A chemical bond is a bond of two or more atoms (molecules) in an organic or inorganic compound. It is formed under the condition of a decrease in the total energy in the system.
Whether all elements can form chemical bonds
All elements of the periodic system have differentability to form a bond. The most stable and, as a result, atoms of noble (inert) gases are chemically inactive, since they contain two or eight electrons in the outer electron shell. They form a small amount of bonds. For example, neon, helium and argon do not form chemical bonds with a single element, whereas xenon, krypton and radon are able to react with fluorine and water molecules.
At atoms of other elements, the external levels are not completed and have from one to seven electrons, therefore, to increase the stability of the shell, they form chemical bonds.
Types of chemical bonds
There are several types of communication:
- Covalent.
- Ionic
- Metallic.
- Hydrogen.
Covalent bond
This type of bond is formed between atoms inmolecule as a result of the socialization or overlap of the valence electron pair. Accordingly, there is an exchange (a) and donor-acceptor (b) mechanisms for the formation of a covalent bond. A separate case is the dative bond, which will be discussed below.
Covalent bond: exchange mechanism
The atoms on the outer level have unpairedelectrons. When interacting, the outer shells overlap. The antiparallel spins of single electrons contained in external levels pair up to form an electron pair common to both atoms. This pair of electrons is, in fact, a covalent bond, which is formed by the exchange mechanism, for example, in a hydrogen molecule.
Covalent bond: donor-acceptor mechanism
Данный механизм заключается в обобществлении two atoms of two electrons at the outer level. In this case, one of the atoms acts as a donor (provides two electrons), and the other - an acceptor (has a vacant orbital for electrons). Atoms of s-and p-elements can be either acceptors or electron donors. Atoms of d-elements can be both donors and acceptors.
To understand what a donor-acceptor mechanism is, consider the two simplest examples - the formation of hydroxonium cations H3ABOUT+ and ammonium NH4+.
An example of a donor-acceptor mechanism is the ammonium cation
Schematically, the reaction of formation of ammonium particles is as follows:
NN3+ X+= NH4+
The electrons in the N atom are distributed in the following order: 1s2 2й2 2p3.
Electronic structure of the cation H: 1s0.
The nitrogen atom on the outer level contains two s- andthree p-electrons. Three p-electrons are involved in the formation of three covalent exchange type nitrogen-hydrogen bonds N-H. As a result, the ammonia molecule NH is formed.3 with covalent bond type. Since the nitrogen atom N on the outer level has a couple more electrons s, the molecule NH3 may also attach a cation of hydrogen. The ammonia molecule is a donor, and the hydrogen cation is H+ - an acceptor that accepts donor electrons from nitrogen to its own free s-orbital.
An example of donor-acceptor mechanism - H3O (hydroxonium ion)
The electrons in the oxygen atom are distributed in the following order: 1s2 2й2 2p4.
Атом кислорода на внешнем уровне имеет два s и four p-electrons. On this basis, two free p-electrons and two s-electrons from two H atoms take part in the formation of H-O bonds. That is, there are 2 bonds in the H molecule2O - covalent, formed by the exchange mechanism.
Electronic structure of the hydrogen cation: 1s0.
Since the oxygen atom on the outer levelthere are still two electrons (s-type), it can form a third covalent bond by a donor-acceptor mechanism. The acceptor may be an atom having a free orbital, in this example it is a particle H+. The free s-orbital of the cation H+ occupy two electrons (s) of the oxygen atom.
Donor-acceptor mechanism for the formation of covalent bonds between inorganic molecules
Донорно-акцепторный механизм ковалентной связи possible not only in interactions of the type "atom-atom" or "molecule-atom", but also in the reactions occurring between molecules. The only condition for donor-acceptor interaction of kinetically independent molecules is a decrease in entropy, in other words, an increase in the orderliness of the chemical structure.
Consider the first example - the formation of aprotic acid (Lewis acid) NH3Bf3. This inorganic complex is formed in the addition reaction of the ammonia molecule and boron fluoride.
NN3+ Bf3= NH3Bf3
The electrons in the boron atom are distributed in the following order: 1s2 2й2 2p1.
When a B atom is excited, one s-type electron moves to the p-sublevel (1s2 2й1 2p2). Thus, at the external level of the excited boron atom there are two s- and two p-electrons.
In the BF molecule3 three covalent bonds boron-fluorine B-F are formedexchange type (boron and fluorine atoms provide one electron each). After the formation of three covalent bonds at the boron atom on the outer electron shell, there remains a free p-sublevel, due to which the boron fluoride molecule can act as an electron acceptor.
The electrons in the nitrogen atom are distributed in the following order: 1s2 2й2 2p3.
Three electrons from atoms N and H are involved innitrogen-hydrogen bond formation. After that, nitrogen still has two more s-type electrons that it can provide for the formation of a bond by the donor-acceptor mechanism.
In the reaction between boron trifluoride and ammonia, the NH molecule3 plays the role of an electron donor, and the BF molecule3 - acceptor. A pair of nitrogen electrons occupies the free orbital of boron fluoride and the chemical compound NH is formed.3Bf3.
Another example of the donor – acceptor bond formation mechanism is the preparation of a polymer of beryllium fluoride.
Schematically, the reaction is as follows:
Bef2+ BeF2+ ... + BeF2-> (BeF2)Mr.
Electrons in a Be atom are located like this - 1s2 2й2, and in atom F - 1s2 2й2 2p5.
Two beryllium-fluorine bonds in a beryllium fluoride molecule are covalent exchange type (two p-electrons from two fluorine atoms and two electrons of the s-sublevel of beryllium are involved).
Between a pair of beryllium (Be) and fluorine (F) atomstwo more covalent bonds are formed by the donor-acceptor mechanism. In the polymer of beryllium fluoride, the fluorine atom is an electron donor, the beryllium atom is their acceptor having a vacant orbital.
Donor-acceptor mechanism for the formation of covalent bonds between organic molecules
When is the formation of communicationto the considered mechanism between molecules of organic nature, more complex compounds are formed - complexes. Any covalently bonded organic compound contains both occupied (non-bonding and bonding) and empty orbitals (loosening and non-bonding). The possibility of donor-acceptor formation of complexes is determined by the degree of stability of the complex, which depends on the strength of the bond.
Consider an example - interaction reactionmethylamine molecules with hydrochloric acid to form methyl ammonium chloride. In the methylamine molecule, all covalent bonds formed by the exchange mechanism are two H – N bonds and one N – CH bond3. After combining with hydrogen and methylgroup of the nitrogen atom has a pair of s-type electrons. Being a donor, it provides this electron pair for a hydrogen atom (acceptor), which has a free orbital.
Donor-acceptor mechanism without forming a chemical bond
Not in all cases of donor-acceptorinteraction is the socialization of the electron pair and the formation of communication. Some organic compounds can be combined with each other due to the overlapping of the donor’s orbital filled with the acceptor’s empty orbital. There is a charge transfer - electrons are delocalized between the acceptor and the donor, located very close to each other. Formed complex compounds with charge transfer (CRP).
This interaction is typical for pi-systemsorbitals of which easily overlap, and electrons easily polarize. Metallocenes, unsaturated amino compounds, TDAE (tetrakis (dimethylamino) ethylene) can act as donors. Acceptors are often fullerenes, chinodimethanes, with acceptor substituents.
Charge transfer can be either partial or full. Full charge transfer occurs during photoexcitation of the molecule. In this case, a complex is formed, which can be observed spectrally.
Независимо от полноты переноса заряда, такие complexes are unstable. To increase the strength and lifetime of such a condition, a bridge group is additionally introduced. As a result, donor-acceptor systems are successfully used in solar energy conversion devices.
In some organic molecules, bond bydonor-acceptor mechanism is formed inside the molecule between the donor and acceptor groups. This type of interaction is called the transannular effect, which is typical, for example, of atranes (organometallic compounds with N-> B, N-> Si bonds).
Semipolar communication, or the Dative mechanism of communication formation
In addition to the exchange and donor-acceptor existsthe third is the dative mechanism (other names are semipolar, semi-polar, or coordination). The donor atom donates a pair of electrons to the free orbital of a neutral atom, which needs two electrons to complete the outer level. A peculiar transition of electron density from the acceptor to the donor occurs. In this case, the donor becomes positively charged (cation), and the acceptor becomes negatively charged (anion).
The chemical bond itself is formed bythe bonding shell (the overlapping of two paired electrons of one of the atoms by the outer free orbital of the other) and the electrostatic attraction that arises between the cation and anion. Thus, the covalent and ionic types are combined in the semipolar bond. A semi-polar relationship is characteristic of d-elements, which in different compounds can play the role of both the acceptor and the donor. In most cases, it is found in complex and organic substances.
Examples of Dative Communication
The simplest example is the chlorine molecule.One Cl atom donates a pair of electrons to another chlorine atom, which has a free d-orbital. In this case, one Cl atom is positively charged, the other is negative, and electrostatic attraction occurs between them. Due to the large length, the dative bond has lower strength compared to the covalent exchange and donor-acceptor type, but its presence increases the strength of the chlorine molecule. That is why the Cl molecule2 more durable than F2 (the fluorine atom has no d-orbitals, the fluorine-fluorine bond is only covalent exchange).
Молекула монооксида углерода CO (угарный газ) formed by three C-O bonds. Since the atoms of oxygen and carbon at the external level have two single electrons, between them formed two covalent exchange bonds. After that, the carbon atom has a vacant orbital, and the O atom has two pairs of electrons on the outer level. Therefore, in the molecule of carbon monoxide (II) there is a third bond — a semipolar bond formed by two valence paired electrons of oxygen and a free carbon orbitals.
Consider a more complex example - the formation of this type of communication on the example of the interaction of dimethyl ether (H3C-O-CH3) with aluminum chloride AlCl3. Атом кислорода в диметиловом эфире связан двумя covalent bonds with methyl groups. After that, he still has two electrons in the p-sublevel, which he gives to the acceptor atom (aluminum) and becomes a positive cation. In this case, the acceptor atom acquires a negative charge (turns into an anion). The cation and anion interact electrostatically with each other.
The value of donor-acceptor connection
The mechanism of formation of donor-acceptor communicationis important in human life and is widely distributed in chemical compounds of both organic and inorganic nature, which is confirmed by the above examples. Ammonia, in which there is an ammonium cation, has been successfully used in everyday life, medicine and industrial production of fertilizers. The hydroxonium ion plays a major role in the dissolution of acids in water. Carbon monoxide is used in industry (for example, in the production of fertilizers, laser systems) and is of great importance in the physiological systems of the human body.
