Presentation on photolysis

Discussion on photolysis, Norrish rearrangement, photolysis of azo compounds, isomerization and rearrangement, Fries rearrangement, chemiluminescence and bioluminescence

1. Shahjalal University of Science and Technology, Sylhet “A presentation on Photochemistry ” Course no: CHE 441 Course title: Advance O rganic Chemistry Submitted to: Mr. Md. Masum Talukder Assistant Professor, Department of Chemistry. Presented by: Supri yo Sharma Reg. No.: 20131310 3 3 Semester: 4/1

2. Contents: Photolysis Norrish Type I Cleavage Norrish Type II Cleavage Photolysis of Azo Compound Isomerization and Rearrangement Photo-Fries rearrangement Chemiluminescence Bioluminescence

3. Ronald George Wreyford Norrish: Ronald George Wreyford Norrish was a British chemist who was awarded the Nobel Prize in Chemistry in 1967 for the development of flash photolysis. One of his accomplishments is development of the Norrish Reaction. The Norrish reaction in organic chemistry describes the photochemical reactions taking place with ketones and aldehydes. This type of reaction is subdivided in Norrish type I and Norrish type II reactions. The reaction is named after Ronald George Wreyford Norrish.

4. Photolysis: The cleavage of bonds as a result of photoactivation and intramolecular rearrangement. Irradiation of a molecule often lead to a hemolytic bond cleavage and produce free radical intermediate. Norrish Type I Cleavage: Photolytic cleavage at the C-C bond alpha to the carbonyl group leads to an alkyl and acyl free radical. This process is often referred to as a Norrish type I cleavage. Acetone h ϑ

5. In the gas phase above 100 C this process followed by decarboxylation . Example: h ϑ h ϑ h ϑ (Ethane) Alkane

6. Norrish Type II Cleavage: Photolysis reaction of ketones bearing a γ -H in a photoelimination, the carbonyl group abstracts the γ -H to give an alkene and a new ketone. This process is often referred to as a Norrish type II cleavage . α β γ

7. Esters possessing a γ -H or β-H also undergo Norrish type II. h ϑ h ϑ

8. Photolysis of Azo Compound: When a diazomethane is irradiated, cleavage of C-N bond produces methylene( the carbene). Photolysis of diazomethane in the presence of 2-butene produces cis-1,2-dimethyl cyclopropane, the product of syn addition. When an inert gas or liquid is added to the reaction stereoselectivity decreases. Both the cis and trans cyclopropane are obtained. Inert gas or Liq. cis trans h ϑ - N 2

9. Isomerization and Rearrangement: E isomers absorb energy more effectively than does the Z isomer. Therefore it is possible to convert E isomer to it’s thermodynamically less stable Z form. The technique is known as optical pumping. E- butenedioic acid ( Fumeric acid) Z- butenedioic acid ( Maleic acid ) hϑ hϑ E- Azobenzene Z- Azobenzene

10. Karl Theophil Fries was a German chemist. Fries is noted for the discovery of the Fries rearrangement which is an important reaction used in organic synthesis. The Fries rearrangement is a rearrangement reaction of a phenolic ester to a hydroxyl aryl ketone by catalysis of Lewis acids. It involves migration of acyl group of phenol ester to the aryl ring. The reaction is ortho and para selective and one of the two products can be favoured by changing reaction conditions, such as temperature and solvent. Photo-Fries rearrangement:

11. Photo-Fries rearrangement: Molecular rearrangement also can be promoted photochemically. Phenolic esters rearrange on irradiation to give ortho and para acyl phenols known as Photo-Fries rearrangement.

12. An interesting photorearrangement occurs when 2,5-cyclohexadienones are irradiated. hϑ 4,4-diphenyl-2,5,- cyclohexadiene 6,6-diphenylbicyclo[3,1,0]-3-hexen-2-one

13. Mechanism: hϑ Intersystem crossing Rearrangement Relaxation Rearrangement

14. Chemiluminescence: Chemiluminescence is the process in which the promotion of molecules to an excited state by the excess energy of a non-photochemical reaction leads to the emission of visible light. The species excited initially may emit light itself on relaxation to the ground state or transfer it’s energy to another molecule which then emits. Example: When perylene mixed with oxalyl chloride and H 2 O 2 in a hydrocarbon solvent, blue light is emitted, which is referred as ‘cold light’. The reaction sequence involves. Oxalyl chloride 1,2-Dioxacyclobutanedione

15. Perylene

16. hϑ (light)

17. Bioluminescence: Chemiluminescence that take place in some living organism is generally known as bioluminescence. The lightf emitted by a firefly is the result of emission by an excited product derived from luciferin . Firefly Luciferin Emisson from derived product of luciferin

18. Thank You