What does a double bond mean in a molecule. double bond

Ethylene for organic chemistry is, perhaps, not a brick, but a whole block. The ethylene molecule consists of two carbon atoms and four hydrogen atoms. How is ethylene built? Indeed, in all organic compounds, carbon must be tetravalent, and in the ethylene molecule each carbon atom is associated with another carbon and two hydrogens, i.e., it is, as it were, trivalent.

No, there is no violation of the principle of quadrivalent carbon in the ethylene molecule: two carbon atoms are not connected to each other by a simple one, as in ethane, but double bond. Each valence is indicated by a dash, and if we connect two carbon atoms with two dashes, then we will keep carbon tetravalent:

But what is hidden behind such designations, how does the connection represented by one line differ from the connection represented by two lines?

Recall how the ethane molecule is formed. Around each carbon atom as a result of hybridization, i.e. mixing, averaging one s- and three R-orbitals form four absolutely identical hybridized sp 3-orbitals.

In the case of ethylene, the bonds between carbon atoms are built differently. There are only two mixed here. R-orbitals with one orbital s. As a result, three hybridized sp 2-orbitals that lie in the same plane: two of them overlap with s-orbitals of two hydrogen atoms and bind these hydrogens to carbon, and the third orbital sp 2 overlaps with exactly the same orbital of the second carbon atom. This bond accounts for one of the dashes between two carbon atoms. What does the second line symbolize?

Recall that we have one more p-electron left. It forms a cloud in the form of a volume eight, which is directed perpendicular to the plane of three sp 2-orbitals. These electron clouds (one figure eight from each carbon) can also overlap with each other, but not head-on, as two sp 2-orbitals, but "sideways". This overlap is indicated by the second dash. The connection of the first type ("foreheads") is denoted by the Greek letter o (sigma), and the connection in which electron clouds

overlapping "sides" is called a π-bond (and such electrons themselves are called π-electrons). Together, this is a double bond. The double bond is shorter than a single bond, its length is 0.133 nm.

So, we have dismantled the device of another part from which you can build "buildings" of organic compounds. What are these buildings?

Let us first take such combinations: one molecule of ethylene and several molecules of methane. If one hydrogen atom in an ethylene molecule is replaced by a methyl group (i.e., a methane residue), then we get propylene (otherwise called propene) CH 2 \u003d CH-CH 3.

Now let's construct the next member of the homologous series (i.e., the member having one CH 2 group more). To do this, we replace one of the hydrogen atoms in propylene with a methyl group. There are several possibilities for such a substitution, as a result we will get three different butylenes (butene).

Replacing the hydrogen of the methyl group, we come to the normal butene-1: CH 2 = CH-CH 2 -CH 3. Substitution of hydrogen at the other end will give butene-2: CH 3 -CH=CH-CH 3 . Finally, substituting the single hydrogen in the double bond, we get iso-butylene: CH 2 \u003d C (CH 3) 2. These are three different substances with different boiling and melting points. The composition of all these hydrocarbons is reflected by the general formula C n H 2n. Similarly, formulas for all possible pentenes, hexenes, etc. can be derived.

So, we have learned how to get unsaturated hydrocarbons on paper. How are they actually obtained?

Main source of protozoa alkenes(i.e., unsaturated hydrocarbons) - products, from which, after heating and distillation, ethylene, propylene, butylenes are isolated ... If an alkane (saturated hydrocarbon) is heated to 500-600 ° C under high pressure in the presence of a catalyst, then two hydrogen atoms are cleaved off to form an alkene. From n-butane, for example, a mixture of butene-1 and butene-2 ​​is obtained.

In the laboratory, unsaturated hydrocarbons (for example, ethylene) are obtained by removing water from alcohols; to do this, they are heated with a catalytic amount of acid:

You can also split off a hydrogen halide molecule with alkali from halogen derivatives of saturated hydrocarbons:

The spectrum of reactions in which compounds with a double bond enter is much more diverse, wider than the set of transformations of alkanes. Consider one of these reactions of unsaturated compounds.

Unsaturated substances add hydrogen halides to the double bond, and halogen-substituted saturated hydrocarbons are formed (i.e., the reaction is the reverse of the one just written). But if you add a hydrogen halide to an asymmetric alkene (to one that has different groups on both sides of the double bond), then two different derivatives can be obtained, for example, in the case of propene, either CH 3 CH 2 CH 2 Cl, or CH 3 CHClCH 3 .

This reaction was studied in the last century by the Russian chemist V. V. Markovnikov. He established the rule that now bears his name: the halogen is added to the least hydrogenated carbon atom (that is, the one that is associated with the fewest number of hydrogen atoms). This means that most of the chloride is formed from propylene. iso-propyl CH 3 CHClCH 3 . But why does the reaction go the way it does? Modern theory provides an explanation for Markovnikov's rule. We present this theory in a somewhat simplified form.

The fact is that the mechanisms of even seemingly simple chemical reactions are quite complex, they include several stages. So it is with the reaction of the addition of a hydrogen halide. The hydrogen chloride molecule is attached to the alkene molecule not immediately, but in parts. Hydrogen is added first in the form of an H+ proton. A positively charged proton approaches the propylene molecule. Which of the double bonded carbons will it attack? It turns out - extreme, because it has a small negative charge, denoted δ- (delta minus). But how did this charge, a small excess of electron density, come about?

It's the metal band's fault. It seems to repel electrons from itself, which therefore accumulate at the opposite carbon atom, away from the methyl group. We only emphasize once again that this electron density shift is very small. It is much less than if a whole electron moved from the middle carbon atom to the outer one. Then we would have to put a plus over the middle atom, and a minus over the extreme one (we put the sign δ-, which means a small part of the total negative charge of the electron).

So, now it is clear that a positively charged proton is much more likely to approach the outermost carbon atom, which carries some excess electron density.


A positively charged proton joins an uncharged molecule and transfers its charge to it. Where is this charge located? If a proton joined the middle carbon atom, then the charge would arise on the outermost carbon. In fact, the proton approaches the extreme carbon atom, and the charge arises on the middle carbon .. Is there any difference where the charge is concentrated? Yes, and there is a big difference. Both carbocations (i.e., organic particles that carry a positive charge on the carbon atom) are unstable and do not live very long. But still, the second cation is more stable: the fact is that it is surrounded on both sides by methyl groups; and we already know that methyl groups are capable of supplying electrons, repelling them from themselves. It turns out that the methyl groups partially compensate for the emerging positive charge, and the smaller this charge, the more stable the carbocation. In the first case, a positive charge. extinguished by only one ethyl group, this carbocation will be less stable than the second.

As a rule, the more stable a particle is, the easier it is to form. And this means that the second carbocation will be obtained much more often than the first. The second stage of the reaction is the addition of a negatively charged chlorine ion to the carbocation. Since the second-type carbocation predominates in the products of the first stage, as a result of the entire reaction, for one molecule of 1-chloropropane, there are thousands of molecules of the isomer in which chlorine is attached to the average carbon. Therefore, we say that the addition proceeds mainly according to the Markovnikov rule. Two factors - the place of attack of the proton at the first stage and the stability of the carbocation formed after this - determine the fulfillment of this rule.

Unsaturated compounds easily attach not only hydrogen chloride, but also many other molecules. Typical examples of chemical transformations of ethylene are shown in the diagram.

The reader may have a question: are there organic molecules built only from ethylene blocks? Yes, there are. And the simplest representative is butadiene CH 2 \u003d CH-CH \u003d CH 2. This compound is widely used in the production of synthetic rubber. In tomatoes, fruits found hydrocarbon lycopene - red crystals. There are 13 double bonds in the carbon chain of this substance.

double bond, a four-electron covalent bond between two adjacent atoms in a molecule. D. s. usually denoted by two valent strokes: > C \u003d C<, >C=N -, >C=O, >C=S, - N=N -, - H=O, etc. This implies that one pair of electrons with sp 2 or sp- forms an s-bond with hybridized orbitals (see. rice. one ), the electron density of which is concentrated along the interatomic axis; An s-link is similar to a simple link. Another pair of electrons R-orbitals forms a p-bond, the electron density of which is concentrated outside the interatomic axis. If in the education of D. s. if atoms of the IV or V groups of the periodic system take part, then these atoms and the atoms directly connected with them are located in the same plane; the bond angles are 120°. In the case of asymmetric systems, distortions of the molecular structure are possible. D. s. shorter than a simple bond and is characterized by a high energy barrier of internal rotation; therefore, the positions of the substituents at the atoms associated with D. s. are not equivalent, and this causes the phenomenon of geometric isomerism. Compounds containing D. s. are capable of addition reactions. If D. s. is electronically symmetric, then the reactions are carried out both by radical (by homolysis of the p-bond) and by ionic mechanisms (due to the polarizing effect of the medium). If the electronegativities of the atoms bound by D. s. are different, or if different substituents are bound to them, then the p-bond is highly polarized. Compounds containing polar D. s. are prone to addition by the ionic mechanism: to electron-withdrawing D. s. nucleophilic reagents are easily attached, and s. - electrophilic. Direction of displacement of electrons during polarization D. s. it is customary to indicate with arrows in the formulas, and the resulting excess charges - with symbols d- and d+ . This facilitates understanding of the radical and ionic mechanisms of addition reactions:

In compounds with two D. with., separated by one simple bond, there is a conjugation of p-bonds and the formation of a single p-electron cloud, the lability of which manifests itself along the entire chain ( rice. 2 , left). The consequence of this conjugation is the ability to 1,4-addition reactions:

If three D. with. are conjugated in a six-membered cycle, then the sextet of p-electrons becomes common for the entire cycle and a relatively stable aromatic system is formed (see Fig. rice. 2, on right). The addition of both electrophilic and nucleophilic reagents to such compounds is energetically difficult. (See also chemical bond. )

chemical bond

All interactions leading to the association of chemical particles (atoms, molecules, ions, etc.) into substances are divided into chemical bonds and intermolecular bonds (intermolecular interactions).

chemical bonds- bonds directly between atoms. There are ionic, covalent and metallic bonds.

Intermolecular bonds- bonds between molecules. These are a hydrogen bond, an ion-dipole bond (due to the formation of this bond, for example, the formation of a hydration shell of ions occurs), a dipole-dipole bond (due to the formation of this bond, molecules of polar substances are combined, for example, in liquid acetone), etc.

Ionic bond- a chemical bond formed due to the electrostatic attraction of oppositely charged ions. In binary compounds (compounds of two elements), it is formed when the sizes of the atoms being bonded differ greatly from each other: some atoms are large, others are small - that is, some atoms easily give away electrons, while others tend to accept them (usually these are atoms of elements that form typical metals and atoms of elements forming typical non-metals); the electronegativity of such atoms is also very different.
The ionic bond is non-directional and non-saturable.

covalent bond- a chemical bond that occurs due to the formation of a common pair of electrons. A covalent bond is formed between small atoms with the same or close radii. A necessary condition is the presence of unpaired electrons in both bonded atoms (exchange mechanism) or an unshared pair in one atom and a free orbital in another (donor-acceptor mechanism):

a) H + H H:H H-H H2 (one shared pair of electrons; H is univalent);
b) NN N 2 (three common pairs of electrons; N is trivalent);
in) H-F HF (one common pair of electrons; H and F are univalent);
G) NH4+ (four shared pairs of electrons; N is tetravalent)
    According to the number of common electron pairs, covalent bonds are divided into
  • simple (single)- one pair of electrons
  • double- two pairs of electrons
  • triple- three pairs of electrons.

Double and triple bonds are called multiple bonds.

According to the distribution of electron density between the bonded atoms, the covalent bond is divided into non-polar and polar. A non-polar bond is formed between identical atoms, a polar bond is formed between different ones.

Electronegativity- a measure of the ability of an atom in a substance to attract common electron pairs.
The electron pairs of polar bonds are biased towards more electronegative elements. The very displacement of electron pairs is called bond polarization. The partial (excess) charges formed during polarization are denoted by + and -, for example: .

According to the nature of the overlapping of electron clouds ("orbitals"), the covalent bond is divided into -bond and -bond.
- Bond is formed due to direct overlap of electron clouds (along the straight line connecting the nuclei of atoms), - bond - due to lateral overlap (on both sides of the plane in which the nuclei of atoms lie).

A covalent bond is directional and saturable, as well as polarizable.
To explain and predict the mutual direction of covalent bonds, a hybridization model is used.

Hybridization of atomic orbitals and electron clouds- the proposed alignment of atomic orbitals in energy, and electron clouds in shape during the formation of covalent bonds by an atom.
The three most common types of hybridization are: sp-, sp 2 and sp 3 - hybridization. For example:
sp-hybridization - in C 2 H 2, BeH 2, CO 2 molecules (linear structure);
sp 2-hybridization - in C 2 H 4, C 6 H 6, BF 3 molecules (flat triangular shape);
sp 3-hybridization - in CCl 4, SiH 4, CH 4 molecules (tetrahedral form); NH 3 (pyramidal shape); H 2 O (corner shape).

metal connection- a chemical bond formed due to the socialization of valence electrons of all bonded atoms of a metal crystal. As a result, a single electron cloud of the crystal is formed, which is easily displaced under the action of electrical voltage - hence the high electrical conductivity of metals.
A metallic bond is formed when the bonded atoms are large and therefore tend to donate electrons. Simple substances with a metallic bond - metals (Na, Ba, Al, Cu, Au, etc.), complex substances - intermetallic compounds (AlCr 2, Ca 2 Cu, Cu 5 Zn 8, etc.).
The metallic bond does not have saturation directionality. It is also preserved in metal melts.

hydrogen bond- an intermolecular bond formed due to the partial acceptance of a pair of electrons of a highly electronegative atom by a hydrogen atom with a large positive partial charge. It is formed when in one molecule there is an atom with a lone pair of electrons and high electronegativity (F, O, N), and in the other there is a hydrogen atom bound by a strongly polar bond with one of these atoms. Examples of intermolecular hydrogen bonds:

H—O—H ··· OH 2 , H—O—H ··· NH 3 , H—O—H ··· F—H, H—F ··· H—F.

Intramolecular hydrogen bonds exist in the molecules of polypeptides, nucleic acids, proteins, etc.

A measure of the strength of any bond is the bond energy.
Bond energy is the energy required to break a given chemical bond in 1 mole of a substance. The unit of measurement is 1 kJ/mol.

The energies of the ionic and covalent bonds are of the same order, the energy of the hydrogen bond is an order of magnitude less.

The energy of a covalent bond depends on the size of the bonded atoms (bond length) and on the multiplicity of the bond. The smaller the atoms and the greater the multiplicity of the bond, the greater its energy.

The ionic bond energy depends on the size of the ions and on their charges. The smaller the ions and the greater their charge, the greater the binding energy.

The structure of matter

According to the type of structure, all substances are divided into molecular and non-molecular. Molecular substances predominate among organic substances, while non-molecular substances predominate among inorganic substances.

According to the type of chemical bond, substances are divided into substances with covalent bonds, substances with ionic bonds (ionic substances) and substances with metallic bonds (metals).

Substances with covalent bonds can be molecular or non-molecular. This significantly affects their physical properties.

Molecular substances consist of molecules interconnected by weak intermolecular bonds, these include: H 2, O 2, N 2, Cl 2, Br 2, S 8, P 4 and other simple substances; CO 2 , SO 2 , N 2 O 5 , H 2 O, HCl, HF, NH 3 , CH 4 , C 2 H 5 OH, organic polymers and many other substances. These substances do not have high strength, have low melting and boiling points, do not conduct electricity, some of them are soluble in water or other solvents.

Non-molecular substances with covalent bonds or atomic substances (diamond, graphite, Si, SiO 2 , SiC and others) form very strong crystals (layered graphite is an exception), they are insoluble in water and other solvents, have high melting and boiling points, most of they do not conduct electric current (except for graphite, which has electrical conductivity, and semiconductors - silicon, germanium, etc.)

All ionic substances are naturally non-molecular. These are solid refractory substances whose solutions and melts conduct electric current. Many of them are soluble in water. It should be noted that in ionic substances, the crystals of which consist of complex ions, there are also covalent bonds, for example: (Na +) 2 (SO 4 2-), (K +) 3 (PO 4 3-), (NH 4 + )(NO 3-), etc. The atoms that make up complex ions are bound by covalent bonds.

Metals (substances with a metallic bond) very diverse in their physical properties. Among them are liquid (Hg), very soft (Na, K) and very hard metals (W, Nb).

The characteristic physical properties of metals are their high electrical conductivity (unlike semiconductors, it decreases with increasing temperature), high heat capacity and ductility (for pure metals).

In the solid state, almost all substances are composed of crystals. According to the type of structure and type of chemical bond, crystals ("crystal lattices") are divided into atomic(crystals of non-molecular substances with a covalent bond), ionic(crystals of ionic substances), molecular(crystals of molecular substances with a covalent bond) and metal(crystals of substances with a metallic bond).

Tasks and tests on the topic "Topic 10. "Chemical bond. The structure of matter."

  • Types of chemical bond - The structure of matter 8–9 class

    Lessons: 2 Assignments: 9 Tests: 1

  • Tasks: 9 Tests: 1

After working through this topic, you should learn the following concepts: chemical bond, intermolecular bond, ionic bond, covalent bond, metallic bond, hydrogen bond, single bond, double bond, triple bond, multiple bonds, non-polar bond, polar bond, electronegativity, bond polarization , - and -bond, hybridization of atomic orbitals, bond energy.

You must know the classification of substances according to the type of structure, according to the type of chemical bond, the dependence of the properties of simple and complex substances on the type of chemical bond and the type of "crystal lattice".

You should be able to: determine the type of chemical bond in a substance, the type of hybridization, draw up bond formation patterns, use the concept of electronegativity, a number of electronegativity; to know how electronegativity changes in chemical elements of one period, and one group to determine the polarity of a covalent bond.

After making sure that everything you need is learned, proceed to the tasks. We wish you success.


Recommended literature:
  • O. S. Gabrielyan, G. G. Lysova. Chemistry 11 cells. M., Bustard, 2002.
  • G. E. Rudzitis, F. G. Feldman. Chemistry 11 cells. M., Education, 2001.

double bond

a four-electron covalent bond between two adjacent atoms in a molecule. D. s. usually denoted by two valence strokes: >C=CC=N -, >C=O, >C=S, - N=N -, - H=O, etc. This implies that one pair of electrons with sp 2 or sp- hybridized orbitals forms a σ-bond (see. rice. one ), the electron density of which is concentrated along the interatomic axis; The σ bond is similar to a simple bond. Another pair of electrons R-orbitals forms a π-bond, the electron density of which is concentrated outside the interatomic axis. If in the education of D. s. if atoms of group IV or V of the periodic system take part, then these atoms and the atoms directly connected with them are located in the same plane; the bond angles are 120°. In the case of asymmetric systems, distortions of the molecular structure are possible. D. s. shorter than a simple bond and is characterized by a high energy barrier of internal rotation; therefore, the positions of substituents on atoms bound by D. s. are not equivalent, and this causes the phenomenon of geometric isomerism. Compounds containing D. s. are capable of addition reactions. If D. s. is electronically symmetric, then the reactions are carried out both by radical (by homolysis of the π-bond) and by ionic mechanisms (due to the polarizing effect of the medium). If the electronegativities of the atoms bound by D. s. are different, or if different substituents are associated with them, then the π bond is strongly polarized. Compounds containing polar D. s. are prone to addition by the ionic mechanism: to electron-withdrawing D. s. nucleophilic reagents are easily attached, and s. - electrophilic. Direction of displacement of electrons during polarization D. s. it is customary to indicate with arrows in the formulas, and the resulting excess charges - with symbols δ - and δ + . This facilitates understanding of the radical and ionic mechanisms of addition reactions:

In compounds with two D. s., separated by one simple bond, the conjugation of π-bonds and the formation of a single π-electron cloud, the lability of which manifests itself along the entire chain ( rice. 2 , left). The consequence of this conjugation is the ability to 1,4-addition reactions:

G. A. Sokolsky.

Rice. 1. Double bond scheme >C = C


Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

See what "Double bond" is in other dictionaries:

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    double bond- dvigubasis ryšys statusas T sritis chemija apibrėžtis Du kovalentiniai ryšiai tarp dviejų atomų. atitikmenys: engl. double bond; ethylene bond. double bond; ethylene bond ryšiai: sinonimas - dvilypis ryšys sinonimas - etileninis ryšys ... Chemijos terminų aiskinamasis žodynas

    double bond- dvilypis ryšys statusas T sritis fizika atitikmenys: angl. double bond vok. Doppelbindung, f rus. double bond, f pranc. liaison double, f … Fizikos terminų žodynas

    Chem. a bond between adjacent atoms in a molecule carried out by two pairs of electrons. Characteristic ch. arr. for organic connections. Graphically depicted with two valence strokes, for example Connections with D. s. (see e.g. Ethylene, Butenes,… … Big encyclopedic polytechnic dictionary

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Ethylene for organic chemistry is, perhaps, not a brick, but a whole block. The ethylene molecule consists of two carbon atoms and four hydrogen atoms.
How is ethylene built? Indeed, in all organic compounds, carbon must be tetravalent, and in the ethylene molecule each carbon atom is associated with another carbon and two hydrogens, i.e., it is, as it were, trivalent.
No, there is no violation of the principle of tetravalence of carbon in the ethylene molecule: two carbon atoms are connected to each other not by a simple one, as in ethane, but by a double bond. Each valence is indicated by a dash, and if we connect two carbon atoms with two dashes, then we will keep carbon tetravalent:
But what is hidden behind such designations, how does the connection represented by one line differ from the connection represented by two lines?
Recall how the ethane molecule is formed. Around each carbon atom, as a result of hybridization, i.e., mixing, averaging one 5- and three p-orbitals, four completely identical hybridized 5p3-orbitals directed in different directions are formed.

In the case of ethylene, the bonds between carbon atoms are built differently. Here, only two orbitals with one 5 orbital mix. As a result, three hybridized 5p2 orbitals are formed that lie in the same plane: two of them overlap with the 5-orbitals of two hydrogen atoms and bind these hydrogens to carbon, and the third $p2 orbital overlaps with exactly the same orbital of the second carbon atom. This bond accounts for one of the dashes between two carbon atoms. What does the second line symbolize?
Recall that we have one more p-electron left. It forms a cloud in the form of a volume eight, which is directed perpendicular to the plane of three orbitals. These electron clouds (one figure eight from each carbon) can also overlap with each other, but not head-on, as two $p2-orbitals overlap, and "sideways". This overlap is indicated by the second dash. The connection of the first type ("foreheads") is denoted by the Greek letter a (sigma), and the connection in which electron clouds overlap "sideways" is called n-bond (and such electrons themselves are called n-electrons). Together, this is a double bond. The double bond is shorter than the single bond, its length is 0.133 mm.
So, we have dismantled the device of another part from which you can build "buildings" of organic compounds. What are these buildings?
Let us first take such combinations: one molecule of ethylene and several molecules of methane. If one hydrogen atom in an ethylene molecule is replaced by a methyl group (i.e., by a methane residue), then we get propylene (otherwise called propene) CH2=CH-CH3.
Now let's construct the next member of the homologous series (i.e., the member with one CH2 group more). To do this, we replace one of the hydrogen atoms in propylene with a methyl group. There are several possibilities for such a substitution, as a result we will get three different butylenes (butene).
Substituting the hydrogen of the methyl group, we arrive at normal butene-1: CH2=CH—CH2—CH3. Replacing the hydrogen at the other end will give butene-2: CH3-€H=CH-CH3. Finally, substituting the only hydrogen in the double bond, we get mso-butylene: CH2=C(CH3)2. These are three different substances with different boiling and melting points. The composition of all these hydrocarbons is reflected by the general formula CnH2n. Similarly, one can derive formulas for all possible pentenes, hexenes, etc.
So, we have learned how to get unsaturated hydrocarbons on paper. How are they actually obtained?
The main source of the simplest alkenes (i.e., unsaturated hydrocarbons) is petroleum products, from which ethylene is isolated after heating and distillation.
propylene, butylenes... If an alkane (saturated hydrocarbon) is heated to 500-600°C under high pressure in the presence of a catalyst, two hydrogen atoms are split off and an alkene is formed. From n-butane, for example, a mixture of butene-1 and butene-2 ​​is obtained.
In the laboratory, unsaturated hydrocarbons (for example, ethylene) are obtained by removing water from alcohols; to do this, they are heated with a catalytic amount of acid:
IDO 200 °С CH3—CH2—OH ----- CH2=CH2
It is also possible to split off a hydrogen halide molecule with alkali from halogen derivatives of saturated hydrocarbons:
NaON
CH3—CH3—CH2C1 SH CH3—CH=CH2—HC!
The spectrum of reactions in which compounds with a double bond enter is much more diverse, wider than the set of transformations of alkanes. Consider one of these reactions of unsaturated compounds.
Unsaturated substances add halogen-hydrogens to the double bond, and halogen-substituted saturated hydrocarbons are formed (i.e., the reaction is the reverse of the one just written). But if you add a hydrogen halide to an unsymmetrical alkene. (to one that has different groups on both sides of the double bond), then two different derivatives can be obtained, for example, in the case of propene, either CH3CH2CH2C1 or CH3CHSNCHUN3.
This reaction was studied in the last century by the Russian chemist V. V. Markovnikov. He established the rule that now bears his name: the halogen is attached to the least hydrogenated carbon atom (i.e., the one that is associated with the fewest number of hydrogen atoms). This means that mainly iso-propyl chloride CH3CH1CH3 is formed from propylene. But why does the reaction go the way it does? Modern theory provides an explanation for the Markovikov rule. We present this theory in a somewhat simplified form.
The fact is that the mechanisms of even seemingly simple chemical reactions are quite complex, they include several stages. So it is with the reaction of the addition of a hydrogen halide. The hydrogen chloride molecule is attached to the alkene molecule not immediately, but in parts. Hydrogen is added first in the form of a P1+ proton. A positively charged proton approaches the propylene molecule. Which of the double bonded carbons will it attack? It turns out - extreme, because it has a small negative charge, denoted b- (delta minus). But how did this charge, a small excess of electron density, come about?
The methyl group is "guilty" of this. It seems to repel electrons from itself, which therefore accumulate at the opposite carbon atom, away from the methyl group. We only emphasize once again that this electron density shift is very small. It is much less than if a whole electron moved from the middle carbon atom to the outer one. Then we would have to put a plus over the middle atom, and a minus over the extreme one (we put the sign q-, which means a small part of the total negative charge of the electron).
So, now it is clear that a positively charged proton is much more likely to approach the outermost carbon atom, which carries some excess electron density.
A positively charged proton joins an uncharged molecule and transfers its charge to it. Where is this charge located? If a proton joined the middle carbon atom, then the charge would arise on the outermost carbon. In fact, the proton approaches the outermost carbon atom, and the charge arises on the middle carbon. Does it matter where the charge is concentrated? Yes, and there is a big difference. Both carbocations (i.e., organic particles that carry a positive charge on the carbon atom) are unstable and do not live very long. But still, the second cation is more stable: the fact is that it is surrounded on both sides by methyl groups; and we already know that methyl groups are able to donate electrons, repel them from themselves. It turns out that methyl groups partially compensate for the resulting positive charge. And the smaller this charge, the more stable the carbocation. In the first case, the positive charge is extinguished by only one ethyl group, this carbocation will be less stable than the second.
As a rule, the more stable a particle is, the easier it is to form. And this means that the second carbocation will be obtained much more often than the first. The second stage of the reaction is the addition of a negatively charged chlorine ion to the carbocation. Since the second-type carbocation predominates in the products of the first stage, as a result of the entire reaction, for one molecule of 1-chloropropane, there are thousands of molecules of the isomer in which chlorine is attached to the average carbon. Therefore, we say that the addition proceeds mainly according to the Markovnikov rule. Two factors - the site of the attack of the proton in the first stage and the stability of the carbocation formed after this - determine the fulfillment of this rule.
Unsaturated compounds easily attach not only hydrogen chloride, but also. many other molecules. Typical examples of chemical transformations of ethylene are shown in the diagram.
The reader may have a question: are there organic molecules built only from ethylene blocks? Yes, there are. And the simplest representative is butadiene CH2=CH-CH=CH2. This compound is widely used in the production of synthetic rubber. The hydrocarbon lycopene, red crystals, was found in tomatoes and fruits. There are 13 double bonds in the carbon chain of this substance.

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