Alkanes :
Alkanes are saturated hydrocarbons, in the molecules of which all atoms are linked by single bonds. Formula -
Physical properties :
- Melting and boiling points increase with molecular weight and carbon backbone length
- Under normal conditions, unbranched alkanes from CH 4 to C 4 H 10 are gases; from C 5 H 12 to C 13 H 28 - liquids; after C 14 H 30 - solids.
- Melting and boiling points decrease from less branched to more branched. So, for example, at 20 ° C n-pentane is a liquid and neopentane is a gas.
Chemical properties:
· Halogenation
this is one of the substitution reactions. First of all, the least hydrogenated carbon atom is halogenated (tertiary atom, then secondary, primary atoms are halogenated last). Halogenation of alkanes takes place 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).
Under the influence of light, the chlorine molecule decomposes into radicals, then they attack the alkane molecules, taking away a hydrogen atom from them, as a result of which methyl radicals · CH 3 are formed, which collide with chlorine molecules, destroying them and forming new radicals.
· Combustion
The main chemical property of saturated hydrocarbons, which determine their use as fuel, is the combustion reaction. Example:
CH 4 + 2O 2 → CO 2 + 2H 2 O + Q
In the event of a lack of oxygen, carbon monoxide or coal is produced instead of carbon dioxide (depending on the oxygen concentration).
In general, the reaction of alkane combustion can be written as follows:
FROM n H 2 n +2 +(1,5n +0.5) O 2 \u003d n CO 2 + ( n +1) H 2 O
· Decomposition
Decomposition reactions occur only under the influence of high temperatures. An increase in temperature leads to the breaking of the carbon bond and the formation of free radicals.
Examples:
CH 4 → C + 2H 2 (t\u003e 1000 ° C)
C 2 H 6 → 2C + 3H 2
Alkenes :
Alkenes are unsaturated hydrocarbons containing in the molecule, in addition to single bonds, one double carbon-carbon bond. Formula- C n H 2n
The belonging of a hydrocarbon to the class of alkenes is reflected by the generic suffix –en in its name.
Physical properties :
- Melting and boiling points of alkenes (simplified) increase with molecular weight and length of the main carbon chain.
- Under normal conditions, alkenes with C 2 H 4 to C 4 H 8 are gases; from C 5 H 10 to C 17 H 34 - liquids, after C 18 H 36 - solids. Alkenes are insoluble in water, but readily soluble in organic solvents.
Chemical properties :
· Dehydration is the process of splitting off a water molecule from an organic compound molecule.
· Polymerization is a chemical process of combining many of the original molecules of a low molecular weight substance into large polymer molecules.
Polymer is a high molecular weight compound, the molecules of which consist of many identical structural units.
Alkadienes :
Alkadienes are unsaturated hydrocarbons containing in the molecule, in addition to single bonds, double carbon-carbon bonds.
... Dienes are structural isomers of alkynes.Physical properties :
Butadiene is a gas (bp −4.5 ° C), isoprene is a liquid boiling at 34 ° C, dimethylbutadiene is a liquid boiling at 70 ° C. Isoprene and other diene hydrocarbons are capable of polymerizing to rubber. Natural rubber in a purified state is a polymer with the general formula (C5H8) n and is obtained from the milky sap of some tropical plants.
The rubber is readily soluble in benzene, gasoline, carbon disulfide. It becomes brittle at low temperatures, sticky when heated. To improve the mechanical and chemical properties of rubber, it is converted into rubber by vulcanization. To obtain rubber products, they are first molded from a mixture of rubber with sulfur, as well as with fillers: soot, chalk, clay and some organic compounds that serve to accelerate vulcanization. Then the products are heated - hot vulcanization. When vulcanized, sulfur chemically bonds with rubber. In addition, in vulcanized rubber, sulfur is contained in a free state in the form of tiny particles.
Diene hydrocarbons polymerize easily. The polymerization reaction of diene hydrocarbons underlies the synthesis of rubber. Enter into addition reactions (hydrogenation, halogenation, hydrohalogenation):
H 2 C \u003d CH-CH \u003d CH 2 + H 2 -\u003e H 3 C-CH \u003d CH-CH 3
Alkyne :
Alkines are unsaturated hydrocarbons whose molecules contain, in addition to single bonds, one triple carbon-carbon bond. Formula-C n H 2n-2
Physical properties :
Alkines are similar in physical properties to the corresponding alkenes. Lower (up to C 4) - gases without color and odor, having higher boiling points than analogs in alkenes.
Alkines are poorly soluble in water, better in organic solvents.
Chemical properties :
Halogenation reactions
Alkynes are able to add one or two halogen molecules to form the corresponding halogen derivatives:
Hydration
In the presence of mercury salts, alkynes add water to form acetaldehyde (for acetylene) or ketone (for other alkynes)
Hydrocarbons are the simplest organic compounds. They are made up of carbon and hydrogen. The compounds of these two elements are called saturated hydrocarbons or alkanes. Their composition is expressed by the general formula for alkanes CnH2n + 2, where n is the number of carbon atoms.
Alkanes are the international name for these compounds... These compounds are also called paraffins and saturated hydrocarbons. The bond in the alkane molecules is simple (or single). The rest of the valencies are saturated with hydrogen atoms. All alkanes are saturated with hydrogen to the limit, its atoms are in a state of sp3 hybridization.
Homologous series of saturated hydrocarbons
Methane is the first in the homologous series of saturated hydrocarbons. Its formula is CH4. The ending -an in the name of saturated hydrocarbons is a distinctive feature. Further, in accordance with the above formula, ethane - C2H6, propane C3H8, butane - C4H10 are located in the homologous series.
From the fifth alkane in the homologous series, the names of the compounds are formed as follows: a Greek number indicating the number of hydrocarbon atoms in the molecule + the ending -an. So, in Greek, the number 5 is pendé, respectively, after butane is pentane - C5H12. Next is hexane C6H14. heptane - C7H16, octane - C8H18, nonane - C9H20, decane - C10H22, etc.
The physical properties of alkanes change markedly in the homologous series: the melting and boiling points increase, and the density increases. Methane, ethane, propane, butane under normal conditions, that is, at a temperature of about 22 degrees Celsius, are gases, from pentane to hexadecane inclusive - liquids, with heptadecane - solids. Starting with butane, alkanes have isomers.
There are tables reflecting changes in the homologous series of alkanes, which clearly reflect their physical properties.
Nomenclature of saturated hydrocarbons, their derivatives
If a hydrogen atom is abstracted from a hydrocarbon molecule, then monovalent particles are formed, which are called radicals (R). The name of the radical is given by the hydrocarbon from which this radical is derived, and the ending -an changes to the ending -yl. For example, when a hydrogen atom is removed from methane, a methyl radical is formed, from ethane - ethyl, from propane - propyl, etc.
Radicals are also formed by inorganic compounds. For example, by subtracting the OH hydroxyl group from nitric acid, one can obtain the monovalent radical -NO2, which is called the nitro group.
When detached from the molecule alkane of two hydrogen atoms, divalent radicals are formed, the names of which are also formed from the names of the corresponding hydrocarbons, but the ending changes to:
- orien, in the event that the hydrogen atoms are torn off from one carbon atom,
- ylene, in the event that two hydrogen atoms are separated from two adjacent carbon atoms.
Alkanes: chemical properties
Let's consider the reactions typical for alkanes. All alkanes share common chemical properties. These substances are inactive.
All known reactions involving hydrocarbons are divided into two types:
- cleavage of the CH bond (an example is the substitution reaction);
- breaking the C-C bond (cracking, formation of separate parts).
Radicals are very active at the moment of formation. By themselves, they exist for a fraction of a second. Radicals easily react with each other. Their unpaired electrons form a new covalent bond. Example: CH3 + CH3 → C2H6
Radicals react easily with molecules of organic substances. They either attach to them, or tear off an atom with an unpaired electron from them, as a result of which new radicals appear, which, in turn, can enter into reactions with other molecules. This chain reaction produces macromolecules that stop growing only when the chain breaks (example: combining two radicals)
Free radical reactions explain many important chemical processes, such as:
- Explosions;
- Oxidation;
- Oil cracking;
- Polymerization of unsaturated compounds.
In detail you can consider the chemical properties saturated hydrocarbons by the example of methane. Above, we have already considered the structure of the alkane molecule. Carbon atoms in the methane molecule are in a state of sp3 hybridization, and a fairly strong bond is formed. Methane is a base gas of odor and color. It is lighter than air. Slightly soluble in water.
Alkanes can burn. Methane burns with a bluish pale flame. In this case, the result of the reaction will be carbon monoxide and water. When mixed with air, as well as in a mixture with oxygen, especially if the volume ratio is 1: 2, these hydrocarbons form explosive mixtures, which makes it extremely dangerous for use in everyday life and mines. If the methane is not completely burned, soot is formed. In industry, this is how they get it.
Formaldehyde and methyl alcohol are obtained from methane by oxidation in the presence of catalysts. If methane is heated strongly, then it decomposes according to the formula CH4 → C + 2H2
Methane decay can be carried out up to the intermediate product in specially equipped ovens. The intermediate is acetylene. The reaction formula is 2CH4 → C2H2 + 3H2. The separation of acetylene from methane cuts production costs by almost half.
Hydrogen is also obtained from methane by reforming methane with steam. Substitution reactions are characteristic of methane. So, at ordinary temperatures, in the light, halogens (Cl, Br) displace hydrogen from the methane molecule in stages. Thus, substances called halogen derivatives are formed. Chlorine atomsreplacing hydrogen atoms in a hydrocarbon molecule, they form a mixture of different compounds.
This mixture contains chloromethane (CH3 Cl or methyl chloride), dichloromethane (CH2Cl2 or methylene chloride), trichloromethane (CHCl3 or chloroform), carbon tetrachloride (CCl4 or carbon tetrachloride).
Any of these compounds can be isolated from the mixture. In production, chloroform and carbon tetrachloride are of great importance, due to the fact that they are solvents of organic compounds (fats, resins, rubber). Methane halogenated derivatives are formed by a free radical chain mechanism.
Light affects chlorine molecules as a result of which they disintegrate to inorganic radicals that rip off a hydrogen atom with one electron from the methane molecule. This forms HCl and methyl. Methyl reacts with a chlorine molecule, resulting in a halogen derivative and a chlorine radical. Further, the chlorine radical continues the chain reaction.
At normal temperatures, methane is sufficiently resistant to alkalis, acids, and many oxidants. The exception is nitric acid. In reaction with it, nitromethane and water are formed.
Addition reactions for methane are not typical, since all valencies in its molecule are saturated.
Reactions in which hydrocarbons are involved can take place not only with the cleavage of the C-H bond, but also with the cleavage of the C-C bond. Such transformations occur in the presence of high temperatures. and catalysts. These reactions include dehydrogenation and cracking.
Acids are obtained from saturated hydrocarbons by oxidation - acetic (from butane), fatty acids (from paraffin).
Methane production
Natural methane widely distributed. It is the main constituent of most combustible natural and artificial gases. It stands out from the coal seams in the mines, from the bottom of the swamps. Natural gases (which is very noticeable in the associated gases of oil fields) contain not only methane, but also other alkanes. The use of these substances is varied. They are used as fuel in various industries, in medicine and technology.
Under laboratory conditions, this gas is released by heating a mixture of sodium acetate + sodium hydroxide, as well as by the reaction of aluminum carbide and water. Also, methane is obtained from simple substances. For this, the prerequisites are the heating and the catalyst. The production of methane by synthesis based on water vapor is of industrial importance.
Methane and its homologues can be obtained by calcining the salts of the corresponding organic acids with alkalis. Another way to obtain alkanes is the Würz reaction, in which monohalogen derivatives with metallic sodium are heated.
Alkanes (methane and its homologues) have the general formula C nH 2 n+2. The first four hydrocarbons are called methane, ethane, propane, butane. The names of the highest members of this series consist of a root - a Greek numeral and the suffix -an. The alkane names are the basis for the IUPAC nomenclature.
Systematic nomenclature rules:
- Main chain rule.
The main circuit is selected based on the following successive criteria:
- The maximum number of functional substituents.
- The maximum number of multiple links.
- Maximum length.
- The maximum number of side hydrocarbon groups.
- Rule of least numbers (locants).
The main chain is numbered from one end to the other in Arabic numerals. Each substituent receives the number of the carbon atom of the main chain to which it is attached. The numbering sequence is chosen in such a way that the sum of the substituent (locant) numbers is the smallest. This rule also applies to the numbering of monocyclic compounds.
- The radical rule.
All hydrocarbon side groups are considered monovalent (simply connected) radicals. If the side radical itself contains side chains, then according to the above rules, an additional main chain is selected in it, which is numbered starting with the carbon atom attached to the main chain.
- Rule of alphabetical order.
The name of a compound begins with a listing of the substituents, indicating their names in alphabetical order. The name of each substituent is preceded by its number in the main chain. The presence of several substituents is indicated by numerator prefixes: di-, tri-, tetra-, etc. After that, the hydrocarbon corresponding to the main chain is called.
Table 12.1 lists the names of the first five hydrocarbons, their radicals, possible isomers and their corresponding formulas. The names of radicals end with the suffix -il.
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Table 12.1. Alkanes of the acyclopean series C n H 2 n +2 . |
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Example. Name all hexane isomers.
Example. Name the next alkane
In this example, of the two twelve-atomed chains, the one in which the sum of the numbers is the smallest is chosen (rule 2).
Using the names of the branched radicals given in table. 12.2,
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Table 12.2. The names of branched radicals. |
the name of this alkane is somewhat simplified:
10-tert-butyl-2,2- (dimethyl) -7-propyl-4-isopropyl-3-ethyl-dodecane.
When a hydrocarbon chain is closed into a cycle with the loss of two hydrogen atoms, monocycloalkanes are formed with the general formula C nH 2 n... Cyclization starts from C 3, names are derived from C n with a cyclo prefix:
Polycyclic alkanes. Their names are formed by the prefix bicyclo-, tricyclo-, etc. Bicyclic and tricyclic compounds contain, respectively, two and three cycles in the molecule, to describe their structure in square brackets indicate in decreasing order the number of carbon atoms in each of the chains connecting the nodal atoms ; under the formula the name of the atom:
This tricyclic hydrocarbon is usually called adamantane (from the Czech adamant - diamond), because it is a combination of three condensed cyclohexane rings in a form that leads to such an arrangement of carbon atoms in the crystal lattice, which is characteristic of diamond.
Cyclic hydrocarbons with one common carbon atom are called spirains, for example, spiro-5,5-undecane:
Planar cyclic molecules are unstable; therefore, various conformational isomers are formed. Unlike configurational isomers (the spatial arrangement of atoms in a molecule without regard to orientation), conformational isomers differ from each other only by rotation of atoms or radicals around formally simple bonds while maintaining the configuration of molecules. The energy of formation of a stable conformer is called conformational.
The conformers are in dynamic equilibrium and transform into each other through unstable forms. The instability of planar cycles is caused by a significant deformation of the bond angles. While maintaining tetrahedral bond angles for cyclohexane C 6H 12, two stable conformations are possible: in the form of a chair (a) and in the form of a bath (b):
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The chemical properties of saturated hydrocarbons are due to the presence of carbon atoms, hydrogen atoms and $ C-H $ and $ C-C $ bonds in their molecules. In the molecule of the simplest alkane methane, chemical bonds form 8 valence electrons (4 electrons of a carbon atom and 4 - hydrogen atoms), which are located on four linking molecular orbitals. So, in a methane molecule, four $ sp3-s (C-H) $ covalent bonds are formed from four $ sp3 $ -hybridized orbitals of the carbon atom and s-orbitals of four hydrogen atoms (Fig. 1). The ethane molecule is formed from two carbon tetrahedra - one $ sp3-sp3 (C-C) $ covalent bond and six $ sp3-s (C-H) $ covalent bonds (Fig. 2).
Figure 2. The structure of the ethane molecule: a - placement of $ \\ sigma $ -bonds in the molecule; b - tetrahedral model of the molecule; c - ball-rod model of the molecule; z - scale model of a molecule according to Stewart - Brigleb Features of chemical bonds in alkanesIn the considered types of covalent bonds, the regions of the highest electron density are located on the line connecting the atomic nuclei. These covalent bonds are formed by localized $ \\ sigma $ - $ (\\ rm M) $$ (\\ rm O) $ and are called $ \\ sigma $ -bonds. An important feature of these bonds is that the electron density in them is distributed symmetrically about the axis passing through the nuclei of atoms (cylindrical symmetry of the electron density). Thanks to this, atoms or groups of atoms that are connected by this bond can rotate freely without causing deformation of the bond. The angle between the directions of the valences of carbon atoms in alkane molecules is $ 109 ^ \\ circ 28 "$. Therefore, in the molecules of these substances, even with a straight carbon chain, carbon atoms are actually not arranged in a straight line. This chain has a zigzag shape, which is associated with the preservation of the intervalent angles of atoms carbon (Fig. 3).
Figure 3. Scheme of the structure of the carbon chain of a normal alkane In alkane molecules with a sufficiently long carbon chain, this angle is increased by $ 2 ^ \\ circ $ due to the repulsion of valence unconnected carbon atoms. Remark 1 Each chemical bond is characterized by a certain energy. It has been experimentally established that the bond energy of $ C-H $ in the methane molecule is 422.9 kJ / mol, ethane - 401.9 kJ / mol, and other alkanes - about 419 kJ / mol. The bond energy of $ C-C $ is 350 kJ / mol. The relationship between the structure of alkanes and their reactivityThe high energy of the $ C-C $ and $ C-H $ bonds results in the low reactivity of saturated hydrocarbons at room temperature. So, alkanes do not discolor bromine water, potassium permanganate solution, do not interact with ionic reagents (acids, alkalis), do not react with oxidants, with active metals. Therefore, for example, sodium metal can be stored in kerosene, which is a mixture of saturated hydrocarbons. Even concentrated sulfuric acid, which charring many organic substances, does not affect alkanes at room temperature. Given the relatively low reactivity of saturated hydrocarbons, they were once called paraffins. Alkanes do not have the ability to add hydrogen, halogens and other reagents. Therefore, this class of organic substances was called saturated hydrocarbons. Chemical reactions of saturated hydrocarbons can occur due to the cleavage of $ C-C $ or $ C-H $ bonds. The cleavage of $ C-H $ -bonds is accompanied by the elimination of hydrogen atoms with the formation of unsaturated compounds or the subsequent replacement of the elimination of hydrogen atoms by other atoms or groups of atoms. Depending on the structure of the alkane and the reaction conditions in the molecules of saturated hydrocarbons, the $ C-H $ bond can break homolytically:
Figure 4. Chemical properties of alkanes And heterolytic with the formation of anions and cations: Figure 5. Chemical properties of alkanes In this case, free radicals can be formed that have an unpaired electron, but do not have an electric charge, or carbocations or carbanions, which have corresponding electric charges. Free radicals are formed as intermediate particles in reactions of the radical mechanism, while carbocations and carbanions are formed in reactions of the ionic mechanism. Due to the fact that the $ C-C $ bonds are non-polar, and the $ C-H $ -bonds are low-polarity and these $ \\ sigma $ -bonds have low polarizability, the heterolytic cleavage of $ \\ sigma $ -bonds in alkane molecules with the formation of ions requires a lot of energy. Hemolytic cleavage of these bonds requires less energy. Therefore, for saturated hydrocarbons, reactions proceeding by a radical mechanism are more characteristic. The splitting of the $ \\ sigma $ -bond $ C-C $ requires less energy than the splitting of the $ C-H $ bond, since the energy of the $ C-C $ -bond is less than the energy of the $ C-H $ -bond. However, chemical reactions more often occur with the cleavage of $ C-H $ -bonds, since they are more accessible to reagents. Effect of branching and size of alkanes on their reactivityThe reactivity of the $ C-H $ -bond changes when going from linear alkanes to branched alkanes. For example, the dissociation energy of the $ C-H $ bond (kJ / mol) during the formation of free radicals changes as follows:
Figure 6. Chemical properties of alkanes In addition, the value of the ionization energy (EI) for alkanes shows that an increase in the total number of $ \\ sigma $ -bonds increases their donor properties and it becomes easier to split off an electron for compounds with a higher molecular weight, for example: Figure 7. Chemical properties of alkanes So, in free radical processes, reactions occur mainly at the tertiary carbon atom, then at the secondary and last of all at the primary, which coincides with the stability series of free radicals. However, as the temperature rises, the observed tendency decreases or even completely disappears. Thus, alkanes are characterized by two types of chemical reactions:
Acyclic hydrocarbons are called alkanes. There are 390 alkanes in total. Nonacontatrictan has the longest structure (C 390 H 782). Halogens can be attached to carbon atoms to form haloalkanes. Structure and nomenclatureBy definition, alkanes are saturated or saturated hydrocarbons with a linear or branched structure. Also called paraffins. Alkane molecules 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 with the fifth representative, the names are made up of the prefix denoting the number of carbon atoms and the suffix -an. For example, octa (eight) forms octane. For branched chains, the names are added:
Example: 4-methylpropane - there is a radical (methyl) at the fourth carbon atom in the propane chain.
Figure: 1. Structural formulas with the names of alkanes. Every tenth alkane gives the name to the next nine alkanes. After the decan there are undecane, dodecane and further, after eicosane - heneicosan, docosane, tricosan, etc. Homological seriesThe first representative is methane, therefore alkanes are also called the homologous series of methane. The first 20 representatives are indicated in the alkane table.
Starting with butane, all alkanes have structural isomers. The prefix iso- is added to the name: isobutane, isopropane, isohexane.
Figure: 2. Examples of isomers. Physical propertiesThe 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 a solid state.
Gaseous alkanes burn with blue or colorless flames. ReceivingAlkanes, like other classes of hydrocarbons, are obtained from oil, gas, coal. For this, laboratory and industrial methods are used:
Alkanes can be obtained by hydrogenation of alkenes and alkynes in the presence of a catalyst - platinum, nickel, palladium. Chemical propertiesAlkanes react with inorganic substances:
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