Photochemistry is the study of chemical reactions caused by light. Key points include:
- Photochemistry involves light interacting with matter, causing physical or chemical changes.
- Photolysis is the process of carrying out a photochemical reaction using light, usually infrared, visible, or ultraviolet light.
- Important natural photochemical reactions include photosynthesis, photography, ozone formation, and solar energy conversion.
- The photochemical process involves light absorption promoting an electron to a higher energy state, followed by primary processes like isomerization, dissociation, or secondary processes like chain reactions.
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
THE PERICYCLIC REACTION THE MOST COMMON TOPIC INCLUDE THE SYLLABUS OF MANY SCIENCE STUDY INCLUDING BSC, MSC , PHARMA STUDY, AND MORE HENCE WE ARE COVERED ALL THE DATA OF IT HOPE THIS WILL MAKE READER EASY.
Photochemistry
ELECTROMAGNETIC SPECTRUM
LAW GOVERNING ABSORPTION OF LIGHT
LAW OF PHOTOCHEMISTRY
Grotthurs-Drapper law.
Einstein Stark law of photochemical equivalence
ELECTRONIC TRANSITIONS
Jablonski Diagram
QUANTUM YIELD
Use Of Photochemistry
Chemistry of vision
Photosynthesis in plant
Formation of Vitamin D
Fluorescent dyes in traffic
Photodynamic therapy
Quantum yield, experimental arrangement, reasons for high and low Quantum yield, problems, photochemical reactions, kinetics of photochemical decomposition of HI, photosensitized reaction, mechanism of photosensitization,
It contains the basic principle of Mossbauer Spectroscopy.
Recoil energy, Dopler shift.
The instrumentation of Mossbauer Spectroscopy.
Hyperfine interactions.
NQR - DEFINITION - ELECTRIC FIELD GRADIENT - NUCLEAR QUADRUPOLE MOMENT - NUCLEAR QUADRUPOLE COUPLING CONSTANT - PRINCIPLE OF NQR - ENERGY OF INTERACTION - SELECTION RULE - FREQUENCY OF TRANSITION - APPLICATIONS
Introductory PPT on Metal Carbonyls having its' classification,structure and applications.This is a basic level PPT specially prepared for UG/PG Chemistry students.
Crown ethers
NOMENCLATURE
GENERAL SYNTHESIS OF CROWN ETHER
AZA CROWN
CRYPTAND
APPLICATIONS
1. SYNTHETIC APPLICTION
Esterification
Saponification
Anhydride formation
Potassium permanganate oxidation
Aromatic substitution reactions
Elimination reactions
Displacement reaction
Generation of carbenes
Superoxide anion
Alkylations – 1. o-alkylations
2. c-alkylations
3. n-alkylations
2. ANALYTICAL APPLICATION
Determination of gold in geological samples
Super critical fluid extraction of trace metal from solid and liquid materials
Application of ionic liquids in analytical chemistry
Oxidation and determination of aldehydes
Crown ethers are used in the laboratory as phase transfer catalyst
OTHER APPLICATION
It is used in photocynation
Resolution of racemic mixture
Benzoin condensation
Hetrocyclisation
Synthesis of furanones
Acetylation of secondary amines in presence of primary amine
This presentation describes about the preparation, properties, bonding modes, classification and applications of metal Dioxygen Complexes. Also explains the MO diagram of molecular oxygen.
The homolytic cleavage of covalent bonds in carbonyl compound under photochemical conditions known as Norrish Type Reactions
They are divided into two types
Norrish Type I
Norrish Type II reaction
PHOTOCHEMISTRY BASIC PRINCIPLE AND JABLONSKI DIAGRAMsuriyachem27
Photochemistry is the branch of chemistry in which study of chemical reactions take place by
the absorption of electromagnetic radiation or by molecules absorb light radiation
(electromagnetic radiation) particularly the visible (wavelength from (400-750) and ultra violet region (wavelength from 100-400nm ), the molecules generally get activated due to
electronic excitation. When electromagnetic radiation is absorb in the ultraviolet/visible
region the molecules get excited to higher electronic state. This involves the promotion of an
electron from bonding molecular orbital to antibonding molecular orbital. According to the quantum theory, both matter and light are quantised, and only certain
specific energies of light are absorbed by specific organic molecule for its excitation. The
absorption or emission of light occurs by the transfer of energy as photons. All photochemical and photo physical processes are initiated by the absorption of a photon of visible or ultraviolet radiation leading to the formation of an electronically - excited state. The molar absorptivity (formerly called the extinction coefficient) of a compound constant
that is characteristic of the compound at a particular wavelength. The Jablonski diagram is a pictorial illustrated of different energy states which are absorbed by molecules. This
partial energy diagram represents the energy of a photo luminescent molecule in its different energy states.The life time of singlet excited state S1 is long hence in this state has done many physical and chemical processes. Molecules returns to its ground state, S0 from excited singlet S1,/ S2 state by release energy as heat, but this is generally quite slow because the amount of energy is large between S0 and S1. This process is called internal conversion. When molecules return to its ground state S0 from excited state S1,/ S2 by giving off
energy in the light form within 10-9 seconds. This process is known as Fluorescence.
This pathway is not very common because it is relatively slow. For smaller, diatomic
and rigid molecules (mainly aromatic compounds) show fluorescence. This is because
emitted fluorescent light is of lower energy than absorbance light. Most molecules in the S1 state may drop to triplet state (T1) (S1→T1). This is
energetically slow process. However, if the singlet state S1 is long lived, the S1 →
T1 conversion occurs by a process called intersystem crossing. It is important
phenomenon in photochemistry. For every excited singlet state there exist
corresponding triplet states. Since transition from ground state singlet (S0) to triplet
state (T1) is forbidden, intersystem crossing is the main source of excited triplet
state. This is one way of populating the triplet state. The efficiency in intersystem
crossing depends on the S1 → T1 energy gap.
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
THE PERICYCLIC REACTION THE MOST COMMON TOPIC INCLUDE THE SYLLABUS OF MANY SCIENCE STUDY INCLUDING BSC, MSC , PHARMA STUDY, AND MORE HENCE WE ARE COVERED ALL THE DATA OF IT HOPE THIS WILL MAKE READER EASY.
Photochemistry
ELECTROMAGNETIC SPECTRUM
LAW GOVERNING ABSORPTION OF LIGHT
LAW OF PHOTOCHEMISTRY
Grotthurs-Drapper law.
Einstein Stark law of photochemical equivalence
ELECTRONIC TRANSITIONS
Jablonski Diagram
QUANTUM YIELD
Use Of Photochemistry
Chemistry of vision
Photosynthesis in plant
Formation of Vitamin D
Fluorescent dyes in traffic
Photodynamic therapy
Quantum yield, experimental arrangement, reasons for high and low Quantum yield, problems, photochemical reactions, kinetics of photochemical decomposition of HI, photosensitized reaction, mechanism of photosensitization,
It contains the basic principle of Mossbauer Spectroscopy.
Recoil energy, Dopler shift.
The instrumentation of Mossbauer Spectroscopy.
Hyperfine interactions.
NQR - DEFINITION - ELECTRIC FIELD GRADIENT - NUCLEAR QUADRUPOLE MOMENT - NUCLEAR QUADRUPOLE COUPLING CONSTANT - PRINCIPLE OF NQR - ENERGY OF INTERACTION - SELECTION RULE - FREQUENCY OF TRANSITION - APPLICATIONS
Introductory PPT on Metal Carbonyls having its' classification,structure and applications.This is a basic level PPT specially prepared for UG/PG Chemistry students.
Crown ethers
NOMENCLATURE
GENERAL SYNTHESIS OF CROWN ETHER
AZA CROWN
CRYPTAND
APPLICATIONS
1. SYNTHETIC APPLICTION
Esterification
Saponification
Anhydride formation
Potassium permanganate oxidation
Aromatic substitution reactions
Elimination reactions
Displacement reaction
Generation of carbenes
Superoxide anion
Alkylations – 1. o-alkylations
2. c-alkylations
3. n-alkylations
2. ANALYTICAL APPLICATION
Determination of gold in geological samples
Super critical fluid extraction of trace metal from solid and liquid materials
Application of ionic liquids in analytical chemistry
Oxidation and determination of aldehydes
Crown ethers are used in the laboratory as phase transfer catalyst
OTHER APPLICATION
It is used in photocynation
Resolution of racemic mixture
Benzoin condensation
Hetrocyclisation
Synthesis of furanones
Acetylation of secondary amines in presence of primary amine
This presentation describes about the preparation, properties, bonding modes, classification and applications of metal Dioxygen Complexes. Also explains the MO diagram of molecular oxygen.
The homolytic cleavage of covalent bonds in carbonyl compound under photochemical conditions known as Norrish Type Reactions
They are divided into two types
Norrish Type I
Norrish Type II reaction
PHOTOCHEMISTRY BASIC PRINCIPLE AND JABLONSKI DIAGRAMsuriyachem27
Photochemistry is the branch of chemistry in which study of chemical reactions take place by
the absorption of electromagnetic radiation or by molecules absorb light radiation
(electromagnetic radiation) particularly the visible (wavelength from (400-750) and ultra violet region (wavelength from 100-400nm ), the molecules generally get activated due to
electronic excitation. When electromagnetic radiation is absorb in the ultraviolet/visible
region the molecules get excited to higher electronic state. This involves the promotion of an
electron from bonding molecular orbital to antibonding molecular orbital. According to the quantum theory, both matter and light are quantised, and only certain
specific energies of light are absorbed by specific organic molecule for its excitation. The
absorption or emission of light occurs by the transfer of energy as photons. All photochemical and photo physical processes are initiated by the absorption of a photon of visible or ultraviolet radiation leading to the formation of an electronically - excited state. The molar absorptivity (formerly called the extinction coefficient) of a compound constant
that is characteristic of the compound at a particular wavelength. The Jablonski diagram is a pictorial illustrated of different energy states which are absorbed by molecules. This
partial energy diagram represents the energy of a photo luminescent molecule in its different energy states.The life time of singlet excited state S1 is long hence in this state has done many physical and chemical processes. Molecules returns to its ground state, S0 from excited singlet S1,/ S2 state by release energy as heat, but this is generally quite slow because the amount of energy is large between S0 and S1. This process is called internal conversion. When molecules return to its ground state S0 from excited state S1,/ S2 by giving off
energy in the light form within 10-9 seconds. This process is known as Fluorescence.
This pathway is not very common because it is relatively slow. For smaller, diatomic
and rigid molecules (mainly aromatic compounds) show fluorescence. This is because
emitted fluorescent light is of lower energy than absorbance light. Most molecules in the S1 state may drop to triplet state (T1) (S1→T1). This is
energetically slow process. However, if the singlet state S1 is long lived, the S1 →
T1 conversion occurs by a process called intersystem crossing. It is important
phenomenon in photochemistry. For every excited singlet state there exist
corresponding triplet states. Since transition from ground state singlet (S0) to triplet
state (T1) is forbidden, intersystem crossing is the main source of excited triplet
state. This is one way of populating the triplet state. The efficiency in intersystem
crossing depends on the S1 → T1 energy gap.
Compilation of various notes
Define what is photosynthesis
Explain the light dependent reactions
Explain the Calvin cycle
Define what is photorespiration
Compare the photorespiration in C3, C4 and CAM plants
Photochemical Reactions M Pharm Chemistry.pptxDiwakar Mishra
Photochemical reaction is included in the syllabus Advance Organic Chrmitry M Pharm (Pharmaceutical Chemistry) which discribes thode chemical reactions which are takes place by the help of light
The Roman Empire A Historical Colossus.pdfkaushalkr1407
The Roman Empire, a vast and enduring power, stands as one of history's most remarkable civilizations, leaving an indelible imprint on the world. It emerged from the Roman Republic, transitioning into an imperial powerhouse under the leadership of Augustus Caesar in 27 BCE. This transformation marked the beginning of an era defined by unprecedented territorial expansion, architectural marvels, and profound cultural influence.
The empire's roots lie in the city of Rome, founded, according to legend, by Romulus in 753 BCE. Over centuries, Rome evolved from a small settlement to a formidable republic, characterized by a complex political system with elected officials and checks on power. However, internal strife, class conflicts, and military ambitions paved the way for the end of the Republic. Julius Caesar’s dictatorship and subsequent assassination in 44 BCE created a power vacuum, leading to a civil war. Octavian, later Augustus, emerged victorious, heralding the Roman Empire’s birth.
Under Augustus, the empire experienced the Pax Romana, a 200-year period of relative peace and stability. Augustus reformed the military, established efficient administrative systems, and initiated grand construction projects. The empire's borders expanded, encompassing territories from Britain to Egypt and from Spain to the Euphrates. Roman legions, renowned for their discipline and engineering prowess, secured and maintained these vast territories, building roads, fortifications, and cities that facilitated control and integration.
The Roman Empire’s society was hierarchical, with a rigid class system. At the top were the patricians, wealthy elites who held significant political power. Below them were the plebeians, free citizens with limited political influence, and the vast numbers of slaves who formed the backbone of the economy. The family unit was central, governed by the paterfamilias, the male head who held absolute authority.
Culturally, the Romans were eclectic, absorbing and adapting elements from the civilizations they encountered, particularly the Greeks. Roman art, literature, and philosophy reflected this synthesis, creating a rich cultural tapestry. Latin, the Roman language, became the lingua franca of the Western world, influencing numerous modern languages.
Roman architecture and engineering achievements were monumental. They perfected the arch, vault, and dome, constructing enduring structures like the Colosseum, Pantheon, and aqueducts. These engineering marvels not only showcased Roman ingenuity but also served practical purposes, from public entertainment to water supply.
2024.06.01 Introducing a competency framework for languag learning materials ...Sandy Millin
http://sandymillin.wordpress.com/iateflwebinar2024
Published classroom materials form the basis of syllabuses, drive teacher professional development, and have a potentially huge influence on learners, teachers and education systems. All teachers also create their own materials, whether a few sentences on a blackboard, a highly-structured fully-realised online course, or anything in between. Despite this, the knowledge and skills needed to create effective language learning materials are rarely part of teacher training, and are mostly learnt by trial and error.
Knowledge and skills frameworks, generally called competency frameworks, for ELT teachers, trainers and managers have existed for a few years now. However, until I created one for my MA dissertation, there wasn’t one drawing together what we need to know and do to be able to effectively produce language learning materials.
This webinar will introduce you to my framework, highlighting the key competencies I identified from my research. It will also show how anybody involved in language teaching (any language, not just English!), teacher training, managing schools or developing language learning materials can benefit from using the framework.
Students, digital devices and success - Andreas Schleicher - 27 May 2024..pptxEduSkills OECD
Andreas Schleicher presents at the OECD webinar ‘Digital devices in schools: detrimental distraction or secret to success?’ on 27 May 2024. The presentation was based on findings from PISA 2022 results and the webinar helped launch the PISA in Focus ‘Managing screen time: How to protect and equip students against distraction’ https://www.oecd-ilibrary.org/education/managing-screen-time_7c225af4-en and the OECD Education Policy Perspective ‘Students, digital devices and success’ can be found here - https://oe.cd/il/5yV
Unit 8 - Information and Communication Technology (Paper I).pdfThiyagu K
This slides describes the basic concepts of ICT, basics of Email, Emerging Technology and Digital Initiatives in Education. This presentations aligns with the UGC Paper I syllabus.
This is a presentation by Dada Robert in a Your Skill Boost masterclass organised by the Excellence Foundation for South Sudan (EFSS) on Saturday, the 25th and Sunday, the 26th of May 2024.
He discussed the concept of quality improvement, emphasizing its applicability to various aspects of life, including personal, project, and program improvements. He defined quality as doing the right thing at the right time in the right way to achieve the best possible results and discussed the concept of the "gap" between what we know and what we do, and how this gap represents the areas we need to improve. He explained the scientific approach to quality improvement, which involves systematic performance analysis, testing and learning, and implementing change ideas. He also highlighted the importance of client focus and a team approach to quality improvement.
Synthetic Fiber Construction in lab .pptxPavel ( NSTU)
Synthetic fiber production is a fascinating and complex field that blends chemistry, engineering, and environmental science. By understanding these aspects, students can gain a comprehensive view of synthetic fiber production, its impact on society and the environment, and the potential for future innovations. Synthetic fibers play a crucial role in modern society, impacting various aspects of daily life, industry, and the environment. ynthetic fibers are integral to modern life, offering a range of benefits from cost-effectiveness and versatility to innovative applications and performance characteristics. While they pose environmental challenges, ongoing research and development aim to create more sustainable and eco-friendly alternatives. Understanding the importance of synthetic fibers helps in appreciating their role in the economy, industry, and daily life, while also emphasizing the need for sustainable practices and innovation.
Ethnobotany and Ethnopharmacology:
Ethnobotany in herbal drug evaluation,
Impact of Ethnobotany in traditional medicine,
New development in herbals,
Bio-prospecting tools for drug discovery,
Role of Ethnopharmacology in drug evaluation,
Reverse Pharmacology.
The French Revolution, which began in 1789, was a period of radical social and political upheaval in France. It marked the decline of absolute monarchies, the rise of secular and democratic republics, and the eventual rise of Napoleon Bonaparte. This revolutionary period is crucial in understanding the transition from feudalism to modernity in Europe.
For more information, visit-www.vavaclasses.com
1. Photochemistry
Photochemistry is the study of the interaction of
electromagnetic radiation with matter resulting into a
physical change or into a chemical reaction.
Photochemistry is the branch of chemistry that deals with the
chemical processes that are caused
by the absorption of light energy.
2. Photolysis
The process by which a photochemical reaction is
carried out is called photolysis.
Photolysis is usually initiated by infrared, visible, or
ultraviolet light.
3. Photochemical reactions
In nature, there are a number of light excited
photochemical reactions such as:
Photosynthesis involves the absorption of light by the
chlorophyll in plants to produce carbohydrates from
carbon dioxide and water.
Photography uses the action of light on grains of silver
chloride or silver bromide to produce an image.
Ozone formation in the upper atmosphere results
from action of light on oxygen molecules.
Solar cells, which are used to power satellites and
space vehicles, convert light energy from the sun to
chemical energy and then release that energy in the
form of electrical energy.
4. The Photochemical Process
Photochemica process, occur when an atom or molecule
absorb a quantum of light energy from a photon.
If a quantum of visible or ultraviolet light is absorbed,
then an electron in a low energy state is excited into a
higher energy state.
If infrared radiation is absorbed by a molecule, then the
excitation energy affects the motions of the nuclei in
the molecule.
M +light ---------M*
5. Photochmical process
Two types
Primary process such as:
• Isomerization
• rearrangement,
• dissociation or
• Photo-ionization etc
Secondary process occur after the
primary
7. 1. The highly energized or excited molecule may return to
its initial state on releasing its excitation energy by
emitting luminescent radiation.
If the process is quick, the process is known as
fluorescence and if it is delayed it is known as
phosphorescence.
8. It may transfer its energy to some other molecule, C, with which it
collides, without emitting light. The latter energy transfer process
results in a normal molecule, M, and an excited molecule, C*. This
process called quenching
M* -------------- M + C*
As a result of the initial light absorption step, an electron (e−) in
the atom or molecule may absorb so much energy that it may
escape from the atom or molecule, leaving behind the positive M+
ion. This process is called photoionization
9. If the excited M* molecule does react, then it may undergo
any of the following chemical processes:
Photodissociation or
Rearrangement or photoisomerization
React with another molecule C
Photodissociation may result when the excited molecule
breaks apart into atomic and/or molecular fragments A and B.
The excited state species may fragment to a pair of radicals or, in the
case of nitrogen dioxide to nitric oxide and oxene.
10. A rearrangement (or photoisomerization) reaction involves the
conversion of molecule M into its isomer N
The conversion of trans-1,2-dichloroethylene into the cis
isomer is an example of intramolecular rearrangement. The
reaction is shown below:
In the trans isomer the chlorine atoms lie on opposite sides of the
double bond, whereas in the cis isomer they are on the same side
of the double bond.
11. Secondary Photochemical Processes
Secondary processes may occur upon
completion of the primary step. Several
examples of such
processes includes Formation of Ozone,
Chain Reactions etc
12. Formation of Ozone
Ozone (O3) is formed in the upper atmosphere from
ordinary oxygen (O2) gas molecules according to the
reaction:
step 1. A quantum of ultraviolet light is absorbed
step 2. The excited oxygen molecule dissociates into two oxygen
atoms.
step 3 An oxygen atom then reacts with O2 to form ozone.
13. Destruction of Ozone in the Upper Stratosphere
Certain chlorofluoromethanes, such as CCl3F and
CCl2F2, are used as refrigerants. These compounds
eventually diffuse into the stratosphere, where the
molecules undergo photodissociation to produce
chlorine (Cl) atoms, which then react with ozone
molecules according to the reaction below
Cl + O3 → ClO + O2
This decrease in the ozone content of the upper
atmosphere allows more ultraviolet radiation to reach
the surface of the earth.
14. Chain Reactions
If the primary photochemical process involves the
dissociation of a molecule into radicals (unstable
fragments of molecules), then the secondary process
may involve a chain reaction.
A chain reaction is a cyclic process whereby a reactive
radical attacks a molecule to produce another unstable
radical.
This new radical can now attack another molecule,
thereby reforming the original radical, which can now
begin a new cycle.
15. According to the above mechanism,
a suitable quantum of light dissociates a chlorine molecule
into atoms (step 1).
The reactive Cl atom attacks a hydrogen molecule to yield
hydrogen chloride and a hydrogen atom (step 2).
The reactive hydrogen atom attacks a chlorine molecule,
which regenerates the Cl atom (step 3).
This chlorine atom can then react with another H2
molecule according to step 2, beginning a new cycle of steps.
16. Like in all chemical operations there are risks in photochemistry.
Irradiation. Low-pressure mercury lamps have their main output
at 254 nm. This light severely damages cells, eyes and skin.
Shield reactors; turn lamps off before checking the reaction.
Never look into the beam of a high power LED; the lights very
high intensity damage your eyes.
Ozone generation: Short wavelength light may generate ozone
from oxygen. Perform reactions always in a well ventilated fume
hood.
Lamps: Most lamps operate at high temperature and at high vapor
pressure. Never move or touch lamps during operation. Never
switch of the cooling right after switching of the lamp.
Hazards of photochemistry
17. Cyclisation reactions
Pericyclic or cyclization reactions ; “Any concerted reaction in which bonds
are formed or broken in a cyclic transitions state”.
i.e. there is a single transition state from start to finish, in contrast to a
stepwise reaction.
Transition state
reaction co-ordinate
Energy
starting
material
product
Concerted reaction
Transition states
reaction co-ordinate
Energy
starting
material product
Multistep reaction
inter-
mediate inter-
mediate
18. Types of Pericyclic or cyclization reactions
electrocyclic reaction
Cycloaddition reaciton
Sigmatropic rearrangement
19. An intramolecular reaction in which a new s bond is
formed between the ends of a conjugated p system
called electrocyclic reaction
Electrocyclic reaction
21. Two different p bond-containing molecules react
to form a cyclic compound called cycloaddition
reaction
Cycloaddition reaction
22. A [2 + 2] Cycloaddition Reaction
• Cycloaddition reactions – Two linear conjugated polyenes converted
onto a cyclic product in one step.
• The formation of cyclobutane rings from alkene components is the
most common photchemical cyclo addition reactions
25. A sigmatropic is a reaction in which a s bond is broken in
the reactant and a new s bond is formed in the product,
and the p bonds rearrange called sigmatropic
rearrangement
Sigmatropic rearrangement
26.
27. Photochemical Reactions of Carbonyl Compounds
The absorption properties of ketones and aldehyds are
convenient for irradiation around 300 nm.
The n → π* excited states of carbonyl compounds display a
rich chemistry in their own right. Since the oxygen has an
unpaired electron, it behaves in much the same way as an
alkoxy radical.
28. Intramolecular reactions of carbonyl Compounds
Following are the typical reaction of carbonyl compounds
Type I Cleavage (α- cleavage)
Hydrogen Abstraction Reaction
Type II Cleavage(β-Cleavage)
Cyclobutanol formation (Yang reaction)
α-β unsaturated ketone
Oxetene Formation (Paterno-Buchi Reaction)
29. Type I Cleavage (α- cleavage)
This reaction type dominates gas phase photochemistry of
many aldehydes and ketones.
30.
31. Hydrogen Abstraction Reaction
Beside carbon monoxide extrusion acyl radicals formed
in a α-cleavage can be stabilized by subsequent
hydrogen migration.
32. Type II Cleavage(β-Cleavage)
Beside carbon monoxide extrusion acyl radicals formed in
a α-cleavage can be stabilized by subsequent hydrogen
migration.
33. Cyclobutanol formation (Yang reaction)
With an appropriate alignment of C=O and C-H
groups and no secondary transformation prevents
cyclization of the 1,4-diradical leads to cyclobutanols.
35. Oxetene Formation (Paterno-Buchi Reaction)
The photochemical [2+2] cycloaddition of an alkene and a
carbonyl group which results the formation of oxeten called
Paternó Büchi reaction. Inter and intramolecular examples
are known.
38. The cis-trans (Z-E) isomerization in alkene
E,Z-Isomerizations of 1,2-disubstituted alkenes are well
documented. A typical example is the photoisomerization
of stilbene, where because of different absorption spectra
the Z-isomere can be enriched in the photostationary
state.
A more complex example is the sensitized preparation of
enantiomerically enriched trans-cyclooctene.
39. It was the first theory to explain the outcome of several pericyclic
reactions. The method is based on the ‘principle of orbital symmetry
conservation’ throughout the reaction.
These rules states that, A pericyclic reaction can take place only if
the symmetries of molecular orbital's of the reactant are the same as
the symmetries of the molecular orbital's of the product.
If the symmetries of both reactant and product orbitals match up,
or “correlate”, then the reaction is said to be symmetry-allowed.
If the symmetries of reactant and product orbitals don’t correlate,
then the reaction is symmetry-disallowed.
Woodward-Hoffmann rules
41. Conservation of orbital symmetry and woodward-
hoffman rules for electrcyclic reactions
The Woodward–Hoffmann rules were developed to explain the
electrocyclic ring-opening and ring-closing reactions of the open
chain conjugated polyenes either by application of heat or application
of light.
According to Woodward and Hoffmann, overlaping or constructive
interaction between two terminal p- orbital will developed only if
both are in same phase or same symmetry.
In order to complete bonding, the terminal p-orbitals may rotate in
the same (conrotation) or opposite directions (disrotation) as shown
below:
42. For development of new σ bonding interactions in the
transition state, the termini must move in conrotatory and
disrotatory fashion in 4π- and 6π-electron systems,
respectively, as shown below.
43. Now, let us look at the symmetry observed for some open-
chain conjugated systems. A particular molecular orbital can
have two possible symmetry elements―a two-fold axis of
symmetry (C2) and a mirror plane of symmetry (σ).
44. Now Let us consider the electrocyclic ring closure of 1,3-
butadiene. In this case the reaction may proceed via either
conrotatory or disrotatory pathway. In the conrotatory mode, the
transition state is associated with a C2 axis of symmetry, while
mirror symmetry is associated with the transition state (TS) in the
disrotatory mode.
45. The molecular orbitals of the reactants and products are correlated in
terms of being symmetric or antisymmetric.
On applying the symmetry element on a MO. The MO is said to be
symmetric if it give rise to same MO in the product.
If the symmetry operation leads to a different MO, then the MO is
said to be antisymmetric. For example, Ψ2 of 1,3-butadiene is
symmetric with respect to 180° rotation about the C2 axis and
antisymmetric with respect to reflection in the mirror plane (σ).
46. Woodward realized that the symmetry elements of all orbitals were
preserved in the MOs of the product. Thus, the symmetry of the
transition state (TS) also must likewise be conserved.
If a symmetry element is present throughout the course of the
reaction, then it is considered as a conserved symmetry element.
Similarly, antisymmetry is said to be conserved, if it is maintained
throughout the course of the reaction, that is, in the reactant,
transition state and the product.
For instance, in the HOMO of a 4π-conjugated system, C2 axis of
symmetry is conserved in reactant, TS and product as:
Conserved symmetry element
47. Now, let us examine the correlation diagram for photochemical
ring-closure of 1,3-butadiene. In this case, one of the electrons in
reactant is excited from Ψ2 to Ψ3. As a result, the electrons in
reactant are now filled in three orbitals -Ψ1, Ψ2 and Ψ3―which are
represented as Ψ1
2Ψ2
1 Ψ3
1. The overall symmetry corresponds to
S2A1S1
The latter matches with S2S1A1―the overall symmetry of product
MOs, namely,σ,π and π*corresponding to σ2π1π*1 The σ-bond is
formed by the overlap of the terminal p-orbitals of Ψ2. This is
possible by disrotatory closure which leads to bonding interactions
between the two p-orbitals under consideration. Thus, the
disrotatory closure of 4π-conjugated substrates occurs
photochemically as can be understood from the figure shown below.
48. Correlation diagram for 4-electron conrotatory and disrotatory closure under
photochemical Conditions
49. The orbital correlation diagrams for cycloaddition reactions can be drawn
as discussed for electrocyclic reactions. To construct the correlation
diagrams for cycloadditions, certain rules have been put forward by
Woodward and Hoffmann, which are as follows:
1. Define geometry for the proposed interaction.
2. Draw MOs of the reactants and the products in the order of increasing
energy.
3. Define each MO as being either symmetric or antisymmetric with
respect to the symmetry elements.
4. Populate the MOs with electrons, beginning with the ground state MO.
5. Correlate reactant MOs with those of the product MOs of like
symmetry. If all the bonding MOs of the reactant correlate with bonding
MOs of the product, the reaction is said to be ‘allowed’. If a bonding MO
in the reactant becomes an antibonding MO in the product, the reaction
then is said to be ‘forbidden’.
50. [2+2] Cycloaddition is the simplest case, and let’s consider
its occurrence under thermal conditions to begin with. There
are 4 possible ways in which the two components may in
principle react to give the cycloadduct. They are: [π2s+π2s],
[π2a+π2s], [π2s+π2a], [π2a+π2a]. However, the geometric
constraints preclude any antarafacial addition between the
two components, thereby limiting the approach of the two
components to suprafacial mode. Let’s analyze the
symmetry elements present in the molecular orbitals of the
two reactants:
51. Let us look at the photochemical [2+2] cycloaddition. In
this case, one electron is promoted from π2 to π3* orbital,
as a result of which the orbital picture becomes completely
different. Now, the π1, π2 and π3* orbitals of the reactants
are related by symmetry to σ1, σ2 and σ3* orbitals of the
cyclized product. As a result, the energy barrier is drastically
reduced and the reaction now becomes allowed with the
formation of product in the excited state. Therefore, [2+2]
cycloaddition can be said to be photochemically allowed.