π bond refers to the chemical bond formed by overlapping atomic orbitals perpendicular to the bond axis in a "side by side" manner. When π bonds are formed, the overlapping parts of atomic orbits are equivalently distributed on the upper and lower sides of the plane including the bond axis, with the same shape, opposite signs and mirror anti symmetry. The Greek letter π in the name represents the p orbital, because the orbital symmetry of the π bond is the same as the p orbital field. P orbitals usually participate in the formation of π bonds, however, d orbitals can also participate in the formation.
The π bond is usually more than σ bond Weak, because its Electronic cloud Distance positively charged atom The nucleus is farther away and needs more energy. quantum mechanics It is believed that the weak bond strength is mainly due to the insufficient overlap between parallel p orbitals. When the orbitals (p orbitals) of two atoms approach from the direction perpendicular to the connecting line between the nuclei of the bonding atoms, the electron clouds overlap to form bonds. The covalent bonds thus formed are called π bonds. π bond usually appears with σ bond, and the electron cloud of π bond is distributed above and below the σ bond. The electrons of the σ bond are tightly localized between the two atoms forming the bond, while the electrons of the π bond are opposite. They can move freely in the molecule and are often distributed among several atoms. If the molecule is a conjugated π bond system, then π electron These π electrons are distributed on the atoms that form molecules. These π electrons are called delocalized π electrons, and π orbitals are called delocalized orbitals. In some cyclic organic compounds, the conjugated π bond extends to the whole molecule, such as polycyclic aromatic hydrocarbons. Since the electron cloud of π electrons is not concentrated between the two atoms forming the bond, their bond is far less strong than the σ bond. Therefore, their absorption spectra appear in the optical region with a longer wavelength than that generated by the σ bond. The absorption spectrum produced by single π bond electronic transition is located in the vacuum ultraviolet region or near ultraviolet region; The molecule with conjugated π bond depends on the degree of conjugation. The electronic spectrum generated by the π electron transition of the molecule with low degree of conjugation is in the ultraviolet region, while the molecule with high degree of conjugation is in the visible light region or near infrared region.
For example, if the parts of px and px with the same orbital symmetry approach and overlap in the direction of the x axis in a "side by side" (side by side) manner, their overlapping parts are equally located on the upper and lower sides of the xy plane containing the bond axis (here referred to as the x axis), with the same shape but opposite signs, that is, they are antisymmetrical to the xy plane. The bond formed by such overlapping is the π bond. The electrons that form π bond are called π electron 。 Overlap ratio of π bond σ bond Small, so π bond is not as stable as σ bond. When the two atoms forming the π bond rotate relative to each other on the internuclear axis, the overlap of p orbitals will be reduced, and finally the π bond will be broken. according to Molecular orbital theory , two atom P orbital of linear combination Can form two molecular orbitals. The energy is lower than the original Atomic orbital Of Bonding orbital π and the anti bond orbital π whose energy is higher than the original atomic orbital, and the corresponding bonds are called π bond and π * bond respectively. Molecules in ground state The two p electrons (π electrons) are in the bonding orbit, while the anti bonding orbit is empty. [1] (1) Two p orbitals form a π bond Although the π bond itself is weaker than the σ bond, the π bond and the σ bond still coexist in multiple bonds, because the combined bonds are stronger than each other. This can be seen by comparing the bond lengths of ethane (154 pm), ethylene (133 pm) and acetylene (120 pm).
(2) From the point of view of atomic orbital overlap, the overlap of π bond is smaller than that of σ bond, so the bond energy of π bond is less than that of σ bond, so the bond stability is lower than that of σ bond, bond electrons are more active than those of σ bond, and they are active participants in chemical reactions.
(3) When two atoms form a covalent single bond, the atomic orbitals always overlap to the maximum extent along the bond axis, so the single bond is a σ bond; When covalent double bonds are formed, there is a σ bond and a π bond; When covalent triple bonds are formed, there are one σ bond and two π bonds. [2]
1. 2 atom Or multiple atoms coplanar; 2. Atoms all provide parallel orbits;
3. The total number of electrons provided is less than twice the number of orbits.
Localized π bond: organic molecules containing only σ bond and isolated π bond are called non Conjugated molecule 。 These σ bonds and isolated π bonds are conventionally regarded as Local key That is, a pair of σ electrons constituting a σ bond and a pair of π electrons in an isolated π bond are approximately fixed between bonding atoms in pairs. Such keys are called localized keys. For example, C two H four Any C-H σ bond of molecule and CH two =CH two The π bond of a molecule, whose electronic motion is limited between two bonding atoms, is a localized bond. Delocalized π bond: in this type of molecule, all π electrons involved in the conjugation system are not limited to between two carbon atoms, but extend to all carbon atoms constituting the conjugation system. This phenomenon is called delocalization. Conjugate π bond is also called Off field key Or not Local key 。 Due to the delocalization of conjugated π bond Conjugate system When the atom of is affected by external reagent, it will immediately affect other parts of the system. Conjugated molecule The conjugated π bond or delocalized bond of is the core of chemical reaction. [2] N two : In the nitrogen molecule, after two N atoms each form a σ bond with one electron on one p orbital, the remaining two mutually perpendicular p orbitals are parallel, forming two Key. C six H six : In benzene molecule, C atom sp2 Hybridization Then, the remaining one p orbital that does not participate in hybridization Parallel to each other There is one electron above each, so six atoms and six electrons are formed（ ）Large pi bond. O three : In ozone molecule, O-O-O is in zigzag shape, and the O atom in the middle is sp two Hybridization , provide a p orbital that does not participate in hybridization, with two electrons on it, and the O atoms on both sides provide one unpaired electron, so three atoms and four electrons are formed（ ）Large pi bond. 
Illustration N two O is isoelectronic and has a similar structure. [1]
The π bond does not necessarily connect several atoms. The π interaction between the metal atom and the σ bond of the hydrogen molecule is Organometallic compound Plays an important role in the restoration of. The π bond in alkynes and alkenes is often combined with metals, and the bond formed contains a high π component. Only in some molecules, σ bond is more active than π bond: for example And ethane 。 In these compounds, Central atom It only contains π bonds. In order to create the maximum overlap between orbits, the bond length is much smaller than expected. [2] 
