# Norish type I reaction proceeds much faster for n-π* excited states compared to π-π* excited states. # For n-π* states, overlap of the α-bond with the half-vacant n-orbital of oxygen weakens the bond, increasing reactivity. # The rate of cleavage increases with increasing radical stability in the order: primary < secondary < tertiary.

1. Norish Type I Reaction
R
O
C
O
C
h
+

2. O O O
O O
O
O
OMe
O
O
2 X 106 3 X 107
1 X 108
2 X 108
2 X 107
1 X 107
7 X 105
not measured
>109
# Norish type I reaction is much faster for n-* compared to * excited states
# n-* reactivity is due to the weakening of the -bond by overlap of this bond with the half
vaccant n-orbital of oxygen.
# This overlap is not possible for * excited states
The rate of cleavage increase with increase radical stability prim<sec<ter radical
The rate constt r given above

3. O O O
O* O*
O*
K
3.3 x 107/s 4.7 x 108/s 1.8 X 109/S
Rate of  cleavage increasing ring strain
< <
Reactvty direct related to triplet energy increase with increase eng
increase with increase ring strain
cycl-pentone has less butanone has more

4. O
C
C
O O
Me
NO
O
C
C
O
C
C
O O
h NO
h
-780C
acyl-alkyl diradical
Intermediate trapping experiment

5. O
Me
H
Ph
C
O
C Me
H
Ph
C
O
C H
Me
Ph
O
Me
H
Ph
O
H
H H
HPh
h
retention
racemization
disproportionation

6. O
C
C
O
C
C
O O
O
C
C
O
C
C
O O
h
h

7. O
C
O
C
O
C C (CH2)n-1CH3
(CH2)n-1CH3
O
H
(CH2)n-1
O
n
O
OR
n
O
C
O
OR
(CH2)n
h
(CH2)n
recombination
(CH2)n
decarbonylation
(CH2)n (CH2)n
+
disproportionation
enal
ROH
ketene
ring expansion
(CH2)n
ROH
oxacarbene
(CH2)n

8. O
C
C O
C
C
O
C
C
O
C
O C
OO
h
-cleavage
ring opening
cyclization
ring opening and cyclization
decarbonyltion
Homolytic
Cyclo prpanyl radical
1-9 di radical
Cylo propyl alpa substiuent
Homolytic in same condition give 1-12 diradical

9. O O*
C
C
O*
O
..
O OR
C
C
O
h
S1 (n-*)
ROH
Ring expansion
Oxa carbene
+ CO
cycloelimination
+

10. R
R'
O
R
R'
1O*
R
R'
1O*
R
R'
OH
n
R
OH R'
R
R'
O
R
R'
1O*
R
R'
1O*
R
R'
3O*
R
R'
O
R
R'
3O*
R
R'
OH
n
R
OH R'
R'
OH
R
R
R'
O
h
1KH
a
1Kd
Kisc
3Kd
3KH
Norish II photoelimination of ketones: Cleavage of 1,4-biradicals formed by γ-H abstraction
reversal
cyclize
Gama H

11. O
Ph
H
C
O
Ph
C
H
C
O
Ph
C
H
O
Ph
OH
Ph
O
Ph
H
optically active
S1 (n-*) T1 (n, *)
racemic
# Yang cyclization
# cleavage
# hydrgen reversal
1-4 diradical
Mclfherty rearangmnt
Eficent from triplet n to pi
Gama H accesbl for 6 member TS
CH must In plane with n orbital of O to atack

12. R
O
R2
X
R1
H
R
C
OH C
R2
X
R1 R C
O C
R2
R1
R
O
R1
R2
O
OMs
H
Ph
O
Ph
O
OTs
Ph
H O
Ph
h
ISC
-HX
Spin center shift
X = OAc, OTs, OMs, ONO2
h
h