- Chinese name
- Photochemical reaction
- Foreign name
- photochemical reaction [2]
- Fundamental law
- First law, second law, etc. of photochemistry
- Category
- photosynthesis; Decomposition
- Also called
- Photochemical action
- Meaning
- Chemical reaction caused by the action of light
The so-called photochemical reaction refers to the chemical reaction triggered by the absorption of a photon by an atom, molecule, free radical or ion.
Reactions in organic synthesis
The common photochemical reactions in organic synthesis include photooxidation, photoreduction, photopolymerization and photo substitution.
Photooxidation is the addition reaction of organic molecules and oxygen under the action of light irradiation and photosensitizer. The photooxidation reaction condition is mild, without chemicals and heavy Metal catalyst , in medicine, essence, detergent and dye, etc fine chemicals There have been many reports on the synthesis of. Photoreduction is a reduction reaction in which organic molecules extract hydrogen molecules from oxygen donors under photocatalysis. More research Hydroxy compound Photoreduction reaction of, for example, benzophenone in Diphenyl carbinol Tetraphenylethanol can be obtained by photoreduction reaction. The common light substitution reaction is the light filtration of aliphatic hydrocarbons to produce chlorinated hydrocarbons, such as CH4 chlorination to monochloromethane dichloromethane , chloroform and carbon tetrachloride 。 Through light substitution reaction, a variety of cleaning agents, pesticides, antioxidants and intermediates can be prepared. These reactions are characterized by gentleness, high recovery rate and good selectivity compared with other methods. Some products can not even be produced by non photo chlorination reaction, so they have great potential in industrial production. [3] Reaction in environmental chemistry
The environmental pollution caused by human life and production is puzzling mankind, such as the oil pollution floating in the sea is endangering our marine resources: waste plastic products are bringing a white pollution to mankind; The exhaust gas emitted by cars always threatens our health. Photochemical reactions in environmental chemistry mainly include photooxidation, photodegradation and photooxygen- Microbial degradation Reaction. Photooxidative degradation is the oxidation of organic molecules such as aromatic aldehydes, alcohols and aromatics to hydrogen under the action of light peroxide 。 Hydroperoxide is easily soluble in water and has been digested due to its high biochemical activity. Aliphatic organic compounds can also be oxidized into small molecular acids, aldehydes and even deeply degraded into CO2 and H2O, directly entering the biological cycle of nature. Microbial chemistry is also an important branch of environmental chemistry, which has been widely used in organic synthesis, waste treatment and other fields with remarkable results. Photooxygen- Microbial degradation There is a narrow application area Degradable plastics It needs to have photosensitive groups or structures easy to interact with microorganisms, so its application is also limited by certain conditions. However, from the perspective of the development trend of the plastic industry, it is inevitable that degradable plastics will be used in packaging industry and agriculture instead of traditional plastics. [3] Photochemical reaction in the environment is mainly exposed to sunlight. Pollutants absorb photons and make the material molecules in an electron excited state And cause chemical reaction with other substances. as Photochemical smog The initial reaction is Nitrogen dioxide （ NO2 ）Under sunlight, it absorbs ultraviolet light (wavelength 2900~4300A) and decomposes into nitric oxide (NO) and Atomic state Oxygen (O, Triplet state ）The photochemical reaction of chain reaction , which leads to a series of reactions between ozone and other organic hydrocarbon compounds and finally produces toxic products of photochemical smog, such as Peroxyacetyl nitrate (PAN), etc. air pollution Its chemical principle is relatively complex, except for common chemical reactions law In addition, the matter in the atmosphere absorbs the radiant energy from the sun（ photon ）Photochemical reaction occurs, making pollutants more toxic (called Secondary pollutants ）。 Photochemical reaction is the reaction triggered by the absorption of photons by the molecules of substances. After the molecule absorbs the photon, the internal electrons occur Energy level transition , forming unstable excited state , and then further dissociation Or other reactions. The general photochemical process is as follows: (1) Initiate reaction to produce excited molecules (A *)
A (molecule)+hv → A*
(2) A * dissociation produces new substances (C1, C2...)
A*→C1+C2+…
(3) A * reacts with other molecules (B) to produce New substances （D1，D2…） A*+B→D1+D2+…
(4) A * Loss of energy returns ground state And emit light (fluorescent or phosphorescence ） A*→A+hv
(5) A * loses its activity due to collision with other chemically inert molecules (M)
A*+M→A+M′
Reaction (1) is to initiate a reaction, which is a molecule or atomic absorption Photon formation excited state A * reaction. The photon energy absorbed by the initiation reaction (1) needs to be adapted to the energy of the electronic energy level difference of the molecule or atom. Matter molecular Electronic energy level The difference is large, only far ultraviolet , UV and visible light Only the medium and high energy parts can valence electron Excited to high energy state. That is to say, only when the wavelength is less than 700 nm, it is possible to induce luminescence chemical reaction. Generated excited state Molecular activity Large, may produce a series of (2)~(4) above Complex reaction 。 Reactions (2) and (3) are two forms of chemical reactions caused by excited molecules. Among them, reaction (2) is the most important one of photochemical reactions in the atmosphere, and excited molecules dissociation It is more than two molecules, atoms or free radicals that transform or migrate pollutants in the atmosphere. Reactions (4) and (5) are excited state There are two forms of energy loss for molecules, and the result is that they return to their original state. Selective absorption of N2, O2 and O3 in the atmosphere solar radiation High energy photons in（ Shortwave radiation ）And cause molecular dissociation: N2+hv→N+N λ<120 nm
O2+hv→O+O λ<240 nm
O3+hv→O2+O λ=220～290 nm
Obviously, the photons with wavelength less than 290 nm in the high-energy part of solar radiation cannot reach the ground because they are absorbed by O2, O3 and N2. Greater than 800 nm Long wave radiation (Infrared part) is almost completely absorbed by water vapor and CO2 in the atmosphere. Therefore, only visible light waves with a wavelength of 300~800 nm are not absorbed and reach the ground through the atmosphere. The low-level pollutants NO2, SO2, RONO, aldehyde, ketone and ROOR 'in the atmosphere can also generate photochemical reactions:
NO2+bv→NO·+O
HNO2（HONO）+hv→NO+HO·
RONO+hv→NO·+RO·
CH2O+hv→H·+HCO
ROOR′+hv→RO·+R′O·
The above photochemical reaction light absorption Generally 300~400 nm. These reactions and the light absorption characteristics of reactants, Absorb light The wavelength and other factors. It should be pointed out that most photochemical reactions are complex and often contain a series of processes. Photochemical reaction can cause combination, decomposition ionization 、 Redox And so on. It can be divided into two categories: one is photosynthesis, for example, green plants can absorb light energy and synthesize carbon dioxide and water under sunlight with the help of plant chlorophyll carbohydrate 。 The other is light Decomposition , such as Upper atmosphere Middle molecular oxygen absorbs ultraviolet light and decomposes into Atomic oxygen ； The fading of dyes in the air, the photosensitivity of film, etc. Only the light absorbed by the molecules in the system can effectively cause the photochemical reaction of the molecules in the system. Although this law is qualitative, it is modern photochemistry The important foundation of. This law was proposed by Grotthus and Draper in 1818, so it is also called Grotthus Draper law In the primary process, only one molecule is activated by one absorbed photon. This law was proposed by Einstein and Stark in 1908-1912, so it is also called Einstein Stark law
Beer Lambert law
parallel Monochromatic light Unabsorbed Transmitted light The relationship between the intensity It and the incident light intensity I0 is (e is Molar extinction coefficient ) 
