Photochemical reactions are chemical reactions initiated by the absorption of light energy. These reactions involve an organic molecule absorbing a photon which causes electronic excitation from a lower to a higher orbital. The excited molecule may then undergo various chemical reactions, including photoaddition, photocycloaddition, and photo-oxidation reactions. Photochemical reactions differ from thermochemical reactions in their requirement of light to initiate the reaction.

1. By:- Vishal Kumar
Integral university
lucknow

2. Photochemical reaction
 Photochemical reactions: Some reactions do not take place in the
dark but take place only in the presence of light or some other
radiation. Such reactions are known as ‘photochemical reactions’.
 These reactions involve absorption of light, highly specific,
energy absorb depends on type of electronic transition..
 Some of these are initiated by the presence of a photosensitizer.
“Photochemical reaction a chemical reaction initiated
by the absorption of energy in the form of light. The consequence
of molecule absorbing light is the creation of transient state whose
chemical and physical properties differ greatly from the original
molecule”.

3. Definition and terms
Light -: electromagnetic field vibration
spreading in quanta
Photon -: the smallest amount of light carrying
energy

4. .

5. .

6. The distance and size scales of the electromagnetic spectrum

7. .

8. LIGHT
Organic
molecule
Sigma, pi,& non-
bonding e-
The energy of the radiation is nearly equal to or
greater than the energy difference between ground
state and excited state.
Orbital of lower
energy(G.S.)
Electronic
excitation
Orbital higher
energy (E.S.)
excited state or alternative excited
may take part in the chemical
reaction
Photochemical
Reactions
Primary process
Secondary process
S1 T1
S0 S0
S0
Intramolecular reactions
(rearrangement,
dissociation, etc.)
Intermolecular
(addition reaction)

9. Difference between photochemical and
thermochemical reactions

10. What Happens When Radiation
Hits a Molecule?

11. Absorbance and emission
Absorbtion/Absorbance :- An electron absorbs the
energy of a photon,and jumps from a lower into a higher
energy orbital.
Emission:-An electron jumps from a higher into a lower
orbital,and releases(emits) a photon of energy.

12. Jablonski energy diagram

13. Photiaddition reaction
 Photoaddition forming 1:1 adduct by the reaction of an
excited state of one molecule with ground state of
another.
 The molecule which is excited state is carbonyl
compound, quinine, aromatic compound, or alkene
molecule and the molecule which is in ground state is
commonly alkene.
 Majority of the Photoaddition reactions forming ring
product

14. Photocycloaddition reaction

15. Photo oxidation
 Oxidation under the influence of the light or
radiant energy.
 Formation of radicals that induces oxidation
of the material.
 Excited singlet or triplet states

16. Steps in photo oxidation reaction
1. Initiation
2. Propagation
3. Branching
4. Termination

17.  .

18. Formation of polycyclic compound
.

19. Photo oxidation of cholesterol
 .

20. .

21. Photo oxidation of polymers
 Light is absorbed by a polymer- photochemical
reaction can occur as a result of activation of a
polymer macromolecule to its excited singlet
or triplet state.
 Chemical change that reduce the polymer’s
molecule weight.

22. .

23. Photo-fragmentation
 The process of breaking up of molecular ion into smaller or
daughter ions in the presence of light known as photo-
fragmentation.
GENERAL MODES OF FREGAMENTATION -:
 Simple cleavage
1.Homolytic
2.Heterolytic
3.Semiheterolytic
 Rearrangement reaction
1.Elimination reaction
2.Ortho reaction
3.Mc-lafferty rearrangement

24. 1. Simple cleavage –:
Transfer of electron take place.
1.Homolytic cleavage– In homolytic cleavage the e-
are equally transferred to both atom and it forms
radical and cation.

25. 2.Heterolytic cleavage -:
 Fragmentation by movement of two electron.
 In this type of cleavage both the electron of the
bond are take over by one of the atoms.
 The fragment are an even electron cation and a
radical with the positive charge residing on the
alkyl group.

26. 3. Semiheterolytic
 In this one electron bond cleavage takes place
resulting in formation of radical and cation.

27. 2. Rearrangement reaction
 It involve the cleavage of bonds and formation of new
bonds i.e. it requires changes to at least two bonds.
 So the produced ions are not structural units of
precursors.
 Generally rearrangement lead to loss of smaller
molecule.

28. 1.Elimination reaction-:
 In this fragmentation hydrogen is obustracted by
hydroxyl group or halogen or acetate F.G. result
in the elimination of water or neutral molecule.

29. 2.Ortho effect reaction -:
 In this case of sustituted aromatic compounds the
substitute and the carbon come in to proximity
has help in elimination of neutral molecule.
 This effect is called ortho effect.

30. 3.Mc-lafferty rearrangement-:
 It involve the migration of gama hydrogen atom
followed by cleavage beta bond rearrangement
leads to the elimination of neutral molecule.

31. Mechanism
Fragmentation take place only at gamma hydrogen
Then this hydrogen is abstracted by oxygen of carbonyl
group of unsaturated compound
Carbonyl group is converted to hydroxyl group
Bond breakdown between alpha and beta carbon
resulting in liberation of 2e-s then rearrangement take
place to form a simple alkene.

35. Reference :-
 Principles of organic synthesis – R.O.C. Normann.
 Organic reation and mechanism – V.K ahluwalia.
 https://www.researchgate.net/figure/Plausible-
photodegradation-mechanisms-occurring-in-PLA-
Mechanism- a-photolysis_fig5_28972767