| Parameter name | Meaning |
| Article topic: | Chemical bonds in organic compounds |
| Rubric (thematic category) | Education |
Most organic compounds contain only a few basic elements: carbon, hydrogen, nitrogen, oxygen, sulfur and, much less frequently, other elements. However, the whole variety of organic compounds is determined, on the one hand, by their qualitative and quantitative composition, and on the other, by the order and nature of the bonds between atoms.
1.1 Electronegativity of elements
The electronegativity of an atom is its ability to attract elements. Electronegativity values are not significant constants, but only show the relative ability of atoms to attract electrons more or less when forming with other atoms.
Atoms located in the electronegativity series in front of carbon and having an electronegativity value less than 2.5 increase the electron density on the carbon atom when forming a bond with it. On the contrary, atoms whose electronegativity value exceeds 2.5 reduce the electron density on the carbon atom when forming a bond.
1.2 Ionic bonding
The electronic configuration for any atom can be formed in two different ways. One of them is electron transfer: atoms of one element give up electrons, which go to the atoms of another element. In this case, a so-called ionic (electrovalent, heteropolar) bond:
An atom that donates electrons becomes a positive ion ( cation); an atom that has accepted an electron becomes a negative ion ( anion).
Distinctive features of ionic compounds are the instantaneous occurrence of reactions, dissociation and solvation of ions in aqueous solutions, high melting and boiling points, solubility in polar solvents, electrical conductivity of solutions and melts.
A heteropolar bond occurs between atoms that differ greatly in electronegativity.
1.3 Covalent bond
When atoms of equal or similar electronegativity interact, electron transfer does not occur. The formation of an electronic configuration for such atoms occurs due to the generalization of two, four or six electrons by interacting atoms. Each of the generalized pairs of electrons forms one covalent (homeopolar) bond:
The most important physical parameters of a covalent bond are those that characterize their symmetry, size, electrical and thermochemical properties.
Link length- ϶ᴛᴏ the equilibrium distance between the centers of nuclei and it depends on what other atoms they are associated with. Thus, the length of the C-C bond, depending on the environment, varies within 0.154 – 0.14 nm.
Bond angles– angles between lines connecting bonded atoms. Knowledge of bond lengths and bond angles is extremely important for constructing a correct spatial model, an idea of the electron density distribution, and is used in quantum chemical calculations.
Energy of breaking a chemical bond- ϶ᴛᴏ energy spent on breaking this bond or released during its formation per mole of particles. In the case of molecules containing two or more identical bonds, the energy of breaking one of these bonds or the average energy of breaking these bonds is distinguished. The higher the chemical bond energy, the stronger the bond. A bond is considered strong, or strong, if its energy exceeds 500 kJ/mol, weak - if its energy is less than 100 kJ/mol. If the interaction of atoms releases energy less than 15 kJ/mol, then it is considered that a chemical bond is not formed, but an intermolecular interaction is observed. Bond strength generally decreases as bond length increases.
Polarity of chemical bonds– characteristic of a chemical bond, showing a change in the distribution of electron density in the space around the nuclei in comparison with the distribution of electron density in the neutral atoms forming this bond. Knowing the polarity of a bond is extremely important for judging the distribution of electron density in a molecule, and therefore the nature of its reactivity.
Bond polarizability is expressed in the displacement of bond electrons under the influence of an external electric field, incl. and another reacting particle. Polarizability is determined by electron mobility. Electrons are more mobile the further they are from the nuclei.
1.4 Breaking ties
Breaking a covalent bond between two atoms can occur in different ways:
When A each atom is separated with one electron, resulting in the formation of particles called radicals, which are highly reactive due to the presence of an unpaired electron; such a gap is called homolytic cleavage communications. In cases b And V one atom can hold both electrons, leaving the other atom without electrons, resulting in negative and positive ions respectively. If the R and X atoms are not identical, splitting can occur along one of these paths based on which atom - R or X - holds a pair of electrons. These kinds of gaps are called heterolytic cleavage and lead to the formation of an ion pair.
Chemical bonds in organic compounds - concept and types. Classification and features of the category "Chemical bonds in organic compounds" 2017, 2018.
The reactivity of organic compounds is determined by the type of chemical bonds and the mutual influence of atoms in the molecule. These factors, in turn, are determined by the interaction of atomic orbitals (AO).
The part of space in which the probability of finding an electron is maximum is called an atomic orbital.
In organic chemistry, the concept of hybrid orbitals of the carbon atom and other elements is widely used. The concept of orbital hybridization is necessary in cases where the number of unpaired electrons in the ground state of an atom is less than the number of bonds it forms. It is postulated that different atomic orbitals of similar energy interact with each other to form hybrid orbitals of the same energy. Hybrid orbitals, due to their large overlap, provide a stronger bond than non-hybridized orbitals. Depending on the number of orbitals that have entered into hybridization, a carbon atom can be in three types of hybridization:
1. First valence state, sp3 hybridization (tetrahedral)
As a result of a linear combination (mixing) of four AOs of an excited carbon atom (one 2s and three 2p), four equivalent sp 3 hybrid orbitals arise, directed in space to the vertices of the tetrahedron at angles of 109.5?. The shape of the hybrid orbital is a three-dimensional figure eight, one of the blades of which is much larger than the other.
2. Second valence state, sp2 - hybridization (triangular)
It arises as a result of the displacement of one 2s and two 2p atomic orbitals. The resulting three sp 2 hybrid orbitals are located in the same plane at an angle of 120? to each other, and non-hybridized p - AO - in a plane perpendicular to it. In the state of sp 2 hybridization, the carbon atom is located in alkene molecules, carbonyl and carboxyl groups
3. Third valence state, sp - hybridization
It arises as a result of mixing one 2s and one 2p AO. The resulting two sp hybrid orbitals are located linearly, and the two p orbitals are located in two mutually perpendicular planes. The carbon atom in the sp hybrid state is found in molecules of alkynes and nitriles
There are three possible types of bonds connecting individual atoms of elements in a compound - electrostatic, covalent and metallic.
Electrostatic bonds primarily include ionic bonds, which occur when one atom transfers an electron or electrons to another, and the resulting ions are attracted to each other.
Organic compounds are characterized mainly by covalent bonds. A covalent bond is a chemical bond formed by sharing the electrons of the bonded atoms.
For the quantum mechanical description of covalent bonds, two main approaches are used: the valence bond (VB) method and the molecular orbital (MO) method. chemical covalent molecule
The BC method is based on the idea of electron pairing that occurs when atomic orbitals overlap. A generalized pair of electrons with opposite spins forms a region with increased electron density between the nuclei of two atoms, attracting both nuclei. A two-electron covalent bond occurs. According to the BC method, atomic orbitals retain their individuality. Therefore, both paired electrons remain in the atomic orbitals of the bonded atoms, i.e., they are localized between the nuclei.
In the initial stage of development of electronic theory (Lewis), the idea of a covalent bond as a socialized pair of electrons was put forward. To explain the properties of different atoms to form a certain number of covalent bonds, the octet rule was formulated. According to him, during the formation of molecules from atoms of the 2nd period of the periodic system, D.I. Mendeleev, the outer shell is filled with the formation of a stable 8-electron system (shell of an inert gas). Four electron pairs can form covalent bonds or exist as lone pairs.
When moving to the elements of the third and subsequent periods, the rule of the octet loses its force, since d-orbitals appear that are quite low in energy. Therefore, atoms of higher periods can form more than four covalent bonds. Lewis's assumptions about a chemical bond as a social pair of electrons were of a purely qualitative nature.
According to the MO method, bond electrons are not localized on AOs of certain atoms, but are located on MOs, which are a linear combination of atomic orbitals (LCAO) of all atoms that make up the molecule. The number of formed MOs is equal to the number of overlapping AOs. A molecular orbital is usually a multicenter orbital and the electrons that fill it are delocalized. The filling of MOs with electrons occurs in compliance with the Pauli principle. A MO obtained by adding the wave functions of atomic orbitals and having a lower energy than the AOs that form it is called bonding. The presence of electrons in this orbital reduces the overall energy of the molecule and ensures the bonding of atoms. A high-energy MO obtained by subtracting wave functions is called antibonding (antibonding). For an antibonding orbital, the probability of finding electrons between nuclei is zero. This orbital is vacant.
In addition to binding and antibonding MOs, there are also non-binding MOs, designated n-MOs. They are formed with the participation of AOs carrying a pair of electrons that are not involved in the formation of the bond. Such electrons are also called free lone pairs or n-electrons (they are found on nitrogen, oxygen, and halogen atoms).
There are two types of covalent bonds: y- (sigma) and p- (pi) bonds.
A y-bond is a bond formed by the axial overlap of any (s-, p- or hybrid sp- atomic orbitals) with the maximum overlap located on the straight line connecting the nuclei of the bonded atoms.
According to the MO method, y-overlap leads to the appearance of two MOs: a bonding y-MO and an antibonding y*-MO.
A p-Bond is a bond formed by lateral (lateral) overlap of a p-AO, with the maximum electron density located on both sides of the straight line connecting the nuclei of atoms. According to the MO method, as a result of a linear combination of two p-AOs, a binding p-MO and an antibonding p*-MO are formed.
A double bond is a combination of y- and p-bonds, and a triple bond is one y- and two p-bonds.
The main characteristics of a covalent bond are energy, length, polarity, polarizability, directionality and saturability.
Bond energy is the amount of energy released during the formation of a given bond or required to separate two bonded atoms. The greater the energy, the stronger the connection.
Bond length is the distance between the centers of bonded atoms. A double bond is shorter than a single bond, and a triple bond is shorter than a double bond.
Bond polarity is determined by the uneven distribution (polarization) of electron density, the reason for which is the difference in electronegativity of bonded atoms. As the difference in electronegativity between bonded atoms increases, the polarity of the bond increases. Thus, one can imagine the transition from a non-polar covalent bond through a polar to an ionic bond. Polar covalent bonds are prone to heterolytic cleavage.
Bond polarizability is a measure of the displacement of bond electrons under the influence of an external electric field, including that of another reacting particle. Polarizability is determined by electron mobility. Electrons are more mobile the further they are from the nuclei.
In organogens (carbon, nitrogen, oxygen, sulfur, halogens) in the formation of y-bonds, the participation of hybrid orbitals is energetically more favorable, providing more efficient overlap.
The overlap of two one-electron AOs is not the only way to form a covalent bond. A covalent bond can be formed by the interaction of a filled two-electron orbital (donor) with a vacant orbital (acceptor). Donors are compounds containing either orbitals with a lone pair of electrons or p - MO. A covalent bond formed by an electron pair of one atom is called donor-acceptor or coordination.
A type of donor-acceptor bond is a semipolar bond. For example, in a nitro group, simultaneously with the formation of a covalent bond due to the lone pair of nitrogen electrons, charges of opposite sign appear on the bonded atoms. Due to electrostatic attraction, an ionic bond occurs between them. The resulting combination of covalent and ionic bonding is called a semipolar bond. The donor-acceptor bond is characteristic of complex compounds. Depending on the type of donor, n- or p-complexes are distinguished.
A hydrogen atom bonded to a strongly electronegative atom (N, O, F) is electron-deficient and is capable of interacting with the lone pair of electrons of another strongly electronegative atom, located either in the same or in another molecule. As a result, a hydrogen bond occurs. Graphically, a hydrogen bond is indicated by three dots.
The hydrogen bond energy is low (10-40 kJ/mol) and is mainly determined by electrostatic interaction.
Intermolecular hydrogen bonds cause the association of organic compounds, which leads to an increase in the boiling point of alcohols (t? boiling point C 2 H 5 OH = 78.3? C; t? boiling point CH 3 OCH 3 = -24? C), carboxylic acids and many other physical (t? melting point, viscosity) and chemical (acid-base) properties.
Intramolecular hydrogen bonds can also occur, for example in salicylic acid, which leads to an increase in its acidity.
The ethylene molecule is flat, the angle between the H - C - H bond is 120? C. In order to break the p - p - double bond and make it possible to rotate around the remaining sp 2 - y - bond, it is necessary to expend a significant amount of energy; Therefore, rotation around the double bond is difficult and the existence of cis- and trans-isomers is possible.
A covalent bond is nonpolar only when bonding atoms that are identical or similar in electronegativity. When electrons bond, the covalent bond density shifts toward the more electronegative atom. This relationship is polarized. Polarization is not limited to just one y-bond, but spreads along the chain and leads to the appearance of partial charges (y) on the atoms
Thus, the “X” substituent causes polarization not only of its y-bond with the carbon atom, but also transmits its influence (exhibits an effect) to neighboring y-bonds. This type of electronic influence is called inductive and is denoted j.
The inductive effect is the transfer of the electronic influence of a substituent along a chain of y-bonds.
The direction of the inductive effect of a substituent is usually assessed qualitatively by comparison with the hydrogen atom, the inductive effect of which is taken to be 0 (the C-H bond is considered practically non-polar).
Substituent X, which attracts the electron density of the y-bond more strongly than the hydrogen atom, exhibits a negative inductive effect -I. If, compared to the hydrogen atom, the substituent Y increases the electron density in the chain, then it exhibits a positive inductive effect, +I. Graphically, the inductive effect is represented by an arrow coinciding with the position of the valence line and pointing towards the more electronegative atom. The +I effect is exerted by alkyl groups, metal atoms, and anions. Most substituents have an -I effect. And the greater, the higher the electronegativity of the atom forming a covalent bond with the carbon atom. Unsaturated groups (all without exception) have an -I effect, the magnitude of which increases with increasing multiple bonds.
The inductive effect, due to the weak polarizability of the y-bond, decays after three or four y-bonds in the circuit. Its effect is strongest on the first two carbon atoms closest to the substituent.
If a molecule contains conjugated double or triple bonds, the conjugation effect (or mesomeric effect; M-effect) occurs.
The conjugation effect is the transfer of the electronic influence of a substituent through a system of p-bonds. Substituents that increase the electron density in a conjugated system exhibit a positive conjugation effect, the +M effect. The +M effect is exhibited by substituents containing atoms with a lone pair of electrons or a whole negative charge. Substituents that withdraw electron density from the conjugated system exhibit a negative (mesomeric) conjugation effect, the -M effect. These include unsaturated groups and positively charged atoms. The redistribution (displacement) of the total electron cloud under the influence of the M effect is graphically depicted by curved arrows, the beginning of which shows which p- or p-electrons are displaced, and the end - the bond or atom to which they are displaced
The mesomeric effect (conjugation effect) is transmitted through a system of conjugated bonds to significantly greater spreads.
A covalent bond can be polarized and delocalized.
Localized covalent bond - the bonding electrons are shared between the two nuclei of the atoms being bonded.
A delocalized bond is a covalent bond whose molecular orbital spans more than 2 atoms. These are almost always p-connections.
Conjugation (mesomerism, mesos - average) is the phenomenon of alignment of bonds and charges in a real molecule (particle) in comparison with a real, but non-existent structure.
Resonance theory - a real molecule or particle is described by a set of specific, so-called resonance structures, which differ from each other only in the distribution of electron density.
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1. Hybridization of carbon atomic orbitals
Atomic orbital is a function that describes the density of the electron cloud at each point in space around the nucleus of an atom. An electron cloud is a region of space in which an electron can be detected with a high probability.
To harmonize the electronic structure of the carbon atom and the valence of this element, concepts about the excitation of the carbon atom are used. In the normal (unexcited) state, the carbon atom has two unpaired 2 R 2 electrons.
In an excited state (when energy is absorbed) one of 2 s 2 electrons can go to free R-orbital. Then four unpaired electrons appear in the carbon atom. At the second energy level except 2 s-there are three orbitals 2 R-orbitals. These 2 R-orbitals have an ellipsoidal shape, similar to dumbbells, and are oriented in space at an angle of 90° to each other. 2 R-Orbitals denote 2 R X, 2R y and 2 R z in accordance with the axes along which these orbitals are located.
When chemical bonds are formed, the electron orbitals acquire the same shape.
Thus, in saturated hydrocarbons one s-orbital and three R-orbitals of the carbon atom to form four identical (hybrid) sR 3-orbitals:
This - sR 3 -hybridization.
Hybridization- alignment (mixing) of atomic orbitals ( s And R) with the formation of new atomic orbitals called hybrid orbitals.
TETRAHEDRON (angles = 109°28?
sR 2 -Hybridization- mixing one s- and two R-orbitals. As a result, three hybrids are formed sR 2 -orbitals.
These sR 2-orbitals are located in the same plane (with axes X, at) and are directed to the vertices of the triangle with an angle between the orbitals of 120°.
Unhybridized R-the orbital is perpendicular to the plane of the three hybrid sR 2-orbitals (oriented along the axis z).
Upper half R-orbitals are above the plane, the lower half is below the plane.
Type sR 2-carbon hybridization occurs in compounds with a double bond:
C=C, C=O, C=N.
Moreover, only one of the bonds between two atoms (for example, C=C) can be an - bond. (The other bonding orbitals of the atom point in opposite directions.)
The second bond is formed as a result of overlapping non-hybrid R-orbitals on both sides of the line connecting the atomic nuclei.
Covalent bond formed by lateral overlap R-orbitals of neighboring carbon atoms is called pi( R)-connection .
sR-Hybridization s- and one R sR-orbitals. sR-The orbitals are located on the same line (at an angle of 180°) and directed in opposite directions from the nucleus of the carbon atom. Two R at- connections. On the image sR-orbitals are shown along the axis y, and the unhybridized two R-orbitals- along axes X And z.
The carbon-carbon triple bond C?C consists of a y-bond that occurs when sp-hybrid orbitals, and two p-bonds.
2. Reactions of electrophilic substitution of hydrogen atoms in the benzene series
1. Halogenation reaction. The halogenation reaction of the benzene ring is carried out in the presence of catalysts (most often iron or aluminum halides). The role of the catalyst is to form a highly polarized complex with a halogen: FORMULA. The leftmost chlorine atom in the complex becomes electron unsaturated as a result of polarization of the Cl - Cl bond and is capable of interacting with nucleophilic reagents (in this case, benzene):
d - the complex abstracts a proton and turns into a substitution product (chlorobenzene). The proton interacts with - with the regeneration of aluminum chloride, forming hydrogen chloride:
In the case of an excess of halogen, di- and polyhalogen-substituted compounds can be obtained, up to the complete replacement of all hydrogen atoms in benzene.
Direct iodination in the aromatic ring cannot be carried out due to the low reactivity of iodine. Direct fluorination of aromatic hydrocarbons proceeds so vigorously that a complex mixture of products is formed, in which the target fluorinated derivatives are contained in small quantities. Depending on the conditions of the halogenation reaction of alkylbenzenes, a halogen can replace hydrogen atoms in the benzene ring (“in the cold” in the presence of Lewis acids) or in the side chain (when heated or in the light). In the latter case, the reaction proceeds by a free radical mechanism, similar to the substitution mechanism in alkanes.
2. Nitration reaction. Benzene reacts slowly with concentrated nitric acid. The rate of nitration increases significantly if the nitration reaction is carried out with a mixture of concentrated nitric and sulfuric acids (usually in a ratio of 1:2); this mixture is called nitrating.
The process occurs due to the fact that sulfuric acid, being stronger, protonates nitric acid, and the resulting protonated particle decomposes into water and an active electrophilic reagent - nitronium cation (nitronium cation).
The nitration reaction of benzene is an electrophilic substitution reaction and is ionic in nature. First, a p-complex is formed as a result of the interaction of electrons of the benzene ring with a positively charged particle of the nitronium cation.
Then the transition of the p-complex to the y-complex occurs. In this case, two p-electrons out of six go to the formation of a covalent bond C-NO2+. The remaining four -electrons are distributed among the five carbon atoms of the benzene ring. A y-complex is formed in the form of an unstable carbocation.
The unstable y-complex, under the influence of the HSO4- ion, loses a proton to form the aromatic structure of nitrobenzene.
3. Sulfonation reaction. To introduce a sulfo group into the benzene ring, fuming sulfuric acid is used, i.e., containing an excess of sulfuric anhydride (SO3). The electrophilic particle is SO3. The mechanism of sulfonation of aromatic compounds includes the following stages:
4. Alkylation reaction according to Friedel-Crafts. The role of the catalyst (usually AlCl3) in this process is to enhance the polarization of the alkyl halide to form a positively charged species that undergoes an electrophilic substitution reaction: FORMULA
3. Anthracene: structure and basic chemical properties
Anthracene - a compound whose molecule consists of three aromatic rings lying in the same plane. It is obtained from the anthracene fraction of coal tar, boiling at 300...350 °C. In laboratory practice, anthracene can be obtained
a) according to the Friedel-Crafts reaction:
b) according to the Fittig reaction:
In the anthracene molecule, the ninth and tenth positions, which are under the influence of the two outer rings, are the most active. Anthracene easily undergoes addition reactions at these positions:
When exposed to oxidizing agents, anthracene easily forms anthraquinone, which is widely used for the synthesis of dyes:
4. Conjugated dienes and methods for their synthesis
Diene hydrocarbons (dienes) are unsaturated hydrocarbons having two double bonds, the general formula CnH2n-2.
The two double bonds in a hydrocarbon molecule can be arranged in different ways. If they are concentrated at one carbon atom, they are called cumulated: -C=C=C- If two double bonds are separated by one simple bond, they are called conjugated: -C=C - C=C- If double bonds are separated by two or more simple bonds bonds, then they are called isolated: -C=C- (CH2)n - C=C-
5. Rules for orientation in the benzene ring
When studying substitution reactions in the benzene ring, it was discovered that if it already contains a substituent, then, depending on its nature, the second one enters a certain position. Thus, each substituent on the benzene ring exhibits a specific directing or orienting effect. The position of the newly introduced substituent is also influenced by the nature of the substituent itself, i.e., whether the active reagent has an electrophilic or nucleophilic nature. All substituents, by the nature of their directing action in are divided into two groups.
Substituents of the first kind direct the introduced group to ortho- and para-positions:
Substituents of this kind include the following groups, arranged in descending order of their orienting strength: N(CH3)2, NH2, OH, CH3 and other alkyls, as well as Cl, Br, I.
Substituents of the second kind V electrophilic substitution reactions direct the input groups to the meta position. Substituents of this kind include the following groups: - NO2, - CN, - SO3H, - CHO, - COOH.
6. The nature of the double bond and chemical properties of ethylene compounds
According to modern concepts, the two bonds connecting two unsaturated carbon atoms are not the same: one of them is a y-bond, and the other a p-bond. The latter bond is less strong and is “broken” during addition reactions.
The inequality of two bonds in unsaturated compounds is indicated, in particular, by comparing the energies of formation of single and double bonds. The energy of formation of a single bond is 340 kJ/mol (about 82 kcal/mol), and a double bond is 615 kJ/mol (about 147 kcal/mol). Naturally, it takes less energy to break a p-bond than to break a y-bond. Thus, the fragility of a double bond is explained by the fact that one of the two bonds forming a double bond has a different electronic structure than ordinary - bonds and is less strong.
Names of olefins usually derived from the names of the corresponding saturated hydrocarbons, but the ending is en replaced by the ending - Ilen. According to the international nomenclature, instead of ending - Ilen olefins are given a shorter ending - en.
Isomerism olefins depends on the isomerism of the chain of carbon atoms, i.e., on whether the chain is straight or branched, and on the position of the double bond in the chain. There is also a third reason for the isomerism of olefins: the different arrangement of atoms and atomic groups in space, i.e. stereoisomerism. Isomerism, depending on the different arrangement in space of atoms and atomic groups, is calledspatial isomerism , orstereoisomerism .
Geometric , orcis- Andtrans isomerism , is a type of spatial isomerism depending on different locations atoms relative to the plane of the double bond.
To indicate the location of the double bond (as well as branches in the chain), according to the international IUPAC nomenclature, the carbon atoms of the longest chain are numbered, starting from the end to which the double bond is closest. Thus, the two straight-chain isomers of butylene will be called butene-1 and butene-2:
1. Hydrogenation reaction. Unsaturated hydrocarbons easily add hydrogen at the double bond in the presence of catalysts 67 (Pt, Pd, Ni). With a Pt or Pd catalyst the reaction occurs at 20...100 °C, with Ni - at higher temperatures:
2. Halogenation reaction. Alkenes under normal conditions add halogens, especially chlorine and bromine. As a result, dihalogen derivatives of alkanes are formed containing halogens at neighboring carbon atoms, the so-called vicinal dihaloalkanes: CH
3CH=CH2 + Cl2> CH3CHClCH2Cl
3. Reaction of addition of hydrogen halides. Hydrohalogenation
4. Alkene hydration reaction. Under normal conditions, alkenes do not react with water. But in the presence of catalysts, under heat and pressure, they add water and form alcohols:
5. Reaction of addition of sulfuric acid. The interaction of alkenes with sulfuric acid proceeds similarly to the addition of hydrogen halides. As a result, acidic esters of sulfuric acid are formed:
6. Alkylation reaction of alkenes. Catalytic addition of alkanes with a tertiary carbon atom to alkenes is possible (catalysts - H2SO4, HF, AlCl3 and BF3):
7. Alkene oxidation reaction. Alkenes are easily oxidized. Depending on the oxidation conditions, different products are formed. When burned in air, alkenes are converted to carbon dioxide and water: CH2 = CH2 + 3O2 > 2CO2 + 2H2O.
When alkenes react with atmospheric oxygen in the presence of a silver catalyst, organic oxides are formed:
Acyl hydroperoxides act similarly on ethylene (Prilezhaev reaction):
One of the most characteristic oxidation reactions is the interaction of alkenes with a weakly alkaline solution of potassium permanganate KMnO4 with the formation of dihydric alcohols - glycols (Wagner reaction). The reaction proceeds in the cold as follows:
Concentrated solutions of oxidizing agents (potassium permanganate in an acidic environment, chromic acid, nitric acid) break the alkene molecule at the double bond to form ketones and acids:
8. Ozonation reaction of alkenes. It is also widely used to determine the structure of alkenes:
9. Substitution reactions. Alkenes are also capable of substitution reactions under certain conditions. Thus, during high-temperature (500...550 °C) chlorination of alkenes, hydrogen is replaced in the allylic position:
10. Alkene polymerization reaction
CH2 = CH2 > (-CH2 - CH2 -) n it turns out to be polyethylene
11. Isomerization reaction. At high temperatures or in the presence of catalysts, alkenes are capable of isomerizing, which either changes the structure of the carbon skeleton or moves the double bond:
7. Naphthalene and its structure. Hückel's rule
Naphthalene hydrocarbons are the main aromatic hydrocarbon of coal tar. There are a large number of polycyclic aromatic compounds in which the benzene rings share ortho carbon atoms. The most important of them are naphthalene, anthracene and phenanthrene. In anthracene, the rings are connected linearly, while in phenanthrene they are connected at an angle, unlike the benzene molecule, not all bonds in the naphthalene core have the same length:
Hückel's rule : aromatic is a planar monocyclic conjugated system containing (4n + 2) p-electrons (where n = 0,1,2...).
Thus, planar cyclic conjugated systems containing 2, 6, 10, 14, etc. will be aromatic. p-electrons.
8. Alkynes and sp-hybridization of the carbon atom. Methods for producing alkynes
Hydrocarbons of the acetylene series have the general formula
WITH n H2 n-2
The first simplest hydrocarbon in this series is acetylene C2H2. The structural formula of acetylene, like other hydrocarbons of this series, contains a triple bond:
N - S? S - N.
sR-Hybridization- this is mixing (alignment in shape and energy) of one s- and one R-orbitals to form two hybrid sR-orbitals. sR-The orbitals are located on the same line (at an angle of 180°) and directed in opposite directions from the nucleus of the carbon atom.
Two R-orbitals remain unhybridized. They are placed mutually perpendicular to the directions at- connections.
On the image sR-orbitals are shown along the axis y, and the unhybridized two R-orbitals- along axes X And z.
The triple carbon-carbon bond C?C consists of a y-bond, which arises from the overlap of sp-hybrid orbitals, and two p-bonds.
Calcium carbide is produced on an industrial scale by heating coal in electric furnaces with quicklime at a temperature of about 2500 ° C according to the reaction
CaO + 3C > CaC2 + CO.
If calcium carbide is exposed to water, it rapidly decomposes with the release of gas - acetylene:
A newer industrial method for producing acetylene is the pyrolysis of hydrocarbons, in particular methane, which at 1400 °C gives a mixture of acetylene with hydrogen:
2CH4>H-C=C-H + 3H2.
1. Dehydrohalogenation of vicinal dihaloalkanes
2. Reaction of sodium acetylenides with primary alkyl halides:
3. Dehalogenation of vicinal tetrahaloalkanes:
9. Preparation methods and chemicalsproperties of alcohols
Alcohols are derivatives of hydrocarbons in which one or more hydrogen atoms are replaced by the corresponding number of hydroxyl groups (-OH).
General formula of alcohols
where R is an alkyl or substituted alkyl group.
The nature of the radical R to which the hydroxyl group is associated determines the saturated or unsaturated state of alcohols, and the number of hydroxyl groups determines its atomicity: alcohols are monoatomic, diatomic, triatomic and polyatomic.
Preparation: 1. Hydration of alkenes
2. Enzymatic hydrolysis of carbohydrates. Enzymatic hydrolysis of sugars by yeast - the most ancient synthetic chemical process - is still of great importance for the production of ethyl alcohol.
When using starch as a starting material, in addition to ethyl alcohol, fusel oil is also formed (in smaller quantities), which is a mixture of primary alcohols, mainly isopentyl, isopropyl and isobutyl.
3. Synthesis of methyl alcohol:
4. Reaction of hydroboration-oxidation of alkenes:
5. Syntheses of alcohols using the Grignard reagent:
Properties: The chemical properties of alcohols are determined both by the structure of the alkyl radical and the reactive hydroxyl group. Reactions involving the hydroxyl group can occur either with the cleavage of the C-OH bond (360 kJ/mol) or with the cleavage of the O-H bond (429 kJ/mol) A. Cleavage of the C-OH bond
1. Reaction with hydrogen halides:
ROH + HX >RX + H2O.
Reactivity decreases in the series: HI > HBr > HCl
2. Reaction with phosphorus trihalides:
3. Dehydration of alcohols in the presence of water-removing agents:
B. Disconnection HE
4. Reaction of alcohols with metals(Na, K, Mg, Al)
5. Formation of esters:
Esterification reaction
6. Oxidation reactions When alcohols are oxidized with a chromium mixture or KMnO4 in a sulfuric acid solution, the composition of the products depends on the nature of the carbon atom (primary, secondary or tertiary) to which the hydroxyl group is associated: primary alcohols form aldehydes, secondary alcohols form ketones.
9. Alkadienes and methods for their preparation
Diene hydrocarbons (dienes) are unsaturated hydrocarbons having two double bonds with the general formula
The two double bonds in a hydrocarbon molecule can be arranged in different ways.
If they are concentrated at one carbon atom, they are called cumulated:
If two double bonds are separated by one single bond, they are called conjugate:
If double bonds are separated by two or more simple bonds, then they are called isolated: -C=C- (CH2)n - C=C-
Dienes are usually prepared by the same methods as simple alkenes. For example, the most important diene, butadiene-1,3 (used to produce synthetic rubber), is obtained in the USA by dehydrogenation of butane:
In the USSR, industrial synthesis of 1,3 butadiene was used according to the method of S.V. Lebedev (1933) from ethyl alcohol at 400...500 °C over a MgO-ZnO catalyst:
The reaction includes the following stages: dehydrogenation of the alcohol to an aldehyde, aldol condensation of acetaldehyde, reduction of the aldol to 1,3-butanediol, and finally dehydration of the alcohol:
10. Electronegativity of elements and types of chemical bonds
Electronegativity (h) (relative electronegativity) is a fundamental chemical property of an atom, a quantitative characteristic of the ability of an atom in a molecule to displace common electron pairs toward itself, that is, the ability of atoms to attract electrons of other atoms.
The highest degree of electronegativity is for halogens and strong oxidizing agents (p-elements of group VII, O, Kr, Xe), and the lowest for active metals (s-elements of group I).
Ionic. The electron configuration of an inert gas for any atom can be formed due to the transfer of electrons: atoms of one of the elements give up electrons, which go to the atoms of another element.
In this case, a so-called ionic (electrovalent, heteropolar) bond is formed between these atoms.
This type of bond occurs between atoms of elements that have significantly different electronegativity (for example, between a typical metal and a typical non-metal).
Covalent bond. When atoms of equal (atoms of the same element) or similar electronegativity interact, electron transfer does not occur. The electron configuration of the inert gas for such atoms is formed due to the sharing of two, four or six electrons by the interacting atoms. Each of the shared pairs of electrons forms one covalent (homeopolar) bond:
Covalent bonding is the most common type of bonding in organic chemistry. It's quite durable.
A covalent bond and therefore a molecule can be nonpolar when both bonded atoms have the same electron affinity (for example, H:H). It can be polar when an electron pair, due to the greater affinity for the electron of one of the atoms, is pulled towards it:
With this method, the designations + and - mean that the atom with the symbol - has excess electron density, and the atom with the + symbol has slightly reduced electron density compared to isolated atoms.
Donor-acceptor bond. When atoms that have lone electron pairs interact with a proton or another atom that lacks two electrons before forming an octet (doublet), the lone electron pair becomes shared and forms a new covalent bond between these atoms.
In this case, the atom that donates electrons is called a donor, and the atom that accepts electrons is called an acceptor:
chemical covalent benzene naphthalene
In the emerging ammonium ion, the covalent bond formed differs from the bonds that existed in the ammonia molecule only in the method of formation; in physical and chemical properties, all four N-H bonds are absolutely identical.
Semipolar connection. This type of donor-acceptor bond is often found in molecules of organic compounds (for example, nitro compounds, sulfoxides, etc.).
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7. Reaction mechanisms. Homo- and heterolytic bond cleavage. Mechanism of electrophilic addition. Radical accession.
Basic reaction mechanisms
We have identified three main types of reacting particles - free radicals, electrophiles, nucleophiles and three corresponding types of reaction mechanisms:
Free radicals;
electrophilic;
nucleophilic.
In addition to classifying reactions according to the type of reacting particles, in organic chemistry there are four types of reactions based on the principle of changing the composition of molecules: addition, substitution, elimination, or elimination (from English to eliminate - remove, split off), and rearrangement. Since addition and substitution can occur under the influence of all three types of reactive particles, several main reaction mechanisms can be distinguished.
In addition, we will consider elimination reactions, which occur under the influence of nucleophilic particles - bases.
7. Elimination
Homolytic bond cleavage is a cleavage where each atom loses one electron. Characteristic of the exchange mechanism of covalent bond formation.
Heterolytic bond cleavage is a rupture when positively and negatively charged particles are formed as a result, because both electrons from a common electron pair remain with one of the atoms. Characteristic of the donor-acceptor mechanism of covalent bond formation.
Electrophilic addition reactions addition electrophilic reaction) - addition reactions in which the attack at the initial stage is carried out by electrophile- a particle that is positively charged or has a deficiency of electrons. At the final stage, the resulting carbocation undergoes nucleophilic attack.
In organic chemistry, the most often attacking electrophilic particle is the proton. H+.
Despite the commonality of the mechanism, addition reactions are distinguished by bond carbon-carbon And carbon heteroatom.
General view of addition reactions at a double bond carbon-carbon:
Electrophilic addition reactions are common among alkenes and alkynes and are widely used in industrial chemical production and laboratory syntheses.
Radical addition reactions are addition reactions in which the attack is carried out free radicals- particles containing one or more unpaired electrons. In this case, radicals can attack both other radicals and neutral particles.
Radical addition reactions are denoted AdR.
Free radical addition reactions are characteristic of alkenes, which often undergo them instead of electrophilic addition reactions in the presence of a source of free radicals
The mechanism of the radical addition reaction includes the following stages:
First stage - circuit initiation. It may begin spontaneously, photochemically, electrochemically, by heating, or by chemical initiation.
Second stage - chain development. At this stage, radicals react with molecules, forming reaction products and new radicals.
Third stage - open circuit or recombination of free radicals.
Radical substitution reactions are accelerated in conditions where free radicals are generated and slowed down in the presence of free radical scavengers.
Radical accession goes against Markovnikov's rule (Kharasch effect). This is caused by the increased stability of tertiary, allylic and some other radicals formed when an attacking radical attaches to a certain position in the molecule.
8. Types of hybridization in organic compounds: alkanes, alkynes, alkenes. SP-,SP 2,-SP 3 – hybrid orbitals
sp 3 – hybridization
All four valence orbitals are involved in hybridization. Bond angle 109 o 28’ (tetrahedron). Carbon atoms form only simple (σ) bonds - the compound is saturated.
sp 2 – hybridization
Three hybrid and one non-hybrid orbital are formed. Bond angle 120° (flat structures, regular triangle). Hybrid orbitals form σ bonds. Non-hybrid orbitals form p-bonds. sp 2–Hybridization is typical for unsaturated compounds with one p-bond.
sp – hybridization
Two hybrid and two non-hybrid orbitals are formed. Bond angle 180° (linear structures). The carbon atom is in the state sp-hybridization takes part in the formation of two double bonds or one triple bond.
Each carbon atom in alkane molecules is in a state of sp 3 hybridization - all 4 hybrid orbitals of the C atom are identical in shape and energy, 4 bonds are directed to the vertices of the tetrahedron at angles of 109°28".
Alkynes (aka acetylene hydrocarbons) - hydrocarbons containing a triple bond between carbon atoms, forming a homologous series with the general formula C n H 2n-2. The carbon atoms at the triple bond are in a state of sp-hybridization.
Alkenes ( olefins, ethylene hydrocarbons) - acyclic unsaturated hydrocarbons containing one double bond between carbon atoms, forming a homologous series with the general formula C n H 2n. The carbon atoms at the double bond are in a state of sp² hybridization and have a bond angle of 120°.
1. Types of chemical bonds in organic compounds.
2. Covalent bond, its main characteristics.
3. Hydrogen bond, intermolecular interactions.
4. Electronic effects: inductive, mesomeric.
The problem of chemical bonding is the most important in chemistry, since the properties of substances and their reactivity depend on the composition, structure and type of chemical bond between atoms.
A chemical bond is the result of the interaction of two or more atoms, leading to a decrease in energy and the formation of a stable polyatomic system, such as a molecule.
Depending on the nature of the distribution of electron density in the region of bonding of atoms, three main types of chemical bonds are distinguished - covalent, ionic and metallic.
Molecules of organic compounds are built from atoms, usually connected to each other through covalent bonds. Ionic bonds are rare in individual organic compounds.
To explain the properties of chemical bonds, two theories are currently used - the valence bond method (VBC) and the molecular orbital method (MMO).
According to the valence bond method, a chemical bond is formed by a pair of electrons having opposite spins. In this case, the electron density increases in the space between the nuclei, which leads to their contraction. Covalent bonds are localized bonds. A chemical bond is formed in the direction where the possibility of overlapping atomic orbitals is greatest. The greater the overlap of atomic orbitals, the stronger the bond.
There are two known mechanisms for the formation of shared electron pairs: the sharing of unpaired electrons with opposite spins of two atoms (exchange mechanism) and the sharing of the lone pair of one of the atoms (donor-acceptor mechanism).
Main characteristics of a chemical bond:
1. Energy of communication. A chemical bond occurs only if the total energy of the interacting atoms decreases, i.e. When a bond is formed, energy is always released. Bond energy is a measure of bond strength. The more energy is released during the formation of a bond, the more energy must be spent on breaking it, therefore, the greater the bond energy, the more stable the connection. The binding energy varies over a very wide range - from 10 to 1000 kJ/mol.
2. Link length- the distance between the nuclei of bonded atoms allows one to judge the equivalence or non-equivalence of chemical bonds and their multiplicity. It depends on the size of the electron shells and the degree of their overlap. As the bond length decreases, the bond energy and stability of the molecules usually increases.
3. Saturation - the ability of atoms to form a limited number of covalent bonds; Due to the saturation of bonds, molecules have a certain composition.
4. Focus- determines the spatial structure of molecules. Atomic orbitals are spatially oriented, so their overlap occurs in certain directions, which determines the direction of the covalent bond. Directivity is expressed quantitatively in the form of bond angles between the directions of chemical bonds in molecules.
5. Polarity- displacement of a common electron pair towards the nucleus of one of the atoms; electronegativity (EO) can serve as a criterion for the ability of an atom to attract an electron. The bond is nonpolar if the difference in electronegativity of the atoms (Δ) is less than 0.5; if Δ = 0.5-2.0 - the connection is polar; if Δ > 2.0, then the bond is ionic.
Due to the displacement of the electron pair to one of the nuclei, the negative charge density of this atom increases, and the atom receives a charge called the effective charge δ-; the negative charge density of the other atom decreases, its effective charge δ+.
A measure of bond polarity is the bond dipole moment μ = qƖ (Cl . m), equal to the product of the effective charge and the length of the dipole.
The dipole moment of a molecule is equal to the vector sum of all dipole moments of chemical bonds and is determined by the geometry of the molecule. The greater the dipole moment of a molecule, the more polar it is. The polarity of a molecule largely determines the physical and chemical properties of a substance.
6 . Polarizability - the ability of the electronic shell of an atom or molecule to deform under the influence of an external field, for example an ion, a polar molecule, etc. Polarizability is a temporary polarization that disappears when the field is removed. Polarizability determines the reactivity of a molecule and depends on the bond length.
7 . Multiplicity. When a covalent bond is formed, there are two types of overlap of atomic orbitals. The overlap of atomic orbitals along the axis connecting the atomic nuclei is called σ-overlap or σ-bond, symmetrical about the bond axis. The lateral overlap of p-atomic orbitals with parallel axes is called π overlap or π bond, which does not have axial symmetry and is weaker than the σ bond. By multiplicity, single (σ-bond), double (combination of σ and π-bonds), triple (combination of σ and 2π-bonds) bonds are distinguished. As the bond multiplicity increases, the bond length decreases and its energy increases.
The carbon atom forms bonds due to electrons of different energy states - s-R-states, but, despite the difference in the shapes of the initial atomic orbitals, the bonds formed by them, for example in methane, turn out to be equivalent. To solve this problem, L. Pauling formulated two postulates - directed valence and hybridization of orbitals. Valence orbitals, for example 2s, 2p x, 2p y, 2p z of carbon, when forming a bond, hybridize (mix) and form equivalent (identical in shape and energy) atomic hybrid orbitals. The electron density of hybridized orbitals is concentrated on one side of the nucleus, which ensures maximum overlap of orbitals, which means the formation of a stronger chemical bond.
sp 3 - hybridization. One s- and three p-orbitals participate in the formation of a hybrid orbital. 4sp 3 - hybridized orbitals of an atom form 4 σ bonds with neighboring atoms. This is typical for saturated carbon compounds - hydrocarbons and their derivatives.
sp 2 - hybridization. In unsaturated compounds, the carbon atom is in an sp 2 -hybridized state; in this case, one s-orbital mixes with two p-orbitals to form three equivalent sp 2 -hybridized orbitals, when overlapping with the orbitals of neighboring atoms, 3 σ-bonds are formed.
The overlap of unhybridized p-orbitals (lateral overlap) leads to the formation of another type of covalent bond - a π bond. A double bond between two carbon atoms is described in the framework of hybridization theory as a combination of one σ and one π bond.
sp hybridization- a combination of an s-orbital and one p-orbital. In this case, two equivalent hybrid sp orbitals are formed, and when they overlap with the orbitals of neighboring atoms, 2 σ bonds are formed. Each carbon atom has two unhybridized p-orbitals, which, overlapping, form two π-bonds. Thus, a triple bond between two carbon atoms is a combination of one σ and two π bonds.
The difference in the shape and orientation of the hybridized orbitals is manifested in bond lengths, bond angles and other characteristics.
Below is the dependence of the structure of compounds on the hybridization of the carbon atom.
Hybridization Molecule Geometry Examples
sp 3 tetrahedral alkanes and their derivatives
sp 2 trigonal ethylene and its homologues, benzene,
carbonyl and carboxyl
groups, etc.
sp linear acetylene and its homologues,
nitrile, cumulated
hydrocarbons, etc.
Ionic bond occurs during the electrostatic interaction of negatively and positively charged ions in a chemical compound. This bond occurs only in the case of a large difference in the electronegativity of the atoms.
Hydrogen bond. A hydrogen atom bonded to a strongly electronegative atom (fluorine, oxygen, nitrogen) is capable of interacting with the lone electron pair of another strongly electronegative atom of the same or another molecule to form an additional weak bond called a hydrogen bond. If a hydrogen bond is formed between different molecules, it is called intermolecular; if a hydrogen bond is formed between two groups of the same molecule, then it is called intramolecular. An intramolecular bond is formed when closure of a five-membered or six-membered ring is possible. A hydrogen bond is indicated by three dots ···. The formation of intermolecular hydrogen bonds causes the association of molecules, which leads to a significant change in the physical properties of substances: an increase in viscosity, dielectric constant, melting and boiling temperatures, heats of vaporization and fusion.
Hydrogen bonds play an important role in proteins, in which helical polymer structures are linked by N–H···O bonds. Double helices of nucleic acids are connected by intermolecular hydrogen bonds N–H···N and N–H···O.
Van der Waals interactions occur between molecules of organic compounds - orientational, inductive, dispersed, which determine the physical properties of substances.
Mutual influence of atoms in a molecule. Deviations from the constant properties of chemical bonds in a molecule are associated with the manifestation of the mutual influence of atoms. The use of concepts of mutual influence makes it possible to predict the properties of stable molecules and determine the stability of organic ions and radicals. This influence is transmitted mainly through a system of covalent bonds, using the so-called electronic effects.
The mutual influence transmitted through a chain of σ bonds is called the inductive electronic effect. The inductive electronic effect (denoted by the letter I) can be positive or negative.
Most functional groups exhibit an -I effect: halogens, amino group, hydroxyl, carbonyl, carboxyl groups. The +I effect is exhibited by aliphatic hydrocarbon radicals, i.e. alkyl radicals (methyl, ethyl, etc.).
The inductive effect is transmitted through the circuit with attenuation. The direction of shift of the electron density of all σ bonds is indicated by straight arrows (→).
The influence of a substituent on the distribution of electron density transmitted through π bonds is called the mesomeric effect (denoted by the letter M). The mesomeric effect can be negative and positive. In structural formulas it is depicted as a curved arrow starting at the center of the electron density and ending at the place where the electron density shifts.
The presence of electronic effects leads to a redistribution of electron density in the molecule and the appearance of partial charges on individual atoms. This determines the reactivity of the molecule and the direction of chemical reactions with its participation.