A covalent bond in organic compounds will help you get an idea about the topic “Covalent bonds in organic compounds”. You will remember the nature of chemical bonds. Learn about how a covalent bond is formed, which is the basis of this bond. This lesson also discusses the principle of building Lewis formulas, talks about the characteristics of a covalent bond (polarity, length, and strength), explains A. Butlerov’s theory, and talks about what an inductive effect is.
Formation of a covalent bond
The chemical bond is mainly electrostatic in nature. For example, a hydrogen molecule is formed from two atoms, because it is energetically favorable for two electrons to be in the field of attraction of two nuclei (protons). This state, in the form of an H 2 molecule, has less energy than two separate hydrogen atoms.
Most organic substances contain covalent bonds.
To form a covalent bond between two atoms, each atom usually shares one electron.
The simplified model uses the two-electron approximation, i.e. all molecules are built on the basis of the summation of two electronic bonds characteristic of the hydrogen molecule.
From the point of view of the law of interaction of electric charges (Coulomb’s law), electrons cannot approach each other due to the enormous forces of electrostatic repulsion. But, according to the laws of quantum mechanics, electrons with oppositely directed spins interact with each other and form an electron pair.
If a covalent bond is denoted as a pair of electrons, we get another form of writing the formula of a substance – the electronic formula or the Lewis formula (Amer. J. Lewis, 1916) (Fig. 1).
Rice. 1. Lewis formulas
In organic molecules, there are not only single bonds but also double and triple. In the Lewis formulas, they are denoted, respectively, by two or three pairs of electrons (Fig. 2).
Rice. 2. Designation of double and triple bonds
Chemical bond polarity
Rice. 3. Covalent non-polar bond
An important characteristic of a covalent bond is its polarity. The bond between identical atoms, for example, in a hydrogen molecule or between carbon atoms in an ethane molecule, is nonpolar – in it, electrons equally belong to both atoms (see Fig. 3).
Rice. 4. Covalent polar bond
If the covalent bond is formed by different atoms, then the electrons in it are shifted to a more electronegative atom. For example, in the hydrogen chloride molecule, the electrons are shifted to the chlorine atom. Small partial charges arise on the atoms, which are denoted by d+ and d- (Fig. 4).
The greater the difference between the electronegativity of the atoms, the more polar the bond.
Mutual influence of atoms in a molecule
The mutual influence of atoms in a molecule leads to the fact that a displacement of bond electrons can occur, even if they are between identical atoms.
For example, in 1,1,1-trifluoroethane CH 3 CF 3, electronegative fluorine atoms “pull” the electron density from the carbon atom onto themselves. Often this is indicated by an arrow instead of a valence dash.
As a result, the carbon atom bound to the fluorine atoms has a lack of electron density, and it pulls valence electrons towards itself. Such a shift in the electron density along the chain of bonds is called the inductive effect of substituents (Fig. 5).
Rice. 5. Electron density shift in 1,1,1-trifluoroethane
Bond length and strength
Important characteristics of a covalent bond are its length and strength. The length of most covalent bonds is from 1 * 10 -10 m to 2 * 10 -10 m, or from 1 to 2 in angstroms (1 A \u003d 1 * 10 -10 m).
The strength of a bond is the energy it takes to break that bond. Typically, rupture values of 1 mol or 6.023 * 10 23 bonds are given (see Table 1).
The mutual arrangement of chemical bonds
At one time it was thought that molecules could be represented by structural formulas lying in the plane of the paper, and these formulas reflect, almost reflect, the true structure of the molecule. But around the middle of the 19th century, it turned out that this was not the case. For the first time, as I said in previous lessons, I came to this conclusion when I was still a student of Can’t Hoff. And he did this on the basis of the experiments of the outstanding French biologist and chemist Pasteur.
The fact is that Pasteur studied the salts of tartaric acid. And you can say he was lucky. Crystallizing a mixed salt of tartaric acid, he discovered under a microscope that he obtained, in general, a set of completely identical, very pretty crystals. But these crystals can easily be divided into two groups that are in no way compatible with each other, namely: all crystals are divided into two parts, one of which is a mirror image of the other.
Thus, optical, or mirror, isomerism was first discovered. Pasteur was able to manually separate these crystals with tweezers under a microscope and found that all chemical properties were practically the same. Only one, rather, the physical property does not coincide, namely: solutions of one type of crystal and a mirror of another type of crystal differently rotated the plane of polarization of light passing through them.
Rice. 6. Models of the methane molecule
In order to explain the results of Pasteur’s experiments, can’t Hoff assume that the carbon atom is always in a non-planar environment, and this non-planar environment has neither a center nor a plane of symmetry? Then the carbon atom, connected to 4 other different fragments of the molecule, which are not identical to each other, must have mirror symmetry. It was then that can’t Hoff suggested the tetrahedral structure of the carbon atom. Optical isomerism followed this assumption. As a result, it was possible to explain the spatial structure of organic compounds (Fig. 6).
But scientists are faced with another mystery that has not been solved so far. The fact is that in nature, organic compounds that are actually formed in organic living matter, as a rule, contain left-handed, meaning the plane of polarization of transmitted light, amino acids, and right-handed sugars. While in any organic synthesis a mixture of such isomers is necessarily obtained.
The reason for this selectivity of living nature is still not clear. But this does not prevent scientists from continuing to synthesize more and more new organic compounds and study their properties.
The formulas drawn on the plane do not reflect the spatial arrangement of atoms relative to each other. However, the tetrahedral structure of the carbon atom in molecules with single bonds leads to the existence of optical isomerism.
Summing up the lesson
You have gained an understanding of the topic “Covalent Bond in Organic Compounds”. You remembered the nature of chemical bonds. We learned about how a covalent bond is formed, which is the basis of this bond. Considered the principle of constructing Lewis formulas. We learned about the characteristics of a covalent bond (polarity, length, and strength), what is the inductive effect.
- Rudzitis G.E. Chemistry. Fundamentals of General Chemistry. Grade 10: textbook for educational institutions: basic level / G. E. Rudzitis, F. G. Feldman. – 14th edition. – M.: Enlightenment, 2012.
- Chemistry. Grade 10. Profile level: textbook. for general education institutions / V. V. Eremin, N. E. Kuzmenko, V. V. Lunin, et al. – M.: Bustard, 2008. – 463 p.
- Chemistry. Grade 11. Profile level: textbook. for general education institutions / V. V. Eremin, N. E. Kuzmenko, V. V. Lunin, et al. – M.: Bustard, 2010. – 462 p.
- Khomchenko G. P., Khomchenko I. G. Collection of problems in chemistry for applicants to universities. – 4th ed. – M.: RIA “New Wave”: Publisher Umerenkov, 2012. – 278 p.
Additional recommended links to Internet resources
- Internet portal “interneturok.ru” ( Source )
- Internet portal “Organic Chemistry” ( Source )
- Internet portal “Chemist” ( Source )
- No. 12, 15 (p. 11) Rudzitis G. E., Feldman F. G. Chemistry: Organic chemistry. Grade 10: textbook for educational institutions: basic level / G. E. Rudzitis, F. G. Feldman. – 14th edition. – M.: Enlightenment, 2012.
- Compose the structural and electronic formulas of ethane C 2 H 6, ethene C 2 H 4, and propyne C 3 H 8.
- Give examples from inorganic chemistry showing that the atoms in a molecule affect each other and their properties change.
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