photochemistry.pptx
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CONTENTS
Transitions
Norrish-I cleavage
Norrish-II cleavage
Paterno-buchi reaction Photoreduction
Calculation of quantum yield
Photochemistry of enones
Rearrangement of,-unsaturated ketones
Photo fries rearrangement
Barton reaction
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Transitions
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Norrish type-1 cleavage(- cleavage)
Norrish type-1 process is given by three types of ketones
Saturated acyalic ketones
Saturated cyclic ketones
,- unsaturated ketones
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Norrish type 1 process given by acyclic saturatedketones:
Norish type-1 clevage
R1
R
O
R2
hR C
O
+R
1
CH R2
acyl radical alkyl radical
Primary process:
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Norish type-1 clevage
R C
O
Decarbonylation
+ CO2(a)
acyl radical alkyl radical
R CH2CH2
R CH2 + R CH2R
R
(Recombination)
R CH2 +R
1
CH R2
R CH3R
1
R2+
intermolecular hydrogen abstraction
:
Secondary process
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Norish type-1 clevage
O
R C + RCH R
2
OR+
R1
R2
scheme 2(c)
R1
CH R2
R1
R2
O
R C
+ +
O
R
H
scheme 2(b)
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Norish type-1 clevage
O
n
h
S1
ISC
T1
O
CH2
C
n
Acyl-alkyl biradical
CH2
CH2n
Radical recombination
Intermolecular
hy drogen bonding
n
CH3
CH2n
Cy cloalkane Alkene
Decarbonylation
(2) Norrish type-1 reaction of saturated cyclic ketones:
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O
C
CH2n
O
CH3
ketene
Scheme 4(c)
Norish type-1 clevage(2) Norrish type-1 reaction of saturated cyclic ketones:
n
O
C
CH2
O
CH2n
unsaturated aldehyd
Scheme 4 (c)
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3) Norrish type-1 process given by cyclopentanones:
Norish type-1 clevage
O O
CH2
C CH2CH2h
gas phase
-CO
CH2 CH2 CH2 CH2+
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-cleavage given by cyclobutanones:
Norish type-1 clevage
O OC
CH2S1
-COCH2
CH2
CH3
CH21,4-acyl
-alkyl di radical
2) By a subsequent -cleavage and formation of ethylene and ketene.
Loss of CO and formation of 1,3-diradical
O
C
CH2
CH2 O CH2 CH2+
I2 2I cycloaddition+
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O
C
CH2
O O H
RROHring
expansion
Oxacarbene Acetal
3) Rebonding to oxygen
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Norish type-1 clevage-cleavage reaction:
O
CH3
h
breaking of
CCbond
OH
CH2CH CH3
(b) (a)
(a) (b)
1,2-Hydrogen shift
O
CH3 CH3
O
CH2CH3
(Major)(Minor)3-penten-2-one
4-penten-2-one
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Norrish type-II cleavage i.e. -Hydrogen abstraction:
(intramolecular hydrogen abstraction)
HO
R
HO
CR
CH2OH
CR
h
n-> *S1 or T 1
.
cyclic transition state 1,4-diradical
CHOH
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CH2OH
CCH3
R
OH
HH
12
3
4
CH3
OH
R
CH3
H
H
. .
sp orbitals
cleavage of -bond
cyclisation
OH
R CH2
CH2OHR
CH2
OR
CH3
ketoformalkenol alkene
+
..
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PATTERNO-BUCHI REACTIONS
H5C6
C6
H5
O + CH2CH
3
CH3 hO
H5C6
H5C6
CH3
CH3
(90% yield)
Addition to Election-rich alkenes
Addition to electron-deficient alkenes
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Photoreduction
OH5C6
H5C62 +
CH3
CH3 OH
hH
5C
6
C6H5
OH
C6H
5
OH
C6H5
+ CH3 CH3
O
Benzophenone 2-propanol
Benzpinacolacetone
1)
S1 (n ->)
ISCT1(n ->
)
H5C6
C
H5C6O
Ph 2 CH O(H3C)2 OH
+ CHPh 2
OH
(H3C)2 CHOH
+
(H3C)2 CH OH (H5C6)2
O(H5C6)2 CH
OH(H3C)2 O
(H5C6)2 O
+ +
H5C6 C
OH
C6H5
OH
C C6H5
C6H5H5C6
OH
H5C6
OH
C6H5
C6H5
+
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Calculation of quantum yieldThe efficiency of photochemical process is often expressed in terms of quantum yield () which is
defined as the number of molecules reacting per quantum of light absorbed
Mathematically
The energy E absorbed per mole of the reacting substance is called one Einstein
The quantum yield of the product formation
If the law is correct then the quantum yield should be unity. This however is very rare.
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Photochemistry of enones
O
h
O O
ISC
1 3
n *
R t f t t d k t
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Rearrangement of,-unsaturated ketones
O
R
R
O
CC
R
R
O
CH
C
R
R
O
R
R
O
R
R
OR
n -> S1
ISC
recombination
ROH
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Photo-Fries rearrangement
https://en.wikipedia.org/wiki/File:Fries_rearrangement_(photo).svg -
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Barton reaction
Barton reaction involves the photolysis of a nitrite to form a -nitroso alcohol.It is nammed for the british chemist Sir Derek Harold Richard-Barton. The
mechanism is belived to involve a homolytic cleavage of N-O bond followedby-hydrogen abstraction and free radical recombination.
http://en.wikipedia.org/wiki/File:Barton_reaction_mechanism.svg -
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PHOTOCEHMISTRY AND PERICYCLICREACTIONS BY
JAGDAMBA SINGH &JAYA SINGH
NEW AGE INTERNATIONAL PUBLISHERS
SECOND EDITION
WIKIPEDIA.ORG
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