Thermal reactions involve absorption or evolution of heat, while photochemical reactions require light to occur. Thermochemical reactions can take place in dark conditions, while photochemical reactions only occur in the presence of light. Temperature significantly affects thermochemical reaction rates, while light intensity mainly influences photochemical reaction rates. The free energy change of a thermochemical reaction is always negative, but a photochemical reaction's free energy change may not be negative. Photochemical reactions involve electronic excitation from light absorption. Excited states can undergo chemical reactions or transfer energy through intersystem crossing to more stable triplet states. Photochemical reactions include photoreduction, photoaddition, and photo-rearrangement reactions of carbonyl compounds and alkenes.

1. 08/04/18 1
Shri Bapu R. Thorat
Assit. Professor of Chemistry
Govt. of Maharashtra, I. Y. College,
Jogeshwari (E),
Maharashtra 400060

2. Thermal and photochemical reactions
Thermal reactions: Thermochemistry is the study of the energy and heat
associated with chemical reactions. If a reaction releases energy and heat, it is
called exothermic reaction. The opposite is the endothermic reaction, when energy and
heat is absorbed.
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’
Thermochemical reactions Photochemical reactions
These reactions involve absorption or evolution of
heat.
These reactions involve absorption of light, highly specific, energy
absorb depends on type of electronic transition.
They can take place even in absence of light i.e.
dark.
The presence of light is the primary requisite for the reaction to take
place.
Temperature has significant effect on the rate of a
thermochemical reaction.
Temperature has very little effect on the rate of a photochemical
reaction. Instead, the intensity of light has a marked effect on the
rate of a photochemical reaction.
The free energy change ∆G of a thermochemical
reaction is always negative.
The free energy change ∆G of a photochemical reaction may not be
negative.
They are accelerated by the presence of a catalyst. Some of these are initiated by the presence of a photosensitizer.
However a photosensitizer acts in a different way than a catalyst.
2B R Thorat

3. The distance and size scales of the electromagnetic spectrum

4. 08/04/18 4
Sigma, Pi and non-
bonding electrons
Organic
molecule
Light
Electronic
excitation
orbital of higherorbital of higher
energy (E.S.)energy (E.S.)
orbital of lowerorbital of lower
energy (G.S.)energy (G.S.)
The energy of the radiation is nearly equal to or greater than
the energy difference between ground state and excited state
excited state or alternative excited
may take part in the chemical
reaction
Photochemical
Reactions
Intramolecular reactions
(rearrangement, dissociation, etc.)
Intermolecular
(addition reaction)
The use of uv-radiations below 200 nm wavelength is inconvenient because of it
absorbed by air (excitation of air).
At room temperature, initially almost all molecules are present in the ground state
(vibrational) So. The excited states are denoted as S1 and T1. It is denoted as S1 (excited
single state- the state in which the excited electron and its original partner have opposite
spin) and T1 (excited electron and its original partner have same spin).
S1 T1
So
So So
Primary process
Secondary process

5. Terms used in photochemistryTerms used in photochemistry
Triplet state is normally are more stable than the
corresponding singlet states because, by Hund's rule,
less inter-electronic repulsion is expected with unpaired
than paired electrons.
States with paired electrons are called singlet statesStates with paired electrons are called singlet states
Spin multiplicity (2S+1) (S1
)
Spin multiplicity = 1
Excited states also can have unpaired electrons
with parallel spins
triplet states (T)
Spin multiplicity = 3
Paramagnetic, low energy,
stable
5B R Thorat
The ground states of most molecules
all electrons are paired with opposite
spin (S0)
The excited states also can have
unpaired electrons with opposite spin
The excited states also can have
unpaired electrons with opposite spin
(2x (+1/2-1/2)+1=1
Diamagnetic, high energy,
less stable

6. 08/04/18 6

7. 08/04/18 7
Photosensitization
DonorAcceptor
Chemical
reaction

8. 08/04/18 8
Jablonski diagram
The electronic excitation and de-excitation of the organic molecules
AB*
AB
hv
excited
molecule
Different chemical
species
Chemical
reaction
Dissociation
Ionization
Luminescence
Fluoroscence
Phosphorescence
Quenching
Intermolecular
energy transfer
Intramolecular
energy transfer
+M
+CD
AB + CD*
AB$
Photosensitizatio
AB + hv'
AB+
+ e-
A + B Fragmentation
(free radicals)
AB + M
S0
1. Excitation or absorption
2. Vibrational relaxation
3. Internal conversion (come to Lower
energy state without emission of light)
4. Inter system crossing (ISC)
5. Fluorescence
6. Phosphorescence
7. Photosensitization
8. Photochemical reaction

9. 08/04/18 9
S0
hv
S1
singlet
Flurescence or
Internal conversion
ChemicalChemical
ReactionReaction
+ Y
ISC
Fragmentation
S0
S0 + Y (S1)Collision
Sensitization
T1
triplet
Two species
ChemicalChemical
ReactionReaction
+ Y
S0 + Y (T1)Collision
Sensitization
Fragmentation
Two species
Phosphorescence or
Intersystem crossing
S0
The life time of excited singlets are very short (10-6
to 10-9
) so that chemical reactions
occurs via these species is very uncommon due to their high energy.
Therefore it is converted to its triplet state which is more stable having life time (> 10-4
)
so that the probability of the triplet state taking part in the reaction is much higher
than for a singlet state.

10. Photochemical reactions
A reaction which takes place by absorption of the visible and ultraviolet radiations is
called a photochemical reaction
Ground
state
Visible/
UV
Occurs in
10−16
–10−15
s
first-order
photochemical
reaction is about 1016
s−1
Occurs after absorption
in 10−12
–10−6
s
intersystem crossing
(ISC) in 10−12
–10−4
s
Phosphorescenc
e in 10−6
–10−1
s
As a consequence, excited triplet states are photochemically important.
Phosphorescence decay is several orders of magnitude slower than most typical reactions.
A energy of excitation is transferred to another molecule by collision so that original
molecule comes to ground state with excitation of another molecule (sensitizer). 10B R Thorat

11. 08/04/18 Spectroscopy 11
σ → σ* Transitions
Below 200 nm
n → σ* Transitions
Saturated
hydrocarbon Saturated
hydrocarbon
containing heteroatom
– N, O, S, P, X
π → π* Transitions
Unsaturated /aromatic
compounds with
heteroatom : C=S, C=O,
C=N, etc
n → π* Transitions
Unsaturated /aromatic
compounds : C=O,
C=O, C≡N, etc
Carried in vacuum
Transition energy
decrease with decrease
in electronegativity
The hydrogen bonding also affects the n → σ* transition because of hydrogen bonding, the non-bonding
electron density of heteroatom decreases, thus required higher transition energy.
The hydrogen bonding also affects the n → σ* transition because of hydrogen bonding, the non-bonding
electron density of heteroatom decreases, thus required higher transition energy.

12. 400 nm 700 nm500 nm
71.5 kcal/mol 57.2 kcal/mol 40.8 kcal/mol
Ultraviolet Region
Chemical Bonds of
DNAand Proteins
Damaged
Infrared Region
Chemical Bonds Energy
too lowto make or break
chemical bonds.
X-Rays
0.1 nm
300,000 kcal/mol
Microwaves
1,000,000 nm
0.03 kcal/mol
Huge energies
per photon.
Tiny energies
per photon.Themal energies
at room temperature
ca 1 kcal/mole
Energy Scales: Why the visible region works for vision

13. 08/04/18 13
ReactionsReactions
Photoreduction of carbonyl compoundsPhotoreduction of carbonyl compounds
Carbonyl compounds are converted into 1,2-diols and alcohols by irradiation in presence of a
hydrogen donating compounds such as isopropyl alcohol.
R
R
O
(Ketone)
(S1) (T1)
hv
R
R
O
R
R
O
R
R
O
R1OH
R
R
OH + R1O
R1OH
Dimerise
R
R
OH
R
R
OH
R
HO
R
(alcohol)
(diol)
(Ketone)

14. 08/04/18 14
The alcohol formation is effective when R-radical of alcohol (proton source) is stable (tertiary) and
the coupling reaction is dominant when R is the radical stabilizing group. (e.g.-aryl). The coupling
reaction is more effective when the reduced alcohol of the starting carbonyl compound is less stable.
The reaction occurs in good state only when the triplet is of the (n, π*) type. The triplet
state abstracts hydrogen atom from hydrogen donor/second reactant molecule.
O
R1
R
hv O
R1
R
O
R1
R
T1 state
Excited states
S0 state S1 state
(n,π∗
) transition
The resulting radical
either combine- R1
R
OH
HO
R
R1
Diol
O
R1
R
T1 state
Alcohol
O
H
H3C H
H3C+
propan-2-ol Ketyl radical
OH
R1
R
O
H3C
H3C
+
propan-2-one
2
solvent
OH
R1
R
2
The resulting radical abstract
hydrogen from solvent

15. 08/04/18 15
Photoreduction of carbonyl compoundsPhotoreduction of carbonyl compounds
Photoreduction of benzophenone to benzpinacole is carried out by irradiation of a solution
benzophenone and isopropyl alcohol with wavelength 340 nm.
The benzophenone forms n → π* triplet state at these wavelength but isopropyl alcohol
does not shows absorption. In addition with isopropyl alcohol, toluene; cyclohexane;
methanol are used as solvent.
O
Ph
Ph hv (340 nm)
(CH3)2CHOH
Ph
Ph
OH
HO
Ph
Ph
(Benzophenone) (Benzopinacole)
O
Ph
Ph
hv (340 nm)
(Benzophenone)
O
Ph
Ph
O
Ph
Ph
S1 state T1 state
Excited statesS0 state
MechanismMechanism
Ph
Ph
OH
HO
Ph
Ph
(Benzopinacole)
Ketyl radical (A)
O
Ph
Ph
T1 state
O
H
H3C H
H3C+
propan-2-ol
OH
Ph
Ph
O
H
H3C
H3C+ O
Ph
Ph
T1 state
+
Ketyl radical
OH
Ph
Ph
O
H3C
H3C
+
propan-2-one
(B)

16. 08/04/18 16
Photoaddition ReactionsPhotoaddition Reactions
 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

17. 08/04/18 17
Photoaddition of alkenes to carbonyl compounds (Paterno-Buchi reaction)
The photoaddition of the triplet state of the carbonyl compound with ground state of the
alkene forming four membered oxetane rings is called as Peterno-Buchi reaction.
It is [2+2] photoaddition reaction.
The carbonyl compound is irradiated first rather than olefins. The triplet state of the
carbonyl compound (due to n →π* transition) is react with alkene.
Ph-CHO Ph-CHO Ph-CHO 3[ ] [ ]
1hv ISC
S0 state S1 state T1 state
O
Ph
H
O
Ph
H
T.S.
(oxetane)
O
Ph
PhH O
Ph
Ph
(oxetane)
O
Ph
Ph
+
(9 part) (1 part)
O
Ph
Ph H
(oxetane)

18. 08/04/18 18
It is not stereospecific reaction because the rotation about the single bond occurs
faster than the spin inversion of electrons. e.g. When benzophenone is irradiated with
cis- or trans-2-butene gives same mixture of both isomeric oxetane product.
31
hv ISC
S0 state S1 state T1 state
O
Ph
Ph
+
O
Ph
Ph
O
Ph
Ph
OPh
Ph
O
Ph
Ph1
11
1
stable
(a)
spin
inversion
(b)
O
Ph
Ph
O
Ph
Ph
S0 state
spin inversion
The ring is formed in two stages. The excited carbonyl triplet state first adds through its
oxygen atom to alkene forming mainly more stable biradical species. The biradical species
under goes spin inversion forming second bond.
T1 state
O
Ph
Ph
S0 state
O
PhPh
CH3CH=CHCH3
OPh
Ph
+ hv C-C
bond rot.
1
1
C-C
bond rot.
O
Ph
Ph
When carbonyl compound reacts with unsymmetrical alkene forming more stable biradical
species which forming major product (a).

19. 08/04/18 19
The γ,δ-unsaturated carbonyl compound under goes intramoleculer Peterno-Buchi
reaction.
O
O
+
O
O
O
COCH3
(cis)
When alkene containing electron withdrawing group, it react with carbonyl compound without
loss of its stereochemistry.
The conjugated diene is also added across the carbonyl compound but yield of the product is
less because of competitive triplet sensitized dimerisation of diene.
S0 stateT1 state
(oxetane)
O
CH3
CH3
O
H3C CH3
CH311
stable
NC
O
H3C
H3C
+
CN
CH3
3
H3C
NC
S0 state
T1 state
3
O
H
H
+
hv
(cis)
H
O
AcO
OAc
hv
T1 stateDimer
The carbonyl compounds is also added to allenes in high regeoselective manner as-
31
hv ISC
S0 state S1 state
(oxetane)
O
Ph
Ph
+O
Ph
Ph
O
Ph
Ph O
Ph
Ph
hv
The biacetyl is also reacting with unsaturated ether regeosectively and stereoselectively to
give cis-2-alkoxyoxetane ring.

20. 08/04/18 20
Drawback:-
i) The excited energy of ketone is higher than the olefin causes energy transformation
from ketone to alkene. The excited alkene undergoes dimerisation. In such cases
ketones are called as sensitizer.
Therefore to overcome this drawback, the ketone having low triplet state than the
alkene is used. E.g. benzophenone.
ii) The reaction between the ketone and conjugated diene (whose triplet excited state
energy is less than the ketone) is not occurs.
iii) The acetylenes also participating in Peterno-Buchi reaction forming unstable
oxetene which undergoes ring opening reaction forming unsaturated carbonyl
compounds.
CH3COCH3 CH3COCH3
[ ]
3 CH3COCH3 +[ ]
3
dimer
Ph2CO Ph2CO[ ]3 O
Ph
Ph
+hv hv
Ph2CO+
hv
CH3 CH3
O
Ph
Ph
O
Ph
Ph
O
Ph
Ph
..

21. 08/04/18 21
Ph
Ph
Ph Ph
H
HPh
Ph
. .hv hv
Photo-Rearrangement reaction
Some compounds undergo rearrangement reaction in presence of light forming structural
isomers (where groups or atoms in same molecule occupied different positions) or to
valence bond isomers (where groups or atoms have same positions but having different
bonding framework).
Cis-Trans isomerism:
When olefins are irradiated by ultra-violet light, they under goes cis-trans isomerism. This
transformation has been carried out by direct irradiation of olefins or irradiation in
presence of sensitizer.
The isomerism has been occurs through singlet or triplet excited state species. The triplet
state has lower barrier to rotation around carbon-carbon bond.
The isomerism has been occurs because π bond lost its character in the excited state, here
the two sets of the substitutions tend to occupied mutually perpendicular planes so as to
minimize the repulsive forces between them.

22. 08/04/18 22
hv
Ph2CO Ph2CO Ph2CO
Ph2CO+cis-stilbene
trans-stilbene
Ph2CO + [PhCH=CHPh cis- + trans-
93% 7%
[ ]
[ ]1 [ ]3
]3[ ]3
Br2
Br
H
RR
H H
RR
Br
H
H
RR
H R
HR
H
Br
Br.
hv
+ +
2
.
.
.-
The simple olefin absorbs light at 200 nm which is not experimentally convenient
therefore isomerism has been carried out by using triplet sensitizer. e.g. Photoisomerism
of stilbene has been carried out by using benzophenone triplet sensitizer.
There are some photochemical cis-trans isomerism has been carried out by using
halogens. Halogen undergoes photochemical decomposition to halogen atom which adds to
olefinic double bond to form radical. Elimination of halogen atom from radical gives raise to
constant ratio of cis and trans isomers

23. 08/04/18 23
hv
H
H
hv
Intramolecular Photocyclisation
Many dienes and polyenes are converted photochemically into cyclic isomers. The
reactions are stereospecific and electrocyclic type when proceeds through singlet excited
state.
In some examples the given organic compounds has been converted into highly strained
ring compounds proceeds via triplet sensitized state
e.g Irradiation of trans, trans-1,4-dimethyl-1,3-buadiene gives cis-3,4-dimethyl cyclobutene
by dis-rotatory ring closer. It explains by orbital symmetry principle
e.g. trans,trans-1,6-dimethyl-2,4,6-hexatriene cyclised with con rotatory motion

24. 08/04/18 24
For acyclic and monocyclic compounds, the rearrangement occurs via singlet transition
state. The rearrangement can involved either singlet or triplet excited state and can be
proceeds via diradical which are formed due to bonding between C2 and C4. The reaction is
more efficient when the substituents are present on the C3 (sp3
) carbon atom. The
reaction is highly stereospecific and occurs with retention in configuration at C1 and C5.
hv
sensitizer
spin
inversion
.
... .
O
PhPh
O
Ph
Ph
O
Ph
Ph
O
Ph
Ph
OH
Ph
Ph
OH
Ph
Ph
+ +
hv
1,4-Diene are also under goes photochemical rearrangement to vinyl cyclopropane. If diene
containing useless conjugated substituents, reaction proceeds by using triplet sensitizer. It is
called as Zimmermann rearrangement
This rearrangement is also called as Di-π methane rearrangement. This process involves
(1,2)-shift. The mechanism and efficiency of the process is depends on the nature of the
system

25. 08/04/18 25
The same rearrangement is occurs in the β,γ-unsaturated aldehydes and ketones.
There are some reactions which forming cage compounds e.g. cyclopentadiene
adds readily to para-benzoquinone thermally (π4+π2) manner. The product
formed under goes ring closer by irradiation.
Nonbornadiene and cyclopentadiene gives cage like compound on irradiation.
R
O
R
O
R
O. .
O O O
O
.
.
O
O
+
O
O
O
O
heat
+
hv

26. 08/04/18 26
+
hv
CH2
n
CH2
CH2
CO CH2
n
CH2
CH2
CO
.
. -CO CH2
n
CH2
CH2
.
.
CH2
n
CH2
CH2
(a)
n-3
CH3(CH2) CH=CH2
CH2=CH2 CH3(CH2) CH=CH2
n-1
(b)(c)
Norrish Type-I and Norrish Type-II Reaction
The bond dissociation energy of a carbon-carbon bond adjacent to a carbonyl group is small
and photo-chemically undergoes hemolytic fission is called Norrish type-I reaction or α-
cleavage
The –CO-C bond under goes fission forming alkyl and acyl radical. The acyl radical under
goes decarboxylation followed by recombination of two radicals or abstract of hydrogen
atom or secondary fission to get hydrocarbon and olefin. The carbonyl compound undergoes
fission in gaseous or liquid state through (n π*) singlet or triplet state.
(a) Recombination, (b) abstract of hydrogen, (c) secondary fission.

27. 08/04/18 27
CH3-CO-CH(CH3)2
CH(CH3)2
CH3CO+
..
CH3
.+(CH3)2CHCO
.
hv (A)
hv (B)
+..
+
hv
O O O
H
O
H
O
hydrogen
transfer
During decomposition more stable radical is formed (alkyl radical). The stability of the alkyl
radicals are- 30
> 20
> 10
> methyl > H. The stability of alkyl radical can be important in the
unsymmetrical ketones.
In above example, the route (a) is currect because of stability of the radical.

28. 08/04/18 28
R
O
H R1
R2
R
O
H R1
R2
R
O H
R2
R1
R
O
CH3
R1
R2
+ +hv
CH3
OH
R
O
H
hv
O
CH3
OD
CH3OD
O
D H
O
H H
hv
O
Ph
OH
Ph
H
OH
Ph
COOCH3
COOCH3
Ph
COOCH3
COOCH3
+
COOCH3
COOCH3
Type-II acid
A ketone having γ-hydrogen atom is under goes Norrish type-II reaction forming an olefin
and enol of a smaller ketone.
It is characterized by the transfer of γ-hydrogen atom to a carbonyl group. This
transformation has been occurred through six membered transition state forming methyl
carbonyl compound and olefin.
Both (n π*) singlet and (n π*) triplet are participate in the intramoleculer hydrogen
abstraction reaction will the ketone having low lying (π π*) state does not under goes
these reaction.
The α,β-unsaturated carbonyl compound under goes Norrish Type-II reaction through six
membered transition state to get enol (containing γ-hydrogen atom) as-