an attack by chlorine on a tertiary carbon atom is 3 times less than the primary one and two times less than the secondary one. Thus, the chlorination of alkanes is a non-stereoselective reaction, except in cases where only one monochlorination product is possible.
Halogenation is one of the substitution reactions. Halogenation of alkanes obeys the Markovnik rule (Markovnikov's Rules) - the least hydrogenated carbon atom is halogenated first. Halogenation of alkanes takes place in stages - no more than one hydrogen atom is halogenated 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 (tetrachloromethane).
Under the action of light, the chlorine molecule decomposes into atoms, then they attack the methane molecules, tearing off their hydrogen atom, as a result of which methyl radicals CH 3 are formed, which collide with chlorine molecules, destroying them and forming new radicals.
Nitration (Konovalov's reaction)
Alkanes react with a 10% solution of nitric acid or nitrogen oxide N 2 O 4 in the gas phase at a temperature of 140 ° and low pressure to form nitro derivatives. The reaction also obeys Markovnikov's rule.
RH + HNO 3 \u003d RNO 2 + H 2 O
i.e., one of the hydrogen atoms is replaced by a NO 2 residue (nitro group) and water is released.
The structural features of the isomers strongly affect the course of this reaction, since it most easily leads to the substitution of a nitro group for a hydrogen atom in the SI residue (available only in some isomers), hydrogen is less easily replaced in the CH 2 group and even more difficult - in the CH 3 residue.
Paraffins are fairly easily nitrated in the gas phase at 150-475°C with nitrogen dioxide or nitric acid vapor; at the same time occurs partially and. oxidation. Nitration of methane produces almost exclusively nitromethane:
All available data point to a free radical mechanism. As a result of the reaction, mixtures of products are formed. Nitric acid at ordinary temperature has almost no effect on paraffinic hydrocarbons. When heated, it acts mainly as an oxidizing agent. However, as M. I. Konovalov (1889) found, when heated, nitric acid acts in part in a “nitrating” way; the nitration reaction with weak nitric acid proceeds especially well when heated and at elevated pressure. The nitration reaction is expressed by the equation.
The homologues following methane give a mixture of different nitroparaffins due to the accompanying cleavage. When ethane is nitrated, nitroethane CH 3 -CH 2 -NO 2 and nitromethane CH 3 -NO 2 are obtained. From propane, a mixture of nitroparaffins is formed:
Nitration of paraffins in the gas phase is now carried out on an industrial scale.
Sulfachlorination:
A practically important reaction is the sulfochlorination of alkanes. When an alkane interacts with chlorine and sulfur dioxide during irradiation, hydrogen is replaced by a chlorosulfonyl group:
The steps for this reaction are:
Cl+R:H→R+HCl
R + SO 2 → RSO 2
RSO 2 + Cl:Cl→RSO 2 Cl+Cl
Alkanesulfonic chlorides are easily hydrolyzed to alkanesulfoxylates (RSO 2 OH), whose sodium salts (RSO 3 ¯ Na + - sodium alkane sulfonate) exhibit properties similar to soaps and are used as detergents.
DEFINITION
Alkanes saturated hydrocarbons are called, the molecules of which consist of carbon and hydrogen atoms, linked to each other only by σ-bonds.
Under normal conditions (at 25 o C and atmospheric pressure), the first four members of the homologous series of alkanes (C 1 - C 4) are gases. Normal alkanes from pentane to heptadecane (C 5 - C 17) - liquids, starting from C 18 and above - solids. As the relative molecular weight increases, the boiling and melting points of alkanes increase. With the same number of carbon atoms in a molecule, branched alkanes have lower boiling points than normal alkanes. The structure of the alkanes molecule using methane as an example is shown in fig. 1.
Rice. 1. The structure of the methane molecule.
Alkanes are practically insoluble in water, since their molecules are of low polarity and do not interact with water molecules. Liquid alkanes mix easily with each other. They dissolve well in non-polar organic solvents such as benzene, carbon tetrachloride, diethyl ether, etc.
Obtaining alkanes
The main sources of various saturated hydrocarbons containing up to 40 carbon atoms are oil and natural gas. Alkanes with a small number of carbon atoms (1 - 10) can be isolated by fractional distillation of natural gas or gasoline fraction of oil.
There are industrial (I) and laboratory (II) methods for obtaining alkanes.
C + H 2 → CH 4 (kat = Ni, t 0);
CO + 3H 2 → CH 4 + H 2 O (kat \u003d Ni, t 0 \u003d 200 - 300);
CO 2 + 4H 2 → CH 4 + 2H 2 O (kat, t 0).
— hydrogenation of unsaturated hydrocarbons
CH 3 -CH \u003d CH 2 + H 2 →CH 3 -CH 2 -CH 3 (kat \u003d Ni, t 0);
— reduction of haloalkanes
C 2 H 5 I + HI → C 2 H 6 + I 2 (t 0);
- alkaline melting reactions of salts of monobasic organic acids
C 2 H 5 -COONa + NaOH → C 2 H 6 + Na 2 CO 3 (t 0);
- interaction of haloalkanes with metallic sodium (Wurtz reaction)
2C 2 H 5 Br + 2Na → CH 3 -CH 2 -CH 2 -CH 3 + 2NaBr;
– electrolysis of salts of monobasic organic acids
2C 2 H 5 COONa + 2H 2 O → H 2 + 2NaOH + C 4 H 10 + 2CO 2;
K (-): 2H 2 O + 2e → H 2 + 2OH -;
A (+): 2C 2 H 5 COO - -2e → 2C 2 H 5 COO + → 2C 2 H 5 + + 2CO 2.
Chemical properties of alkanes
Alkanes are among the least reactive organic compounds, which is explained by their structure.
Alkanes under normal conditions do not react with concentrated acids, molten and concentrated alkalis, alkali metals, halogens (except fluorine), potassium permanganate and potassium dichromate in an acidic environment.
For alkanes, reactions proceeding according to the radical mechanism are most characteristic. Energetically more favorable homolytic gap C-H bonds and C-C than their heterolytic gap.
Radical substitution reactions most easily proceed at the tertiary carbon atom, more easily at the secondary, and last of all, at the primary carbon atom.
All chemical transformations of alkanes proceed with splitting:
1) C-H bonds
- halogenation (S R)
CH 4 + Cl 2 → CH 3 Cl + HCl ( hv);
CH 3 -CH 2 -CH 3 + Br 2 → CH 3 -CHBr-CH 3 + HBr ( hv).
- nitration (S R)
CH 3 -C (CH 3) H-CH 3 + HONO 2 (dilute) → CH 3 -C (NO 2) H-CH 3 + H 2 O (t 0).
– sulfochlorination (S R)
R-H + SO 2 + Cl 2 → RSO 2 Cl + HCl ( hv).
– dehydrogenation
CH 3 -CH 3 → CH 2 \u003d CH 2 + H 2 (kat \u003d Ni, t 0).
— dehydrocyclization
CH 3 (CH 2) 4 CH 3 → C 6 H 6 + 4H 2 (kat = Cr 2 O 3, t 0).
2) C-H and C-C bonds
- isomerization (intramolecular rearrangement)
CH 3 -CH 2 -CH 2 -CH 3 →CH 3 -C (CH 3) H-CH 3 (kat \u003d AlCl 3, t 0).
- oxidation
2CH 3 -CH 2 -CH 2 -CH 3 + 5O 2 → 4CH 3 COOH + 2H 2 O (t 0, p);
C n H 2n + 2 + (1.5n + 0.5) O 2 → nCO 2 + (n + 1) H 2 O (t 0).
Application of alkanes
Alkanes have found application in various industries. Let us consider in more detail, using the example of some representatives of the homologous series, as well as mixtures of alkanes.
Methane is the raw material basis of the most important chemical industrial processes for producing carbon and hydrogen, acetylene, oxygen-containing organic compounds - alcohols, aldehydes, acids. Propane is used as an automotive fuel. Butane is used to produce butadiene, which is a raw material for the production of synthetic rubber.
A mixture of liquid and solid alkanes up to C 25, called vaseline, is used in medicine as the basis for ointments. A mixture of solid alkanes C 18 - C 25 (paraffin) is used to impregnate various materials (paper, fabrics, wood) to give them hydrophobic properties, i.e. water impermeability. In medicine, it is used for physiotherapeutic procedures (paraffin treatment).
Examples of problem solving
EXAMPLE 1
| Exercise | When chlorinating methane, 1.54 g of the compound was obtained, the vapor density in air of which is 5.31. Calculate the mass of manganese dioxide MnO 2 that will be required to produce chlorine if the ratio of the volumes of methane and chlorine introduced into the reaction is 1:2. |
| Solution | The ratio of the mass of a given gas to the mass of another gas taken in the same volume, at the same temperature and the same pressure, is called the relative density of the first gas over the second. This value shows how many times the first gas is heavier or lighter than the second gas. The relative molecular weight of air is taken equal to 29 (taking into account the content of nitrogen, oxygen and other gases in the air). It should be noted that the concept of "relative molecular weight of air" is used conditionally, since air is a mixture of gases. Let's find the molar mass of the gas formed during the chlorination of methane: M gas \u003d 29 × D air (gas) \u003d 29 × 5.31 \u003d 154 g / mol. This is carbon tetrachloride - CCl 4 . We write the reaction equation and arrange the stoichiometric coefficients: CH 4 + 4Cl 2 \u003d CCl 4 + 4HCl. Calculate the amount of carbon tetrachloride substance: n(CCl 4) = m(CCl 4) / M(CCl 4); n (CCl 4) \u003d 1.54 / 154 \u003d 0.01 mol. According to the reaction equation n (CCl 4) : n (CH 4) = 1: 1, then n (CH 4) \u003d n (CCl 4) \u003d 0.01 mol. Then, the amount of chlorine substance should be equal to n(Cl 2) = 2 × 4 n(CH 4), i.e. n(Cl 2) \u003d 8 × 0.01 \u003d 0.08 mol. We write the reaction equation for the production of chlorine: MnO 2 + 4HCl \u003d MnCl 2 + Cl 2 + 2H 2 O. The number of moles of manganese dioxide is 0.08 moles, because n (Cl 2) : n (MnO 2) = 1: 1. Find the mass of manganese dioxide: m (MnO 2) \u003d n (MnO 2) × M (MnO 2); M (MnO 2) \u003d Ar (Mn) + 2 × Ar (O) \u003d 55 + 2 × 16 \u003d 87 g / mol; m (MnO 2) \u003d 0.08 × 87 \u003d 10.4 g. |
| Answer | The mass of manganese dioxide is 10.4 g. |
EXAMPLE 2
