photochemistry.pptx

7/27/2019 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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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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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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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:


O O

CH2

C CH2CH2h

gas phase

-CO

CH2 CH2 CH2 CH2+


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-cleavage given by cyclobutanones:


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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photochemistry.pptx

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