 Good morning. Myself, Dr. Sanjay Sarsamkar, working as an assistant professor in the Department of Humanities and Sciences, Walchand Institute of Technology, Sulapur. Today we are going to learn about organic reactions and synthesis of drug molecules part first. Learning outcome. At the end of this session, students will be able to explain the different types of organic reactions. Introduction. Organic reactions are chemical reactions involving organic compounds. Obviously, there are two types of reactions. One involving carbon called as organic reaction and the other. Other than carbon elements are involved, such reactions are known as inorganic reactions. So, we are interested only in this organic type of reactions. The basic organic reactions are addition reactions, elimination reactions, substitution reactions, rearrangement reactions, photochemical reactions, pericyclic reactions and redox reactions. Our today's content, it is, we are going to learn about addition reactions, substitution reactions, elimination reactions and rearrangement reactions. So first, addition reaction. When two molecules combines to form one product molecule, it is called as an addition reaction. For example, addition of hydrochloric acid to ethylene molecule. This ethylene molecule and hydrochloric acid react with each other and it gives us the product that is ethyl chloride. How this reaction proceeds? We know in case of this ethylene, carbon-carbon double bond is present, that is unsaturation point is available. This bond break opens and hydrochloric acid gets added up against the double bond to each carbon. So, pi bond opening gives site for addition of a hydrogen and chlorine from hydrochloric acid and that ends with the formation of a ethyl chloride containing only one single bond. This can be explained with the help of this schematic representation. This is ethylene molecule, this is hydrochloric acid, which gives us ethyl chloride. We can see this video downloaded from the YouTube itself regarding addition reaction. When an addition reaction happens, a carbon-carbon double bond or triple bond of an organic compound is broken. The electrons shift to connect to hydrogen atoms, halogens, hydroxides or other compounds. The connecting with hydrogen is called hydrogenation. Connecting with chlorine is chlorination and when it is flooring the bonds with the organic compound we call it a fluorination reaction. The number of double bonds in an organic compound can be calculated by the amount of gas consumed in addition reactions with hydrogen molecules. These reactions require methyl catalysts to break the covalent bonds between hydrogen atoms. Bromine molecules are dark reddish brown in color. When participating in an addition reaction, bromine is required to be dissolved in a non-polar solvent like carbon tetrachloride. As bromine is used up, the color of the solution will fade. This reaction has a higher chance of happening at room temperature than the previous hydrogenation reaction, so it doesn't need a metal catalyst to boost the reaction's speed and probability. The reason to this is that a bromine atom has more electron shells which results in higher bond length and lower bond energy. Here's an example of an addition reaction. The reactants are propene and hydrogen bromide in the sulfane of alcohol. Bromine has much higher electronegativity than hydrogen. In other words, it really wants to take hydrogen's electron. So when hydrogen bumps into the unstable pi bond between the carbons and its focus is no longer fully on its own electron, bromine takes a chance and grabs hydrogen's electron and runs off. At the same time, hydrogen takes an electron from the pi bond and bonds with carbon. We see that now one carbon is missing an electron. It has a partial positive charge as a carbon cation. Bromine on the other hand has an extra electron, forming a negatively charged bromine anion. When the carbon cation tries to get an electron back, it seizes an electron held by the bromine anion, thus bonding and creating two bromopropane. This process of breaking a pi bond is an addition reaction. Next reaction is substitution reaction, a reaction in which an attacking species that is nucleophile, electrophile or free radical replaces another atom or group in the substrate is called as a substitution reaction. For example, chlorination of methane. When methane reacts with chlorine in presence of ultraviolet radiation, it gives methyl chloride and hydrochloric acid is also formed. This can be explained once again by means of the schematic presentation. This is ethane molecule. This is chlorine molecule when they combine with each other that forms ethyl chloride in presence of ultraviolet radiations. At this juncture pause the video and answer the question. An organic reaction yielding only one product is A, addition reaction, B, elimination reaction, C, substitution reaction and D, rearrangement reaction. So the answer for this question that is an organic reaction yielding only one product is A that is addition type of reaction. Next reaction is elimination reaction. When molecule is split into fragment molecule it is called as an elimination reaction. Example for this it is ethyl chloride when refluxed with alcoholic potassium hydroxide it gives ethylene molecule and hydrochloric acid. Again this can be explained with the help of this presentation. This is ethyl chloride, potassium hydroxide and ethyl chloride reacts with each other and that forms ethylene molecule also this forms potassium hydroxide and water molecule. Next type of reaction is rearrangement reaction. A reaction in which either the carbon skeleton or the functional group or both are modified is known as rearrangement reaction. Example isobutyl alcohol in presence of acid gives a tertiary butyl alcohol. So rearrangement of species occurs hydroxyl group it is transferred to the car this particular carbon and it forms COH bond over here and at the same time the carbon which is carrying hydrogen it is promoted to this particular carbon and it forms a CH3 group. Second example for this it is allyl phenyl ether and when it is heated at about 473 degree Kelvin it gives allyl phenyl. Again we can represent this by this particular diagrammatic presentation isobutyl alcohol gets converted into tertiary butyl alcohol in presence of H plus ions that is acid and this is for allyl phenyl ether when it is heated it gives allyl phenyl this rearrangement of a molecule it is known as rearrangement type of reaction itself. For this particular presentation I have used a textbook of organic chemistry by Morrison and Boyd and for YouTube video this particular link. Thank you.