Photochemical reaction [Examples, Types]

Photochemical reaction

A photochemical reaction is a chemical reaction that is started or affected  by the absorption of light. When a molecule receive light, it goes through a photochemical reaction, which resulted in the

  • Production of excited states
  • Creation of radical intermediates,
  • or other products that vary from the reactants.

Examples of Photochemical reaction

Example of photochemical reaction are given below;

  • changing of ozone to oxygen in the Earth’s atmosphere is one of the example of photochemical reaction. This reaction happen when
  1. ozone (O3) gets ultraviolet (UV) radiation from the sun, resulted in the production of an excited state molecule of O3.
  2. The excited molecule of O3 can then interact with another molecule of O3 to produce two molecules of oxygen (O2) and an oxygen atom (O). Equation of the reaction is given below
  3. hv+ O3 → O2 + O

photodissociation of chlorine

Another example is the photodissociation of chlorine (Cl2) in the Earth’s atmosphere. When chlorine molecules gets UV radiation

  1. the Cl-Cl bond can disrupt ,
  2. leading to the production of two chlorine atoms (2Cl)
  3. that can react with other molecules to start a chain reactions, such as the destruction of ozone. equation of the reaction is given below
  4. hν +  Cl2→ 2Cl

Types of photo chemical reaction


  1. Photodissociation:
  2. Photoisomerization:
  3. Energy transfer:
  4. Electron transfer:
  5. Photocycloaddition:
  6. Photopolymerization:


There are many kinds of photochemical reactions, every single engaging the absorption of light by a molecule and consequent to a specific outcome. photochemical reactions include following types:


In this kind of reaction, absorption of light cause to breakdown a molecule into its fragment. For example,upon uncovered to UV light halogenated hydrocarbons, the C-Cl bond can disrupted, resulting in the production of a free radical.


In this sort of reaction, the absorption of light triggers a change

  • in the geometry of a molecule
  • or the position of a double bond, leading to the formation of an isomer.

For example, when a molecule containing a double bond absorbs light, the double bond can alter its position, outcome in the formation of an isomer.

Energy transfer:

In this sort of reaction, the excited state of a molecule give its energy to a ground state molecule, resulting in the formation of a fresh product.

For example, in photosynthesis, light energy is trapped by chlorophyll molecules, which then hand over this energy to other molecules to form glucose.

Electron transfer:

In this type of reaction, an electron is shift from one molecule to another when it absorb a light.

For example, in the process of photosynthesis,

  • electrons are shifted from water molecules to carbon dioxide molecules
  • resulting in the production of glucose.


In this type of reaction, two molecules add up to get a shape of cyclic compound upon absorption of light.

For example, when benzene and ethene are reveal to UV light, they can go through a photocycloaddition reaction to form cyclohexene.


In this sort of reaction, the absorption of light results in the production of polymer chains.

For example, in the formation of plastics, monomers on reveal to UV light start a photopolymerization reaction that create a polymer chains.


The mechanism and outcome of a photochemical reaction effected by

  • the properties of the reacting molecules
  • and the nature of the absorbed light.