The spatial structure of organic compounds. (1975)

Samples of organic substances having the same molecular composition, but differing in physical and chemical properties.

They are called isomers.

Experience with tertiary butyl alcohol and diethyl ether.

The first is crystals that dissolve in water.

The second is an immiscible liquid with water.

In the 60s of the XIX century, A.M. Butlerov, a professor at Kazan University, created the concept of chemical structure as the order of bonding of atoms.

Books written by A.M. Butlerov.

Tertiary butyl alcohol and diethyl ether differ in the order of atomic bonding.

Another pair of isomers is ethyl alcohol and dimethyl ether.

These are different substances, because they have the same atoms connected to each other in different sequences.

Flat structural formulas of different substances.

The molecules are three-dimensional.

A.M. Butlerov depicted one of the forms in the form of a triangular prism.

Butlerov's manuscripts.

In 1874, the Dutch chemist Jacob Hendrik Van Hoff laid the foundations of stereochemistry.

View of Rotterdam.

Portrait of Jacob Hendrik Van Hoff.

The basis of his hypothesis was the idea of the tetrahedral spatial structure of the carbon atom.

Image of the structure of a carbon atom.

One of the methods for determining the geometric structure of molecules is X-ray diffraction analysis.

Equipment for carrying out such an analysis.

Radiographs allow us to find interatomic distances with an accuracy of hundredths of an angstrom, valence angles with an accuracy of fractions of a degree.

The necessary calculations are performed on a computer.

Periodic table of Mendeleev.

The basis of organic compounds is carbon.

When bonds are formed, the carbon atom is excited, acquiring a new electronic configuration.

A cartoon explaining the hybridization of carbon, the creation of carbon chains and their forms.

The concept of rotary isomers or conformers.

Each substance is usually a mixture of different conformers in mobile equilibrium.

A cartoon explaining the conformations of butane that occur during internal rotation around the central carbon-carbon bond.

Newman's formula.

Diagram of changes in the potential energy of a butane molecule.

The concept of conformational energy.

Normal butane is a mixture of three conformers.

Most organic compounds exist in the form of equilibrium mixtures of inhibited conformations.

Conformational features largely determine the physical and chemical properties of substances.

A cartoon showing the spatial orientation of the valence.

A model of a molecule at the Brussels exhibition in the form of a large metal structure.

In fact, the atoms inside the molecule are packed very tightly.

A sample of such a structure, a hemispherical model.

Van der Waals radius.

A cartoon explaining the transformation of the chemical bond of atoms and obtaining a model in which the bond length of atoms is correctly transmitted.

No atom that is not chemically bound to this atom can penetrate into the sphere.

A cartoon explaining the creation of models of multivalent atoms.

Samples of models of various molecules.

Hemispherical models clearly show that during the transition from one conformation to another, individual groups of atoms not only approach or move away, but can even touch each other.

This increases the energy of such conformations and makes their existence unprofitable.

Let's consider other features of the structure of molecules based on the fluorochlorobromethane molecule.

We will replace the hemispherical model with a spherical rod.

Let's swap two atoms.

A comparison of the resulting model with the original one shows that they relate to each other as an object and its incompatible mirror image.

The molecules of such compounds do not have a single element of symmetry.

The concept of an asymmetric atom.

Such a pair of spatial isomers is called enantiomers or optical antipodes.

Characteristics of enantiomers.

A cartoon explaining the operation of the polarimeter.

Experience with optical antipodes.

The concept of racemic modification.

Substances that have optical isomerism.

Fischer projection formulas.

Depending on the orientation of the tetrahedron of the asymmetric carbon atom relative to the observer, the same model can be represented by 12 external source formulas.

They can be translated into each other using an even number of permutations of substituents.

The name of optical antipodes is constructed in several ways.

Variants of constructions.

According to the R,S-system, each of the substituents is put in the name depending on its atomic number.

Then the model is oriented so that the junior deputy is removed from the observer.

Then the seniority of the other three deputies will fall either clockwise, which is indicated by the symbol R, or counterclockwise, indicated by the symbol S.

After determining the seniority, the formulas are preceded by the symbols R or S.

Definition of the symbols R or S according to the Fisher formula.

The number of isomers depending on the asymmetric atoms.

Examples.

The concept of diastereomers.

Diastereomers differ from each other not only in the sign and magnitude of optical rotation, but also in other physical and chemical properties.

These differences are due to the fact that the distances between atoms are not the same for diastereomers, therefore the nature of the mutual influence of chemically unrelated atoms is also different.

In compounds with two identical asymmetric atoms, the number of diastereomers decreases.

Example, three sterioisomers of tartaric acid.

Consideration of Fischer's formulas of optically active tartaric acids shows that they have a certain element of symmetry - the second-order axis.

When rotating 180 degrees around this axis, we return to the original structure.

Consequently, the existence of optical antipodes is also possible for connections with axes of symmetry.

Chiral molecules.

Achiral molecules.

Modern methods make it possible to detect subtle features of the spatial structure of molecules and even notice the difference between structurally identical groups.

Examples.

The concept of enantiotopic groups.

Experience in the use of nuclear magnetic resonance for the determination of enantiotope substituents.

Experience in determining the differences between diastereotopic groups.

In the second valence state, the sp2-hybridization state, two carbon atoms are bound by a double bond.

Four substituents adjacent to them lie in the same plane with unsaturated carbon atoms.

With certain types of substitution, geometric isomers or cis-trans isomers arise in homologues and ethylene derivatives.

They differ in the position of the substituents relative to the plane of the double bond.

Rotation around the double bond is difficult.

For the mutual transition of geometric isomers, it is necessary to expend considerable energy.

Geometric isomers are stable, separately existing substances.

They have different chemical and physical properties.

The concept of cis form.

The concept of a trans form.

Z,E are notation systems of geometric isomers.

Examples of notation.

In the third valence state during sp hybridization, the valence angle is 180 degrees.

In the acetylene molecule, all four atoms lie on the same straight line and, therefore, geometric isomerism is not observed here.

Examples of organic reactions.

Formation of a racemate.

Walden appeal.

Bimolecular cleavage of E2.

Bimolecular anti-cleavage.

Without taking into account stereochemical factors, it is impossible to make a complete picture of the structure of the substance and the processes occurring during chemical reactions.