The spatial structure of the molecules of inorganic and organic substances is of great importance in describing their chemical and physical properties. If we consider a substance as a set of letters and numbers on paper, it is not always possible to come to the right conclusions. To describe many phenomena, especially those related to organic chemistry, it is necessary to know the stereometric structure of the molecule.
What is stereometry
Stereometry is a branch of chemistry that explains the properties of the molecules of a substance based on its structure. Moreover, the spatial representation of molecules plays an important role here, since it is the key to many bioorganic phenomena.
Stereometry is a set of basic rules by which almost any molecule can be represented in volumetric form. The disadvantage of the gross formula written on a regular piece of paper is its inability to reveal the full list of properties of the substance under study.
An example would be fumaric acid, which belongs to the dibasic class. It is poorly soluble in water,poisonous and can be found in nature. However, if you change the spatial arrangement of COOH groups, you can get a completely different substance - maleic acid. It is highly soluble in water, can only be obtained artificially, and is dangerous to humans due to toxic properties.
Vant Hoff's stereochemical theory
In the 19th century, M. Butlerov's ideas about the flat structure of any molecule could not explain many properties of substances, especially organics. This was the impetus for van't Hoff to write the work "Chemistry in Space", in which he supplemented the theory of M. Butlerov with his research in this area. He introduced the concept of the spatial structure of molecules, and also explained the importance of his discovery for chemical science.
Thus, the existence of three types of lactic acid was proved: meat-lactic, dextrorotatory and fermented lactic acid. On a piece of paper for each of these substances, the structural formula will be the same, but the spatial structure of the molecules explains this phenomenon.
The result of Van't Hoff's stereochemical theory was the proof of the fact that the carbon atom is not flat, because its four valence bonds face the vertices of an imaginary tetrahedron.
Pyramidal spatial structure of organic molecules
Based on the findings of van't Hoff and his research, each carbon in the skeleton of organic matter can be represented as a tetrahedron. That's how wewe can consider 4 possible cases of the formation of C-C bonds and explain the structure of such molecules.
The first case is when the molecule is a single carbon atom that forms 4 bonds with hydrogen protons. The spatial structure of methane molecules almost completely repeats the outlines of a tetrahedron, however, the bond angle is slightly changed due to the interaction of hydrogen atoms.
The formation of one chemical C-C bond can be represented as two pyramids, which are interconnected by a common vertex. From such a construction of the molecule, it can be seen that these tetrahedra can rotate around their axis and freely change position. If we consider this system using the example of an ethane molecule, the carbons in the skeleton are indeed able to rotate. However, of the two characteristic positions, preference is given to the energetically favorable one, when the hydrogens in the Newman projection do not overlap.
The spatial structure of the ethylene molecule is an example of the third variant of the formation of C-C bonds, when two tetrahedra have one common face, i.e. intersect at two adjacent vertices. It becomes clear that because of such a stereometric position of the molecule, the movement of carbon atoms relative to its axis is difficult, because requires breaking one of the links. On the other hand, the formation of cis- and trans-isomers of substances becomes possible, since two free radicals from each carbon can be either mirrored or crisscrossed.
Cis- and transposition of the molecule explains the existence of fumaric and maleicacids. Two bonds are formed between the carbon atoms in these molecules, and each of them has one hydrogen atom and a COOH group.
The last case, which characterizes the spatial structure of molecules, can be represented by two pyramids that have one common face and are interconnected by three vertices. An example is the acetylene molecule.
Firstly, such molecules do not have cis or trans isomers. Secondly, carbon atoms are not able to rotate around their axis. And thirdly, all atoms and their radicals are located on the same axis, and the bond angle is 180 degrees.
Of course, the cases described can be applied to substances whose skeleton contains more than two hydrogen atoms. The principle of stereometric construction of such molecules is retained.
Spatial structure of molecules of inorganic substances
The formation of covalent bonds in inorganic compounds is similar in mechanism to that of organic substances. To form a bond, it is necessary to have unshared electron pairs in two atoms, which form a common electron cloud.
The overlapping of orbitals during the formation of a covalent bond occurs along one line of atomic nuclei. If an atom forms two or more bonds, then the distance between them is characterized by the magnitude of the bond angle.
If we consider a water molecule, which is formed by one oxygen atom and two hydrogen atoms, the bond angle should ideally be 90 degrees. Howeverexperimental studies have shown that this value is 104.5 degrees. The spatial structure of molecules differs from the theoretically predicted one due to the presence of interaction forces between hydrogen atoms. They repel each other, thereby increasing the bond angle between them.
Sp-hybridization
Hybridization is the theory of the formation of identical hybrid orbitals of a molecule. This phenomenon occurs due to the presence of unshared electron pairs at different energy levels in the central atom.
For example, consider the formation of covalent bonds in the BeCl2 molecule. Beryllium has unshared electron pairs at the s and p levels, which in theory should cause the formation of an uneven corner molecule. However, in practice they are linear and the bond angle is 180 degrees.
Sp-hybridization is used in the formation of two covalent bonds. However, there are other types of formation of hybrid orbitals.
Sp2 hybridization
This type of hybridization is responsible for the spatial structure of molecules with three covalent bonds. An example is the BCl3 molecule. The central barium atom has three unshared electron pairs: two at the p-level and one at the s-level.
Three covalent bonds form a molecule that is located in the same plane, and its bond angle is 120 degrees.
Sp3 hybridization
Another option for the formation of hybrid orbitals, when the central atom has 4 unshared electron pairs: 3 at the p-level and 1 at the s-level. An example of such a substance is methane. The spatial structure of methane molecules is a tetraerd, the valence angle in which is 109.5 degrees. The change in the angle is characterized by the interaction of hydrogen atoms with each other.
