Photochemistry of alkenes

Alkenes by absorption of light activated to higher energy singlet & triplet state and undergoes chemical reaction. These reactions are mainly:- 1. Cis - trans isomerization
2. Dimerization
3. Cycloaddition

1. A Power point presentation On “Photochemistry of Alkenes” Presented By:- Geeta Class:-M.Sc. chemistry (3rd sem) Roll no. 180000701021. (1)

2. Content:- Introduction Electronic states of ethylene Cis-trans isomerisation of alkenes 3.1 By direct irradiation 3.2 By sensitisation 4. Dimerisation of alkenes 5. Intramolecular dimerisation 6. Photochemistry of conjugated dienes 6.1 Excited states of butadiene 6.2 direct irradiation of butadiene 6.3 irradiation in presence of sensitiser 7. References. (2)

3. Introduction Alkenes by absorption of light activated to higher energy singlet & triplet state and undergoes chemical reaction. These reactions are mainly:- Cis-trans isomerisation Dimerisation Cycloaddition. (3)

4. Different electronic states of ethylene

5. Cis-trans isomerisation:- The most commonly observed photochemical reaction of alkenes is cis-trans isomerisation which can occur by both direct and sensitised irradiation. This isomerisation reaction is usually associated with π-π* excited states. (5)

6. Direct irradiation:- Direct irradiation for simple alkenes is difficult. Most common substrate is stilbene. Irradiation of either cis or trans stilbene at 313nm results in the formation of 93% cis & 7% trans stilbene. (6)

7. Mechanism by which cis-trans isomerisation occurs:- (7)

8. Some examples of cis-trans isomerisation of alkenes by direct irradiation:- (8)

9. Sensitised cis-trans isomerisation:- This can be brought about by use of a triplet sensitiser such as ketone Mechanism:- (9)

10. Continued…….. Sometimes photochemical cis-trans isomerisation takes place in the presence halogens. For example:- (10)

11. Dimerisation of alkenes:- An alkene in its S 1 or T 1 state reacts with same (photodimerisation) or different (photocycloaddition) alkenes to produce a cyclobutane derivative. Dimerisation of simple alkenes is difficult. (11)

12. Continued….. Dimerisation reaction is stereospecific (12)

13. Intramolecular dimerisation:- Non-conjugated dienes undergoes intramolecular dimerisation to give bicyclic products. The product depends on the number of -CH 2 - groups between two double bonds. (13)

14. Continued……… If number n is even the major product is formed via cross cycloaddition. This feature is more prominent in cyclic dienes in which cycloaddition is the only product. (14)

15. The photoreaction of 1,5-hexadiene from the T 1 state follows the rule of five. (15)

16. Photochemistry of conjugated dienes (butadiene):- Butadiene is known to exist in solution as a mixture of s-trans (95%) & s-sis (5%) confirmers In the irradiation of butadiene an elecron is excited from π-π* which give rise to the increased bonding between C 2 and C 3 at the expense of bonding between C 1 and C 2 or C 3 and C 4. (16)

17. Excited states of butadiene:- (17)

18. Direct irradiation of butadiene:- This give rise to formation of cyclobutane or bicyclobutane. (18)

19. Continued.. If concentrated solution of 1,3 - butadiene is irradiated some dimer formation can be detected. Dimerisation give four different dimers. (19)

20. Sensitised irradiation of butadiene:- When butadiene is irradiated in the presence of sensitiser,the product is formed by triplet state & differ from that of formed by singlet state. (20)

21. References:- V.P. Sharma & Rakesh Kumar , Pericyclic reactions & organic photochemistry,first edition, Pragati Prakashan,2008. Jagdamba singh & Jaya singh, Photochemistry & Pericyclic reactions,Third edition,New academic science limited,2012. Biswanath Dinda, Essentials of pericyclic & photochemical reactions, Springer,2017. (22)

22. Conclusion:- Alkenes gives reactions when irradiated. Undergoes cis tran isomerisation ,cycloaddition,etc. Many synthetic utality. (21)