Saturated hydrocarbons are compounds that are molecules consisting of carbon atoms in a state of sp 3 hybridization. They are connected to each other exclusively by covalent sigma bonds. The name "saturated" or "saturated" hydrocarbons comes from the fact that these compounds do not have the ability to attach any atoms. They are extreme, completely saturated. The exception is cycloalkanes.
What are alkanes?
Alkanes are saturated hydrocarbons, and their carbon chain is open and consists of carbon atoms connected to each other using single bonds. It does not contain other (that is, double, like alkenes, or triple, like alkyls) bonds. Alkanes are also called paraffins. They received this name because well-known paraffins are a mixture of predominantly these saturated hydrocarbons C 18 -C 35 with particular inertness.
General information about alkanes and their radicals
Their formula: C n P 2 n +2, here n is greater than or equal to 1. The molar mass is calculated using the formula: M = 14n + 2. Characteristic feature: the endings in their names are “-an”. The residues of their molecules, which are formed as a result of the replacement of hydrogen atoms with other atoms, are called aliphatic radicals, or alkyls. They are designated by the letter R. The general formula of monovalent aliphatic radicals: C n P 2 n +1, here n is greater than or equal to 1. The molar mass of aliphatic radicals is calculated by the formula: M = 14n + 1. A characteristic feature of aliphatic radicals: endings in the names “- silt." Alkane molecules have their own structural features:
- The C-C bond is characterized by a length of 0.154 nm;
- The C-H bond is characterized by a length of 0.109 nm;
- the bond angle (the angle between carbon-carbon bonds) is 109 degrees and 28 minutes.
Alkanes begin the homologous series: methane, ethane, propane, butane, and so on.
Physical properties of alkanes
Alkanes are substances that are colorless and insoluble in water. The temperature at which alkanes begin to melt and the temperature at which they boil increase in accordance with the increase in molecular weight and hydrocarbon chain length. From less branched to more branched alkanes, the boiling and melting points decrease. Gaseous alkanes can burn with a pale blue or colorless flame and produce quite a lot of heat. CH 4 -C 4 H 10 are gases that also have no odor. C 5 H 12 -C 15 H 32 are liquids that have a specific odor. C 15 H 32 and so on are solids that are also odorless.
Chemical properties of alkanes
These compounds are chemically inactive, which can be explained by the strength of difficult-to-break sigma bonds - C-C and C-H. It is also worth considering that C-C bonds are non-polar, and C-H bonds are low-polar. These are low-polarized types of bonds belonging to the sigma type and, accordingly, they are most likely to be broken by a homolytic mechanism, as a result of which radicals will be formed. Thus, the chemical properties of alkanes are mainly limited to radical substitution reactions.
Nitration reactions
Alkanes react only with nitric acid with a concentration of 10% or with tetravalent nitrogen oxide in a gaseous environment at a temperature of 140°C. The nitration reaction of alkanes is called the Konovalov reaction. As a result, nitro compounds and water are formed: CH 4 + nitric acid (diluted) = CH 3 - NO 2 (nitromethane) + water.
Combustion reactions
Saturated hydrocarbons are very often used as fuel, which is justified by their ability to burn: C n P 2n+2 + ((3n+1)/2) O 2 = (n+1) H 2 O + n CO 2.
Oxidation reactions
The chemical properties of alkanes also include their ability to oxidize. Depending on what conditions accompany the reaction and how they are changed, different end products can be obtained from the same substance. Mild oxidation of methane with oxygen in the presence of a catalyst accelerating the reaction and a temperature of about 200 ° C can result in the following substances:
1) 2CH 4 (oxidation with oxygen) = 2CH 3 OH (alcohol - methanol).
2) CH 4 (oxidation with oxygen) = CH 2 O (aldehyde - methanal or formaldehyde) + H 2 O.
3) 2CH 4 (oxidation with oxygen) = 2HCOOH (carboxylic acid - methane or formic) + 2H 2 O.
Also, the oxidation of alkanes can be carried out in a gaseous or liquid medium with air. Such reactions lead to the formation of higher fatty alcohols and corresponding acids.
Relation to heat
At temperatures not exceeding +150-250°C, always in the presence of a catalyst, a structural rearrangement of organic substances occurs, which consists of a change in the order of the connection of atoms. This process is called isomerization, and the substances resulting from the reaction are called isomers. Thus, from normal butane, its isomer is obtained - isobutane. At temperatures of 300-600°C and the presence of a catalyst, C-H bonds are broken with the formation of hydrogen molecules (dehydrogenation reactions), hydrogen molecules with the closure of the carbon chain into a cycle (cyclization or aromatization reactions of alkanes):
1) 2CH 4 = C 2 H 4 (ethene) + 2H 2.
2) 2CH 4 = C 2 H 2 (ethyne) + 3H 2.
3) C 7 H 16 (normal heptane) = C 6 H 5 - CH 3 (toluene) + 4 H 2.
Halogenation reactions
Such reactions involve the introduction of halogens (their atoms) into the molecule of an organic substance, resulting in the formation of a C-halogen bond. When alkanes react with halogens, halogen derivatives are formed. This reaction has specific features. It proceeds according to a radical mechanism, and in order to initiate it, it is necessary to expose the mixture of halogens and alkanes to ultraviolet radiation or simply heat it. The properties of alkanes allow the halogenation reaction to proceed until complete replacement with halogen atoms is achieved. That is, the chlorination of methane will not end in one stage and the production of methyl chloride. The reaction will go further, all possible substitution products will be formed, starting with chloromethane and ending with carbon tetrachloride. Exposure of other alkanes to chlorine under these conditions will result in the formation of various products resulting from the substitution of hydrogen at different carbon atoms. The temperature at which the reaction occurs will determine the ratio of the final products and the rate of their formation. The longer the hydrocarbon chain of an alkane, the easier this reaction will occur. During halogenation, the least hydrogenated (tertiary) carbon atom will be replaced first. The primary one will react after all the others. The halogenation reaction will occur in stages. In the first stage, only one hydrogen atom is replaced. Alkanes do not interact with halogen solutions (chlorine and bromine water).
Sulfochlorination reactions
The chemical properties of alkanes are also complemented by the sulfochlorination reaction (called the Reed reaction). When exposed to ultraviolet radiation, alkanes are able to react with a mixture of chlorine and sulfur dioxide. As a result, hydrogen chloride is formed, as well as an alkyl radical, which adds sulfur dioxide. The result is a complex compound that becomes stable due to the capture of a chlorine atom and the destruction of its next molecule: R-H + SO 2 + Cl 2 + ultraviolet radiation = R-SO 2 Cl + HCl. The sulfonyl chlorides formed as a result of the reaction are widely used in the production of surfactants.
Physical properties. Under normal conditions, 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 number of carbon atoms in the chain increases, i.e. As the relative molecular weight increases, the boiling and melting points of alkanes increase. With the same number of carbon atoms in the molecule, branched alkanes have lower boiling points than normal alkanes.
Alkanespractically insoluble in water, since their molecules are slightly polar and do not interact with water molecules, they dissolve well in non-polar organic solvents such as benzene, carbon tetrachloride, etc. Liquid alkanes are easily mixed with each other.
The main natural sources of alkanes are oil and natural gas. Various petroleum fractions contain alkanes from C5H12 to C 30 H 62. Natural gas consists of methane (95%) with an admixture of ethane and propane.
From synthetic methods for obtaining alkanes The following can be distinguished:/>
1 . Obtained from unsaturated hydrocarbons. The interaction of alkenes or alkynes with hydrogen (“hydrogenation”) occurs in the presence of metal catalysts (/>Ni, Pd ) at
heating:
CH z - C ≡CH+ 2H 2 → CH 3 -CH 2 -CH 3.
2. Receiving from halogen-conducted. When monohalogenated alkanes are heated with sodium metal, alkanes with double the number of carbon atoms are obtained (Wurtz reaction):
C 2 H 5 Br + 2 Na + Br - C 2 H 5 → C 2 H 5 - C 2 H 5 + 2 NaBr.
A similar reaction is not carried out with two different halogenated alkanes, since this produces a mixture of three different alkanes
3. Preparation from salts of carboxylic acids. When anhydrous salts of carboxylic acids are fused with alkalis, alkanes are obtained containing one less carbon atom compared to the carbon chain of the original carboxylic acids:
4.Methane production. An electric arc burning in a hydrogen atmosphere produces a significant amount of methane:
C + 2H 2 → CH 4 .
The same reaction occurs when carbon is heated in a hydrogen atmosphere to 400-500 °C at elevated pressure in the presence of a catalyst.
In laboratory conditions, methane is often obtained from aluminum carbide:
A l 4 C 3 + 12H 2 O = ZSN 4 + 4A l (OH) 3 .
Chemical properties. Under normal conditions, alkanes are chemically inert. They are resistant to the action of many reagents: they do not interact with concentrated sulfuric and nitric acids, with concentrated and molten alkalis, and are not oxidized by strong oxidizing agents - potassium permanganateKMn O 4, etc.
The chemical stability of alkanes is explained by their high strengths—C-C and C-H bonds, as well as their non-polarity. Non-polar C-C and C-H bonds in alkanes are not prone to ionic cleavage, but are capable of homolytic cleavage under the influence of active free radicals. Therefore, alkanes are characterized by radical reactions, which result in compounds where hydrogen atoms are replaced by other atoms or groups of atoms. Consequently, alkanes enter into reactions that proceed through the mechanism of radical substitution, denoted by the symbol S R ( from English, substitution radicalic). According to this mechanism, hydrogen atoms are most easily replaced at tertiary, then at secondary and primary carbon atoms.
1. Halogenation. When alkanes interact with halogens (chlorine and bromine) under the influence of UV radiation or high temperature, a mixture of products from mono- to polyhalogen-substituted alkanes The general scheme of this reaction is shown using methane as an example:
b) Growth of the chain. The chlorine radical removes a hydrogen atom from the alkane molecule:
Cl· + CH 4 →HC/>l + CH 3 ·
In this case, an alkyl radical is formed, which removes a chlorine atom from the chlorine molecule:
CH 3 + C l 2 →CH 3 C l + C l·
These reactions are repeated until the chain breaks in one of the reactions:
Cl· + Cl· → С l/> 2, СН 3 · + СН 3 · → С 2 Н 6, СН 3 · + Cl· → CH 3 С l ·
Overall reaction equation:
| hv | ||
| CH 4 + Cl 2 | → | CH 3 Cl + HCl. |
The resulting chloromethane can be further chlorinated, giving a mixture of products CH 2 Cl 2, CHCl 3, CC l 4 according to the scheme (*).
Development of chain theory free radical reactions is closely connected with the name of the outstanding Russian scientist, Nobel Prize winner N.I. Semenov (1896-1986).
2. Nitration (Konovalov reaction). When dilute nitric acid acts on alkanes at 140°C and low pressure, a radical reaction occurs:
In radical reactions (halogenation, nitration), hydrogen atoms at tertiary carbon atoms are first mixed, then at secondary and primary carbon atoms.This is explained by the fact that the bond between the tertiary carbon atom and hydrogen is most easily broken homolytically (bond energy 376 kJ/mol), then the secondary one (390 kJ/mol), and only then the primary one (415 kJ/mol).
3. Isomerization. Normal alkanes can, under certain conditions, transform into branched-chain alkanes:
4. Cracking is a hemolytic cleavage of C-C bonds, which occurs when heated and under the influence of catalysts.
When higher alkanes are cracked, alkenes and lower alkanes are formed; when methane and ethane are cracked, acetylene is formed:
C/> 8 H 18 → C 4 H 10 + C 4 H 8 ,/>
2CH 4 → C 2 H 2 + ZN 2,
C 2 H 6 → C 2 H 2 + 2H 2.
These reactions are of great industrial importance. In this way, high-boiling oil fractions (fuel oil) are converted into gasoline, kerosene and other valuable products.
5. Oxidation. By mild oxidation of methane with atmospheric oxygen in the presence of various catalysts, methyl alcohol, formaldehyde, and formic acid can be obtained:
 |
Mild catalytic oxidation of butane with atmospheric oxygen is one of the industrial methods for producing acetic acid:
t°
2 C 4/>H/>10 + 5 O/>2 → 4 CH/>3 COOH/>+ 2H 2 O .
cat
Alkanes in air burn to CO 2 and H 2 O:/>
С n Н 2 n +2 + (З n+1)/2O 2 = n CO 2 + (n +1) H 2 O.
DEFINITION
Alkanes– saturated (aliphatic) hydrocarbons, the composition of which is expressed by the formula C n H 2 n +2.
Alkanes form a homologous series, each chemical compound of which differs in composition from the next and previous ones by the same number of carbon and hydrogen atoms - CH 2, and the substances included in the homologous series are called homologues. The homologous series of alkanes is presented in Table 1.
Table 1. Homologous series of alkanes.
In alkane molecules, primary (i.e. connected by one bond), secondary (i.e. connected by two bonds), tertiary (i.e. connected by three bonds) and quaternary (i.e. connected by four bonds) carbon atoms are distinguished.
C 1 H3 – C 2 H 2 – C 1 H 3 (1 – primary, 2 – secondary carbon atoms)
CH 3 –C 3 H(CH 3) – CH 3 (3-tertiary carbon atom)
CH 3 – C 4 (CH 3) 3 – CH 3 (4-quaternary carbon atom)
Alkanes are characterized by structural isomerism (carbon skeleton isomerism). Thus, pentane has the following isomers:
CH 3 -CH 2 -CH 2 -CH 2 -CH 3 (pentane)
CH 3 –CH(CH 3)-CH 2 -CH 3 (2-methylbutane)
CH 3 -C(CH 3) 2 -CH 3 (2,2 – dimethylpropane)
Alkanes, starting with heptane, are characterized by optical isomerism.
The carbon atoms in saturated hydrocarbons are in sp 3 hybridization. The angles between bonds in alkane molecules are 109.5.
Chemical properties of alkanes
Under normal conditions, alkanes are chemically inert - they do not react with either acids or alkalis. This is explained by the high strength of C-C and C-H bonds. Non-polar C-C and C-H bonds can only be cleaved homolytically under the influence of active free radicals. Therefore, alkanes enter into reactions that proceed through the radical substitution mechanism. In radical reactions, hydrogen atoms are first replaced at tertiary carbon atoms, then at secondary and primary carbon atoms.
Radical substitution reactions have a chain nature. The main stages: nucleation (initiation) of the chain (1) - occurs under the influence of UV radiation and leads to the formation of free radicals, chain growth (2) - occurs due to the abstraction of a hydrogen atom from the alkane molecule; chain termination (3) – occurs when two identical or different radicals collide.
X:X → 2X . (1)
R:H+X . → HX + R . (2)
R . + X:X → R:X + X . (2)
R . + R . → R:R (3)
R . +X . → R:X (3)
X . +X . → X:X (3)
Halogenation. When alkanes interact with chlorine and bromine under the action of UV radiation or high temperature, a mixture of products from mono- to polyhalogen-substituted alkanes is formed:
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 (Konovalov reaction). When dilute nitric acid acts on alkanes at 140C and low pressure, a radical reaction occurs:
CH 3 -CH 3 +HNO 3 = CH 3 -CH 2 -NO 2 (nitroethane) + H 2 O
Sulfochlorination and sulfoxidation. Direct sulfonation of alkanes is difficult and is most often accompanied by oxidation, resulting in the formation of alkanesulfonyl chlorides:
R-H + SO 2 + Cl 2 → R-SO 3 Cl + HCl
The sulfonic oxidation reaction proceeds similarly, only in this case alkanesulfonic acids are formed:
R-H + SO 2 + ½ O 2 → R-SO 3 H
Cracking– radical cleavage of C-C bonds. Occurs when heated and in the presence of catalysts. When higher alkanes are cracked, alkenes are formed; when methane and ethane are cracked, acetylene is formed:
C 8 H 18 = C 4 H 10 (butane) + C 3 H 8 (propane)
2CH 4 = C 2 H 2 (acetylene) + 3H 2
Oxidation. The mild oxidation of methane with atmospheric oxygen can produce methanol, formic aldehyde or formic acid. In air, alkanes burn to carbon dioxide and water:
C n H 2 n +2 + (3n+1)/2 O 2 = nCO 2 + (n+1)H 2 O
Physical properties of alkanes
Under normal conditions, C 1 -C 4 are gases, C 5 -C 17 are liquids, and starting from C 18 are solids. Alkanes are practically insoluble in water, but are highly soluble in non-polar solvents, such as benzene. Thus, methane CH 4 (swamp, mine gas) is a colorless and odorless gas, highly soluble in ethanol, ether, hydrocarbons, but poorly soluble in water. Methane is used as a high-calorie fuel in natural gas, as a raw material for the production of hydrogen, acetylene, chloroform and other organic substances on an industrial scale.
Propane C 3 H 8 and butane C 4 H 10 are gases used in everyday life as bottled gases due to their easy liquefaction. Propane is used as a car fuel because it is more environmentally friendly than gasoline. Butane is the raw material for the production of 1,3-butadiene, which is used in the production of synthetic rubber.
Preparation of alkanes
Alkanes are obtained from natural sources - natural gas (80-90% - methane, 2-3% - ethane and other saturated hydrocarbons), coal, peat, wood, oil and rock wax.
There are laboratory and industrial methods for producing alkanes. In industry, alkanes are obtained from bituminous coal (1) or by the Fischer-Tropsch reaction (2):
nC + (n+1)H 2 = C n H 2 n +2 (1)
nCO + (2n+1)H 2 = C n H 2 n +2 + H 2 O (2)
Laboratory methods for producing alkanes include: hydrogenation of unsaturated hydrocarbons by heating and in the presence of catalysts (Ni, Pt, Pd) (1), the interaction of water with organometallic compounds (2), electrolysis of carboxylic acids (3), by decarboxylation reactions (4) and Wurtz (5) and in other ways.
R 1 -C≡C-R 2 (alkyne) → R 1 -CH = CH-R 2 (alkene) → R 1 -CH 2 – CH 2 -R 2 (alkane) (1)
R-Cl + Mg → R-Mg-Cl + H 2 O → R-H (alkane) + Mg(OH)Cl (2)
CH 3 COONa↔ CH 3 COO — + Na +
2CH 3 COO - → 2CO 2 + C 2 H 6 (ethane) (3)
CH 3 COONa + NaOH → CH 4 + Na 2 CO 3 (4)
R 1 -Cl +2Na +Cl-R 2 →2NaCl + R 1 -R 2 (5)
Examples of problem solving
EXAMPLE 1
| Exercise | Determine the mass of chlorine required for the first stage chlorination of 11.2 liters of methane. |
| Solution | Let us write the reaction equation for the first stage of methane chlorination (i.e., in the halogenation reaction, only one hydrogen atom is replaced, resulting in the formation of a monochlorine derivative): CH 4 + Cl 2 = CH 3 Cl + HCl (chloromethane) Let's find the amount of methane substance: v(CH 4) = V(CH 4)/V m v(CH 4) = 11.2/22.4 = 0.5 mol According to the reaction equation, the number of moles of chlorine and the number of moles of methane are equal to 1 mol, therefore, the practical number of moles of chlorine and methane will also be the same and will be equal to: v(Cl 2) = v(CH 4) = 0.5 mol Knowing the amount of chlorine, you can find its mass (which is what is asked in the problem). The mass of chlorine is calculated as the product of the amount of chlorine substance and its molar mass (molecular mass of 1 mole of chlorine; molecular mass is calculated using the table of chemical elements by D.I. Mendeleev). The mass of chlorine will be equal to: m(Cl 2) = v(Cl 2)×M(Cl 2) m(Cl 2) = 0.5×71 = 35.5 g |
| Answer | The mass of chlorine is 35.5 g |
DEFINITION
Alkanes are called saturated hydrocarbons, the molecules of which consist of carbon and hydrogen atoms connected 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) are liquids, starting from C 18 and above are solids. As the relative molecular weight increases, the boiling and melting points of alkanes increase. With the same number of carbon atoms in the molecule, branched alkanes have lower boiling points than normal alkanes. The structure of the alkane 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 low-polar 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.
Preparation of 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 the gasoline fraction of oil.
There are industrial (I) and laboratory (II) methods for producing alkanes.
C + H 2 → CH 4 (kat = Ni, t 0);
CO + 3H 2 → CH 4 + H 2 O (kat = Ni, t 0 = 200 - 300);
CO 2 + 4H 2 → CH 4 + 2H 2 O (kat, t 0).
— hydrogenation of unsaturated hydrocarbons
CH 3 -CH=CH 2 + H 2 →CH 3 -CH 2 -CH 3 (kat = 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 sodium metal (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, the most typical reactions are those that proceed by a radical mechanism. Homolytic cleavage of C-H and C-C bonds is energetically more favorable than their heterolytic cleavage.
Radical substitution reactions most easily occur at the tertiary carbon atom, then at the secondary carbon atom, and lastly 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 =CH 2 + H 2 (kat = 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=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).
Applications 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 forms the raw material basis for the most important chemical industrial processes for the production of carbon and hydrogen, acetylene, oxygen-containing organic compounds - alcohols, aldehydes, acids. Propane is used as automobile 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 of 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. non-wetting with water. In medicine it is used for physiotherapeutic procedures (paraffin treatment).
Examples of problem solving
EXAMPLE 1
| Exercise | When chlorinating methane, 1.54 g of a compound was obtained, the vapor density of which in air 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 to 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 to be 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 mass 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 = 29 × D air (gas) = 29 × 5.31 = 154 g/mol. This is carbon tetrachloride - CCl 4. Let's write the reaction equation and arrange the stoichiometric coefficients: CH 4 + 4Cl 2 = CCl 4 + 4HCl. Let's calculate the amount of carbon tetrachloride substance: n(CCl 4) = m(CCl 4) / M(CCl 4); n(CCl 4) = 1.54 / 154 = 0.01 mol. According to the reaction equation n(CCl 4) : n(CH 4) = 1: 1, which means n(CH 4) = n(CCl 4) = 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) = 8 × 0.01 = 0.08 mol. Let us write the reaction equation for the production of chlorine: MnO 2 + 4HCl = MnCl 2 + Cl 2 + 2H 2 O. The number of moles of manganese dioxide is 0.08 mol, because n(Cl 2) : n(MnO 2) = 1: 1. Find the mass of manganese dioxide: m(MnO 2) = n(MnO 2) ×M(MnO 2); M(MnO 2) = Ar(Mn) + 2×Ar(O) = 55 + 2×16 = 87 g/mol; m(MnO 2) = 0.08 × 87 = 10.4 g. |
| Answer | The mass of manganese dioxide is 10.4 g. |
EXAMPLE 2
| Exercise | Determine the molecular formula of trichloroalkane, the mass fraction of chlorine in which is 72.20%. Draw up the structural formulas of all possible isomers and give the names of the substances according to the IUPAC substitutive nomenclature. |
| Answer | Let's write down the general formula of trichloroalkean: C n H 2 n -1 Cl 3 . According to the formula ω(Cl) = 3×Ar(Cl) / Mr(C n H 2 n -1 Cl 3) × 100% Let's calculate the molecular weight of trichloroalkane: Mr(C n H 2 n -1 Cl 3) = 3 × 35.5 / 72.20 × 100% = 147.5. Let's find the value of n: 12n + 2n - 1 + 35.5×3 = 147.5; Therefore, the formula of trichloroalkane is C 3 H 5 Cl 3. Let's compose the structural formulas of the isomers: 1,2,3-trichloropropane (1), 1,1,2-trichloropropane (2), 1,1,3-trichloropropane (3), 1,1,1-trichloropropane (4) and 1 ,2,2-trichloropropane (5). CH 2 Cl-CHCl-CH 2 Cl (1); CHCl 2 -CHCl-CH 3 (2); CHCl 2 -CH 2 -CH 2 Cl (3); CCl 3 -CH 2 -CH 3 (4); Structure of 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. In alkane molecules, all carbon atoms are in the state sp 3 -hybridization. This means that all four hybrid orbitals of the carbon atom are identical in shape, energy and are directed towards the corners of an equilateral triangular pyramid - tetrahedron. The angles between the orbitals are 109° 28′. Almost free rotation is possible around a single carbon-carbon bond, and alkane molecules can take on a wide variety of shapes with angles at the carbon atoms close to tetrahedral (109° 28′), for example, in the n-pentane molecule.
It is especially worth recalling the bonds in alkane molecules. All bonds in the molecules of saturated hydrocarbons are single. The overlap occurs along the axis connecting the nuclei of atoms, i.e. it σ bonds. Carbon-carbon bonds are non-polar and poorly polarizable. The length of the C-C bond in alkanes is 0.154 nm (1.54 10 10 m). C-H bonds are somewhat shorter. The electron density is slightly shifted towards the more electronegative carbon atom, i.e. the C-H bond is weakly polar. Homologous series of methaneHomologs- substances that are similar in structure and properties and differ in one or more CH groups 2 .
Saturated hydrocarbons constitute the homologous series of methane. Isomerism and nomenclature of alkanesAlkanes are characterized by the so-called structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkane, which is characterized by structural isomers, is butane.
Let us consider in more detail the basic nomenclature for alkanes IUPAC. 1. Main circuit selection. 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. Numbering of main chain atoms. 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 (-CH 3), then propyl (-CH 2 -CH 2 -CH 3), ethyl (-CH 2 -CH 3 ) etc. Please note that the name of the substituent is formed by replacing the suffix -ane with the suffix -yl 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, a hyphen indicates the number of substituents (di - two, three - three, tetra - four, penta - five) and the name of the substituent (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 D: 2-methyl 4-ethylhexane. Absence of saturated hydrocarbons in molecules polar bonds leads to them poorly soluble in water, do not interact with charged particles (ions). The most characteristic reactions for alkanes are those involving free radicals. Physical properties of alkanesThe 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”, when you smell it, you need to call 04, is determined by the smell of mercaptans - sulfur-containing compounds specially added to methane used in household and industrial gas appliances so that people those near them could detect the leak by smell). Hydrocarbons of composition from WITH 5 N 12 before WITH 15 N 32 - 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 of alkanesSubstitution reactions.The most characteristic reactions for alkanes are free radical substitution, during which a hydrogen atom is replaced by a halogen atom or some group. Let us present the characteristic equations halogenation reactions: In case of excess halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms with chlorine:
The resulting substances are widely used as solvents and starting materials in organic syntheses. Dehydrogenation reaction(hydrogen abstraction).When alkanes are passed over a catalyst (Pt, Ni, Al 2 O 3, Cr 2 O 3) at high temperatures (400-600 °C), a hydrogen molecule is eliminated and a alkene: 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. 1. Combustion of saturated hydrocarbons is a free radical exothermic reaction, which is very important when using alkanes as fuel: In general, the combustion reaction of alkanes can be written as follows: 2. Thermal splitting of hydrocarbons. The process proceeds according to free radical mechanism. An increase in temperature leads to homolytic cleavage of the carbon-carbon bond and the formation of free radicals. These radicals interact with each other, exchanging a hydrogen atom, to form a molecule alkane and alkene molecule: Thermal decomposition reactions underlie the industrial process - hydrocarbon cracking. This process is the most important stage of oil refining. 3. Pyrolysis. When methane is heated to a temperature of 1000 °C, methane pyrolysis- decomposition into simple substances: When heated to a temperature of 1500 °C, the formation of acetylene: 4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with branched carbon skeleton:
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:
Alkanes enter into reactions that proceed according to the free radical mechanism, since 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 in which the electron density can shift under the influence of external factors (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since the bonds in alkane molecules are not broken by the heterolytic mechanism. |
