A single covalent bond in a molecule. Simple (single) bond Types of bonds in bioorganic compounds
Atoms of most elements do not exist separately, as they can interact with each other. In this interaction, more complex particles are formed.
The nature of the chemical bond is the action of electrostatic forces, which are the forces of interaction between electric charges. Electrons and atomic nuclei have such charges.
Electrons located at the outer electronic levels (valence electrons), being farthest from the nucleus, interact with it the weakest, and therefore are able to break away from the nucleus. They are responsible for the binding of atoms to each other.
Types of interaction in chemistry
The types of chemical bond can be represented as the following table:
Ionic bond characteristic
The chemical interaction that is formed due to ion attraction having different charges is called ionic. This happens if the bonded atoms have a significant difference in electronegativity (that is, the ability to attract electrons) and the electron pair goes to a more electronegative element. The result of such a transition of electrons from one atom to another is the formation of charged particles - ions. There is an attraction between them.
have the lowest electronegativity typical metals, and the largest are typical non-metals. Ions are thus formed by interactions between typical metals and typical non-metals.
Metal atoms become positively charged ions (cations), donating electrons to external electronic levels, and non-metals accept electrons, thus turning into negatively charged ions (anions).
Atoms move into a more stable energy state, completing their electronic configurations.
The ionic bond is non-directional and not saturable, since the electrostatic interaction occurs in all directions, respectively, the ion can attract ions of the opposite sign in all directions.
The arrangement of ions is such that around each is a certain number of oppositely charged ions. The concept of "molecule" for ionic compounds doesn't make sense.
Examples of Education
The formation of a bond in sodium chloride (nacl) is due to the transfer of an electron from the Na atom to the Cl atom with the formation of the corresponding ions:
Na 0 - 1 e \u003d Na + (cation)
Cl 0 + 1 e \u003d Cl - (anion)
In sodium chloride, there are six chloride anions around the sodium cations, and six sodium ions around each chloride ion.
When an interaction is formed between atoms in barium sulfide, the following processes occur:
Ba 0 - 2 e \u003d Ba 2+
S 0 + 2 e \u003d S 2-
Ba donates its two electrons to sulfur, resulting in the formation of sulfur anions S 2- and barium cations Ba 2+ .
metal chemical bond
The number of electrons in the outer energy levels of metals is small; they easily break away from the nucleus. As a result of this detachment, metal ions and free electrons are formed. These electrons are called "electron gas". Electrons move freely throughout the volume of the metal and are constantly bound and detached from atoms.
The structure of the metal substance is as follows: the crystal lattice is the backbone of the substance, and electrons can move freely between its nodes.
The following examples can be given:
Mg - 2e<->Mg2+
Cs-e<->Cs+
Ca-2e<->Ca2+
Fe-3e<->Fe3+
Covalent: polar and non-polar
The most common type of chemical interaction is a covalent bond. The electronegativity values of the interacting elements do not differ sharply, in connection with this, only a shift of the common electron pair to a more electronegative atom occurs.
Covalent interaction can be formed by the exchange mechanism or by the donor-acceptor mechanism.
The exchange mechanism is realized if each of the atoms has unpaired electrons in the outer electronic levels and the overlap of atomic orbitals leads to the appearance of a pair of electrons that already belongs to both atoms. When one of the atoms has a pair of electrons at the outer electronic level, and the other has a free orbital, then when the atomic orbitals overlap, the electron pair is socialized and the interaction occurs according to the donor-acceptor mechanism.
Covalent are divided by multiplicity into:
- simple or single;
- double;
- triple.
Doubles provide the socialization of two pairs of electrons at once, and triples - three.
According to the distribution of electron density (polarity) between the bonded atoms, the covalent bond is divided into:
- non-polar;
- polar.
A non-polar bond is formed by the same atoms, and a polar bond is formed by electronegativity different.
The interaction of atoms with similar electronegativity is called a non-polar bond. The common pair of electrons in such a molecule is not attracted to any of the atoms, but belongs equally to both.
The interaction of elements differing in electronegativity leads to the formation of polar bonds. Common electron pairs with this type of interaction are attracted by a more electronegative element, but do not completely transfer to it (that is, the formation of ions does not occur). As a result of such a shift in the electron density, partial charges appear on atoms: on a more electronegative one, a negative charge, and on a less electronegative one, a positive one.
Properties and characteristics of covalence
The main characteristics of a covalent bond:
- The length is determined by the distance between the nuclei of the interacting atoms.
- Polarity is determined by the displacement of the electron cloud to one of the atoms.
- Orientation - the property to form space-oriented bonds and, accordingly, molecules that have certain geometric shapes.
- Saturation is determined by the ability to form a limited number of bonds.
- Polarizability is determined by the ability to change polarity under the influence of an external electric field.
- The energy required to break a bond, which determines its strength.
Molecules of hydrogen (H2), chlorine (Cl2), oxygen (O2), nitrogen (N2) and many others can be an example of a covalent non-polar interaction.
H+ + H → H-H molecule has a single non-polar bond,
O: + :O → O=O the molecule has a double nonpolar,
Ṅ: + Ṅ: → N≡N the molecule has a triple non-polar.
As an example of a covalent bond chemical elements you can bring molecules of carbon dioxide (CO2) and carbon monoxide (CO) gas, hydrogen sulfide (H2S), of hydrochloric acid(HCL), water (H2O), methane (CH4), sulfur oxide (SO2) and many others.
In the CO2 molecule, the relationship between carbon and oxygen atoms is covalent polar, since the more electronegative hydrogen attracts electron density to itself. Oxygen has two unpaired electrons in the outer level, while carbon can provide four valence electrons to form an interaction. As a result, double bonds are formed and the molecule looks like this: O=C=O.
In order to determine the type of bond in a particular molecule, it is enough to consider its constituent atoms. Simple substances metals form metallic, metals with non-metals - ionic, simple substances non-metals - covalent non-polar, and molecules consisting of different non-metals are formed through a covalent polar bond.
Simple (single) bond Types of bonds in bioorganic compounds.
| Parameter name | Meaning |
| Article subject: | Simple (single) bond Types of bonds in bioorganic compounds. |
| Rubric (thematic category) | Chemistry |
covalent bond. Multiple connection. non-polar connection. polar connection.
valence electrons. Hybrid (hybridized) orbital. Link length
Keywords.
Characterization of chemical bonds in bioorganic compounds
AROMATICITY
LECTURE 1
CONNECTED SYSTEMS: ACYCLIC AND CYCLIC.
1. Characteristics of chemical bonds in bioorganic compounds. Hybridization of the orbitals of the carbon atom.
2. Classification of conjugate systems: acyclic and cyclic.
3 Types of conjugation: π, π and π, p
4. Criteria for the stability of conjugated systems - ʼʼ conjugation energyʼʼ
5. Acyclic (non-cyclic) conjugate systems, types of conjugation. The main representatives (alkadienes, unsaturated carboxylic acids, vitamin A, carotene, lycopene).
6. Cyclic adjoint systems. Aromatic criteria. Hückel's rule. The role of π-π-, π-ρ-conjugation in the formation of aromatic systems.
7. Carbocyclic aromatic compounds: (benzene, naphthalene, anthracene, phenanthrene, phenol, aniline, benzoic acid) - structure, formation of an aromatic system.
8. Heterocyclic aromatic compounds (pyridine, pyrimidine, pyrrole, purine, imidazole, furan, thiophene) - structure, features of the formation of an aromatic system. Hybridization of electronic orbitals of the nitrogen atom in the formation of five- and six-membered heteroaromatic compounds.
9. Medico-biological significance of natural compounds containing conjugated bond systems, and aromatic.
The initial level of knowledge for mastering the topic ( school course chemistry):
Electronic configurations of elements (carbon, oxygen, nitrogen, hydrogen, sulfur, halogens), the concept of ʼʼorbitalʼʼ, hybridization of orbitals and spatial orientation of orbitals of elements of period 2., types of chemical bonds, features of the formation of covalent σ- and π-bonds, changes in the electronegativity of elements in a period and group, classification and principles of the nomenclature of organic compounds.
Organic molecules are formed through covalent bonds. Covalent bonds arise between two atomic nuclei due to a common (socialized) pair of electrons. This method refers to the exchange mechanism. Non-polar and polar bonds are formed.
Non-polar bonds are characterized by a symmetrical distribution of electron density between the two atoms that this bond connects.
Polar bonds are characterized by an asymmetric (non-uniform) distribution of electron density, it shifts towards a more electronegative atom.
Electronegativity series (composed downwards)
A) elements: F> O> N> C1> Br> I ~~ S> C> H
B) carbon atom: C (sp) > C (sp 2) > C (sp 3)
Covalent bonds are of two types: sigma (σ) and pi (π).
In organic molecules, sigma (σ) bonds are formed by electrons located on hybrid (hybridized) orbitals, the electron density is located between atoms on the conditional line of their binding.
π-bonds (pi-bonds) arise when two unhybridized p-orbitals overlap. Their main axes are parallel to each other and perpendicular to the σ-bond line. The combination of σ and π bonds is called a double (multiple) bond, it consists of two pairs of electrons. A triple bond consists of three pairs of electrons - one σ - and two π -bonds. (It is extremely rare in bioorganic compounds).
σ - Bonds are involved in the formation of the skeleton of the molecule, they are the main ones, and π -bonds can be considered as additional, but imparting special chemical properties to molecules.
1.2. Hybridization of the orbitals of the carbon atom 6 C
Electronic configuration of the unexcited state of the carbon atom
expressed by the distribution of electrons 1s 2 2s 2 2p 2.
At the same time, in bioorganic compounds, as well as in most inorganic substances, the carbon atom has a valency of four.
There is a transition of one of the 2s electrons to a free 2p orbital. Excited states of the carbon atom arise, creating the possibility of the formation of three hybrid states, denoted as С sp 3 , С sp 2 , С sp .
A hybrid orbital has characteristics different from the "pure" s, p, d orbitals and is a "mixture" of two or more types of unhybridized orbitals.
Hybrid orbitals are characteristic of atoms only in molecules.
The concept of hybridization was introduced in 1931 by L. Pauling, Nobel Prize winner.
Consider the arrangement of hybrid orbitals in space.
C sp 3 --- -- -- ---
In the excited state, 4 equivalent hybrid orbitals are formed. The location of the bonds corresponds to the direction of the central angles of a regular tetrahedron, the angle between any two bonds is equal to 109 0 28 , .
In alkanes and their derivatives (alcohols, haloalkanes, amines), all carbon, oxygen, and nitrogen atoms are in the same sp 3 hybrid state. A carbon atom forms four, a nitrogen atom three, an oxygen atom two covalent σ -connections. Around these bonds, the parts of the molecule can freely rotate relative to each other.
In the excited state sp 2, three equivalent hybrid orbitals arise, the electrons located on them form three σ -bonds that are located in the same plane, the angle between the bonds is 120 0 . Unhybridized 2p - orbitals of two neighboring atoms form π -connection. It is located perpendicular to the plane in which they are σ -connections. The interaction of p-electrons carries in this case the name ʼʼ lateral overlapʼʼ. A double bond does not allow free rotation of parts of the molecule around itself. The fixed position of the parts of the molecule is accompanied by the formation of two geometric planar isomeric forms, which are called: cis (cis) - and trans (trans) - isomers. (cis- lat- on one side, trans- lat- across).
π -connection
Atoms linked by a double bond are in a state of sp 2 hybridization and
present in alkenes, aromatic compounds, form a carbonyl group
>C=O, azomethine group (imino group) -CH= N-
With sp 2 - --- -- ---
The structural formula of an organic compound is 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 bonds
A covalent polar bond is characterized by an uneven distribution of electron density. Two conditional images are used to indicate the direction of electron density shift.
Polar σ - bond. The electron density shift is indicated by an arrow along the communication line. The end of the arrow points towards the more electronegative atom. The appearance of partial positive and negative charges is indicated using the letter ʼʼ bʼʼ ʼʼ deltaʼʼ with the desired charge sign.
b + b- b+ b + b- b + b-
CH 3 -\u003e O<- Н СН 3 - >C1 CH 3 -\u003e NH 2
methanol chloromethane aminomethane (methylamine)
Polar π bond. The electron density shift is indicated by a semicircular (curved) arrow above the pi bond, also directed towards the more electronegative atom. ()
b + b- b + b-
H 2 C \u003d O CH 3 - C \u003d== O
methanal |
CH 3 propanone -2
1. Determine the type of hybridization of carbon, oxygen, nitrogen atoms in compounds A, B, C. Name the compounds using the IUPAC nomenclature rules.
A. CH 3 -CH 2 - CH 2 -OH B. CH 2 \u003d CH - CH 2 - CH \u003d O
B. CH 3 - N H - C 2 H 5
2. Make the designations characterizing the direction of polarization of all the indicated bonds in the compounds (A - D)
A. CH 3 - Br B. C 2 H 5 - O- H C. CH 3 -NH- C 2 H 5
G. C 2 H 5 - CH \u003d O
Simple (single) bond Types of bonds in bioorganic compounds. - concept and types. Classification and features of the category "Single (single) bond Types of bonds in bioorganic compounds." 2017, 2018.
covalent chemical bond occurs in molecules between atoms due to the formation of common electron pairs. The type of covalent bond can be understood as both the mechanism of its formation and the polarity of the bond. In general, covalent bonds can be classified as follows:
- According to the mechanism of formation, a covalent bond can be formed by an exchange or donor-acceptor mechanism.
- The polarity of a covalent bond can be non-polar or polar.
- According to the multiplicity of the covalent bond, it can be single, double or triple.
This means that a covalent bond in a molecule has three characteristics. For example, in a molecule of hydrogen chloride (HCl), a covalent bond is formed by the exchange mechanism, it is polar and single. In the ammonium cation (NH 4 +), a covalent bond between ammonia (NH 3) and a hydrogen cation (H +) is formed according to the donor-acceptor mechanism, in addition, this bond is polar, is single. In the nitrogen molecule (N 2), the covalent bond is formed by the exchange mechanism, it is non-polar, it is triple.
At exchange mechanism the formation of a covalent bond, each atom has a free electron (or several electrons). Free electrons of different atoms form pairs in the form of a common electron cloud.
At donor-acceptor mechanism the formation of a covalent bond, one atom has a free electron pair, and the other has an empty orbital. The first (donor) gives a pair for common use with the second (acceptor). So in the ammonium cation, nitrogen has a lone pair, and the hydrogen ion has a free orbital.
Non-polar covalent bond formed between atoms of the same chemical element. So in the molecules of hydrogen (H 2), oxygen (O 2), etc., the bond is non-polar. This means that the common electron pair equally belongs to both atoms, since they have the same electronegativity.
Polar covalent bond formed between atoms of different chemical elements. A more electronegative atom displaces an electron pair towards itself. The greater the difference in the electronegativity of the atoms, the more the electrons will be displaced, and the bond will be more polar. So in CH 4, the shift of common electron pairs from hydrogen atoms to carbon atom is not so large, since carbon is not much more electronegative than hydrogen. However, in hydrogen fluoride, the HF bond is highly polar, since the difference in electronegativity between hydrogen and fluorine is significant.
Single covalent bond formed when atoms share the same electron pair double- if two triple- if three. An example of a single covalent bond can be hydrogen molecules (H 2), hydrogen chloride (HCl). An example of a double covalent bond is an oxygen molecule (O 2), where each oxygen atom has two unpaired electrons. An example of a triple covalent bond is a nitrogen molecule (N 2).
covalent bond. Multiple connection. non-polar connection. polar connection.
valence electrons. Hybrid (hybridized) orbital. Link length
Keywords.
Characterization of chemical bonds in bioorganic compounds
AROMATICITY
LECTURE 1
CONNECTED SYSTEMS: ACYCLIC AND CYCLIC.
1. Characterization of chemical bonds in bioorganic compounds. Hybridization of the orbitals of the carbon atom.
2. Classification of conjugate systems: acyclic and cyclic.
3 Types of conjugation: π, π and π, p
4. Criteria for the stability of conjugated systems - "conjugation energy"
5. Acyclic (non-cyclic) conjugate systems, types of conjugation. The main representatives (alkadienes, unsaturated carboxylic acids, vitamin A, carotene, lycopene).
6. Cyclic adjoint systems. Aromatic criteria. Hückel's rule. The role of π-π-, π-ρ-conjugation in the formation of aromatic systems.
7. Carbocyclic aromatic compounds: (benzene, naphthalene, anthracene, phenanthrene, phenol, aniline, benzoic acid) - structure, formation of an aromatic system.
8. Heterocyclic aromatic compounds (pyridine, pyrimidine, pyrrole, purine, imidazole, furan, thiophene) - structure, features of the formation of an aromatic system. Hybridization of electron orbitals of the nitrogen atom in the formation of five- and six-membered heteroaromatic compounds.
9. Medico-biological significance natural compounds containing conjugated bond systems, and aromatic.
The initial level of knowledge for mastering the topic (school chemistry course):
Electronic configurations of elements (carbon, oxygen, nitrogen, hydrogen, sulfur, halogens), the concept of "orbital", hybridization of orbitals and spatial orientation of the orbitals of elements of the 2nd period., types of chemical bonds, features of the formation of covalent σ- and π-bonds, changes in the electronegativity of elements in period and group, classification and principles of nomenclature of organic compounds.
Organic molecules are formed through covalent bonds. Covalent bonds arise between two atomic nuclei due to a common (socialized) pair of electrons. This method refers to the exchange mechanism. Non-polar and polar bonds are formed.
Non-polar bonds are characterized by a symmetrical distribution of electron density between the two atoms that this bond connects.
Polar bonds are characterized by an asymmetric (non-uniform) distribution of electron density; it shifts towards a more electronegative atom.
Electronegativity series (composed downwards)
A) elements: F> O> N> C1> Br> I ~~ S> C> H
B) carbon atom: C (sp) > C (sp 2) > C (sp 3)
Covalent bonds can be of two types: sigma (σ) and pi (π).
In organic molecules, sigma (σ) bonds are formed by electrons located on hybrid (hybridized) orbitals, the electron density is located between atoms on the conditional line of their binding.
π-bonds (pi-bonds) arise when two unhybridized p-orbitals overlap. Their main axes are parallel to each other and perpendicular to the σ-bond line. The combination of σ and π bonds is called a double (multiple) bond, it consists of two pairs of electrons. A triple bond consists of three pairs of electrons - one σ - and two π -bonds. (It is extremely rare in bioorganic compounds).
σ - Bonds are involved in the formation of the skeleton of the molecule, they are the main ones, and π -bonds can be considered as additional, but imparting special chemical properties to molecules.
1.2. Hybridization of the orbitals of the carbon atom 6 C
Electronic configuration of the unexcited state of the carbon atom
is expressed by the distribution of electrons 1s 2 2s 2 2p 2 .
However, in bioorganic compounds, as well as in most inorganic substances, the carbon atom has a valency of four.
There is a transition of one of the 2s electrons to a free 2p orbital. Excited states of the carbon atom arise, creating the possibility of the formation of three hybrid states, denoted as С sp 3 , С sp 2 , С sp .
A hybrid orbital has characteristics different from the "pure" s, p, d orbitals and is a "mixture" of two or more types of unhybridized orbitals.
Hybrid orbitals are characteristic of atoms only in molecules.
The concept of hybridization was introduced in 1931 by L. Pauling, Nobel Prize winner.
Consider the arrangement of hybrid orbitals in space.
C sp 3 --- -- -- ---
In the excited state, 4 equivalent hybrid orbitals are formed. The location of the bonds corresponds to the direction of the central angles of a regular tetrahedron, the angle between any two bonds is equal to 109 0 28 , .
In alkanes and their derivatives (alcohols, haloalkanes, amines), all carbon, oxygen, and nitrogen atoms are in the same sp 3 hybrid state. A carbon atom forms four, a nitrogen atom three, an oxygen atom two covalent σ -connections. Around these bonds, the parts of the molecule can freely rotate relative to each other.
In the excited state sp 2, three equivalent hybrid orbitals arise, the electrons located on them form three σ -bonds that are located in the same plane, the angle between the bonds is 120 0 . Unhybridized 2p orbitals of two neighboring atoms form π -connection. It is located perpendicular to the plane in which they are σ -connections. The interaction of p-electrons in this case is called "lateral overlap". A double bond does not allow free rotation of parts of the molecule around itself. The fixed position of the parts of the molecule is accompanied by the formation of two geometric planar isomeric forms, which are called: cis (cis) - and trans (trans) - isomers. (cis- lat- on one side, trans- lat- across).
π -connection
Atoms linked by a double bond are in a state of sp 2 hybridization and
present in alkenes, aromatic compounds, form a carbonyl group
>C=O, azomethine group (imino group) -CH= N-
With sp 2 - --- -- ---
The structural formula of an organic compound is 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 bonds
A covalent polar bond is characterized by an uneven distribution of electron density. Two conditional images are used to indicate the direction of electron density shift.
Polar σ - bond. The electron density shift is indicated by an arrow along the communication line. The end of the arrow points towards the more electronegative atom. The appearance of partial positive and negative charges is indicated using the letter "b" "delta" with the desired charge sign.
b + b- b+ b + b- b + b-
CH 3 -\u003e O<- Н СН 3 - >C1 CH 3 -\u003e NH 2
methanol chloromethane aminomethane (methylamine)
Polar π bond. The electron density shift is indicated by a semicircular (curved) arrow above the pi bond, also directed towards the more electronegative atom. ()
b + b- b + b-
H 2 C \u003d O CH 3 - C \u003d== O
methanal |
CH 3 propanone -2
1. Determine the type of hybridization of carbon, oxygen, nitrogen atoms in compounds A, B, C. Name the compounds using the IUPAC nomenclature rules.
A. CH 3 -CH 2 - CH 2 -OH B. CH 2 \u003d CH - CH 2 - CH \u003d O
B. CH 3 - N H - C 2 H 5
2. Make the designations characterizing the direction of polarization of all the indicated bonds in the compounds (A - D)
A. CH 3 - Br B. C 2 H 5 - O- H C. CH 3 -NH- C 2 H 5
170762 0
Each atom has a certain number of electrons.
Entering into chemical reactions, atoms donate, acquire, or socialize electrons, reaching the most stable electronic configuration. The configuration with the lowest energy is the most stable (as in noble gas atoms). This pattern is called the "octet rule" (Fig. 1).
Rice. one.
This rule applies to all connection types. Electronic bonds between atoms allow them to form stable structures, from the simplest crystals to complex biomolecules that eventually form living systems. They differ from crystals in their continuous metabolism. However, many chemical reactions proceed according to the mechanisms electronic transfer, which play an important role in the energy processes in the body.
chemical bond is the force that holds together two or more atoms, ions, molecules, or any combination of them.
The nature of the chemical bond is universal: it is an electrostatic force of attraction between negatively charged electrons and positively charged nuclei, determined by the configuration of the electrons in the outer shell of atoms. The ability of an atom to form chemical bonds is called valence, or oxidation state. The concept of valence electrons- electrons that form chemical bonds, that is, those located in the most high-energy orbitals. Respectively, outer shell an atom containing these orbitals is called valence shell. At present, it is not enough to indicate the presence of a chemical bond, but it is necessary to clarify its type: ionic, covalent, dipole-dipole, metallic.
The first type of connection isionic connection
In accordance with electronic theory Lewis and Kossel valencies, atoms can achieve a stable electron configuration in two ways: first, by losing electrons, becoming cations, secondly, acquiring them, turning into anions. As a result of electron transfer, due to the electrostatic force of attraction between ions with charges of the opposite sign, a chemical bond is formed, called Kossel " electrovalent(now called ionic).
In this case, anions and cations form a stable electronic configuration with a filled outer electron shell. Typical ionic bonds are formed from T and II group cations periodic system and anions of non-metallic elements of groups VI and VII (16 and 17 subgroups - respectively, chalcogens And halogens). The bonds in ionic compounds are unsaturated and non-directional, so they retain the possibility of electrostatic interaction with other ions. On fig. 2 and 3 show examples of ionic bonds corresponding to the Kossel electron transfer model.
Rice. 2.
Rice. 3. Ionic bond in a molecule table salt(NaCl)
Here it is appropriate to recall some of the properties that explain the behavior of substances in nature, in particular, to consider the concept of acids And grounds.
Aqueous solutions of all these substances are electrolytes. They change color in different ways. indicators. The mechanism of action of indicators was discovered by F.V. Ostwald. He showed that the indicators are weak acids or bases, the color of which in the undissociated and dissociated states is different.
Bases can neutralize acids. Not all bases are soluble in water (for example, some organic compounds that do not contain -OH groups are insoluble, in particular, triethylamine N (C 2 H 5) 3); soluble bases are called alkalis.
Aqueous solutions of acids enter into characteristic reactions:
a) with metal oxides - with the formation of salt and water;
b) with metals - with the formation of salt and hydrogen;
c) with carbonates - with the formation of salt, CO 2 and H 2 O.
The properties of acids and bases are described by several theories. In accordance with the theory of S.A. Arrhenius, an acid is a substance that dissociates to form ions H+ , while the base forms ions IS HE- . This theory does not take into account the existence of organic bases that do not have hydroxyl groups.
In line with proton Bronsted and Lowry's theory, an acid is a substance containing molecules or ions that donate protons ( donors protons), and the base is a substance consisting of molecules or ions that accept protons ( acceptors protons). Note that in aqueous solutions, hydrogen ions exist in a hydrated form, that is, in the form of hydronium ions H3O+ . This theory describes reactions not only with water and hydroxide ions, but also carried out in the absence of a solvent or with a non-aqueous solvent.
For example, in the reaction between ammonia NH 3 (weak base) and hydrogen chloride in the gas phase, solid ammonium chloride is formed, and in an equilibrium mixture of two substances there are always 4 particles, two of which are acids, and the other two are bases:
This equilibrium mixture consists of two conjugated pairs of acids and bases:
1)NH 4+ and NH 3
2) HCl And Cl ‑
Here, in each conjugated pair, the acid and base differ by one proton. Every acid has a conjugate base. A strong acid corresponds to a weak conjugate base, and weak acid is a strong conjugate base.
The Bronsted-Lowry theory makes it possible to explain the unique role of water for the life of the biosphere. Water, depending on the substance interacting with it, can exhibit the properties of either an acid or a base. For example, in reactions with aqueous solutions With acetic acid, water is a base, and with aqueous solutions of ammonia, it is an acid.
1) CH 3 COOH + H 2 O ↔ H 3 O + + CH 3 SOO- . Here the acetic acid molecule donates a proton to the water molecule;
2) NH3 + H 2 O ↔ NH4 + + IS HE- . Here the ammonia molecule accepts a proton from the water molecule.
Thus, water can form two conjugated pairs:
1) H 2 O(acid) and IS HE- (conjugate base)
2) H 3 O+ (acid) and H 2 O(conjugate base).
In the first case, water donates a proton, and in the second, it accepts it.
Such a property is called amphiprotonity. Substances that can react as both acids and bases are called amphoteric. Such substances are often found in nature. For example, amino acids can form salts with both acids and bases. Therefore, peptides readily form coordination compounds with the metal ions present.
In this way, characteristic property ionic bond - the complete movement of a bunch of binding electrons to one of the nuclei. This means that there is a region between the ions where the electron density is almost zero.
The second type of connection iscovalent connection
Atoms can form stable electronic configurations by sharing electrons.
Such a bond is formed when a pair of electrons is shared one at a time. from each atom. In this case, the socialized bond electrons are distributed equally among the atoms. An example of a covalent bond is homonuclear diatomic H molecules 2 , N 2 , F 2. Allotropes have the same type of bond. O 2 and ozone O 3 and for a polyatomic molecule S 8 and also heteronuclear molecules hydrogen chloride HCl, carbon dioxide CO 2, methane CH 4, ethanol FROM 2 H 5 IS HE, sulfur hexafluoride SF 6, acetylene FROM 2 H 2. All these molecules have the same common electrons, and their bonds are saturated and directed in the same way (Fig. 4).
For biologists, it is important that the covalent radii of atoms in double and triple bonds are reduced compared to a single bond.
Rice. 4. Covalent bond in the Cl 2 molecule.
Ionic and covalent types of bonds are two limiting cases of many existing types of chemical bonds, and in practice most of the bonds are intermediate.
Compounds of two elements located at opposite ends of the same or different periods of the Mendeleev system predominantly form ionic bonds. As the elements approach each other within a period, the ionic nature of their compounds decreases, while the covalent character increases. For example, the halides and oxides of the elements on the left side of the periodic table form predominantly ionic bonds ( NaCl, AgBr, BaSO 4 , CaCO 3 , KNO 3 , CaO, NaOH), and the same compounds of the elements on the right side of the table are covalent ( H 2 O, CO 2, NH 3, NO 2, CH 4, phenol C6H5OH, glucose C 6 H 12 O 6, ethanol C 2 H 5 OH).
The covalent bond, in turn, has another modification.
In polyatomic ions and in complex biological molecules, both electrons can only come from one atom. It is called donor electron pair. An atom that socializes this pair of electrons with a donor is called acceptor electron pair. This type of covalent bond is called coordination (donor-acceptor, ordative) communication(Fig. 5). This type of bond is most important for biology and medicine, since the chemistry of the most important d-elements for metabolism is largely described by coordination bonds.
Pic. five.
As a rule, in a complex compound, a metal atom acts as an electron pair acceptor; on the contrary, in ionic and covalent bonds, the metal atom is an electron donor.
The essence of the covalent bond and its variety - the coordination bond - can be clarified with the help of another theory of acids and bases, proposed by GN. Lewis. He somewhat expanded the semantic concept of the terms "acid" and "base" according to the Bronsted-Lowry theory. The Lewis theory explains the nature of the formation of complex ions and the participation of substances in nucleophilic substitution reactions, that is, in the formation of CS.
According to Lewis, an acid is a substance capable of forming a covalent bond by accepting an electron pair from a base. A Lewis base is a substance that has a lone pair of electrons, which, by donating electrons, forms a covalent bond with Lewis acid.
That is, the Lewis theory expands the range of acid-base reactions also to reactions in which protons do not participate at all. Moreover, the proton itself, according to this theory, is also an acid, since it is able to accept an electron pair.
Therefore, according to this theory, cations are Lewis acids and anions are Lewis bases. The following reactions are examples:
It was noted above that the subdivision of substances into ionic and covalent ones is relative, since there is no complete transfer of an electron from metal atoms to acceptor atoms in covalent molecules. In compounds with an ionic bond, each ion is in the electric field of ions of the opposite sign, so they are mutually polarized, and their shells are deformed.
Polarizability determined by the electronic structure, charge and size of the ion; it is higher for anions than for cations. The highest polarizability among cations is for cations of larger charge and smaller size, for example, for Hg 2+ , Cd 2+ , Pb 2+ , Al 3+ , Tl 3+. Has a strong polarizing effect H+ . Since the effect of ion polarization is two-way, it significantly changes the properties of the compounds they form.
The third type of connection -dipole-dipole connection
In addition to the listed types of communication, there are also dipole-dipole intermolecular interactions, also known as van der Waals .
The strength of these interactions depends on the nature of the molecules.
There are three types of interactions: permanent dipole - permanent dipole ( dipole-dipole attraction); permanent dipole - induced dipole ( induction attraction); instantaneous dipole - induced dipole ( dispersion attraction, or London forces; rice. 6).
Rice. 6.
Only molecules with polar covalent bonds have a dipole-dipole moment ( HCl, NH 3, SO 2, H 2 O, C 6 H 5 Cl), and the bond strength is 1-2 debye(1D \u003d 3.338 × 10 -30 coulomb meters - C × m).
In biochemistry, another type of bond is distinguished - hydrogen connection, which is a limiting case dipole-dipole attraction. This bond is formed by the attraction between a hydrogen atom and a small electronegative atom, most often oxygen, fluorine and nitrogen. With large atoms that have a similar electronegativity (for example, with chlorine and sulfur), the hydrogen bond is much weaker. The hydrogen atom is distinguished by one essential feature: when the binding electrons are pulled away, its nucleus - the proton - is exposed and ceases to be screened by electrons.
Therefore, the atom turns into a large dipole.
A hydrogen bond, unlike a van der Waals bond, is formed not only during intermolecular interactions, but also within one molecule - intramolecular hydrogen bond. Hydrogen bonds play an important role in biochemistry, for example, for stabilizing the structure of proteins in the form of an a-helix, or for the formation of a DNA double helix (Fig. 7).
Fig.7.
Hydrogen and van der Waals bonds are much weaker than ionic, covalent, and coordination bonds. The energy of intermolecular bonds is indicated in Table. one.
Table 1. Energy of intermolecular forces
Note: The degree of intermolecular interactions reflect the enthalpy of melting and evaporation (boiling). Ionic compounds require much more energy to separate ions than to separate molecules. The melting enthalpies of ionic compounds are much higher than those of molecular compounds.
The fourth type of connection -metallic bond
Finally, there is another type of intermolecular bonds - metal: connection of positive ions of the lattice of metals with free electrons. This type of connection does not occur in biological objects.
From a brief review of the types of bonds, one detail emerges: an important parameter of an atom or ion of a metal - an electron donor, as well as an atom - an electron acceptor is its size.
Without going into details, we note that the covalent radii of atoms, the ionic radii of metals, and the van der Waals radii of interacting molecules increase as their atomic number in the groups of the periodic system increases. In this case, the values of the ion radii are the smallest, and the van der Waals radii are the largest. As a rule, when moving down the group, the radii of all elements increase, both covalent and van der Waals.
The most important for biologists and physicians are coordination(donor-acceptor) bonds considered by coordination chemistry.
Medical bioinorganics. G.K. Barashkov
