Photochemical Reactions

Photochemical reactions are of immense importance as these are the basis of many such processes which are base of sustainable life on earth. For example, photosynthesis, formation of vitamin D with sunlight etc. are photochemical reactions. Photochemical reactions are studied or comes under the branch of chemistry called photochemistry.  It is a branch of chemistry which deals with the chemical effects of light.


What are Photochemical Reactions? 

Trommsdorff described the first photochemical reaction in 1834. He observed the reaction on crystals of -santonin. These crystals when exposed to sunlight turned yellow and burst. 


Those reactions which take place by absorption of light energy are called photochemical reactions. Generally, it takes place by the absorption of ultraviolet light, visible light or infrared radiation. Wavelength of all these radiations are given below in the table –

Radiation 

Wavelength 

Ultraviolet 

100-400 nm 

Visible 

400-750 nm 

Infrared 

750-2500 nm 


Photochemical reactions proceed differently than temperature driven reactions or thermal reactions. In paths of photochemical reactions high energy intermediates are formed which cannot be formed thermally. In these reactions large activation barriers are crossed in a short time. Some photochemical reactions are destructive such as photodegradation of plastics. 


Examples of Photochemical Reactions 

Most common example of a photochemical reaction is photosynthesis. In photosynthesis plants use sunlight and water to convert carbon dioxide into glucose (Carbohydrates) and oxygen. Reaction is given below –

6CO2 + 6H2O sunlight → C6H12O6 + 6O2


Some other examples of photochemical reactions are given below –

  • Formation of vitamin D by exposure to sunlight. Reaction is given below –

  • Bioluminescence reactions which occur mainly in marine animals. 

  • Silver chloride absorbs light and decomposes into silver and chlorine. Reaction is given below –

            2AgCl sunlight→ 2Ag + Cl2

  • Silver bromide also behaves as silver chloride in presence of sunlight. Reaction is given below –

             2AgBr sunlight → 2Ag + Br2 

  • Many polymerization reactions use light energy. These are also photochemical reactions. In many polymerization reactions free radicals are formed by photoinitiation. It is known as photolysis. Reaction is given below –

  • Photodegradation of many substances take place by photochemical reactions. For example, photodegradation of polyvinyl chloride. 

  • Photodynamic therapy is based on photochemical reactions. Photochemical reaction takes place when doctors use light to destroy tumors. 

  • Vision is initiated by photochemical reaction of rhodopsin. 

  • Photochemical reaction takes place or used for production of anti-malaria drug. 

  • Photoalkylation is also an example of photochemical reaction. In these reactions alkyl groups are attached in the molecules by using light energy. 

  • Electrocyclic reactions, radical reactions, photoisomerization and Norrish reaction 1 and 2 are examples of photochemical organic reactions.  

  • Industrial production of benzyl chloride is also a photochemical reaction. Reactions are given below –

            Cl2 + hv 🡪 2Cl·

            C6H5CH3 + 2Cl·🡪 C6H5CH2· + HCl 

            C6H5CH2· + Cl· 🡪 C6H5CH2Cl

  • Free radical halogenation reactions are also example of photochemical reactions. Reaction is given below –

  • Coordination complexes and organometallic compounds are also photoreactive and show photochemical reactions. 


Laws of Photochemistry 

As we know, the arrival of a reactant to an excited state is the 1st step of photochemical processes. Which is called photoexcitation. With this photochemical reactions or processes follows laws of photochemistry as well. There are two laws of photochemistry which are Grotthuss-Draper law and Stark – Einstein law. 

  • Grotthuss - Draper law – This law states that light must be absorbed by a chemical substance in order to take place a chemical reaction. This law was given by chemists Theodor Grotthuss and John W. Drapper.  

  • Stark – Einstein law – This law states that for each photon of light absorbed by a chemical system, no more than one molecule is activated for a photochemical reaction, as defined by the quantum yield. This law was given by physicists Johannes Stark and Albert Einstein. 

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