Photochemical reactions

PHOTOCHEMICALREACTIONS

1. PHOTOCHEMICAL REACTIONS K. N eha

2. Introduction Photochemistry & its law Features of Photochemical reactions Basic principles of Photochemical reactions Types of photochemical reactions Photo-oxidation Photo-addition Photo-fragmentation References Contents

3. Photochemistry is the study of chemical reactions resulting from the exposure of light radiations . Light supplies the required energy to take place the photochemical reactions. The visible and UV radiations (2000-8000Ǻ wavelength) are mainly used in photochemical reactions . Thermochemical reactions (dark reactions) are brought about by molecular collisions . These reactions are spontaneous and are accompanied by a decrease in free energy. But certain photochemical reactions are accompanied by an increase in free energy. Introduction

4. Photochemical reaction , a chemical reaction initiated by the absorption of energy in the form of light. The consequence of molecules' absorbing light is the creation of transient excited states whose chemical and physical properties differ greatly from the original molecules . When a light composing number of colours is used, the photochemical reaction may not be initiated by all colours. Contd...

5. In  Environmental chemistry  , photochemical reactions are of considerable importance to the trace chemistry of the  atmosphere  . The Los Angeles  photochemical smog  is an example of a system of photochemical reactions. Perhaps one of the most important reactions in this system is the photochemically driven conversion of  nitrogen  dioxide, a major component of  automobile  exhaust. Photochemical reactions are also important in natural waters. Here they may be responsible for enhancing the reaction rate of organic compounds or changing the oxidation state of metallic ions in solution. Photochemical reactions also occur in solids. Since solids often lack the transparency required for the light to penetrate the surface, reactions are usually limited to the surface, where incoming photons initiate reactions in the top-most molecules. Contd...

6. Grothurs-Draper Law : The first law of Photochemistry ,known as the Grothurs-Draper law, states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place. This law was modified by Stark & Einstein, states that for each photon of light absorbed by a chemical system, only one molecule is activated for photochemical reaction. This is also known as the photo equivalence law (only one excited molecule per photon absorbed). Basic laws of photochemistry

7. Beer-Lambert Law : The Beer-lambert law is the linear relationship between absorbance & concentration of an absorber of electromagnetic radiation. Lamberts observed that the absorption of light of a single wavelength (monochromatic radiation) passing through homogeneous solutions of equal concentration is proportional to the pathlength . A=k . l Beer’s law states that the absorption of a single wavelength (monochromatic radiation) passing through homogeneous solution of equal pathlength is proportional to the concentration . A=k .c Beer-Lambert law is commonly written as : A= ε .c. l Contd…

8. Photochemical reactions are driven by light or near-visible electromagnetic radiation. In general, incoming units of energy, known as photons, excite effected molecules, raising their energy to a point where they can undergo reactions that would normally be exceedingly difficult. The process is distinguished from thermal reactions, which take place with molecules in their normal energy states. Under sunlight conditions, photochemical processes can generate small amounts of extremely reactive molecules which initiate important chemical reaction sequences. To initiate a photochemical reaction, two requirements need to be met. First, the photon must have enough energy to initiate the photochemical reaction in the molecule . Second, the compound must be colored, in order to be able to react with visible or near-visible photon radiation . Photochemical reaction

9. The characteristics of photochemical reactions are listed below: Each molecule taking part in a photo process absorbs only one photon of radiant energy . Photo processing does not occur in the dark . Each photochemical reaction requires a definite amount of energy which is characteristic of a particular wavelength of the photon. The rate of photochemical reaction depends upon the intensity of radiation absorbed . The ΔG values for light initiated reactions may or may not be negative . The temperature does not have marked effect on the rate of light initiated reactions. Features of Photochemical reactions

10. 1.   Linear addition to an unsaturated molecule , e.g., the pyrimidine base, thymne, in DNA can combine with the amino acid residue, cysteine, in proteins. This is a model for the photochemical crosslinking of DNA and proteins by UV radiation.   2.  Cycloaddition of unsaturated molecules , e.g., two thymines can react to form a ring product, the thymine dimer, an important class of products formed in DNA by UV radiation. Types of Photochemical reactions

11. 3.  Photofragmentation , e.g., the side chain of riboflavin can split off to form lumiflavin . 4.  Photooxidation , e.g., the ring structure of cholesterol can add a peroxy group. 

12. 5.  Photohydration , e.g., uracil can add a molecule of water to it 5-6 double bond when UV irradiated. 6.  Cis-Trans Isomerization , e.g., all- trans  retinal can be converted to 11- cis  retinal.

13. 7.   Photorearrangement , e.g., 7-dehydrocholesterol can be converted to vitamin D 3 . 8.  Energy Transfer , e.g., all photosensitized reactions.

14. Photochemical reactions Thermochemical reactions These involve the absorption of light These involve either absorption or evolution of heat Take place in presence of light Take place in dark or in presence of light They are independent of temperature They are dependent of temperature Rate of reactions is dependent on the intensity of the light absorbed Rate of reactions is not affected by the intensity of light The free energy change is negative or positive The free energy change is always negative Differences between photochemical and thermal reactions:

15. Photosensitizations and Quenching : In some photochemical reactions, the reactant molecules do not absorb radiation and no chemical reaction occurs. However, if a suitable foreign substance (called sensitizer), which absorbs radiation, is added to the reactant, the reaction takes place. The sensitizer gets excited during absorption of radiation and transfers its energy to the reactants and initiates the reaction. Photosensitization : The foreign substance absorbs the radiation and transfers the absorbed energy to the reactants is called a photosensitizer. This process is called photosensitized reaction (or) photosensitization. Examples, Atomic photosensitizers : mercury, cadmium, zinc & Molecular photosensitizers : benzophenone, sulphur dioxide. Quenching : When the excited foreign substance collides with another substance it gets converted into some other product due to the transfer of its energy to the colliding substance. This process is known as quenching. ENERGY TRANSFER IN PHOTOCHEMICAL REACTIONS:

16. Photo-oxidation Incorporation of oxygen in the presence of UV light is known as Photochemical oxidation. In other word Oxidation caused by action of light. There are two mechanisms by which oxygen is incorporated in Photochemical oxidation reactions. The first is Backstrom mechanism which is also known as Photo-sensitisation oxidation mechanism. Its main characteristic is abstraction of hydrogen by the sensitizer in its excited triplet state followed by addition of oxygen to the newly created radical. This type of photooxygenation is known as type I photooxygenation. The second mechanism known as Photosensitised oxygen transfer involves the direct combination of the substrate with oxygen. There are two proposals regarding the state of oxygen involved. This type of photooxygenation is known as type II photooxygenation .

17. Alkenes, dienes and polyenes are attacked by singlet oxygen. The singlet oxygen may be generated by thermal methods, by excitation of ground state oxygen in a microwave discharge or by the use of visible radiation and photochemical sensitizer such as methylene blue, rose bengal, chlorophyll, riboflavin, fluorescein or halo fluorescein. Acyclic or cyclic conjugated dienes gives 1,4-cycloaddition reaction with singlet oxygen to form six membered cyclic adduct , i.e. cyclic peroxide. Photo-oxidation of alkenes and polyenes

18. The formation of hydro peroxides in photosensitised oxygen-transfer reactions that follow the Schenck type of mechanism occurs only when hydrogen is present on allylic carbon. The reaction has, moreover, some definite steric and electronic requirements. Oxygen always becomes attached to one of the double bonded, which then shifted into the allylic position. The reaction is like ene reaction. Contd…

19. The above reactions do not proceed through free radicals. The oxidation with singlet oxygen occurs by concerted mechanism like the ene reaction. Some substituted alkenes behave differently with singlet oxygen and form a dioxetane in a cycloaddition reaction. Some dioxetanes are stable but others readily decompose to two carbonyl compounds. Electron rich alkenes undergoes this reaction and reaction is stereospecific. Contd…

20. Contd… Mechanism of the reaction is as follows:- The oxidative cyclisation of conjugated trienes to form aromatic system is one of the more extensively studied photochemical oxidations. The conversion of cis-stilbene to phenanthrene is a main example of such a ring closure. The reaction takes place in the presence of hydrogen acceptors.

21. A photochemical reaction in which part of a compound is added to one end of a double or triple bond, while the rest adds to the other end, converting it, respectively, to a single or double bond. Photo-addition Reaction Example: Addition reaction

22. Photo-addition Reaction Example: Mechanism: Cyclo-addition reaction

23. 1, 2-photoaddition of benzenes Example: 2 Example: 1

24. Example: 3 Example: 4 1, 2-photoaddition of benzenes

25. 1, 3-photoaddition of benzenes Example: 1 Example: 2

26. 1, 4-photoaddition of benzenes Example: 1 Example: 3 Example: 2

27. Example: 4 Example: 5 1, 4-photoaddition of benzenes

28. This reaction is a [4+2] cycloaddition reaction. Addition of oxygen

29. Biologically active cardenolide ouabagenin Photochemical reaction of an ion that results in two or more fragments of which at least one is an ion. PHOTO-FRAGMENTATION:- Example:

30. Photo-fragmentation step by step Step: 1 Step: 2 At temperature much above room temperature the acetyl radical breaks down to give another methyl radical & carbon monoxide.

31. Some examples of drugs that undergoes photochemical reaction

32. Anti-diabetic agent found from Azadirachta indica (Neem).

33. Santonin in presence of dioxane gives Luminosantonin Steroid also gives the result in the presence of acidic medium

34. Vitamin D

35. Some other example of photochemical reactions: Photochemical reaction is very common in our daily life like below: Photosynthesis, in which most plants use solar energy to convert carbon dioxide & water into glucose, disposing of oxygen as a side-product. Human rely on photochemistry for the formation of vitamin D. Vision is initiated by a photochemical reaction of rhodopsin. In fireflies, an enzyme in the abdomen catalyzes a reaction that results in bioluminescence. In organic reactions are electrocyclic reactions, photoisomerization & Norrish reactions. Many polymerizations are stated by photoinitiator, which decompose upon absorbing light to produce the free radicals for Radical polymerization. In photoresist technology, used in the production of microelectronic components.

36. Photochemistry is attaining an increasing importance in health science, curing some forms of cancers through phototherapy, repairing tissues and performing microsurgery using lasers.  Other important applications of photochemistry include the use of photolithography to manufacture computer chips and photo polymerization to produce protective coatings for a variety of high value materials such as optical fibers.  One of the "Holy Grails" of photochemistry is the discovery of practical ways to convert sunlight into high grade fuels to replace fossil fuels.  Photo physics especially the use of fluorescence as a sensor is currently of enormous importance for applications in the materials sciences and in biological sciences. Applications

37. Conclusion Unlike thermal reaction ,in photochemical reaction the absorption of the light raises an individual molecule to an excited electronic state. Selective excitation of individual molecules is a special character of photochemical reactions. The chemistry of excited molecules (in photochemical reactions) is quite different from the chemistry of the molecules in the ground state(in thermal reactions). The total energy supplied in photochemical reaction is much lower than thermal reactions. Photochemical reactions is used for highly strained & thermodynamically unstable compounds. Photochemical reactions are highly stereospecific . The product obtained by thermal & photochemical process normally differ in stereochemistry.

38. Singh Jagdamba, Singh Jaya, Photochemistry and pericyclic reactions, Revised second edition, The New Age International Publishers, page no: 1-130,141-226 . Ahluwalia.V.K, Parashar.P.K, Organic Reaction Mechanism , Third Edition, Narosa, Page no:632-660. Wikipedia Google Dictionary.com References