PHOTOCHEMICALREACTIONS

1. K. Neha

2. 
 Introduction
 Photochemistry & its law
 Features of Photochemical reactions
 Basic principles of Photochemical reactions
 Types of photochemical reactions
 Photo-oxidation
 Photo-addition
 Photo-fragmentation
 References
Contents

3. 
 Photochemistry is the study of chemical reactions resulting from the
exposure of light radiations. Light supplies the required energy to take
place the photochemical reactions. The visible and UV radiations
(2000-8000Ǻ wavelength) are mainly used in photochemical reactions.
 Thermochemical reactions (dark reactions) are brought about by
molecular collisions. These reactions are spontaneous and are
accompanied by a decrease in free energy. But certain photochemical
reactions are accompanied by an increase in free energy.
Introduction

4. 
 Photochemical reaction, a chemical reaction initiated by the
absorption of energy in the form of light. The consequence of
molecules' absorbing light is the creation of transient excited states
whose chemical and physical properties differ greatly from the original
molecules.
 When a light composing number of colours is used, the photochemical
reaction may not be initiated by all colours.
Contd...

5. 
In Environmental chemistry , photochemical reactions are of
considerable importance to the trace chemistry of the atmosphere .
The Los Angeles photochemical smog is an example of a system of
photochemical reactions.
Perhaps one of the most important reactions in this system is the
photochemically driven conversion of nitrogen dioxide, a major
component of automobile exhaust.
Photochemical reactions are also important in natural waters. Here they
may be responsible for enhancing the reaction rate of organic compounds
or changing the oxidation state of metallic ions in solution.
Photochemical reactions also occur in solids. Since solids often lack the
transparency required for the light to penetrate the surface, reactions are
usually limited to the surface, where incoming photons initiate reactions
in the top-most molecules.
Contd...

6.  Grothurs-Draper Law:
o The first law of Photochemistry ,known as the Grothurs-Draper law,
states that light must be absorbed by a chemical substance in order
for a photochemical reaction to take place.
o This law was modified by Stark & Einstein, states that for each
photon of light absorbed by a chemical system, only one molecule is
activated for photochemical reaction.
o This is also known as the photo equivalence law (only one excited
molecule per photon absorbed).
Basic laws of photochemistry

7.  Beer-Lambert Law:
o The Beer-lambert law is the linear relationship between absorbance &
concentration of an absorber of electromagnetic radiation.
o Lamberts observed that the absorption of light of a single wavelength
(monochromatic radiation) passing through homogeneous solutions of
equal concentration is proportional to the pathlength.
A=k .l
o Beer’s law states that the absorption of a single wavelength
(monochromatic radiation) passing through homogeneous solution of
equal pathlength is proportional to the concentration.
A=k .c
o Beer-Lambert law is commonly written as :
A=ε .c. l
Contd…

8. 
 Photochemical reactions are driven by light or near-visible electromagnetic
radiation.
 In general, incoming units of energy, known as photons, excite effected
molecules, raising their energy to a point where they can undergo reactions
that would normally be exceedingly difficult. The process is distinguished
from thermal reactions, which take place with molecules in their normal
energy states. Under sunlight conditions, photochemical processes can
generate small amounts of extremely reactive molecules which initiate
important chemical reaction sequences.
 To initiate a photochemical reaction, two requirements need to be met.
 First, the photon must have enough energy to initiate the
photochemical reaction in the molecule.
 Second, the compound must be colored, in order to be able to react
with visible or near-visible photon radiation.
Photochemical reaction

9. The characteristics of photochemical reactions are listed below:
 Each molecule taking part in a photo process absorbs only one photon of
radiant energy.
 Photo processing does not occur in the dark.
 Each photochemical reaction requires a definite amount of energy which is
characteristic of a particular wavelength of the photon.
 The rate of photochemical reaction depends upon the intensity of radiation
absorbed.
 The ΔG values for light initiated reactions may or may not be negative.
 The temperature does not have marked effect on the rate of light initiated
reactions.
Features of Photochemical
reactions

10. 1. Linear addition to an unsaturated molecule, e.g., the pyrimidine base,
thymne, in DNA can combine with the amino acid residue, cysteine, in proteins.
This is a model for the photochemical crosslinking of DNA and proteins by UV
radiation.
2. Cycloaddition of unsaturated molecules, e.g., two thymines can react to form a
ring product, the thymine dimer, an important class of products formed in DNA
by UV radiation.
Types of Photochemical reactions

11. 3. Photofragmentation, e.g., the side chain of riboflavin can split off to
form lumiflavin.
4. Photooxidation, e.g., the ring structure of cholesterol can add a peroxy
group.

12. 5. Photohydration, e.g., uracil can add a molecule of water to it 5-6 double
bond when UV irradiated.
6. Cis-Trans Isomerization, e.g., all-trans retinal can be converted to 11-
cis retinal.

13. 7. Photorearrangement, e.g., 7-dehydrocholesterol can be converted to
vitamin D3.
8. Energy Transfer, e.g., all photosensitized reactions.

14. 
Photochemical reactions Thermochemical reactions
 These involve the absorption
of light
 These involve either
absorption or evolution of
heat
 Take place in presence of
light
 Take place in dark or in
presence of light
 They are independent of
temperature
 They are dependent of
temperature
 Rate of reactions is dependent
on the intensity of the light
absorbed
 Rate of reactions is not
affected by the intensity of
light
 The free energy change is
negative or positive
 The free energy change is
always negative
Differences between photochemical
and thermal reactions:

15. Photosensitizations and Quenching: In some photochemical reactions, the
reactant molecules do not absorb radiation and no chemical reaction occurs.
However, if a suitable foreign substance (called sensitizer), which absorbs
radiation, is added to the reactant, the reaction takes place. The sensitizer gets
excited during absorption of radiation and transfers its energy to the reactants and
initiates the reaction.
 Photosensitization: The foreign substance absorbs the radiation and transfers
the absorbed energy to the reactants is called a photosensitizer. This process is
called photosensitized reaction (or) photosensitization. Examples,
i) Atomic photosensitizers : mercury, cadmium, zinc &
ii) Molecular photosensitizers: benzophenone, sulphur dioxide.
 Quenching: When the excited foreign substance collides with another
substance it gets converted into some other product due to the transfer of its
energy to the colliding substance. This process is known as quenching.
ENERGY TRANSFER IN
PHOTOCHEMICAL REACTIONS:

16. 
Photo-oxidation
 Incorporation of oxygen in the presence of UV light is known as
Photochemical oxidation.
 In other word Oxidation caused by action of light.
 There are two mechanisms by which oxygen is incorporated in
Photochemical oxidation reactions.
 The first is Backstrom mechanism which is also known as Photo-
sensitisation oxidation mechanism.
 Its main characteristic is abstraction of hydrogen by the sensitizer in its
excited triplet state followed by addition of oxygen to the newly created
radical. This type of photooxygenation is known as type I
photooxygenation.
 The second mechanism known as Photosensitised oxygen transfer
involves the direct combination of the substrate with oxygen. There are
two proposals regarding the state of oxygen involved. This type of
photooxygenation is known as type II photooxygenation.

17. 
 Alkenes, dienes and polyenes are attacked by singlet oxygen. The singlet
oxygen may be generated by thermal methods, by excitation of ground state
oxygen in a microwave discharge or by the use of visible radiation and
photochemical sensitizer such as methylene blue, rose bengal, chlorophyll,
riboflavin, fluorescein or halo fluorescein.
 Acyclic or cyclic conjugated dienes gives 1,4-cycloaddition reaction with
singlet oxygen to form six membered cyclic adduct , i.e. cyclic peroxide.
Photo-oxidation of alkenes and
polyenes

18. 
The formation of hydro peroxides in photosensitised oxygen-transfer reactions
that follow the Schenck type of mechanism occurs only when hydrogen is
present on allylic carbon. The reaction has, moreover, some definite steric and
electronic requirements. Oxygen always becomes attached to one of the double
bonded, which then shifted into the allylic position. The reaction is like ene
reaction.
Contd…

19. 
The above reactions do not proceed through free radicals. The oxidation with
singlet oxygen occurs by concerted mechanism like the ene reaction.
Some substituted alkenes behave differently with singlet oxygen and form a
dioxetane in a cycloaddition reaction. Some dioxetanes are stable but others
readily decompose to two carbonyl compounds. Electron rich alkenes undergoes
this reaction and reaction is stereospecific.
Contd…

20. Contd…
Mechanism of the reaction is as follows:-
The oxidative cyclisation of conjugated trienes to form aromatic system is one
of the more extensively studied photochemical oxidations. The conversion of
cis-stilbene to phenanthrene is a main example of such a ring closure. The
reaction takes place in the presence of hydrogen acceptors.

21. A photochemical reaction in which part of a compound is added to one end of
a double or triple bond, while the rest adds to the other end, converting it,
respectively, to a single or double bond.
Photo-addition Reaction
Example:
Addition reaction

22. Photo-addition Reaction
Example:
Mechanism:
Cyclo-addition reaction

23. 1, 2-photoaddition of benzenes
Example: 2
Example: 1

24. Example: 3
Example: 4
1, 2-photoaddition of benzenes

25. 1, 3-photoaddition of benzenes
Example: 1
Example: 2

26. 1, 4-photoaddition of benzenes
Example: 1
Example: 3
Example: 2

27. Example: 4
Example: 5
1, 4-photoaddition of benzenes

28. This reaction is a [4+2] cycloaddition reaction.
Addition of oxygen

29. Biologically active cardenolide ouabagenin
Photochemical reaction of an ion that results in two or more fragments of which at
least one is an ion.
PHOTO-FRAGMENTATION:-
Example:

30. Photo-fragmentation step by step
Step: 1
Step: 2
At temperature much above room temperature the acetyl radical breaks down to give
another methyl radical & carbon monoxide.

31. Some examples of drugs that
undergoes photochemical
reaction

32. Anti-diabetic agent found from Azadirachta indica (Neem).

33. Santonin in presence of dioxane gives Luminosantonin
Steroid also gives the result in the presence of acidic medium

34. Vitamin D

35. Some other example of
photochemical reactions:
Photochemical reaction is very common in our daily life like below:
 Photosynthesis, in which most plants use solar energy to convert carbon
dioxide & water into glucose, disposing of oxygen as a side-product.
 Human rely on photochemistry for the formation of vitamin D.
 Vision is initiated by a photochemical reaction of rhodopsin.
 In fireflies, an enzyme in the abdomen catalyzes a reaction that results in
bioluminescence.
 In organic reactions are electrocyclic reactions, photoisomerization &
Norrish reactions.
 Many polymerizations are stated by photoinitiator, which decompose upon
absorbing light to produce the free radicals for Radical polymerization.
 In photoresist technology, used in the production of microelectronic
components.

36. 
 Photochemistry is attaining an increasing importance in health science,
curing some forms of cancers through phototherapy, repairing tissues and
performing microsurgery using lasers.
 Other important applications of photochemistry include the use of
photolithography to manufacture computer chips and photo polymerization
to produce protective coatings for a variety of high value materials such as
optical fibers.
 One of the "Holy Grails" of photochemistry is the discovery of practical
ways to convert sunlight into high grade fuels to replace fossil fuels.
 Photo physics especially the use of fluorescence as a sensor is currently of
enormous importance for applications in the materials sciences and in
biological sciences.
Applications

37. Conclusion
 Unlike thermal reaction ,in photochemical reaction the absorption of the light
raises an individual molecule to an excited electronic state.
 Selective excitation of individual molecules is a special character of
photochemical reactions.
 The chemistry of excited molecules (in photochemical reactions) is quite
different from the chemistry of the molecules in the ground state(in thermal
reactions).
 The total energy supplied in photochemical reaction is much lower than
thermal reactions.
 Photochemical reactions is used for highly strained & thermodynamically
unstable compounds.
 Photochemical reactions are highly stereospecific .
 The product obtained by thermal & photochemical process normally differ in
stereochemistry.

38.  Singh Jagdamba, Singh Jaya, Photochemistry and pericyclic reactions,
Revised second edition, The New Age International Publishers, page
no: 1-130,141-226.
 Ahluwalia.V.K, Parashar.P.K, Organic Reaction Mechanism , Third
Edition, Narosa, Page no:632-660.
 Wikipedia
 Google
 Dictionary.com
References