1.
Photochemistry of Alkene
A Semimar Presented By
LOKESH JANGID
M.Sc. Chemistry Semester - III
Department of Chemistry
S.P.C. Govt. College, Ajmer
2021-2022

2.
Contents
 What is photochemical reaction
 Photochemistry of Alkene
 Geometrical isomerism
Cis-trans isomerisation by direct irradiation
By the use of triplet photosensitizer
 Cyclisation reaction
Concerted cycloaddition reaction
Non-concerted cycloaddition reaction
 Rearrangement reaction of Alkene
In 1,4-dien
In 1,5-dien
 Reference

3.
What is photochemical reaction
Photochemical reaction :- Photochemical reaction is
a chemical reaction which is initiated by the absorption
of energy from the light.
After absorbing light, the molecule become excited and
transient excited state is created whose chemical and
physical properties are differed greatly from the original
molecule.
The transient excited state converted into product.

4.
Photochemistry of alkene
Each sp2 carbon have half filled 2pz orbital which do co-lateral
overlapping with one adjacent c-atom’s 2pz-orbital. Thus π-
framework is formed which is perpendicular to σ-framework.
Example – Ethylene
The Molecular orbital diagram of ethylene is – π π
When this molecule also irradiated, it also undergoes various
photochemical reaction like Isomerisation, pericyclic reaction
and rearrangement.
*
2

5.
Isomerisation reaction
a) cis-trans isomerisation by direct irradiation –
Alkene have a property to display a characteristic
photochemical interconversion of cis-trans isomers.
C6H5 – CH=CH – C6H5 Cis- stilbene 93% + trans-stilbene 7%
hʋ

6.
B) Cis-trans isomerisation by use of sensitizer-
Photoisomerisation can also be carried out in presence of
sensitizers.
If the energy of photosensitizer is more than 60kcal/mol
then cis/trans >1
If energy is between 52-58 kcal/mol
then cis/trans >>>> 1

7.
Cyclisation reaction
The conjugated polyenes form radicals on irradiation,
which leads to a number of products by the
recombination of radicals and also leads to
cyclisation.
The type of cyclisation reaction depends upon the
geometry of alkene and also depends on spin state of
excited sensitizer

8.
A) Concerted cycloaddition reaction : - This
reaction is completed by the formation of
cyclic transition state.

9.
B) Non-concerted cycloaddition reaction :-
The non concerted cycloaddition reaction consist
of two steps.
i. First is the formation of a biradical
intermediate.
ii. Second is non stereospecific or partially
stereospecific cyclic product.

10.
Rearrangement of 1,4-diene
Also known as D-π-methane rearrangement. The
rearrangement of 1,4-diene is a concerted
reaction which involve a 1,2-shift with closing
the ring and thus form a vinyl cyclopropane.
it is stereospecific at all c-centres

11.
Mechanism:-

12.
Rearrangement of 1,5-diene
The thermal rearrangement of 1,5-diene occur by
[3,3] sigma tropic reaction, which also called “cope
rearrangement”.
The reaction is proceed with the formation of cyclic
transition state which is confirm by the negative
change in entropy.

13.
In Cope rearrangement, radicals are formed & both
are allyl radical.
The migration group is allyl radical.
2 2
T.S.

14.
Reference
• Modern photochemistry - VINAY PRABHA
SHARMA (A pragati edition)
• Photochemistry – ALKA L. GUPTA (A pragati
edition)
• http://en.m.wikipedia.org

15.
Thank You….