ORGANIC PHOTOCHEMISTRY -Dr. A.G. Nikalje Pharmaceutical Chemistry Department

1. ORGANIC PHOTOCHEMISTRY -Dr. A.G. Nikalje Pharmaceutical Chemistry Department

2. INTRODUCTION • Law of Photochemical Equivalence:- one molecule reacts for each quantum • absorbed . • Quantum yield (Q) = No. of molecules reacting or produced / No. of quanta absorbed 2) Photochemical Reaction: Two main types • a) Direct light- induced or • Non-sensitized Reaction • b)Photosensitized reaction--- • Donor( should have longer life time and higher excitation energy than acceptor) • Multiplicities of D and A excited states are the same. • Common sensitizer is benzophenone 3) Photolysis 4) Flash photolysis

4. Jablonski diagramme Relaxation mechanism for excited state molecules

5. Photochemical Elimination Reaction For ketones this involves loss of carbon monoxide.eg Photolysis of acetone in vapor phase. Products depend on temp. of the experiment. a) Elevated temperature

6. b) Room temperature

7. Norrish type I process: Photochemical reactions of ald./ ketones which result in fission of C—CO bond followed by elimination of carbon monoxide Norrish type II process: longer chain carbonyl compounds undergo γ H-abstraction And fission of molecule occures to give an alkene and ketone via 6- membered cyclicT.S. involving oxygen of carbonyl group as member of the ring.

8. Cyclic ketone elimination

9. Photochemical reductions • A hydrogen atom is abstracted from the solvent or from another reactant to produce • two free radicals.eg. Irradiation of benzophenone in alcoholic solvent (as isopropalol) • to give benzpinacol and oxidation product of alcohol. • Two molecules of benzophenone are consumed.

10. Barton Reaction

11. Photochemical Oxidation

12. Photochemical Rearrangment Dienone-Rearrangement