1. Organic Photochemistry
Introduction to Photochemistry
Classifications of Photochemical Reactions
Application of Photochemistry in Organic
Synthesis

3. Energies
100 kcal/mol= 4.3 eV= 286 nm= /cm (near UV)nano= 10-9
286 kcal/mol= 12.4 eV= 100 nm= /cm (far UV)
Typical Bond Energies
C-H = 110 kcal/mol
C-C = 80
C=C = 150
C=O = 170
Uv light nm wavelength, so this is sufficient energy to break bonds –knock electrons out of bonding orbitals (electronic excitation).

6. A Jablonski diagram, named after the Polish physicist AleksanderJabłoński, is a
diagram that illustrates the electronic states of a molecule and the transitions
between them.
The states are arranged vertically by energy and grouped horizontally by spin
multiplicity.
Radiative transitions are indicated by straight arrows and nonradiative transitions
by squiggly arrows.
The vibrational ground states of each electronic state are indicated with thick
lines, the higher rotational states with thinner lines.

8. Physical Processes Undergone by Excited Molecules
So + hv --- S1 Excitation
S1v -- S1 + heat Vibrational Relaxation
S  So + hv Fluorescence
S1 ---- So + heat Internal Conversion
S1 --- T1 Intersystem Crossing
T1v -- T1 + heat Vibrational Relaxation
T1v -- So + hv Phosphorescence
T1 --- So + heat Intersystem Crossing
S1 + A (So) --- So + A (S1) Singlet-Singlet Energy Transfer
T1 + A (So) -- So + A (T1) Triplet-Triplet Energy Transfer

9. Why Use Photochemistry
Overcome large kinetic barriers in a short amount of time
Produce immense molecular complexity in a single step
Form thermodynamically disfavored products
Allows reactivity that would otherwise be inaccessible by almost any other synthetic method
The reagent (light) is cheap, easily accessible, and renewable
Drawback
Reactivity is often unpredictable
Many substrates are not compatible
Selectivity and conversion are sometimes low

10. Chemical Processes undergone by Excited Molecules

11. 1) α-Cleavage (Norrish type I reaction)
1) α-Cleavage (Norrish type I reaction). In solution the radicals undergo further reactions to give products.

12. 2) Hydrogen Abstraction followed by cleavage = Norrishtype II cleavage
2) Hydrogen Abstraction followed by cleavage = Norrishtype II cleavage.The radicals can abstract a Hydrogen atom from a donor. The resulting radicals can then undergo further reactions.

13. An intramolecular example:

19. Thank You
P.G.V. Subhash
II M.Sc.