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.
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2. Content:-
1. Introduction
2. Electronic states of ethylene
3. 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

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.
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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.
●
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7. Mechanism by which cis-trans
isomerisation occurs:-

8. Some examples of cis-trans isomerisation of
alkenes by direct irradiation:-
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9. Sensitised cis-trans isomerisation:-
This can be brought about by use of a triplet
sensitiser such as ketone
Mechanism:-

10. Continued……..
Sometimes photochemical cis-trans isomerisation takes
place in the presence halogens.
For example:-
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11. Dimerisation of alkenes:-
● An alkene in its S1 or T1 state reacts with same
(photodimerisation) or different (photocycloaddition)
alkenes to produce a cyclobutane derivative.
● Dimerisation of simple alkenes is difficult.

12. Continued…..
Dimerisation reaction is stereospecific

13. Intramolecular dimerisation:-
Non-conjugated dienes undergoes intramolecular
dimerisation to give bicyclic products. The product
depends on the number of -CH2 - groups between
two double bonds.
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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.
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15. The photoreaction of 1,5-hexadiene from the
T1 state follows the rule of five.
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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 C2 and C3 at the

17. Excited states of butadiene:-
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18. Direct irradiation of butadiene:-
This give rise to formation of cyclobutane or
bicyclobutane.
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19. Continued..
If concentrated solution of 1,3 - butadiene is irradiated
some dimer formation can be detected.
Dimerisation give four different dimers.
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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.
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21. Conclusion:-
● Alkenes gives reactions when
irradiated.
● Undergoes cis tran isomerisation
,cycloaddition,etc.
● Many synthetic utality.
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22. References:-
1. V.P. Sharma & Rakesh Kumar , Pericyclic
reactions & organic photochemistry,first edition,
Pragati Prakashan,2008.
2. Jagdamba singh & Jaya singh, Photochemistry &
Pericyclic reactions,Third edition,New academic
science limited,2012.
3. Biswanath Dinda, Essentials of pericyclic &
photochemical reactions, Springer,2017.
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