Basics of Photochemistry and Norrish Type I Reaction: Presented By: Dr. Nidhi Vashistha

Basics of Photochemistry and
Norrish Type I reaction
Presented by:
Dr. Nidhi Vashistha
Photochemistry
 Photochemistry is the branch of chemistry that deals with the chemical

processes that are caused by the absorption of light energy
or
 Photochemistry is the study of the interaction of electromagnetic radiation
with matter resulting into a physical change or into a chemical reaction .
• Most of the photochemical reactions are temperature independent because

they show radical mechanism
• In nature there are no. of light excited photochemical reactions such as:
Photosynthesis, ozone formation, solar cell etc
Primary Processes
 One molecule is excited into an electronically excited state by absorption of

a photon, it can undergo a number of different primary processes.
 Photochemical processes are those in which the excited species dissociates,

isomerizes, rearranges, or react with another molecule.
 Photophysical processes include radiative transitions in which the excited

molecule emits light in the form of fluorescence or phosphorescence and
returns to the ground state, and intramolecular non-radiative transitions in
which some or all of the energy of the absorbed photon is ultimately
converted to heat
Jablonski diagram
Luminescence
• Luminescence may be defined as process of emission of light by any substance

in the ultraviolet, visible and infrared region by absorbing suitable extraneous
energy or light. It is also called as cold light.
• There are two types of Luminescence
(i) Fluorescence
(ii) Phosphorescence
• Fluorescence is the emission of light by a substance that has absorbed light or

electromagnetic radiation of a different wavelength.
• The fluorescence life time in which molecule stays in its excited state before emitting a
photon is in the order of 10-7 to 10-9sec
• Quantum mechanically, fluorescence occurs between singlet states ie. S 1 S0
• Phosphorescence substances will release significant amounts of light over long

time scales relative to fluorescence
• Quantum mechanically, Phosphorescence occurs between Excited triplet state and
singlet ground states, T1 S0
Photochemistry of carbonyl compounds
 Carbonyl compound undergo chemical reaction in both gas and liquid phase
 The light photon energy must match with the energy required to excite one electron
from HOMO to LUMO of a ground state molecule to its excited state
 It involves n π* excitation of an oxygen lone pair electron (n) to the π* lowest

unoccupied molecular orbital of the C=O chromophore
 The reactive excited states of saturated Ketones are n π* states, whereas conjugated
ketones are of π π* states, both of these transitions may occur by singlet or triplet
excited state.
 Different type of photochemical reactions of carbonyl compound
(i)Norrish type I and II

(ii)Photo reduction reaction
(iii)Cyclo addition reaction
Norrish Type 1 Reactions
 The Norrish type I reaction is the Photochemical cleavage or homolysis of aldehydes and ketones into
two free radical intermediates.
 On cleavage of the α-carbon bond from either state, two radical fragments are obtained – alkyl and
acyl radical .
 Mechanism
H+ transfer
1 O 3
O O
 - bond cleavage O
hv
n- ISC H3C CH3 C CH3
H3C CH3 H3C CH3
H3C
Ketene + methane Dimerisation

Distinctive features of Norrish Type 1
 Singlet and triplet state can takes part in the reaction

 Reaction proceed to form more stable free radicle
 SP2 and SP nature of α-carbon never takes part in the reaction
 Examples
O O
(i) hv
H
n- H C
C CH3
1. H3 C
C
H
C
CH3 C
H2C CH3 CO
H3 C CH3
(ii)  - bond H3 C
cleavage
CH3
CH3
CH
H3C CH3
O O
2.
C
hv
CO
CH3
H3 C CH3 CH3 H3 C CH3
C H
C C
H3C CH3 C
CH3 H3 C CH2 CH3
H transfer major
O O
3. hv Ph
Ph Ph Ph C Ph Ph
H2C CO CH2 H2C
Ph
Ph
major
O O
H H
hv (i)
4. CO
(ii) H
(ii) (i)
O H-transfer
O
O O
H
hv
(i)
5. CO
(ii) H
(ii) (i)
O H-transfer
O

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Basics of Photochemistry and

Norrish Type I reaction

 Photochemistry is the branch of chemistry that deals with the chemical

• Most of the photochemical reactions are temperature independent because

 One molecule is excited into an electronically excited state by absorption of

 Photochemical processes are those in which the excited species dissociates,

 Photophysical processes include radiative transitions in which the excited

• Luminescence may be defined as process of emission of light by any substance

• Fluorescence is the emission of light by a substance that has absorbed light or

• Phosphorescence substances will release significant amounts of light over long

 It involves n π* excitation of an oxygen lone pair electron (n) to the π* lowest

 Different type of photochemical reactions of carbonyl compound

(i)Norrish type I and II

Ketene + methane Dimerisation

 Singlet and triplet state can takes part in the reaction

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