1. CYCLOADDITION REACTION BY
PHOTOCHEMICAL MODE
SAMEEN FATIMA Roll no 908529

2. THEORIES
 FRONTIER
MOLECULAR
ORBITAL (FMO)
THEORY
 WOOD WARD-
HOFFMAN RULE
 MOBIUS-HUCKLE
THEORY
PRESENTATION TITLE 2

3. CYCLOADDITION REACTION
Cyclo-addition reactions that are also done thermodynamically (the 4+2 are
also called Diels Alders reaction) but that which reactions do not are done
photo chemically. This is understood that [2+2] cyclo-addition reactions are
only done by photochemical mode.
octahydrocyclobuta[1,2:3,4]di[5]annulen-1(2H)-one
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Basic_Principles
_of_Organic_Chemistry_(Roberts_and_Caserio)/13%3A_Polyfunctional_Compounds_Al
kadienes_and_Approaches_to_Organic_Synthesis/13.03%3A_Cycloaddition_Reactions

4. ONLY 6 Π ELECTRONS SYSTEM IS STUDIED UNDER THE THERMAL CONTROL. 4 Π
ELECTRONS SYSTEM THAT IS A SUPRAFACIAL IS STUDIED UNDER THE
PHOTOCHEMICAL CONDITIONS. UNDER PHOTO-CHEMICAL CONDITIONS,
THE RULES SWITCH SUCH THAT ALL THE CYCLO-ADDITIONS THAT ARE NOT
ALLOWED THERMALLY ARE ALLOWED PHOTO-CHEMICALLY. THE VAST
MAJORITY OF [2+2] PHOTO-CYCLO-ADDITION REACTIONS INVOLVE ENONE–
ALKENE CYCLO-ADDITION WHICH IS CONVENIENTLY ACHIEVED
THROUGH DIRECT EXCITATION OR SENSITIZATION BY UV IRRADIATION. THIS
ONLY NEEDS BECAUSE OF THE INCOMPATIBLE SYMMETRY BETWEEN THE TWO
DIFFERENT ALKENES.
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Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart
Warren.

5. LIGHT IS ABLE TO BE ABSORBED BY
THE CONJUGATED SYSTEM AND
THE CONJUGATED SYSTEM CAN TURN
THAT RADIATION ENERGY INTO
KINETIC ENERGY AND SHOOT UP AN
ELECTRON TO A HIGHER ENERGY
STATE. UNDER PHOTOCHEMICAL
CONDITIONS AN ELECTRON
UNDERGOES EXCITATION TO THE NEXT
LEVEL SO Π1* WILL THEN BE THE
HOMO.
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Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves
and Stuart Warren.

6. THE FMO THEORY.
THIS THEORY CONSIDERS ONLY THE FRONTIER MOS, THE HOMO OF ONE ETHENE MOLECULE
AND THE LUMO OF ANOTHER.
ONLY IF THE INTERACTING LOBES ARE IN PHASE THE REACTION IS THERMALLY FEASIBLE.
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7. • IN THE MO DIAGRAM THE LOBES ON BOTH SIDES ARE NOT IN PHASE, HENCE
THIS PROCESS IS THERMALLY NOT FEASIBLE.
• ON PHOTO-IRRADIATION ONE OF THE ELECTRONS FROM Π1 UNDER GOES
EXCITATION TO Π1*. THUS UNDER PHOTOCHEMICAL CONDITIONS THE HOMO
IS Π1*.
• THE FMO PICTURE IS:
In the MO diagram the lobes on both sides
are in phase, hence this process is
photochemically feasible.
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http://www.chemvista.org/cycloadditions1.html

8. PRESENTATION TITLE 8
https://www.youtube.com/watch?v=cPz6FW1Hlnw

9. WOOD WARD HOFFMAN RULE: A GENERAL [P+Q]-CYCLOADDITION IS A
CONCERTED ADDITION REACTION BETWEEN TWO COMPONENTS, ONE
WITH P Π-ELECTRONS, AND ONE WITH Q Π-ELECTRONS. THIS REACTION IS
SYMMETRY ALLOWED UNDER THE FOLLOWING CONDITIONS:[13]
• FOR A SUPRA/SUPRA OR ANTARA/ANTARA CYCLOADDITION, IT IS
THERMALLY ALLOWED IF P + Q = 4N + 2 AND PHOTOCHEMICALLY ALLOWED
IF P + Q = 4N
• FOR A SUPRA/ANTARA CYCLOADDITION, IT IS THERMALLY ALLOWED
IF P + Q = 4N AND PHOTOCHEMICALLY ALLOWED IF P + Q = 4N + 2
THIS IS SUMMARIZED IN THE FOLLOWING TABLE:
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p + q Thermally allowed Photochemically allowed
4n ps + qa or pa + qs ps + qs or pa + qa
4n + 2 ps + qs or pa + qa ps + qa or pa + qs
Woodward, R. B.; Hoffmann, Roald (1971). The Conservation of Orbital
Symmetry (3rd printing, 1st ed.). Weinheim, BRD: Verlag Chemie GmbH
(BRD) and Academic Press (USA). pp. 1–178. ISBN 978-1483256153.

10. Q1
Disseminate standardized metrics
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MOBIUS HUCKLE THEORY
Electrons Nodes Aromaticity Thermally
Allowed
Photochemically
Allowed
4n zero Anti-aromatic Supra-supra,
antara-supara
4n one aromatic Supra-antara,
antara-supara
4n+2 zero aromatic Supra-supra,
antara-antara
4n+2 one Anti-aromatic Supra-antara,
antara-supra

11. Examples of 4n electron system
As its’ mechanism is difficult but some chemists perform the diradical system. That is as under:
Mechanism of diradical system:
An unpaired electron in the
sigma-radical is in the
sigma orbital and an
unpaired electron in the
pi radical is in the pi
orbital respectively.
Therefore, the radicals
above are pi radicals
Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart Warren

12. 12
Regioselectivity in [2+2] Cycloaddition reaction: If we combine the HOMO of one alkene and
LUMO of the enone so it gives the cycloaddition by thermal reactivity but if we are talking about the
photochemical reactivity then regioselectivity found in it. Ionic reaction does not give
regioselectivity. It is that the Following reaction only give the combination of LUMO to LUMO
reaction if occur photochemically.
The forbidden state of this reaction when HOMO and LUMO combine then the reaction is as:

13. 13
The orbital diagram of the LUMO to LUMO reaction is:
Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart Warren.

14. Reaction is stereospecific that the least hindered transition state is observed.
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Stereo specificity in Cyclo-addition reaction: In any way some stereo-specificity is also found in
photochemical cyclo addition reaction. For this the following example is best to explain. In which the two
old rings keep out of each other’s way during the reaction and the conformation of the product looks
reasonably unhindered.
Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart Warren.

15. [1]
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Basic_Principles_of_Organic_Che
mistry_(Roberts_and_Caserio)/13%3A_Polyfunctional_Compounds_Alkadienes_and_Approaches_to_Org
anic_Synthesis/13.03%3A_Cycloaddition_Reactions
[2] Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart Warren.
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[3] http://www.chemvista.org/cycloadditions1.html
[4] Woodward, R. B.; Hoffmann, Roald (1971). The Conservation of Orbital Symmetry (3rd printing,
1st ed.). Weinheim, BRD: Verlag Chemie GmbH (BRD) and Academic Press (USA). pp. 1–178. ISBN 978-
1483256153.
[5] https://aklectures.com/lecture/cycloaddition-reactions/photochemical-2-2-cycloaddition-reaction
References
[6] Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart Warren.

16. THANK
YOU
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