Sigma and pi bonds (σ- and π-bonds)
covalent chemical bonds characterized by a specific, but different spatial symmetry of the electron density distribution. As is known, a covalent bond is formed as a result of the sharing of electrons of interacting atoms. The resulting electron cloud of the σ bond is symmetrical with respect to the bond line, that is, the line connecting the nuclei of interacting atoms. Simple bonds in chemical compounds are usually (π bonds (see Simple bond). The electron cloud of a π bond is symmetrical about the plane passing through the bond line ( rice. 1
, b), and in this plane (called the nodal plane) the electron density is zero. The use of Greek letters σ and π is associated with their correspondence to Latin letters s And R in the designation of electrons of the atom, with the participation of which for the first time it becomes possible to form σ- and π-bonds, respectively. Because clouds of atomic R-orbitals ( p x, RU, p z) are symmetrical about the corresponding axes Cartesian coordinates (X, at, z), then if one R-orbital, for example p z, takes part in the formation of the σ bond (axis z- communication line), the remaining two R-orbitals ( p x, p y) can take part in the formation of two π-bonds (their nodal planes will be yz And xz respectively; cm. rice. 2
). Can also take part in the formation of σ and π bonds d- (cm. rice. 1
) And f-electrons of the atom. Lit.: Pimentel G., Spratley R., How quantum mechanics explains the chemical bond, trans. from English, M., 1973; Shustorovich E. M., Chemical bond, M., 1973. E. M. Shustorovich. Rice. 1. Schematic representation of the spatial orientation of orbitals during the formation of a σ bond as a result of s - s-, s - p σ-, p σ - p σ -interactions (a) and π-bond as a result of p π -, p π -, d π - d π - interactions (b). Rice. 2. Schematic representation of clouds of p x -, p y -, p z - electrons. The axes of Cartesian coordinates and the nodal planes of p x - and p y -orbitals are shown.
Big Soviet encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .
See what “Sigma and pi bonds” are in other dictionaries:
- (models) models of field theory, in which m scalar fields (i=1, ..., m) can be considered as defining a mapping of d-dimensional time space (of an arbitrary signature) into a certain manifold M of dimension with a metric... Physical encyclopedia
Fig. 1. Sigma connection ... Wikipedia
Greek alphabet Αα Alpha Νν Nu ... Wikipedia
Sigma (σ) - and pi (π) - connections- a covalent chemical property characterized by a certain, but different spatial symmetry of the electron density distribution. The resulting electron cloud σ communication is symmetrical relative to the communication line,... ... encyclopedic Dictionary in metallurgy
- (from Latin cumulo collect, accumulate) a system of bonds in which at least one atom is connected by double bonds to two neighboring atoms. K. s. in the Sigma and Pi connection group)). σ bonds are formed by two atomic orbitals of the C atom in... ...
Covalent bond using the example of a methane molecule: complete external energy level Hydrogen (H) has 2 electrons and carbon (C) has 8 electrons. Covalent bond is a bond formed by directed valence electron clouds. Neutral... ... Wikipedia
delta-sigma modulator- Modification of a delta modulator, at the input of which an integrator is turned on, and upon reception the reverse operation is performed, i.e. differentiation of the demodulator output signal. From an engineering point of view, the implementation of a delta-sigma modulator is no more difficult than... ... Technical Translator's Guide
project Sigma- A project separated in 1976 from the secret American project Aquarius. The goal of the project is to establish communication with aliens and is probably carried out at one of the Air Force bases in the state. New Mexico. E. Project Sigma D. Project Sigma … Explanatory ufological dictionary with equivalents in English and German
Type Open share... Wikipedia
One of the most important types of intramolecular mutual influence of atoms and bonds in organic compounds; due to interaction electronic systems atoms (primarily valence electrons, see Valence). The main sign... ... Great Soviet Encyclopedia
Books
- Digital PBX for rural communications, Zaporozhchenko N.P., Kartashevsky V.G., Mishin D.V., Roslyakov A.V., Sutyagina L.N., The book presents materials on the principles of construction and basic design of rural telephone networks ( STS), and also considered current state and prospects for rural development... Category: Telecommunications, electroacoustics, radio communications Publisher:
Basic objects of bio.chemistry.
Objects of study bio organic chemistry are proteins and peptides, nucleic acids, carbohydrates, lipids, biopolymers, alkaloids, terpenoids, vitamins, antibiotics, hormones, toxins, as well as synthetic regulators biological processes: medications, pesticides, etc.
Isomerism of organic compounds, its types. Characteristics of types of isomerism, examples.
There are two types of isomerism: structural and spatial (i.e. stereoisomerism). Structural isomers differ from each other by the order of bonds of atoms in the molecule, stereoisomers - by the arrangement of atoms in space with the same order of bonds between them.
The following types of structural isomerism are distinguished: carbon skeleton isomerism, positional isomerism, isomerism of various classes of organic compounds (interclass isomerism).
Isomerism of the carbon skeleton is due to the different order of bonds between the carbon atoms forming the skeleton of the molecule. For example: the molecular formula C4H10 corresponds to two hydrocarbons: n-butane and isobutane. For the C5H12 hydrocarbon, three isomers are possible: pentane, iso-pentane and neopentane. C4H10 corresponds to two hydrocarbons: n-butane and isobutane. For the C5H12 hydrocarbon, three isomers are possible: pentane, iso-pentane and neopentane.
Positional isomerism is due to different positions of the multiple bond, substituent, and functional group with the same carbon skeleton of the molecule
Interclass isomerism is the isomerism of substances belonging to different classes of organic compounds.
Modern classification and nomenclature of organic compounds.
Currently, a systematic nomenclature is widely used - IUPAC - International Unified Chemical Nomenclature. IUPAC rules are based on several systems:
1) radical functional (the name is based on the name of the functional group),
2) connecting (names are made up of several equal parts),
3) substitutive (the basis of the name is the hydrocarbon fragment).
Covalent bonds. Pi and sigma bonds.
Covalent bond is the main type of bond in organic compounds.
It is a bond formed by the overlap of a pair of valence electron clouds.
A pi bond is a covalent bond formed by overlapping p atomic orbitals.
A sigma bond is a covalent bond formed when s-atomic orbitals overlap.
If both s- and p-bonds are formed between atoms in a molecule, then a multiple (double or triple) bond is formed.
6. Modern ideas about the structure of organic compounds. The concept " chemical structure", "configuration", "conformation", their definition. The role of structure in the manifestation of biological activity.
In 1861 A.M. Butlerov proposed a theory of the chemical structure of organic compounds, which underlies modern ideas about the structure of org. connections, which consists of the following basic provisions:
1. In the molecules of substances there is a strict sequence of chemical bonding of atoms, which is called a chemical structure.
2. The chemical properties of a substance are determined by the nature of its elementary components, their quantity and chemical structure.
3.If substances with the same composition and molecular weight different structure, then the phenomenon of isomerism occurs.
4. Since in specific reactions only some parts of the molecule change, studying the structure of the product helps determine the structure of the original molecule.
5. The chemical nature (reactivity) of individual atoms in a molecule changes depending on the environment, i.e. depending on which atoms of other elements they are connected to.
The concept of “chemical structure” includes the idea of a certain order of connection of atoms in a molecule and of their chemical interaction, which changes the properties of the atoms.
SECTION I. GENERAL CHEMISTRY
3. Chemical bond
3.5. Sigma and pi bond
Spatially, two types of bonds are distinguished - sigma and pi bonds.
1. Sigma bond (σ bond) is a simple (single) covalent bond formed by overlapping electron clouds along the line connecting the atoms. The connection is characterized by axial symmetry:
Both ordinary and hybridized orbitals can take part in the formation of a σ bond.
2. Pi bond (π bond). If an atom has unpaired electrons left after forming a σ bond, it can use them to form a second type of bond, which is called a π bond. Let us consider its mechanism using the example of the formation of an oxygen molecule O2.
Electronic formula of the Oxygen atom - 8 O 1 s 2 2 s 2 2 p 2 , or 
Two unpaired p-electrons in an Oxygen atom can form two joint covalent pairs with the electrons of the second Oxygen atom:
One pair goes to form a σ bond:
The other, perpendicular to it, is for the formation of a π bond:
Another p -orbital (p in), as well as s -orbital, in which there are two paired electrons, do not take part in the bond and are not socialized.
Similarly, during the formation of organic compounds (alkenes and alkadienes) after sp 2 -hybridization, each of the two Carbon atoms (between which a bond is formed) remains with one non-hybridized p-orbital.
which are located in a plane that is perpendicular to the axis of the connection of Carbon atoms:
The sum of σ and π bonds gives a double bond.
A triple bond is formed in a similar way and consists of one σ-bond (p x) and two σ-bonds, which are formed by two mutually perpendicular pairs p-orbitals (p y, p z):
Example: formation of a nitrogen molecule N 2.
Electronic formula of the Nitrogen atom - 7 N 1s 2 2s
2 2 p 3 or 
Three p -electrons in the Nitrogen atom are unpaired and can form three joint covalent pairs with the electrons of the second Nitrogen atom:
As a result of the formation of three common electron pairs N≡N each nitrogen atom acquires a stable electronic configuration inert element 2 s 2 2 p 6 (octet of electrons).
A triple bond also occurs during the formation of alkynes (in organic chemistry). As a result s g-hybridization of the outer electron shell of the Carbon atom produces two s p-orbitals located along the 0X axis. One of them goes to form a b-bond with another Carbon atom (the second - to form a σ-bond with a Hydrogen atom). And two non-hybridized p-orbitals (p y, p z ) are placed perpendicular to each other and to the axis of connection of atoms (0X).
With the help of a π bond, a molecule of benzene and other arenes is formed. The length of the bond (aromatic, “one and a half”, affects) 1 is intermediate between the length of a single (0.154 nm) and double (0.134 nm) bond and is 0.140 nm. All six Carbon atoms have a common π-electron cloud, the density of which is localized above and below the plane of the aromatic nucleus and is evenly distributed (delocalized) between all Carbon atoms. According to modern ideas, it has the shape of a toroid:
1 The bond length is understood as the distance between the centers of the nuclei of the Carbon atoms involved in the bond.
Single connection– a covalent bond in which only one shared electron pair is formed between two atoms.
Sigma communication– a covalent bond, during the formation of which the area of overlap of electron clouds is located on the line connecting the nuclei of atoms. Single bonds are always sigma bonds.
Pi connection– a covalent bond, during the formation of which the area of overlapping electron clouds is located on both sides of the line connecting the nuclei. They are formed when two or three common electron pairs appear between two atoms. The number of shared electron pairs between bonded atoms characterizes communication multiplicity.
If a bond between two atoms is formed by two shared electron pairs, then such a bond is called double bond. Any double bond consists of one sigma bond and one pi bond.
If a bond between two atoms is formed by three shared electron pairs, then such a bond is called triple bond. Any triple bond consists of one sigma bond and two pi bonds.
Double and triple bonds have common name: multiple connections.
The overlapping orbitals must have the same symmetry about the internuclear axis. The overlap of atomic orbitals along the line connecting the atomic nuclei leads to the formation of σ - connections. Only one σ bond is possible between two atoms in a chemical particle. All σ bonds have axial symmetry relative to the internuclear axis. Fragments of chemical particles can rotate around the internuclear axis without disturbing the degree of overlap of atomic orbitals forming σ bonds. A set of directed, strictly oriented in space σ-bonds creates the structure of a chemical particle.
With the additional overlap of atomic orbitals perpendicular to the bond line, π bonds. As a result, multiple bonds arise between atoms: Single (σ), Double (σ +π), Triple (σ + π + π).F−F, O=O, N≡N.
With the appearance of a π-bond that does not have axial symmetry, free rotation of fragments of a chemical particle around the σ-bond becomes impossible, since it should lead to the rupture of the π-bond. In addition to σ- and π-bonds, it is possible to form another type of bond - δ-bonds: Typically, such a bond is formed after the formation of σ- and π-bonds by atoms when the atoms have d- and f-orbitals by overlapping their “petals” in four places at once. As a result, the multiplicity of communication can increase to 4-5.
Basic types of structures inorganic compounds. Substances with molecular and
non-molecular structure. Atomic, molecular, ionic and metallic
crystal lattices.
| Type | molecular | ionic | atomic | metal |
| In nodes | molecules | Positively and negatively charged ions (cations and anions) | Atoms | Atoms and positively charged metal ions |
| Character of communication | Intermolecular interaction forces (including hydrogen bonds) | Electrostatic ionic bonds | Covalent bonds | Metallic bond between metal ions and free electrons. |
| Durable | Weak | Durable | Very durable | Various strengths |
| Exc. Phys. Saints | Low-melting, low hardness, many soluble in water. | Refractory, solid, many soluble in water, solutions and melts conduct electric current (type 2 conductors) | Very refractory, very hard, practically insoluble in water | They are diverse in properties: they have shine, have electrical conductivity (conductors of the 1st kind) and thermal conductivity. |
| approx. | Iodine, ice, dry ice. | NaCl, KOH, Ba(NO 3) 2 | Diamond, silicon | Copper, potassium, zinc. |
Molecular and non-molecular substances - one of the signs chemical substances regarding their structure.
Molecular substances- these are substances whose smallest structural particles are molecules
Molecules- the smallest particle of a molecular substance capable of existing independently and preserving it Chemical properties. Molecular substances have low melting and boiling points and exist under standard conditions in a solid, liquid or gaseous state.
Non-molecular substances- these are substances whose smallest structural particles are atoms or ions.
And he is an atom or group of atoms that has a positive or negative charge.
Non-molecular substances are in standard conditions in solid state of aggregation and have high melting and boiling points.
There are substances with molecular and non-molecular structure. All gases and all liquids have a molecular structure. Solids can have a molecular or non-molecular structure. Volatile solids (ice, iodine, white phosphorus, organic substances) have a molecular structure. In nodes crystal lattice highly volatile solids are molecules. Most inorganic solids have a non-molecular structure; the lattice sites contain ions (salts, bases) or atoms (metals, diamond, silicon). Substances with a molecular structure make up more than 95% of all known substances, since organic substances have a molecular structure, and organic matter much more is known than inorganic ones.
Chemical reactions. Classification chemical reactions. The main problems of chemical kinetics and chemical thermodynamics.
Chemical reactions– These are phenomena in which the transformation of one substance into another occurs.
Signs of chemical reactions:
ü Gas release
Na 2 CO 3 +2HCl=2NaCl+H 2 O+CO 2
ü Precipitation or dissolution of sediment
BaCl 2 +H 2 SO 4 =BaSO 4 +2HCl
ü Color change
FeCl 3(yellow) +3KSCN (colorless) =Fe(SCN) 3(red) +3KCl
ü Odor appears.
ü Emission of light and heat
H 2 SO 4 +2NaOH=Na 2 SO 4 +2H 2 O+Q
2Mg+O 2 =2MgO+ hv
For chemical reactions to occur, conditions are necessary: contact of reacting substances, heating, lighting.
Classifications of chemical reactions:
I. According to the number and composition of the starting reagents:
a) Compound reaction- a reaction in which several substances form one substance, more complex than the original ones: A+B=AB
SO 3 +H 2 O=H 2 SO 4
NH 3 +HCl=NH 4 Cl
b) Decomposition reaction- a reaction in which several substances are formed from one complex substance. Final products There can be both simple and complex substances: AB=A+B
2KClO 3 =2KCl+3O 2
c) Substitution reaction- a reaction in which atoms of one element replace atoms of another element in a complex substance and at the same time two new ones are formed - simple and complex: X+AB=AX+B
Fe+CuSO 4 =FeSO 4 +Cu
Zn+2HCl=ZnCl 2 +H 2
d) Exchange reaction- a reaction in which reacting substances exchange their constituent parts, as a result of which two new substances are formed from two complex substances complex substances: AB+CX=AX+CB
BaCl 2 + Na 2 SO 4 = 2NaCl + BaSO 4
AgNO 3 +HCl=HNO 3 +AgCl
II. According to the sign of the thermal effect, reactions are divided into:
a) endothermic- reactions that occur with the absorption of heat
b) exothermic- reactions that result in the release of heat.
III. Taking into account the phenomenon of catalysis:
a) catalytic(flowing with the participation of a catalyst)
b) non-catalytic.
IV. Based on reversibility, reactions are divided into:
a) reversible– flowing simultaneously in the parallel and reverse direction
b) irreversible – flowing in one direction
V. Based on changes in the oxidation states of elements in the molecules of reacting substances:
a) OVR– electron transfer reactions
b) Not OVR– reactions without electron transfer.
VI. Based on the homogeneity of the reaction system:
a) Homogeneous– flowing in a homogeneous system
b) Heterogeneous– occurring in a heterogeneous system
There are two types of covalent bonds: sigma and pi bonds. A sigma bond is a single covalent bond formed when an AO overlaps along a straight line (axis) connecting the nuclei of two bonded atoms with a maximum overlap on this straight line. A sigma bond can arise when any (s-, p-hybrid) AOs overlap. In organogens (carbon, nitrogen, oxygen, sulfur), hybrid orbitals may take part in the formation of sigma bonds, providing more efficient overlap. In addition to axial overlap, another type of overlap is possible - lateral overlap of p-AO, leading to the formation of a pi bond. A pi bond is a bond formed by the lateral overlap of unhybridized p-AOs with a maximum overlap on both sides of the straight line connecting the nuclei of atoms. Multiple bonds often found in organic compounds are a combination of sigma and pi bonds; double - one sigma and one pi, triple - one sigma and two pi bonds.
Bonding energy is the energy released when a bond is formed or required to separate two bonded atoms. It serves as a measure of the strength of the bond: the greater the energy, the stronger the bond.
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. Bonds between carbon atoms in different states of hybridization are characterized by general pattern: As the fraction of s orbital in the hybrid orbital increases, the bond length decreases. For example, in the series of compounds propane CH3-CH2-CH3, propene CH3-CH=CH2, propine CH3-C-=CH, the CH3-C bond length is respectively 0.154, 0.150 and 0.146 nm.
In chemistry, the concept of hybrid orbitals of the carbon atom and other elements is widely used. The concept of hybridization as a way to describe the rearrangement of orbitals is necessary in cases where the number of unpaired electrons in the ground state of an atom is less than the number of bonds formed. It is postulated that different atomic orbitals having similar energy levels interact with each other, forming hybrid orbitals with the same shape and energy. Hybridized orbitals, due to greater overlap, form stronger bonds compared to non-hybridized orbitals.
The type of hybridization determines the orientation of hybrid AOs in space and, consequently, the geometry of the molecules. Depending on the number of orbitals that have entered into hybridization, a carbon atom can be in one of three states of hybridization. sp3-Hybridization. As a result of sp3 hybridization, a carbon atom from the ground state 1s2-2s2-2p2 due to the movement of an electron from the 2s to 2p orbital goes into the excited state 1s2-2s1-2p3. When four external AOs of an excited carbon atom (one 2s and three 2p orbitals) are mixed, four equivalent sp hybrid orbitals arise. They have the shape of a three-dimensional figure eight, one of the blades of which is much larger than the other. Due to mutual repulsion, sp3-hybrid AOs are directed in space towards the vertices of the tetrahedron and the angles between them are equal to 109.5° (the most favorable location). Each hybrid orbital in an atom is filled with one electron. The carbon atom in the state of sp3 hybridization has the electronic configuration 1s2(2sp3)4.
This state of hybridization is characteristic of carbon atoms in saturated hydrocarbons (alkanes) and, accordingly, in alkyl radicals of their derivatives. sp2-Hybridization. As a result of sp2 hybridization, due to the mixing of one 2s and two 2p AOs of an excited carbon atom, three equivalent sp2 hybrid orbitals are formed, located in the same plane at an angle of 120’. Unhybridized 2p-AO is in a perpendicular plane. The carbon atom in the state of sp2 hybridization has the electronic configuration 1s2-(2sp2)3-2p1. This carbon atom is characteristic of unsaturated hydrocarbons (alkenes), as well as some functional groups, for example carbonyl, carboxyl, etc. sp-Hybridization. As a result of sp hybridization, due to the mixing of one 2s and one 2p orbitals of an excited carbon atom, two equivalent sp hybrid AOs are formed, located linearly at an angle of 180°. The two remaining unhybridized 2p-AOs are located in mutually perpendicular planes. The carbon atom in the state of sp-hybridization has the electronic configuration 1s2-(2sp)2-2p2. Such an atom is found in compounds that have a triple bond, for example in alkynes and nitriles. Atoms of other elements can also be in a hybridized state. For example, the nitrogen atom in the ammonium ion NH4+ and, accordingly, alkylammonium RNH3+ is in a state of sp3 hybridization; in pyrrole and pyridine - sp2-hybridization; in nitriles - sp-hybridization.
