Alkaans and their isomers. Alkaans - definition, structure, physical and chemical properties. - cracking and pyrolysis of alkanes oil

Alkaans (limiting hydrocarbons, paraffins)

• Alkana - aliphatic (acyclic) extreme hydrocarbons in which carbon atoms are interconnected by simple (single) connections in unbranched or branched chains.

Alkana - The name of limit hydrocarbons on international nomenclature.
Paraffins- Historically established name, reflecting the properties of these compounds (from lat. parrum affinis - having little affinity, lowactive).
Limit, or saturatedThese hydrocarbons are called due to the full saturation of the carbon chain atoms of hydrogen.

The simplest representatives of Alkanov:

Molecules models:

When comparing these compounds it is clear that they differ from each other for the group. -CH 2 - (methylene). Adding another group to the propane -CH 2 -, I get Bhutan From 4 H 10, then alkana From 5 n 12, From 6 N 14 etc.

Now you can withdraw the general formula of alkanans. The number of carbon atoms in a row of alkanov we will take n. , then the number of hydrogen atoms will be the magnitude 2N + 2. . Consequently, the composition of alkanov corresponds to the general formula C n h 2n + 2.
Therefore, such a definition is used:

Alkana - hydrocarbons whose composition is expressed by the general formula C n h 2n + 2where n. - The number of carbon atoms.

The structure of alkanan

Chemical structure (the order of connecting atoms in molecules) of the simplest alkanes - methane, ethane and propane - show them structural formulasshown in Section 2. From these formulas it can be seen that there are two types of chemical ties in alkanes:

C-S. and C-N..

Communication C-C is a covalent non-polar. Communication C - H is a covalent weaklyolar, because Carbon and hydrogen are close by electronegability (2.5 - for carbon and 2.1 - for hydrogen). The formation of covalent bonds in alkanes due to the general electronic pairs of carbon and hydrogen atoms can be shown using electronic formulas:

Electronic and structural formulas reflect chemical structure but do not give ideas about the spatial structure of moleculeswhich significantly affects the properties of the substance.

Spatial structure. The relative arrangement of the molecule atoms in space depends on the direction of atomic orbitals (AO) of these atoms. In hydrocarbons, the main role is played by the spatial orientation of carbon orbitals, since the spherical 1S-AO of the hydrogen atom is deprived of a certain orientation.

The spatial arrangement of carbon AO in turn depends on the type of hybridization (part I, section 4.3). A rich carbon atom in alkanes is associated with four other atoms. Consequently, its state corresponds to SP 3-hybridization (part I, section 4.3.1). In this case, each of the four SP 3-hybrid carbon Ao is involved in the axial (σ-) overlapping from S-AO hydrogen or with SP 3 -AO of another carbon atom, forming σ-bond with S-H or C-C.

Four σ-bonds of carbon are directed in space at an angle of 109 ° C 28 ", which corresponds to the smallest repulsion of electrons. Therefore, the molecule of the simplest representative of alkanes - methane CH 4 - has the form of a tetrahedron, in the center of which there is a carbon atoms, and at the vertices - hydrogen atoms:

Valenny corner N-sn is equal 109 About 28 ". The spatial structure of methane can be shown using bulk (large-scale) and scale model models.

For recording it is convenient to use a spatial (stereoochemical) formula.

In the molecule of the next homologist - ethane with 2 H 6 - two tetrahedral sp. 3 carbon mats form a more complex spatial structure:

For alkanov molecules containing over 2 carbon atoms, curved forms are characteristic. This can be shown on the example n.-Butan (VRML model) or n.-Penta:

Isomeriya Alkanov

• Isomerius - the appearance of the compounds that have the same composition (the same molecular formula), but a different structure. Such compounds are called isomaers.

Differences in the order of the compound of atoms in molecules (i.e. in the chemical structure) lead to structural isomeria. The structure of structural isomers is reflected by structural formulas. In a number of alkanans, structural isomerism is manifested when the content of 4 and more carbon atoms in the circuit is, i.e. Starting with Bhutan with 4 H 10.
If in molecules of the same composition and the same chemical structure, various mutual arrangement of atoms in space is possible, then observed spatial isomerism (stereoisomeria). In this case, the use of structural formulas is not enough and models of molecules or special formulas should be used - stereochemical (spatial) or projection.

Alkanes, starting with Ethane H 3 C-CH 3, exist in various spatial forms ( conformations) due to intramolecular rotation of σ-links C-C, and manifest the so-called rotary (conformational) isomerism.

In addition, if there is a carbon atom in the molecule associated with 4 different substituents, another type of spatial isomerism is possible, when two stereoisomers belong to each other as an object and its mirror image (just as the left hand refers to the right). Such differences in the structure of molecules are called optical isomeria.

Structural isomeria Alkanov

• Structural isomers - compounds of the same composition, characterized by the procedure for binding atoms, i.e. Chemical structure of molecules.

The cause of the manifestation of structural isomerism in a row of alkans is the equipment of carbon atoms to form circuits of various structures. This type of structural isomerism is called isomeria carbon skeleton.

For example, Alkan composition C 4 H 10 may exist as two Structural isomers:

and Alkan C 5 H 12 - in the form three structural isomers, characterized by the structure of the carbon chain:

With an increase in the number of carbon atoms in the composition of molecules, the possibilities for branching the chain, i.e. The amount of isomers is growing a threshold of the number of carbon atoms.

Structural isomers are distinguished physical properties. Alkanes with a branched structure due to less dense packaging of molecules and, accordingly, smaller intermolecular interactions, boil the temperature than their unbranched isomers.

In the output of the structural formulas of the isomers, the following techniques use.

The purpose of the lesson: Creating conditions for the formation of knowledge among students on the characteristics of the structure, homologous series, isomerism, the nomenclature of alkanans, methods for obtaining them.

Tasks lesson:

Education: consider the homologous series of limit hydrocarbons, structure, physical properties, methods for their preparation in the processing of natural gas, the possibility of their preparation from natural sources: natural and passing oil gases, oil and stone coal.

Developing: Development of cognitive interests, creative and intellectual abilities, the development of independence in the acquisition of new knowledge using new technologies.

Educational: show the unity of the material world on the example of the genetic bond of hydrocarbons of different homologous series, obtained by processing natural and associated petroleum gas, oil and stone coal.

Equipment : Schemery model, computer, multimedia projector, screen, presentation.

During the classes

1. 
   Organizing time.
2. 
   Motivational block.

Teacher.We continue to study organic chemistry. I suggest moving to you 160 years ago, in good old England (Slide 2) 1848 15-year-old Hannah Griner died in Newcastle during a small surgical intervention. Doctor Meggison, who conducted an operation, during the inquiry so outlined facts. "I sat the patient in the chair and brought to her nose a handkerchief moistened with just one teaspoon of this substance. Hannah made two inhales. After a minute I asked my assistant to start the operation. Another minute, I opened her eyes, they remained open, the spool vessels were filled with blood, lips, and the face was very afraid. I decided to perform bloodletting from the veins, but got blood no more spoon. I think that before my attempt to bleed it was dead. Since the first inhalation of this substance, no more than three minutes passed to death. " What caused Hannah's death still a mystery. What do you think about what substance is we talking about? ( All assumptions are listened.)Chloroform. Find a formula. (Use directory, Internet.)SN 3 FROMl.. This substance is derived from CH 4. What class does this substance belong to? Alkana. (Slide 3)(Students write the subject of the lesson in a notebook).Alkaans, first class from all classes of organic substances that we will study. Therefore, I offer you a plan for which all classes of organic substances will be studied. (Slide 4).

Message Topics lesson: "Alkaans". (Slide 1)

Acquaintance with the study plan of alkanes . (Slide 3)

1. Definition. Formula. Structure. Structure. 2.Gomological row. Nomenclature. 3. Wides of isomeria.

4.physical properties. 5. Promotions

6.Chemical properties.

7. Application.

1. 
   Alkana. (Limit hydrocarbons. Paraffins. Saturated hydrocarbons.) (Slide 4)

Alkanes - hydrocarbons in molecules of which all carbon atoms are associated with single connections (-) and have a general formula:

C n h 2n + 2 (Slide 5)

1. 
   What is homologs? (Students answer this question)

Homological series of alkanov

Alkans, having a general formula with N h 2 n +2, are a series of related compounds with a similar structure in which each subsequent member differs from the previous one to the permanent group of atoms (-CH 2 -). Such a sequence of compounds is called homological near (from Greek. homolog - Slot), separate members of this series - homologists, and the group of atoms on which neighboring homologues differ in, - homological difference.

Exercise 1. From the listed hydrocarbons, select Alkans and make a homologous row, starting with the smallest, write their molecular formulas:

Ethan, propane, butane, ethylene, methane, acetylene, propane, butene, propin, hexane, octane, pentane, butene, heptane, dean.(slide 6)

Homological row (answer)(slide7)

Name radicals (Slide 8)

1. 
   The structure of alkanes - methane and homologues (Slide 9 - 10)

The carbon atom in all organic substances is in an "excited" state, i.e., has four unpaired electron at the external level.

Each electronic cloud has a reserve of energy: the S- cloud has a smaller stock of energy than the P-cloud, in the carbon atom they are in different energy states. Therefore, when education chemical bond Hybridization occurs, i.e. alignment of electronic clouds in energy supply. This is displayed on the shape and orientation of the clouds, there is a restructuring (spatial) electronic clouds.

As a result of SP3 - hybridization, all four valence electronic clouds are hybridized: the valence angle between these axes of hybridized clouds 109 ° 28 ", therefore the molecules have a spatial tetrahedral form, the shape of carbon chains is zigzagot; carbon atoms are not on one straight line, because when rotating Atoms valence angles remain the same.

Everything organic substances Built mainly due to covalent bonds. Carbon - carbon and carbon - hydrogen bonds relate to sigma - bonds - this is a connection formed by overlapping atomic orbitals along the line passing through the kernels of atoms. It is possible to rotate around sigma - connections, since this connection has an axial symmetry.

1. 
   Types of isomerism for alkanes: (Slide 11)

Isomerism - the appearance of the compounds that have

the same composition (the same molecular formula), but different

structure. Such compounds are called isomaers.

Differences in the order of the compound of atoms in molecules (i.e. in chemical

structure) lead to structural isomeria. Structural structure

isomers are reflected by structural formulas. In a row of alkanans, structural isomerism is manifested in the content of 4 and more carbon atoms in the chain, i.e., starting with Bhutan with 4 H 10. (Slide 12)

1. 
   Algorithm for drawing the name of alkanov (Slide 13)

Task 2. Give the names of the following hydrocarbons at the International Nomenclature of the Jew: (Slide 14)

CH 3.
but). CH 3 - CH 2 - C - CH 3 B). CH 3 - CH - CH - CH - CH 3
CH 3 CH 3 C 2N 5 CH 3

Task 3. Make the structural formulas of hydrocarbons in the following names of the International Nomenclature of the Republic of Jupak: (Slide 15)

but). 2,3,3 -Thetheyl-4-ethylhexane

b). 2,3,4 - trimethylgptan

1. 
   Physical properties. (Slide 16)

Homologists are distinguished by molecular weight and, therefore, physical characteristics. With an increase in the number of carbon atoms in the alkane molecule (with increasing molecular weight) in the homologous row is observed labor change physical properties of homologues (Transition of quantity in quality): boiling and melting temperatures increase, density increases.

CH 4 -C 4 H 10 - Gaza

T boiling: -161,6: -0,5 ° C

T Melting: -182,5: -138.3 ° C

C 5 H 12 -C 15 H 32 - liquid

T boiling: 36.1: 270.5 ° C

T Melting: -129.8: 10 ° C

T boiling: 287,5 ° C

T Melting: 20 ° C

Methane on Earth (slides 17)

Methane - Product of Life! (Slide 18)

The contribution of various factors in the formation of a total flux of methane into the atmosphere of the Earth (slide 20-21)

1. 
   Industrial methods for producing alkanes

(slide 22-25)

Alkans are isolated from natural sources (natural and associated gases, oil, stone coal).

Synthetic methods are also used.

1. Oil cracking (industrial method)

C n h 2 n +2  C m H 2 m +2 + C n - M H 2 (n - m)

alcan Alken

When cracking, alkanes are obtained along with unsaturated compounds.

(alkenes). This method is important in that the molecules of higher alkanans are obtained very valuable raw materials for organic synthesis: propane, butane, isobutane, isopentane, etc.

2. Isomerization of alkanes:

CH 3 - (CH 2) 6 -CH 3 CH 3 -C (CH 3) 2 -CH 2 -CH (CH 3) -CH 3

otan Isoocan
2. WITHOUT WITHOUT WITHOUT WITHOUT CARBODY:

C n h 2 n  C n h 2 n +2  C n h 2 n -2

alkens Alkana Alkina

3. Gasification of solid fuel (at elevated temperature and pressure, catalyst Ni): C + 2H 2  CH 4

4. From the synthesis gas (CO + H 2), a mixture of alkanes is obtained:

nCO + (2N + 1) H 2  C n h 2 n +2 + NH 2 O

Laboratory methods for producing alkanes (slide 26-35)

5. Synthesis of more complex alkanes from halogen-produced with a smaller number of coal atoms:

2ch 3 Cl + 2NA  CH 3 CH 3 + 2NACL (Synthesis Vup.c.)

chloromethane etan

6. From salts carboxylic acids:

a) alkali fusion (reaction Duma)

CH 3 COONA + NAOH  CH 4 + NA 2 CO 3

sodium acetate

b) electrolysis Colbe

electrolysis

2rCoona + 2H 2 O  RR + 2CO 2 + H 2 + 2NAOH

on the anode on the cathode

7. Determination of carbides of metals (methanides) with water:

Al 4 C 3 + 12H 2 O  4Al (OH) 3  + 3CH 4 

Synthesis of alkanov

Halogen derivatives of alkanes are widely used for alkane synthesis

with a given structure of molecules. For this use the reaction

their interactions with active metals (Nurez reaction):

C 2 H 5 I + 2NA + IC 2 H 5  C 4 H 10 + 2NAI

If in the reaction to use different halogen alkyls, then a mixture of three products is obtained. For example:). CH 3 - CH - CH 3 B). CH 3 - CH 2

IN). CH 2 - CH 2 g). SH 2 - CH 3

CH 3 CH 3 CH 2 - CH 3

3. The name of the hydrocarbon with the formula

CH 3 - CH 2 - CH 2 - CH - CH 3

but). 4-methylpenan b). 2-methylpropan

in). 3-methylpenan d). 2-methylpenan

4. Hydrocarbons with Formulas C 6 H 12 and C 6 H 14 are:

a) isomers,

b) homologists,

c) there is no right answer to the listed.

5. Aluminum carbide hydrolysis can be obtained:

but). Ethan B). Propane c). Methane d). Ethylene

IV. Homework: (Slide 38) Tutorial O.S. Gabrielyan Grade 10 (profile level): 11DO p.75,
Literature.

1. 
   Gorkovovenko M. Yu. Lesson development in chemistry to educational kits O. S. Gabrielyan et al., 10 (11) class. M.: "VECO", 2005
2. 
   Rybnikova ZD, Rybnikov A. V. Organic chemistry. Grade 10: Key topics. Abstract classes. Control and verification. - M.: Iris - Press, 2003
3. 
   Ulyanova G. M. Organic Chemistry. Grade 10: Toolkit. - SPb.: "Parity", 2003
4. 
   Electronic publication "Virtual School Kim, Chemistry Lessons 10 - 11 Classes"
5. 
   Internet resources.

It will not be understood from determining the concept of alkanans. This saturated or limit can also be said that these are carbon, in which the compound of C atoms is carried out by means of simple ties. The general formula has the form: CNH₂N + 2.

It is known that the ratio of the number of H and C atoms in their molecules is maximally, if compared with other classes. Due to the fact that all valences are occupied either C, or H, the chemical properties of alkanans are not pronounced enough, so they are the phrase of the limit or saturated hydrocarbons.

There is also an older name, which best reflects their relative cheerleeness - paraffins, which means "devoid affinations".

So, the topic of our today's conversation: "Alkaans: Homological row, nomenclature, structure, isomerism." The data will also be presented regarding their physical properties.

Alkaans: Building, Nomenclature

In them, atoms C are in such a state as SP3 hybridization. In connection with this molecule of alkanes, you can demonstrate as a set of tetrahedral structures C, which are associated not only among themselves, but also with H.

There are strong, very low-polar s-links between the C and H atoms. Atoms around the simple bonds always rotate, in view of which the alkanes molecules take various forms, and the length of the communication, the angle between them - constant values. Forms that are transformed into each other due to the rotation of the molecule occurring around σ-links, is customary called it by conformations.

In the process of separating the H atom from the considered molecule, 1-valence particles are formed, called hydrocarbon radicals. They appear as a result of compounds not only but also inorganic. If you take 2 hydrogen atoms from the limiting hydrocarbon molecule, 2-valence radicals are obtained.

Thus, alkanov nomenclature can be:

• radial (old option);
• replacement (international, systematic). It is offered Jew.

Features of radial nomenclature

In the first case, the nomenclature of alkanov is characterized as follows:

1. Consideration of hydrocarbons as methane derivatives, which substituted 1 or several H atoms with radicals.
2. High degree of convenience in case of not very complex connections.

Features of replacement nomenclature

Alkanov replacement nomenclature has the following features:

1. The basis for the name is 1 carbon chain, the remaining molecular fragments are considered as substituents.
2. If there are several identical radicals, the number (strictly in words) is specified before their name, and the radical numbers are separated by commas.

Chemistry: Alkanov Nomenclature

For convenience, information is presented as a table.

Name of substance	The basis of the name (root)	Molecular formula	The name of the carbon deputy	Formula carbon substituent

The above alkanov nomenclature includes names that have developed historically (the first 4 members of a number of limit hydrocarbons).

The names of non-verminated alkanans with 5 and more atoms C are formed from Greek numerals, which reflect this number of atoms C. So, suffix -an says that a substance from a number of saturated compounds.

When drawing up the names of the deployed alkanes as the main chain, the one is selected that contains the maximum number of C atoms. It is numbered so that the substituents are with the smallest number. In the case of two and more chains of the same length, the main thing becomes the largest number of substituents.

Isomeriya Alkanov

As a hydrocarbon-ancestor, their row is methane CH₄. With each subsequent representative of the methane row, there is a difference from the previous one on the methylene group - CH₂. This pattern is traced in the whole range of alkans.

German Scientist Schil put forward a proposal to name this row homologous. Translated from Greek means "similar, like".

Thus, the homologous series is a set of related organic compounds having a similar structure with close chemicals. Homologists - members of this series. Homological difference is a methylene group on which 2 adjacent homologues are distinguished.

As mentioned earlier, the composition of any saturated hydrocarbon can be expressed by the general formula of CNH₂n + 2. So, as follows from a methane a member of the homologous series, Ethan is C₂H₆. To derive its structure from methane, it is necessary to replace 1 H atom on the CH₃ (figure below).

The structure of each subsequent homologue can be removed from the previous one in the same way. As a result, propane - C₃H₈ is formed from the etha.

What is isomers?

These are substances that have identical qualitative and quantitative molecular composition (identical molecular formula), but a different chemical structure, as well as possessing different chimperations.

The elevated hydrocarbons differ in such a parameter as the boiling point: -0.5 ° - butane, -10 ° isobutane. This type of isomeria is referred to as the isomerism of a carbon skeleton, it refers to the structural type.

The number of structural isomers is growing rapidly with an increase in carbon atoms. Thus, C₁₀H₂₂ will correspond to 75 isomers (not including spatial), and 4347 isomers are already known for C₁₅H₃₂, for C₂₀H₄₂ - 366 319.

So, it has already become clear what is alkanes, a homologous series, isomerism, a nomenclature. Now it is worth moving to the rules for drawing the names of the Jew.

Nomenclature Jupak: Rules for the formation of titles

First, it is necessary to find a carbon chain in the hydrocarbon structure, which is the longest and contains the maximum number of substituents. Then it is necessary to numbered the atoms C chains, starting from the end to which the deputy is closest.

Secondly, the basis is the name of the unbranched saturated hydrocarbon, which in the number of C atoms corresponds to the main chain.

Thirdly, before the basis, you must specify the numbers of the lockers, near which the substituents are located. They are recorded through Defis the names of the substituents.

Fourth, in the case of the presence of identical substituents at different atoms, the lokes are combined, while the multiplying prefix appears before the title: Di - for two identical substituents, three - for three, tetra - four, penta - for five, etc. Numbers Must be separated from each other with a comma, and from words - a hyphen.

If the same atom C contains two deputies at once, the locker is also recorded twice.

According to these rules, an international nomenclature of alkanov is formed.

Newman's projections

This American scientist proposed for a graphic demonstration of conformations special projection formulas - Newman's projections. They correspond to the forms A and B and are presented in the figure below.

In the first case, this is a-obscured conformation, and in the second - b-injected. In positions, the H atoms are located at a minimum distance from each other. This form corresponds to the most great importance Energy, due to the fact that the repulsion between them is the largest. This is an energetically unprofitable state, as a result of which the molecule seeks to leave it and move to a more stable position B. Here atoms h are removed as much as possible from each other. Thus, the energy difference of these provisions is 12 kJ / mol, thanks to which free rotation around the axis in the molecule of ethane, which connects the methyl groups, it turns out to be uneven. After entering the energetically, the favorable position of the molecule is delayed there, in other words, "brakes". That is why it is called the inhibited. The result is 10 thousand. Ethal molecules are in the inhibited form of conformation under the condition of room temperature. Only one has another form - obscure.

Obtaining limit hydrocarbons

It has already been known from the article that these are alkanes (the structure, the nomenclature is described in detail before). It will not be worth considering ways to receive them. They are distinguished from natural sources such as oil, natural, stone coal. Synthetic methods also apply. For example, H₂ 2H₂:

1. CNH₂N hydrogenation process (alkenes) → CNH₂N + 2 (alkanes) ← CNH₂N-2 (alkina).
2. From a mixture of monooxide C and H - Synthesis gas: NCO + (2N + 1) H₂ → CNH₂N + 2 + NH₂O.
3. From carboxylic acids (their salts): Electrolysis on the anode, on the cathode:

• electrolysis Colle: 2rCoona + 2H₂o → R-R + 2CO₂ + H₂ + 2NAOH;
• duma reaction (alkali alloy): CH₃COONA + NaOH (T) → CH₄ + NA₂CO₃.

1. Correct oil: CNH₂n + 2 (450-700 °) → CMH₂M + 2 + CN-MH₂ (N-M).
2. Fuel gasification (solid): C + 2H₂ → CH₄.
3. Synthesis of complex alkanes (halogen production), which have a smaller number of atoms C: 2Ch₃Cl (chloromethane) + 2NA → CH₃-CH₃ (ethane) + 2NACL.
4. Decomposition of water methanides (metal carbides): Al₄c₃ + 12H₂o → 4Al (OH₃) ↓ + 3CH₄.

Physical properties of limiting hydrocarbons

For convenience, the data is grouped into a table.

Formula	Alkan	Melting temperature in ° C	Boiling temperature in ° C	Density, g / ml
				0.415 at t \u003d -165 ° C
				0,561 at t \u003d -100 ° C
				0,583 at T \u003d -45 ° C
				0.579 at t \u003d 0 ° C
	2-methylpropan			0.557 at t \u003d -25 ° C
	2,2-dimethyl propane
	2-methylbutan
	2-methylpenan
	2,2,3,3-tetra-methylbutan
	2,2,4-trimethyl pentan
n-C₁₀h₂₂.
n-C₁₁h₂₄.	n-Undekan
n-C₁₂h₂₆.	n-dodecan
n-C₁₃h₂₈.	n-triedekan
n-C₁₄h₃₀.	n-tetradecan
n-C₁₅h₃₂.	n-pentadecan
n-C₁₆h₃₄.	n-hexadekan
n-C₂₀h₄₂.	n-Eicosan
n-C₃₀h₆₂.	n-triakontan		1 mm Hg. Art
n-C₄₀h₈₂.	n-Tetracotan		3 mm RT. Art.
n-C₅₀h₁₀₂.	n-pentocontanu		15 mm RT. Art.
n-C₆₀h₁₂₂.	n-hexacontan
n-C₇₀h₁₄₂.	n-heptakontan
n-C₁₀₀h₂₀₂.

Conclusion

The article described such a concept as alkanes (structure, nomenclature, isomerism, homologous series, etc.). A little told about the peculiarities of radial and substitution nomenclatures. Methods for obtaining alkanes are described.

In addition, the article lists the entire nomenclature of alkanov in detail (the test can help assimilate the information received).

Hydrocarbons in whose molecules atoms are associated with single bonds and which correspond to the general formula C n h 2 n +2.
In alkanes molecules, all carbon atoms are in a state of SP 3-hybridization. This means that all four hybrid orbitals of the carbon atom are the same in shape, energy and are directed into the angles of the equilateral triangular pyramid - tetrahedron. The corners between orbital are equal to 109 ° 28 '.

Around the single carbon-carbon relationship is possible practically free rotation, and alkane molecules can acquire the most diverse shape with angles with carbon atoms close to tetrahedral (109 ° 28 '), for example, in a molecule N.- Ready.

It is especially worth recalling links in alkanan molecules. All connections in elevated hydrocarbon molecules are single. Overlapping occurs along the axis,
Connecting nuclei atoms, i.e. it is σ-bond. Combines carbon - carbon are non-polar and poorly polarizable. Length C-due Alkanes are 0.154 nm (1.54 10 - 10 m). Communication S-H is somewhat shorter. Electronic density is slightly shifted towards a more electronegative carbon atom, i.e. c-n connection is a weaklyolar.

Lack in molecules of limiting hydrocarbons polar connections It leads to the fact that they are poorly dissolved in water, do not interoperade with charged particles (ions). The most characteristic of alkanans are reactions flowing with the participation of free radicals.

Homological row of methane

Homologies - Substances similar to the structure and properties and differing in one or more groups of CH 2.

Isomeria and nomenclature

For alkans, the so-called structural isomeria is characteristic. Structural isomers differ from each other by the structure of the carbon skeleton. The simplest alkan for which structural isomers are characteristic is Bhutan.

Basics of nomenclature

1. Selecting the main chain. The formation of the name of the hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in the molecule, which is like its basis.
2. Numbering atoms of the main chain. Atoms of the main chain are assigned numbers. The numbering of the main chain atoms begins with the end to which the substituent is closer (structure A, b). If the substituents are equal to the removal from the end of the chain, the numbering begins on the end, in which they are larger (structure B). If different substituents are at an equal distance from the ends of the chain, the numbering begins with the end to which the older is closer (structure d). The seniority of hydrocarbon substituents is determined by the order in the alphabet of the letter with which their name begins: methyl (-sn 3), then ethyl (-CH 2 -CH 3), propyl (-CH 2 -CH 2 -CH 3 ) etc.
Note that the name of the substituent is formed by the replacement of suffix -an on suffix - il. In the title of the corresponding alkane.
3. Name formation. At the beginning of the name indicate numbers - numbers of carbon atoms at which substituents are located. If there are several substituents with a given atom, the corresponding number in the title is repeated twice through the comma (2.2-). After the number through the hyphen indicate the number of substituents ( di - two, three - three, tetra - four, penta - Five) and the name of the deputy (methyl, ethyl, propyl). Then without spaces and hyphens - the name of the main chain. The main chain is called a hydrocarbon - a member of the homologous series of methane ( methane CH 4, ethane From 2 H 6, propane C 3 H 8, with 4 H 10, pentane From 5 n 12, hexane From 6 H 14, heptane C 7 H 16, octane C 8 H 18, nonan. From 9 H 20, dean From 10 H 22).

Physical properties of alkanov

The first four representatives of the homologous series of methane - gases. The simplest of them - methane - gas without color, taste and smell (the smell of "gas", feeling that, it is necessary to call 04, is determined by the smell of mercaptans - sulfur-containing compounds specifically added to methane used in household and industrial gas devices to ensure that people located next to them, could have defined a leak by smell).
Hydrocarbons of the composition from 4 H 12 to C 15 H 32 - fluid; Heavy hydrocarbons - solids. Boiling and melting temperatures of alkanes are gradually increased with an increase in the length of the carbon chain. All hydrocarbons are poorly dissolved in water, liquid hydrocarbons are common organic solvents.

Chemical properties of alkanov

Reaction reaction.
The most characteristic of alkanans are the reactions of free radical substitution, during which the hydrogen atom is replaced by a halogen atom or any group. We present the equations of characteristic reactions halogenation:


In the case of excess halogen chlorination can go further, up to the complete replacement of all hydrogen atoms on chlorine:

The substances obtained are widely used as solvents and starting materials in organic synthesis.
Dehydrogenation reaction (hydrogen cleavage).
During the transmission of alkanes over the catalyst (Pt, Ni, A1 2 0 3, SG 2 0 3) high temperatures (400-600 ° C) The hydrogen molecule and the formation of alkene occurs:


Reactions accompanied by the destruction of the carbon chain.
All limit hydrocarbons are lit with the formation of carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain ratios can explode.
1. The burning of limit hydrocarbons - This is a free radical exothermic reaction, which is very important when using alkanes as fuel:

IN general Alkanan burning reaction can be written as follows:

2. Thermal splitting of hydrocarbons.

The process proceeds through a free radical mechanism. Increased temperature leads to a homolitical breaking of carbon-carbon bonds and the formation of free radicals.

These radicals interact with each other by exchanging the hydrogen atom, to form an alkane molecule and alkene molecule:

The thermal cleavage reactions underlie the industrial process - cracking of hydrocarbons. This process is the most important stage of oil refining.

3. Pyrolysis. When the methane is heated to a temperature of 1000 ° C, the methane pyrolysis begins - decomposition on simple substances:

When heated to a temperature of 1500 ° C, acetylene is possible:

4. Isomerization. When the linear hydrocarbons are heated with isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton occurs:

5. Aromatization. Alkanes with six or more carbon atoms in the chain in the presence of the catalyst are cyclized with the formation of benzene and its derivatives:

Alkans are in reactions flowing through a free radical mechanism, since all carbon atoms in alkanan molecules are in a state of SP 3-hybridization. Molecules of these substances are built using covalent non-polar sch (carbon - carbon) connections and weaklyolar C-H (carbon - hydrogen) connections. They do not have sections with an increased and reduced electron density, easily polarized bonds, i.e. such bonds, the electron density in which can be shifted under the action of external factors (electrostatic fields of ions). Consequently, the alkanes will not react with charged particles, since the bonds in the molecules of alkanans do not break through the heterolitical mechanism.

Alkans are saturated hydrocarbons, in whose molecules all carbon atoms are occupied by means of simple ties of hydrogen atoms. Therefore, for homologues of a series of methane, the structural isomeric of alkanans is characteristic.

Isomerius of carbon skeleton

For homologs with four and more carbon atoms, a structural isomeric is characterized by a carbon skeleton change. Metyl groups -CH 2 can be connected to any carbon of the chain, forming new substances. The more carbon atoms in the chain, the greater the isomologies can form homologs. The theoretical number of homologues is calculated mathematically.

Fig. 1. Approximate number of isomers of methane homologs.

In addition to methyl groups, long carbon chains can be connected 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-pentan (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-methylpenan (CH 3 -CH (CH 3) -CH 2 -CH 2 -CH 3), 3-methylpenan ( CH 3 -CH 2 -CH (CH 3) -CH 2 -CH 3), 2,3-dimethylbutan (CH 3 -CH (CH 3) -CH (CH 3) -CH 3), 2,2-dimethylbutane ( CH 3 -C (CH 3) 2 -CH 2 -CH 3).

Fig. 2. Examples of structural isomers.

Branched isomers differ from linear molecules with physical properties. Alkans with an extensive structure melted and boiled at lower temperatures than linear analogs.

Nomenclature

The International Nomenclature of Jewead has set the rules for the names of branched chains. To call a structural isomer, follows:

• find the longest chain and call it;
• number carbon atoms, starting from the end, where the most substituents;
• specify the number of identical substituents with numeric consoles;
• give names to substituents.

The name consists of four parts coming with each other:

• numbers denoting chains atoms who have substituents;
• numerical consoles;
• name of the deputy;
• name of the main chain.

For example, in the molecule of CH 3 -CH (CH 3) -CH 2 -C (CH 3) 2 -CH 3, the main chain has five carbon atoms. So it is Pentan. At the right end there are more branching, therefore the numbering of atoms begins from here. In this case, the second atom has two identical substituents, which is also reflected in the title. It turns out that this substance has the name of 2,2,4-trimethylpentane.

Different deputies (methyl, ethyl, propyl) are listed in the title alphabetically: 4,4-dimethyl-3-ethylGepan, 3-methyl-3-ethyl liter.

Numerical consoles are usually used from two to four: di- (two), three- (three), tetra- (four).

What did we know?

For alkans, structural isomeria is characteristic. Structural isomers are peculiar to all homologs, starting with Bhutan. With structural isomerism, substituents join carbon atoms in a carbon chain, forming complex branched chains. The name of the isomer consists of the names of the main chain, substituents, verbal designation of the number of substituents, the digital designation of carbon atoms to which the substituents are attached.