Photochemistry 1

study about photochemistry and wood ward and hoffman diagram

1. Photochemistry
Photochemistry is the study of the interaction of
electromagnetic radiation with matter resulting into a
physical change or into a chemical reaction.
Photochemistry is the branch of chemistry that deals with the
chemical processes that are caused
by the absorption of light energy.

2. Photolysis
The process by which a photochemical reaction is
carried out is called photolysis.
Photolysis is usually initiated by infrared, visible, or
ultraviolet light.

3. Photochemical reactions
In nature, there are a number of light excited
photochemical reactions such as:
Photosynthesisinvolves the absorption of light by the
chlorophyll in plants to produce carbohydrates from
carbon dioxide and water.
Photographyuses the action of light on grains of silver
chloride or silver bromide to produce an image.
Ozone formationin the upper atmosphere results
from action of light on oxygen molecules.
Solar cells,which are used to power satellites and
space vehicles, convert light energy from the sun to
chemical energy and then release that energy in the
form of electrical energy.

4. The Photochemical Process
Photochemica process, occur when an atom or molecule
absorb a quantum of light energy from a photon.
If a quantum of visible or ultraviolet light is absorbed,
then an electron in a low energy state is excited into a
higher energy state.
If infrared radiation is absorbed by a molecule, then the
excitation energy affects the motions of the nuclei in
the molecule.
M +light ---------M*

5. Photochmical process
Two types
Primary process such as:
•Isomerization
•rearrangement,
•dissociation or
•Photo-ionization etc
Secondary process occur after the
primary

6. Primary processes

7. 1.The highly energized or excited molecule may return to
its initial state on releasing its excitation energy by
emitting luminescent radiation.
If the process is quick, the process is known as
fluorescence and if it is delayed it is known as
phosphorescence.

8. It may transfer its energy to some other molecule, C, with which it
collides, without emitting light. The latter energy transfer process
results in a normal molecule, M, and an excited molecule, C*. This
process called quenching
M* --------------M + C*
As a result of the initial light absorption step, an electron (e−) in
the atom or molecule may absorb so much energy that it may
escape from the atom or molecule, leaving behind the positive M+
ion. This process is called photoionization

9. If the excited M* molecule does react, then it may undergo
any of the following chemical processes:
Photodissociation or
Rearrangement or photoisomerization
React with another molecule C
Photodissociationmay result when the excited molecule
breaks apart into atomic and/or molecular fragments A and B.
The excited state species may fragment to a pair of radicals or, in the
case of nitrogen dioxide to nitric oxide and oxene.

10. A rearrangement (or photoisomerization)reaction involves the
conversion of molecule M into its isomer N
The conversion of trans-1,2-dichloroethylene into the cis
isomer is an example of intramolecular rearrangement. The
reaction is shown below:
Inthetransisomerthechlorineatomslieonoppositesidesofthe
doublebond,whereasinthecisisomertheyareonthesameside
ofthedoublebond.

11. Secondary Photochemical Processes
Secondary processes may occur upon
completion of the primary step. Several
examples of such
processes includes Formation of Ozone,
Chain Reactions etc

12. Formation of Ozone
Ozone (O3) is formed in the upper atmosphere from
ordinary oxygen (O2) gas molecules according to the
reaction:
step1.Aquantumofultravioletlightisabsorbed
step2.Theexcitedoxygenmoleculedissociatesintotwooxygen
atoms.
step3AnoxygenatomthenreactswithO2toformozone.

13. Destruction of Ozone in the Upper Stratosphere
Certain chlorofluoromethanes, such as CCl3F and
CCl2F2, are used as refrigerants. These compounds
eventually diffuse into the stratosphere, where the
molecules undergo photodissociation to produce
chlorine (Cl) atoms, which then react with ozone
molecules according to the reaction below
Cl + O3 →ClO + O2
This decrease in the ozone content of the upper
atmosphere allows more ultraviolet radiation to reach
the surface of the earth.

14. Chain Reactions
If the primary photochemical process involves the
dissociation of a molecule into radicals (unstable
fragments of molecules), then the secondary process
may involve a chain reaction.
A chain reaction is a cyclic process whereby a reactive
radical attacks a molecule to produce another unstable
radical.
This new radical can now attack another molecule,
thereby reforming the original radical, which can now
begin a new cycle.

15. According to the above mechanism,
a suitable quantum of light dissociates a chlorine molecule
into atoms (step 1).
The reactive Cl atom attacks a hydrogen molecule to yield
hydrogen chloride and a hydrogen atom (step 2).
The reactive hydrogen atom attacks a chlorine molecule,
which regenerates the Cl atom (step 3).
This chlorine atom can then react with another H2
molecule according to step 2, beginning a new cycle of steps.

16. Likeinallchemicaloperationstherearerisksinphotochemistry.
Irradiation.Low-pressuremercurylampshavetheirmainoutput
at254nm.Thislightseverelydamagescells,eyesandskin.
Shieldreactors;turnlampsoffbeforecheckingthereaction.
NeverlookintothebeamofahighpowerLED;thelightsvery
highintensitydamageyoureyes.
Ozonegeneration:Shortwavelengthlightmaygenerateozone
fromoxygen.Performreactionsalwaysinawellventilatedfume
hood.
Lamps:Mostlampsoperateathightemperatureandathighvapor
pressure.Nevermoveortouchlampsduringoperation.Never
switchofthecoolingrightafterswitchingofthelamp.
Hazardsofphotochemistry

17. Cyclisation reactions
Pericyclic or cyclization reactions ; “Any concerted reaction in which bonds
are formed or broken in a cyclic transitions state”.
i.e. there is a single transition state from start to finish, in contrast to a
stepwise reaction.Transition state
reaction co-ordinate
Energy
starting
material
product
Concerted reaction
Transition states
reaction co-ordinate
Energy
starting
material
product
Multistep reaction
inter-
mediateinter-
mediate

18. Types of Pericyclic or cyclization reactions
electrocyclic reaction
Cycloaddition reaciton
Sigmatropic rearrangement

19. An intramolecular reaction in which a new sbond is
formed between the ends of a conjugated psystem
called electrocyclic reaction
Electrocyclic reaction

20. Electrocyclic Reactions Are Reversible

21. Two different pbond-containing molecules react
to form a cyclic compound called cycloaddition
reaction
Cycloaddition reaction

22. A [2 + 2] Cycloaddition Reaction
•Cycloaddition reactions–Two linear conjugated polyenes converted
onto a cyclic product in one step.
•The formation of cyclobutane rings from alkene components is the
most common photchemical cyclo addition reactions

23. MO Analysis of the
[2 + 2] Cycloaddition Reaction

24. Intramolecular [2+2] photocycloaddition
Cycloaddition process when taking place within the
same molecule. Such cycloaddition reaciotn known as
intramolecular cycloaddition.

25. A sigmatropic is a reaction in which a sbond is broken in
the reactant and a new sbond is formed in the product,
and the pbonds rearrange called sigmatropic
rearrangement
Sigmatropic rearrangement

27. Photochemical Reactions of Carbonyl Compounds
The absorption properties of ketones and aldehyds are
convenient for irradiation around 300 nm.
The n → π* excited states of carbonyl compounds display a
rich chemistry in their own right. Since the oxygen has an
unpaired electron, it behaves in much the same way as an
alkoxy radical.

28. Intramolecular reactions of carbonyl Compounds
Following are the typical reaction of carbonyl compounds
Type I Cleavage (α-cleavage)
Hydrogen Abstraction Reaction
Type II Cleavage(β-Cleavage)
Cyclobutanol formation (Yang reaction)
α-β unsaturated ketone
Oxetene Formation (Paterno-Buchi Reaction)

29. Type I Cleavage (α-cleavage)
This reaction type dominates gas phase photochemistry of
many aldehydes and ketones.

31. Hydrogen Abstraction Reaction
Beside carbon monoxide extrusion acyl radicals formed
in a α-cleavage can be stabilized by subsequent
hydrogen migration.

32. Type II Cleavage(β-Cleavage)
Beside carbon monoxide extrusion acyl radicals formed in
a α-cleavage can be stabilized by subsequent hydrogen
migration.

33. Cyclobutanol formation (Yang reaction)
With an appropriate alignment of C=O and C-H
groups and no secondary transformation prevents
cyclization of the 1,4-diradical leads to cyclobutanols.

34. α-β unsaturated ketone

35. Oxetene Formation (Paterno-Buchi Reaction)
The photochemical [2+2] cycloaddition of an alkene and a
carbonyl group which results the formation of oxeten called
Paternó Büchi reaction. Inter and intramolecular examples
are known.

36. Summary of carbonyl reactions

37. Alkene cyclizations
Alkene generally requires short wavelength irradiation
extending to about 200 –210 nm.

38. The cis-trans (Z-E) isomerization in alkene
E,Z-Isomerizations of 1,2-disubstituted alkenes are well
documented. A typical example is the photoisomerization
of stilbene, where because of different absorption spectra
the Z-isomere can be enriched in the photostationary
state.
A more complex example is the sensitized preparation of
enantiomerically enriched trans-cyclooctene.

39. Itwasthefirsttheorytoexplaintheoutcomeofseveralpericyclic
reactions.Themethodisbasedonthe‘principleoforbitalsymmetry
conservation’throughoutthereaction.
Theserulesstatesthat,Apericyclicreactioncantakeplaceonlyif
thesymmetriesofmolecularorbital'softhereactantarethesameas
thesymmetriesofthemolecularorbital'softheproduct.
Ifthesymmetriesofbothreactantandproductorbitalsmatchup,
or“correlate”,thenthereactionissaidtobesymmetry-allowed.
Ifthesymmetriesofreactantandproductorbitalsdon’tcorrelate,
thenthereactionissymmetry-disallowed.
Woodward-Hoffmann rules

40. Woodward-Hoffmann rules
Robert Burns Woodward 1917-1979
American, Nobel 1965
Roald Hoffmann1937
American, Nobel 1981

41. Conservation of orbital symmetry and woodward-
hoffman rules for electrcyclic reactions
The Woodward–Hoffmann rules were developed to explain the
electrocyclic ring-opening and ring-closing reactions of the open
chain conjugated polyenes either by application of heat or application
of light.
According to Woodward and Hoffmann, overlaping or constructive
interaction between two terminal p-orbital will developed only if
both are in same phase or same symmetry.
In order to complete bonding, the terminal p-orbitals may rotate in
the same (conrotation) or opposite directions (disrotation) as shown
below:

42. Fordevelopmentofnewσbondinginteractionsinthe
transitionstate,theterminimustmoveinconrotatoryand
disrotatoryfashionin4π-and6π-electronsystems,
respectively,asshownbelow.

43. Now, let us look at the symmetry observed for some open-
chain conjugated systems. A particular molecular orbital can
have two possible symmetry elements―a two-fold axis of
symmetry (C
2) and a mirror plane of symmetry (σ).

44. Now Let us consider the electrocyclic ring closure of 1,3-
butadiene. In this case the reaction may proceed via either
conrotatory or disrotatory pathway. In the conrotatory mode, the
transition state is associated with a C
2axis of symmetry, while
mirror symmetry is associated with the transition state (TS) in the
disrotatory mode.

45. The molecular orbitals of the reactants and products are correlated in
terms of being symmetric or antisymmetric.
On applying the symmetry element on a MO. The MO is said to be
symmetric if it give rise to same MO in the product.
If the symmetry operation leads to a different MO, then the MO is
said to be antisymmetric. For example, Ψ
2of 1,3-butadiene is
symmetric with respect to 180°rotation about the C
2axis and
antisymmetric with respect to reflection in the mirror plane (σ).

46. Woodward realized that the symmetry elements of all orbitals were
preserved in the MOs of the product. Thus, the symmetry of the
transition state (TS) also must likewise be conserved.
If a symmetry element is present throughout the course of the
reaction, then it is considered as a conserved symmetry element.
Similarly, antisymmetry is said to be conserved, if it is maintained
throughout the course of the reaction, that is, in the reactant,
transition state and the product.
For instance, in the HOMO of a 4π-conjugated system, C
2axis of
symmetry is conserved in reactant, TS and product as:
Conserved symmetry element

47. Now, let us examine the correlation diagram for photochemical
ring-closure of 1,3-butadiene. In this case, one of the electrons in
reactant is excited from Ψ
2to Ψ
3. As a result, the electrons in
reactant are now filled in three orbitals -Ψ
1, Ψ
2and Ψ
3―which are
represented as Ψ
1
2
Ψ
2
1
Ψ
3
1
. The overall symmetry corresponds to
S
2
A
1
S
1
The latter matches with S
2
S
1
A
1
―the overall symmetry of product
MOs, namely,σ,π and π*corresponding to σ
2
π
1
π*
1
The σ-bond is
formed by the overlap of the terminal p-orbitals of Ψ
2. This is
possible by disrotatory closure which leads to bonding interactions
between the two p-orbitals under consideration. Thus, the
disrotatory closure of 4π-conjugated substrates occurs
photochemically as can be understood from the figure shown below.

48. Correlation diagram for 4-electron conrotatory and disrotatory closure under
photochemical Conditions

49. The orbital correlation diagrams for cycloaddition reactions can be drawn
as discussed for electrocyclic reactions. To construct the correlation
diagrams for cycloadditions, certain rules have been put forward by
Woodward and Hoffmann, which are as follows:
1. Define geometry for the proposed interaction.
2. Draw MOs of the reactants and the products in the order of increasing
energy.
3. Define each MO as being either symmetric or antisymmetric with
respect to the symmetry elements.
4. Populate the MOs with electrons, beginning with the ground state MO.
5. Correlate reactant MOs with those of the product MOs of like
symmetry. If all the bonding MOs of the reactant correlate with bonding
MOs of the product, the reaction is said to be ‘allowed’. If a bonding MO
in the reactant becomes an antibonding MO in the product, the reaction
then is said to be ‘forbidden’.

50. [2+2] Cycloaddition is the simplest case, and let’s consider
its occurrence under thermal conditions to begin with. There
are 4 possible ways in which the two components may in
principle react to give the cycloadduct. They are: [
π2
s+
π2
s],
[
π2
a+
π2
s], [
π2
s+
π2
a], [
π2
a+
π2
a]. However, the geometric
constraints preclude any antarafacial addition between the
two components, thereby limiting the approach of the two
components to suprafacial mode. Let’s analyze the
symmetry elements present in the molecular orbitals of the
two reactants:

51. Let us look at the photochemical[2+2] cycloaddition. In
this case, one electron is promoted from π
2to π
3* orbital,
as a result of which the orbital picture becomes completely
different. Now, the π
1, π
2and π
3* orbitals of the reactants
are related by symmetry to σ
1, σ
2and σ
3* orbitals of the
cyclized product. As a result, the energy barrier is drastically
reduced and the reaction now becomes allowed with the
formation of product in the excited state. Therefore, [2+2]
cycloaddition can be said to be photochemically allowed.