Photochemical reactions | Advanced organic chemistry II

Photochemical Reactions

1. Photochemical reactions Mayur m. karanjekar ( M.Pharm 1 st year 2 nd semester)

2. content Photochemical Reaction 2 Introduction Photochemistry &its law Features of photochemical reactions Basic principle of photochemical reactions Types of photochemical reactions Photo-oxidation Photo-addition Photo-fragmentation Applications

3. Introduction Photochemistry 3 Photochemistry- It is the study of chemical reactions resulting from exposure of light radiations. Light supplies the required energy to take place the photochemical reaction. The visible and UV radiations 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. 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. 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.

4. Basics laws of photochemistry photochemistry 4 The First Law of Photochemistry states that light must be absorbed for photochemistry to occur. This is a simple concept, but it is the basis for performing photochemical and photobiological experiments correctly. If light of a particular wavelength is not absorbed by a system, no photochemistry will occur, and no photobiological effects will be observed, no matter how long one irradiates with that wavelength of light. The Second Law of Photochemistry states that for each photon of light absorbed by a chemical system, only one molecule is activated for a photochemical reaction. This law is true for ordinary light intensities, however, with high-powered lasers, two-photon reactions can occur, i.e., the molecule is raised to a higher energy state than produced by single-photon absorption. The Bunsen-Roscoe Law of Reciprocity states that a photochemical effect is directly proportional to the total energy dose, irrespective of the time required to deliver the dose. This law is true for chemicals in a test tube, but the response of cells to radiation usually involves a sequence of interacting biological reactions, making a linear "dose x time" relationship highly unlikely. There is no reciprocity when damage is produced, e.g., DNA damage, but there can be reciprocity over a narrow range of doses for photoreceptors that trigger a response, such as phytochrome Grotthus -Draper Law (or) The Principle of Photochemical Activation: Grotthus -Draper law states that only the light which is absorbed by a substance can bring about a photochemical change. However, the absorbed radiation does not necessarily cause a chemical reaction. When the conditions are not favorable for the molecules to react, the light energy may be reemitted as heat or light or it remains unused.

5. Basics laws of photochemistry photochemistry 5 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 x 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 x c Beer-Lambert law is commonly written as: A= E c x l

6. Photochemical Reactions 6 Photochemical reaction 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 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. The characteristics/features 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 (delta)G 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.

7. Types 0f Photochemical Reactions 7 Photochemical reaction Linear addition to an unsaturated molecule - The Pyrimidine base, thiamine in DNA can combine with the amino acid residue, cystine in proteins. This is a model for photochemical crosslinking of DNA and proteins by UV radiations. Cycloaddition of unsaturated molecules – Two thymine can react to form a ring product, the thymine dimer, an important class of product formed in DNA by UV radiation. Photofragmentation – The side chain of riboflavin can split off to form lumiflavin.

8. Types 0f Photochemical Reactions 8 Photochemical reaction 4. Photo-oxidation – The ring structure of cholesterol can add a peroxy group. 5. Photohydration- Uracil can add a molecule of water to it 5-6 double bond when UV irradiated. 6. Cis- Trans Isomerization- All trans retinal are converted to 11 cis retinal.

9. Types 0f Photochemical Reactions 9 Photochemical reaction 7. Photorearrangement – 7- dehydrocholesterol can be converted to vitamin D3. 8. Energy Transfer – It include all photosensitized reactions.

10. Photo- oxidation 10 Photochemical reaction 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- sensitization 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 Photosensitized 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. Photo- oxidation of alkenes and polyenes – 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.

11. Photo- oxidation 11 Photochemical reaction The formation of hydro peroxides in photosensitized 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. The above reactions do not proceed through free radicals. The oxidation with singlet oxygen occurs by concerted mechanism like the ene reaction.

12. Photo- oxidation 12 Photochemical reaction 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. Mechanism of this reaction is as follows-

13. Photo- addition 13 Photochemical reaction 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. Mechanism - 1,2- Photoaddition of benzenes –

14. Photo- addition 14 Photochemical reaction Addition of Oxygen This reaction is a [4+2] cycloaddition reaction.

15. Photo-Fragmentation 15 Photochemical reaction Photochemical reaction of an ion that results into two or more fragments of which at least one is anion. Example: Biologically active cardenolide ouabagenin

16. Photo-Fragmentation 16 Photochemical reaction Step-wise Photo- fragmentation: Step 1- Step 2 – At Temperature above room temperature the acetyl radical breaks down to give another methyl radical and carbon monoxide.

17. Photo-Fragmentation 17 Photochemical reaction Examples – Santonin in presence of dioxane gives luminosantonin . Steroids also gives the result in presence of acidic medium.

18. Photo-Fragmentation 18 Photochemical reaction Some others examples - 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. Recommended 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 photo initiator, which decompose upon absorbing light to produce the free radicals for Radical polymerization. In photoresist technology, used in the production of microelectronic components.

19. applications 19 Photochemical reaction Photochemistry is attaining an increase in importance in health science, curing certain cancers through phototherapy, repairing tissues and performing microsurgery using lasers. Other important application include the use of photolithography to manufacture chips and photo polymerization to produce protective coatings for a variety of high value materials such as optical fibres . One of the ‘ Holy Grials ’ of photochemistry is the discovery of practical ways to convert sunlight into high grade fuels to replace fossils fuels. Used in material science and biological sciences. It is used for the synthesis of anti-malarial drugs. Vitamins, drugs and fragrances are also synthesized by photochemical reactions. For the production of synthetic organic molecules. For the preparation of benzyl chloride.

20. Thank you Mayur Karnjekar