Covalent bonds are represented by unsaturated organic compounds containing double and triple chemical bonds. To describe the nature of unsaturated compounds, L.Poling introduces the concepts of sigma- and π-bonds, hybridization atomic orbitals.

Poling hybridization for two S- and two PH electrons allowed to explain the direction of chemical bonds, in particular the tetrahedral configuration of methane. To explain the structure of ethylene from four equivalent SP3 electrons of the carbon atom, it is necessary to identify one p-electron for the formation of an additional connection called π-communication. In this case, the three remaining SP2-hybrid orbital are located in a plane at an angle of 120 ° and form basic bonds, for example, a flat molecule of ethylene.

In the case of an acetylene molecule in hybridization (by polying), only one S- and one p-orbital are involved, while two SP-orbitals are formed, located at an angle of 180 ° and directed into opposite sides. Two "pure" p-orbital carbon atoms pairly overlap in interdependacular planes, forming two π-bonds of the linear acetylene molecule.

The views of L. Poling were reflected in his book "The Nature of Chemical Communications, for many years who became the desk book of the Chemist. In 1954, L. Polying was awarded the Nobel Prize for Chemistry with the wording "For the study of the nature of the chemical bond and its application for the opposition structure of complex compounds."

However, the physical meaning of selective hybridization of atomic orbitals remained unclear, hybridization was algebraic transformations that physical reality could not be attributed.

Linus Paulong attempted to improve the description of the chemical bond, eliminating the electorality of the hybridization of orbital in the molecules of unsaturated compounds and creating the theory of curved chemical bond. In his report on the symposium on theoretical organic chemistry dedicated to the memory of Kekule (London, September 1958), L. Polying proposed a new way of describing a double bond as a combination of two identical curved chemical bonds, and triple bonds - three curved chemical bonds. On this

the symposium L. Polying argued with all categorical:

There may be chemists, believing that an extremely important innovation ... A description of σ, π- descriptions for double or triple coupling and conjugate systems instead of a description with the help of curved bonds. I argue that σ, π- description is less satisfactory than the description with the help of curved links, that this innovation is only passing and will soon dismiss.

AT new theory Polneg All binding electrons became equivalent and equivalent from the line connecting the kernel of the molecule. The theory of the curved chemical bond of Polneg took into account the statistical interpretation of the wave function of M. Born, the Coulomb electronic correlation of electrons. Physical sense appeared - the nature of the chemical bond is fully determined by the electrical interaction of nuclei and electrons. The larger the binding electrons, the smaller the interstitial distance and the stronger chemical bond between carbon atoms.

Three center chemical communications

The further development of the ideas about the chemical bond gave American physicochemistry of U. Lipskomb, which developed the theory of two-electronic three-center ties and topological theory, allowing the structure of some of some boron hydrides (boronovodov).

Electron vapor in three centered chemical bond becomes common to three nuclear atoms. In the simplest representative of the three-centered chemical bond - the molecular ion of hydrogen H3 + the electronic pair holds three protons in the whole whole.

Four single covalent functions operate in the dive molecule b-H communication and two two-electron three-center connections. The inter-identity distance in single covalent bond is 1.19 Å, while the similar distance in the three-center b-H-B is 1.31 Å. The angle of three-center b-H-B (φ) is 830. The combination of two three-center bonds in the dieboy molecule allows the kernel of the boron atoms at a distance of DB-B \u003d 2 · 1.31 · SIN φ / 2 \u003d 1.736 Å. The core of the binding hydrogen atoms are removed from the plane in which four single covalent bonds are located, at a distance h \u003d 1.31 · cos φ / 2 \u003d 0.981 Å.

Three center connections can be realized not only in the triangle of two boron atoms and one hydrogen atom, but also between the three boron atoms, for example, in frame borocheidors (pentaboran - B 5 H 9, December - B 10 H 4, etc.). In these structures there are conventional (terminal) and included in the three-center bond (bridging) hydrogen atoms and triangles from boron atoms.

The existence of boranians with their two-electron three-center connections with the "bridal" hydrogen atoms violated the canonical doctrine of valence. The hydrogen atom, who previously considered a standard monovalent element, turned out to be associated with the same bonds with two boron atoms and became a formally bivalent element. The works of U. Lilipsm to decipher the structure of Boraganov expanded the ideas about the chemical bond. The Nobel Committee honored William Nanna Lipovsky Prize in Chemistry for 1976 with the wording "for studying the structure of boranes (borogidritis), clarifying the problems of chemical bonds).

Multicenter chemical communications

In 1951, T.kili and P.Poson unexpectedly with a synthesis of dicyclopentadienyl received a completely new iron-organic compound. Obtaining an unknown earlier solely stable yellow-orange crystalline compound of iron immediately attracted attention.

E. Fisher and D. Yuilinson, independently of each other installed the structure of the new compound - two rings of cyclopentadienyl are located in parallel, layers, or in the form of a "sandwich" with an iron atom located between them in the center (Fig. 8). The name "Ferrocene" was proposed by R. Woodvord (or rather, an employee of his group D.Watch). It reflects the presence in the compound of the iron atom and ten carbon atoms (zehn - ten).

All ten bonds (C-Fe) in the ferrocene molecule are equivalent, the value of the inter-identity distance Fe - C - 2.04 Å. All carbon atoms in the ferrocene molecule are structurally and chemically equivalent, each length c-C communication 1.40 - 1.41 Å (for comparison, in benzene C-C 1.39 Å communication length). A 36-electronic shell occurs around the iron atom.

In 1973, Ernst Otto Fisher and Jeffrey Wilkinson were awarded the Nobel Prize in Chemistry with the wording "for the innovative, independently work, in the field of organometallic, so-called sandwich compounds." Indarb Lindquist, member of the Swedish Royal Academy of Sciences, in his speech at the presentation of the laureates, said that "the discovery and proof of the new principles of the relationships and structures available in sandwich compounds are a significant achievement, the practical significance of which is currently impossible to predict."

Currently obtained dicyclopentadienyl derivatives of many metals. Derivatives of transition metals have the same structure and the same nature of communication as ferrocene. Lantanoids form not a sandwich structure, but a design that resembles a three-glaving star [LA, CE, PR atoms, is consequently created, therefore fifteen centered chemical communications.

Soon, after Ferrocene was obtained by Dibenzolch. According to the same scheme, dibenzestumolybdenum and dibenzevalvanady]. In all connections of this class, metal atoms hold two six-sided rings in a single whole. All 12 ties of metal-carbon in these compounds are identical.

Uranian [bis (cyclooktateten) uranium] is also synthesized, in which the uranium atom keeps two eighth rings. All 16 ties of uranium-carbon in the uraniatene are identical. Uranian is obtained by the interaction of UCl 4 with a mixture of cyclooktatetraen and potassium in tetrahydrofuran at minus 300 C.

Simple (single) link types of ties in bioorganic compounds.

Covalent connection. Multiple communication. Non-polar connection. Polar communication.

Valence electrons. Hybrid (hybridized) orbital. Length communication

Keywords.

Characteristics of chemical bonds in bioorganic compounds

Aromatic

Lecture 1.

Conjugated systems: acyclic and cyclic.

1. Characteristics of chemical bonds in bioorganic compounds. Hybridization of carbon atom orbital.

2. Classification of conjugate systems: acyclic and cyclic.

3 Types of pairing: π, π and π, p

4. Criteria of the stability of conjugate systems - 'Energy of the pairing'''''''''''

5. Acyclic (non-cyclic) conjugate systems, pairing types. Major Representatives (Alkadians, unsaturated carboxylic acids, vitamin A, carotene, Licopene).

6. Cyclic conjugate systems. Aromatic criteria. Hyukkel rule. The role of π-π-, π-ρ-conjugation in the formation of aromatic systems.

7. Careciclic aromatic compounds: (benzene, naphthalene, anthracene, phenantrene, phenol, aniline, benzoic acid) - structure, formation of an aromatic system.

8. Heterocyclic aromatic compounds (pyridine, pyrimidine, pyrrolet, purine, imidazole, furan, thiophene) - structure, features of the formation of an aromatic system. Hybridization of electronic orbitals of nitrogen atom in the formation of five and six-membered heteroaromatic compounds.

9. The medical and biological importance of natural compounds containing conjugate bonding systems, and aromatic.

The initial level of knowledge for assimilation of the topic (school chemistry course):

Electronic configurations of elements (carbon, oxygen, nitrogen, hydrogen, sulfitial. Halogens), concept''orbital'', hybridization of orbital and spatial orientation of elements orbitals of elements 2 periods, types of chemical bonds, features of the formation of covalent σ-and π - connections, change of electronegativity elements in the period and a group, classification and principles of the nomenclature of organic compounds.

Organic molecules are formed by covalent ties. Covalent bonds arise between the two nuclei atoms due to the total (common) pair of electrons. This method refers to the exchange mechanism. Formed non-polar and polar bonds.

Non-polar connections Characterized by a symmetric distribution of electron density between two atoms, which this relationship connects.

Polar bonds are characterized by asymmetric (uneven) distribution of electron density, its displacement occurs towards a more electronegative atom.

Electricity series (reduced)

A) Elements: F\u003e O\u003e N\u003e C1\u003e BR\u003e I ~~ S\u003e C\u003e H

B) carbon atom: C (SP)\u003e C (SP 2)\u003e C (SP 3)

Covalent bonds are two types: sigma (σ) and pi (π).

In organic molecules of sigma (σ), communications are formed by electrons located on hybrid (hybridized) orbitals, electron density is located between atoms on the conventional line of their binding.

π - connection (PI-Owls) occur when overlapping two non-mentioned p-orbitals. The main axes of them are located in parallel to each other and perpendicular to the line σ-competition. The combination of σ and π - bonds is called a double (multiple) connection, consists of two pairs of electrons. The triple bond consists of three pairs of electrons - one σ - and two π-means. (In bioorganic compounds it is extremely rare).

σ - Communications are involved in the formation of the skeleton of the molecule, they are the main, and π - Communications can be considered as an additional, but sizing special chemical properties.

1.2. Hybridization of the orbital carbon atom 6 s

Electronic configuration of an unexcited state of carbon atom

it is expressed by the distribution of electrons 1S 2 2S 2 2P 2.

At the same time, in bioorganic compounds, as well, in most inorganic substances, carbon atom has valence equal to four.

There is a transition of one of 2S electrons to the free 2P orbital. The excited states of the carbon atom occur, creating the possibility of forming three hybrid states, designated as SP 3, with SP 2, with SP.

The hybrid orbital has characteristics that are different from '' tidy'ys S, P, D- orbitals and is 'mixture''''' two or more types of non-mentioned orbital.

Hybrid orbitals are peculiar to atoms only in molecules.

The concept of hybridization was introduced in 1931. L.Poling, laureate of the Nobel Prize,.

Consider the location in the space of hybrid orbitals.

With s p 3 --- - ---

In the excited state, 4 equivalent hybrid orbitals are formed. The arrangement of bonds corresponds to the direction of the central angles of the correct tetrahedron, the value of the angle between the two any connections is 109 0 28 ,.

In the alkans and their derivatives (alcohols, halogens, amines) in all carbon atoms, oxygen, nitrogen are located in the same hybrid SP 3 desire. A carbon atom forms four, nitrogen atom three, two covalent oxygen atom σ - Communication. Around these links is possible free rotation of the parts of the molecule relative to each other.

In the excited state SP 2, there are three equivalent hybrid orbital, the electrons arranged on them form three σ - Communications, which are located in the same plane, the angle between connections 120 0. Ungibridized 2p - orbitals of two SOS atoms form π -Ob. It is perpendicular to the plane in which there are σ - Communication. The interaction of P-electrons is in this case the name 'of lateral overlap''''''''''''''''''''''''' A multiple connection does not allow the free rotation of the parts of the molecule. The fixed position of the parts of the molecule is accompanied by the formation of two geometric plane isomeric forms that are called: CIS (CIS) - and TRANS (TRANS) - isomers. (cis lat- one way, trans lat- through).

π -Svyaz.

Atoms associated with double bond are in a state of hybridization SP 2 and

are present in alkens, aromatic compounds, form a carbonyl group

\u003e C \u003d o, azomethine group (imino group) -CH \u003d N-

With SP 2 - --- ---

Structural formula Organic compounds are depicted using Lewis structures (each pair of electrons between atoms is replaced by a dash)

C 2 H 6 CH 3 - CH 3 H H

1.3. Polarization of covalent connections

Covalent polar communication is characterized by uneven distribution of electron density. To refer to the direction of the electron density offset, two conventional images are used.

Polar σ - Communication. Electronic density displacement is denoted by an arrow along the communication line. The end of the arrow is directed toward the more electronegative atom. The appearance of partial positive and negative charges indicate using the letter 'B'''' Delta'' 'with the desired charging mark.

b + B - B + B + B - B + B-

CH 3 -\u003e O<- Н СН 3 - > C1 CH 3 -\u003e NN 2

methanol chloromethane aminomethane (methylamine)

Polar π -Svyaz.. The displacement of the electron density is denoted by a semicircular (curved) arrow over the PI-bonding also directed towards the more electronegative atom. ()

b + B - B + B-

H 2 C \u003d Oh 3 - C \u003d\u003d\u003d

methanal |

CH 3 propanone -2

1. Allow the type of hybridization of carbon atoms, oxygen, nitrogen in the combination of A, B, V. Name the combination, using the IUPAC nomenclature rules.

A. CH 3 -CH 2 - CH 2 - B. CH 2 \u003d CH - CH 2 - CH \u003d O

VH 3 - N - C 2 H 5

2. Make notation characterizing the direction of polarization of the BP specified connections In combination (A - D)

A. CH 3 - RR B. C 2 H 5 - ON VN 3 -NN-C 2N 5

G. C 2 H 5 - CH \u003d O

Simple (single) link types of ties in bioorganic compounds. - Concept and species. Classification and features of the category "Simple (single) link types of links in bioorganic connections." 2014, 2015.

Double bond Covalent four-electron relationship between two adjacent atoms in the molecule. D. s. It is usually indicated by two valence strokes:\u003e C \u003d C<, >C \u003d n -,\u003e c \u003d o,\u003e c \u003d s, - n \u003d n -, - n \u003d o and others. At the same time it is understood that one pair of electrons with sP 2. or sp.- hybridized orbital form s-° C (see fig. one ), whose electron density is concentrated along the interatomic axis; S-° C is similar to a simple connection. Another pair of electrons with r-Evubitals forms p-° C, the electron density of which is focused outside the interatomic axis. If in the formation of D. s. Atoms IV or V groups of the periodic system, these atoms and atoms associated with them are directly located in the same plane; Valence angles are 120 °. In the case of asymmetric systems, the molecular structure is possible. D. s. shorter than a simple connection and is characterized by a high energy barrier of internal rotation; Therefore, the positions of substituents with atoms associated with D., are notaquivalent, and this causes the phenomenon of geometric isomeria. Compounds containing D. with., Are capable of accession reactions. If D. s. electronically symmetric, the reactions are carried out both by radical (by hydraulica p-force) and by ion mechanisms (due to the polarizing action of the medium). If the electronegativity of atoms associated with D. s., Various or if various substituents are associated with them, then P is completely polarized. Compounds containing polar D. s., Prone to attachment over the ionic mechanism: to electronically accurate D. with. The nucleophilic reagents are easily attached, and to the electron donor D. s. - Electrophile. The direction of displacement of electrons during the polarization of D. s. It is customary to indicate arrows in the formulas, and the resulting excess charges - symbols d - and d. +. This facilitates the understanding of the radical and ion mechanisms of reactions of attachment:

In compounds with two D. s., Separated by one simple bond, there is a conjugation of P - bonds and the formation of a single p -electronic cloud, the lability of which manifests itself along the entire chain ( fig. 2. left). The consequence of such a conjugation is the ability to reactions 1,4-attachment:

If three D. s. Caught in a six-membered cycle, then the sextet p -electrons becomes common to the entire cycle and a relatively stable aromatic system is formed (see fig. 2, on right). Accession to such compounds such as electrical and nucleophilic reagents is energetically difficult. (See also Chemical communications. )