Elaboration formationOrganic moleculeIt refers to the subject of the combination and mutual influence of atoms in molecules, as well as the relationship between them and the properties of the compound.
In the early 19th century, the study of structural theory began.1828 F. Weiler and J. vonLiebig Recognize that every compound has a certain composition.Swedish chemist JJ.BezeliusIt was proposed in 1827IsomerismThe concept of phenomenon, and pointed out that the difference of isomers is caused by the different combinations of atoms in the molecule, and this different combination is called structure.This is the beginning of structural theory.
valence
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In 1858, German chemist FA.Kekule And British chemist AS. Cooper also introduced the concept of atomic valence and proposed“Valence theory”。According to the atomic valences of carbon, hydrogen, nitrogen and other elements, the theory that carbon atoms can combine to form chains in organic compounds is proposed.
Kekule and Cooper clarified that the number of carbon atoms always keeps a certain proportion with other atoms in both simple and complex compounds.For example, in methane CH, carbon tetrachloride CCl and chloroform CHCl, one carbon atom is combined with four other atoms.Kekule believed that every atom has a certain "chemical force", and called this chemical force valence.Assuming that hydrogen is univalent, carbon is tetravalent in all four compounds.Kekule proposed to use short lines to express the valence of atoms, for example:
Structure Theory of Organic Chemistry
A longer line is placed between two atoms that are bound together, indicating that each atom uses a valence to bond, for example:
Structure Theory of Organic Chemistry
Kekule and Cooper designed the molecular formula for methane similar to the modern electronic structure formula.In modern language, the valence bond of Kekule and Cooper is a pair of shared electrons:
Structure Theory of Organic Chemistry
On the basis of the concept that carbon atom is tetravalent and the valence bond theory, Kekule and Cooper clarified that carbon atoms can not only bond with other atoms, but also combine with each other to form carbon chains.For example, in ethane, two carbon atoms are linked into a chain, and the remaining six valence bonds are combined with six hydrogen;In propane, three carbon atoms are linked into a chain, and the other eight valence bonds are combined with eight hydrogen:
Structure Theory of Organic Chemistry
As for butane CH, there are two isomers, which can be explained as four carbon atoms can be connected to form a straight chain, or three carbon atoms can be connected to form a straight chain, while the fourth carbon atom is connected to the carbon atom in the middle of the straight chain to form a branch chain. If each carbon atom has the required hydrogen atom to meet its four valence, the structural formula in line with CH can be obtained:
Structure Theory of Organic Chemistry
When carbon atoms combine with each other, they can not only combine with one valence in the form of single bond, but also combine with two valence or three valence in the form of double bond in some compounds.Thus, the structural formulas of CH and CH are:
Structure Theory of Organic Chemistry
When each carbon atom is connected with the other two carbon atoms by two bonds, a ring structure is formed. At the same time, each carbon atom is combined with two hydrogen atoms by its other two valence, which also just meets the requirements of carbon atom's four valence, such as cyclopentane (structural formula is as follows).In 1865, Kekule applied this theory to aromatic compounds and proposed that carbon atoms form a ring structure in aromatic compounds;The Kekuler structural formula of benzene is proposed.
Structure Theory of Organic Chemistry
Because it was impossible to directly determine the static structure of organic molecules at that time, Kekule believed that these formulas learned through chemical reactions only represented the chemical properties of a molecule, and only represented the structural differences between a certain reaction before and after the reaction.A compound can have different reactions, so it should have different formulas.Cooper, when expressing the structure of organic compounds, only starts from the concept of atomic valence of elements, and ignores the chemical properties.Therefore, although Kekule and Cooper have made indelible achievements in structural theory, they have no clear concept of the nature of molecular structure.
Butlerov's structural theory In 1861, Russian chemist A. МButlerovIt is pointed out that each molecule in the material composition is not a simple accumulation of atoms, but a chemical combination of atoms in a certain order;There are complex chemical forces between atoms. The distribution of such chemical forces is called the chemical structure of matter.Butlerov clearly pointed out that a molecule has only one structure, and the chemical structure can be inferred from the chemical properties of the compound;On the contrary, the chemical properties of molecules can also be predicted through the chemical structure.Butelierov also synthesized tert butanol through practice, complementing one of the four isomers of butanol, further verifying the correctness of the structure theory.
The Tetrahedral Structure Hypothesis of Carbon Atoms, 1874, JH. Fantov and J-A. Lebel linked the number of optically active isomers (see optically active isomers) with the spatial structure, proposed the hypothesis that carbon atoms have a tetrahedral structure, and clarified that when carbon atoms are combined with other four atoms (or groups), carbon atoms are located in the center of a regular tetrahedron, and four protons (or groups) are located at the corners of the tetrahedron.If the four atoms (or groups) connected by the carbon atoms are different, a pair of different entities and mirror images will be produced, and there will be optical isomers, which will produce optical isomerism.This hypothesis laid the foundation of stereochemistry.
Geometric isomerism
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In 1887, J.A. Wislitzenus and in 1888, A.von Bayer put forward the theory of geometric isomerism and established the concept of geometric structure according to the characteristics that maleic acid can form anhydride, while fumaric acid cannot form anhydride, but only after it is transformed into maleic acid, according to the observation of Vantov.
In 1885, C. Lal studied the characteristics of ethyl acetoacetate and clarified the tautomerism.
Electronic model diagram
In the 1920s, chemists introduced the electronic theory of physics to organic chemistry.G.N.LouisIt is believed that a line in Kekule's formula represents a pair of electrons, and two carbon atoms each produce an electron to bond, which is called covalent bond, or shared electron pair.The valence bond theory and modern structure theory were formed.The most commonly used approximation methods to deal with molecular structure problems are electron pairing method and molecular orbital method:
① Electron pairing method, also known as valence bond method, is based on WH.HeitlerAnd FW.LondonIt is based on the treatment of hydrogen molecules.According to the electron pairing method, when the single electrons with opposite spins are close to each other, they can pair with each other to form stable chemical bonds.The chemical bonds thus formed are called covalent bonds.If an atom has several unpaired electrons, it can pair with several electrons with opposite spin to form bonds, so an atom has one unpaired electron, and its valence number is.
When two electrons with opposite spins combine to form bonds, the more the atomic orbits of bonding electrons overlap, the greater the density of the electron cloud, and the stronger the covalent bonds formed.In this way, the formation of covalent bonds takes the direction of the highest electron cloud density within the possible range.For example, the 1s electron cloud of H overlaps the 3p electron cloud of Cl, and H and Cl are close to each other along the axis to form stable molecules (Figure 1).The covalent bonds formed in this way are called bonds. As a result of orbital overlap, the electron cloud has cylindrical axial symmetry.The electron pairing method is also applicable to polyatomic molecules, such as tetravalent carbides.
Structure Theory of Organic Chemistry
Structure Theory of Organic Chemistry
According to Pauli principle and quantum mechanics calculation, when the main quantum number is equal to 2 (=2), the valence electron can be divided into two sublayers: two s electrons in one sublayer, and six p electrons in the other sublayer.The electron cloud of s electrons is distributed in spherical symmetry, and their orbits have no selected direction in space.The electron cloud of p-electron has the shape of "8", and their orbits are distributed in space in a fixed way. The direction of the three p-electron orbits is equivalent to the direction of the space vertical coordinate (Figure 2).
Structure Theory of Organic Chemistry
Structure Theory of Organic Chemistry
Structure Theory of Organic Chemistry
Structure Theory of Organic Chemistry
Structure Theory of Organic Chemistry
In its basic state, the carbon atom has a pair of ls electrons, a pair of 2s electrons and two unpaired 2p electrons (Figure 3).When forming a compound, if only these two unpaired p electrons are used as valence electrons, then a compound of bivalent carbon is formed.When carbon and other atoms or atomic clusters form a compound of tetravalent carbon, carbon must have a 2s electron transition to the vacancy in the 2p electron layer under the excited state (Figure 4).Thus, one electron participating in the formation of a covalent bond in a tetravalent carbon compound is an s electron, and the other three are p electrons.When a carbon atom forms a compound with four identical atoms or clusters, its four valence bonds form the four top corners of the tetrahedron symmetrically and uniformly.Accordingly, L.C. Pauling and JC. Slater put forward the famous theory of atomic orbital hybridization in 1928: several atomic orbitals can be mixed, and then they can be redistributed to form the same number of hybrid orbitals.That is to say, when the carbon atom is in the excited state, its s electrons and p electrons are located in the energy level that is quite close to each other. They are no longer equivalent to individual s electrons and p electrons, but rather equivalent to a certain combination state of them, forming a sp electron cloud that is evenly distributed in space (Figures 5 and 6).This phenomenon of several electronic orbits recombining into complex electronic orbits is called the hybridization of atomic orbits, and the new orbits formed in the process of hybridization are called hybrid orbits (see the color diagram).When carbon atoms are combined with four identical atoms or atomic clusters, the included angle is 109 ° 28 '.
Structure Theory of Organic Chemistry
Organic chemical structure theory Organic chemical structure theory Organic chemical structure theory
Structure Theory of Organic Chemistry
Structure Theory of Organic Chemistry
Figure 7 shows four hybrid atomic orbitals of three carbon atoms, which are used to form two C-C and C-C bonds.Here, the orbit on the right of C atom (represented by dotted line) overlaps with that of C atom, while the orbit on the left of C atom (represented by solid line) overlaps with that of C atom.
In the process of forming alkene bonds, one s electron and two p electrons of each carbon atom hybridize to form an sp electron cloud, forming three bonds with the combined atoms or atomic clusters, and forming an included angle of 120 ° with each other in the same plane (Figure 8).The remaining one electron of each of the two carbon atoms, without hybridization, combines with each other according to the two "8" shaped p orbitals to form a π bond.The two p orbitals that form the π bond are parallel to each other and equally distributed above and below the plane occupied by the bond in the direction perpendicular to the bond (Figure 9).The electron orbits of the π bond overlap the most when they are parallel to each other.The rotation along the C-C bond will make the two π - electron orbits leave the parallel state, so the combination of π - electrons will hinder the rotation.
Structure Theory of Organic Chemistry
Organic chemical structure theory Organic chemical structure theory Organic chemical structure theory
Structure Theory of Organic Chemistry
Structure Theory of Organic Chemistry
In the process of forming the alkyne bond, the carbon atom hybridizes with an s electron and a p electron to form an sp electron cloud, and forms two bonds with the combined atoms or atomic clusters. As a result, the bond formed is in a straight line, and the bond angle is 180 °.The other two p electrons of each carbon atom are paired with the two p electrons of the combined carbon atom to form two π bonds, which are perpendicular to each other in the direction perpendicular to the HCCH line (Figure 10).
② Molecular orbital theory holds that after atoms combine to form molecules, valence electrons are no longer localized in individual atoms, but move in the whole molecule.Molecular orbital theory uses linear combinations of atomic orbitals to represent molecular orbitals.The state function ψ is used to describe the motion state of each electron in the molecule, and ψ is the molecular orbital.ψ d represents the probability of the electron in the micro volume product d, and ψ is the probability density (also called the electron cloud density).To represent the corresponding energy of each molecular orbital ψ, it approximately represents the energy required for electron ionization in this orbital.Assuming that the total energy of the molecule is equal to the sum of the energy of the molecular orbital occupied by electrons, if ψ is occupied by two electrons, its energy should be calculated twice, namely:
Structure Theory of Organic Chemistry
Where is the number of electrons in ψ orbit, which can be equal to 0, 1 or 2.
Molecular orbitals are formed by the combination of original orbitals.A filled orbit can have at most two electrons.The combination of two atomic orbitals with the same energy forms two molecular orbitals, one of which has lower energy than the atomic orbital, called bond orbital;The other energy is higher than the atomic orbital energy, called the antibonding orbital.For C-C bonds, the symbols representing bonding and anti bonding between atomic orbitals are: the action between two (+) lobes is bonding (orbital);The action between one (+) and one (-) lobe is anti bonding (orbital).The symbol is discontinuous.
In the description here, the orbits maintain their shape in the hybrid atomic orbits, but they do not quantitatively represent the distribution of electron density in the formed C-C bond.The interaction here is characterized by the difference in symmetry between bonding orbits and anti bonding orbits.For the plane perpendicular to the axis between two carbon atoms, the bonding orbital is symmetric (S), while the anti bonding orbital is antisymmetric (A) (Figure 11).
Structure Theory of Organic Chemistry
Organic chemical structure theory Organic chemical structure theory Organic chemical structure theory
Structure Theory of Organic Chemistry
Structure Theory of Organic Chemistry
Using a similar method, π orbitals can be formed from isolated p orbitals. Figure 12 shows the results of the interaction of two p orbitals.If two (+) symbols overlap, the π bond is bonded and is written as π (C=C), while the anti bond orbital, with discontinuous symbols or nodes, is written as π (C=C).
The difference between these two orbitals (bonding and anti bonding) is also in their symmetry.In Figure 13, there are two mirror surfaces (m), one bisects the orbit (in the molecular plane), the other is perpendicular to the C-C bond (perpendicular to the molecular plane and bisects the molecule), and a double rotation axis c passing through the C-C bond center.π orbitals and π orbitals can be distinguished by two independent symmetry operations.The π orbit is symmetric (S) to the mirror m, while the rotation around the c axis is antisymmetric (A).The corresponding π orbital is antisymmetric (A) for m and symmetric (S) around the c axis.
The overlap between the two 2p orbitals is smaller than that contained in the interaction, so the π bond is also weaker than the bond.At the same time, the specific energy levels of π and π increase and decrease (Figure 14).
Structure Theory of Organic Chemistry
Structure Theory of Organic Chemistry
Other organic molecules, such as 1,3-butadiene and benzene, can also be treated by the same method.
Since the 1960s, on the one hand, on the basis of quantum mechanics, we have continuously improved the calculation method of molecular orbits, and cooperated with computers, many accurate data have been obtained, such as the electronic density of atoms in molecules, the distance between atoms, bond angles, etc;On the other hand, these data measured by modern physical technology are in perfect agreement on some occasions.Since 1935, chemists have also demonstrated the relationship between the atoms in the molecule that are not directly connected by the valence bond from the thermodynamic derivation and experimental data, and proposed that the rotational energy between the two atoms connected by the single bond causes isomerismH. R. Barton summarized in his theory of conformational analysis in 1950.It puts forward a new concept for the image of molecules, clarifies many phenomena that were not understood before, and has a strong predictability. It has extremely far-reaching significance in organic chemistry and its related sciences.
