Free Chemistry NEET Video Lectures for Organic Chemistry: Some Basic Principles and Techniques

00:00:02 Welcome to the lecture on fundamental principles of organic chemistry and some techniques. I am professor Shankara Raman from the department of chemistry IIT Madras. In the first lecture of a series of lectures that I would like to give in organic chemistry. Today we will discuss some fundamental aspects such as hybridization in organic chemistry. To start with let us define what organic chemistry is all about. Organic chemistry is a very fascinating subject in chemistry. It is a subset of chemistry. It deals with compounds of

00:00:37 carbon. So essentially you can define organic chemistry as chemistry of carbon compounds. Carbon not only forms bonds with itself in forming long chains of hydrocarbon. For example, the first member of the hydrocarbon is methane. The second member is ethane where the two carbons are bonded to each other. The structure should be like this. Then go to the next homal series. You have propane, butane, pentane and so on. So carbon has the ability to form carbonarbon bonds quite elaborately. So much so that the polymer polyethylene

00:01:53 is a linear chain of CH2s where the CH2s hundreds of them are connected together. I'll just put N here. N can be 100, 120, 150 and so on. So carbon is capable of forming such longchain hydrocarbons such as polyethylene for example. It also forms compounds with other elements of the periodic table with hydrogen, nitrogen, sulfur, oxygen, phosphorus and hogens. So where the carbon is directly attached to nitrogen, sulfur, phosphorus, hogen, they are all considered to be organo organic compounds.

00:02:48 Now organic chemistry is essentially a life sustaining chemistry in the world of biology. If you look at biological molecules like DNA for example, proteins, carbohydrates, lipids and so on, they are all organic compounds, organic molecules. So one can call for example DNA, proteins, carbohydrates, lipids, in other words fat. Now these are polymeric compounds that are available in nature from biological systems and all of them belong to the category of organic compounds. Not only that these organic molecules

00:03:52 are essential for the sustenance of life, they occur very widely in nature. Early on during the mid of 18th century, there was a theory called vital force theory. This was proposed by a scientist by name Bcilius. He was a Swedish scientist. In 1780, Mercilius proposed the theory called the vital force theory. Now, according to this theory, if you want to make an organic compound, you have to have a living system like a plant or an animal and so on. The reason for believing such a theory is that early on during the

00:04:51 development of organic chemistry, organic molecules were typically isolated from nature. What do I mean by nature? It the organic compounds were isolated from plant material or from animals or from living organisms for example. And such a theory had the support from the isolation of the material from nature where the living creatures created the organic compounds. So the vital force theory was believed to be in existence for several decades originally proposed from 1780 by basilius until 1828. Another

00:05:41 scientist came along by the name Frederick Schler. He sort of did an experiment which amount to disproving the concept that a living organism is necessary for producing organic compounds. This is a very part-breaking experiment, very important experiment in organic chemistry. experiment itself is essentially taking ammonium chloride which is an inorganic substance. There is no carbon present in the system. Then taking potassium cyanide which is also an inorganic salt for example. Mixing together you get ammonium cyanate.

00:06:32 Ammonium cyanate is also an ionic inorganic compound. And what oler did was to heat this compound. The molecule this compound under goes a rearrangement reaction in producing a molecule known as ura. URA is considered to be an organic compound. This was the first lab synthesized organic compound. So that was a blow to the so-called vital force theory because now it is possible to generate an organic compound from purely inorganic substances. Inorganic substances are typically substances which are obtained from

00:07:30 minerals from earth's crust for example. So they don't need a life form to generate inorganic molecules. Such an inorganic molecule is heated and converted into an organic molecule in the laboratory for the first time which gave the vital blow to the it gave the blow to the vital force theory as proposed by berscelius. Since then organic chemists are involved in synthesis of organic compounds. Let us define organic synthesis. This is essentially making organic compound in the laboratory.

00:08:27 without the involvement of any kind of microorganism or a living matter. For example, where laboratory techniques are used for producing organic compound that is called organic synthesis. By mid 19th century, organic chemistry is a developed subject. Organic chemistry is only about 200 to 225 years old. Right now it is a full-fledged part of chemistry and it is a fully developed aspect of chemistry at the present time. In addition to 1828 synthesis of the ura by oler in 1845 cobbay synthesized acidic acid in the

00:09:18 laboratory. This is the first time acetic acid was synthesized in the laboratory. Acetic acid was called vinegar which was obtained from natural sources. For example, however, for the first time in the laboratory, synthesis of acidic acid was demonstrated by Colbeay. In 18 56, Berthalot synthesized methane from aluminium carbide. Aluminium carbide upon hydraysis gives methane. Again, a laboratory synthesis of an organic compound from what is considered to be an inorganic substance, namely the aluminium carbide, which is

00:10:01 an ionic inorganic substance. So the original belief that vital force is necessary for synthesis of organic compound has been disproved by several of this kind of synthesis by the mid 19th century or so. And currently organic synthesis is a very wellestablished discipline of chemistry. One can synthesize simple molecules like for example aspirine to very complex molecules like asteroid molecule in the laboratory without the involvement of any kind of a living microorganism or living plant or such

00:10:43 materials. Now, organic chemistry is applicable in for example in medicines, in food, in clothing and in fuels. For example, medicines such as for example simple compound like aspirine which is a structure that is shown here. This is acetyl salicylic acid and that is called aspirin. It's a headache medicine. This is an organic compound. Ibuprofen for example is an organic compound. This is ibuprofen for example neproxine, ibuprofen, aspirin, paracetamol for example they are all organic compounds. used in

00:12:09 a day-to-day medicine. Starch is an organic compound which is a major constituent of rice and other grains. For example, which is a source of this is a class of compounds known as carbohydrates. These are the source of energy. Clothing for example, nylon, polyester, even cotton which is available in nature for example is a form of organic compound. They are all polymeric material. Nevertheless, they are organic compounds. Fuels for example gasoline, petrol, diesel, they are all hydrocarbon compounds,

00:13:05 organic compounds. So it essentially shows that organic compound essentially is present everywhere around you. So organic chemistry is a very important subject not only for life sustenance. It is also important in biology in terms of the kind of molecules that one deals with in biochemistry for example. So till now what I have been trying to impress upon is the importance of organic chemistry and the kind of theories that existed which were later on disproved by means of other scientists. For example,

00:13:52 now we will go into shapes of organic molecules in organic chemistry. Organic molecules can be either three-dimensional, two-dimensional or one-dimensional depending upon the kind of carbon that we are dealing with. Let us start with the simple molecule methane. Methane has four hydrogens that are attached to a carbon. And one order to in order to explain the size and shape of methane, one invokes the theory of hybridization. Before we go into hybridization, let us for the sake of argumentative

00:14:50 purposes write methane in two different format. This is one structure of methane. One can write in this particular this particular structure the carbon is connected to four hydrogen and all the hydrogens's as well as carbon are in one plane which is the plane of the blackboard. In other words, this is a square plane or structure of methane. I'm not saying this is the right structure but there is a possibility that one can have the square plane or structure. Alternatively, one can consider for example the hydrogen,

00:15:42 carbon and hydrogen to be in one plane. So these three atoms are in one plane. The third hydrogen is behind the plane of the blackboard and the fourth hydrogen is in front of the plane of the blackboard. This is another structure of methane. This is called the tetrahedrron structure until the early 20th century. The structure of methane was known or the tetrahedral carbon of methane was known. It is by two scientist vanhoff and label. He is a Dutch scientist and label is a French scientist. Independently during the early

00:16:46 1900s they simultaneously proposed the carbon of a hydrocarbon to be tetrahedral in shape. They had their own reasons based on stereochemistry of organic compounds which we will deal with later. Nevertheless, this was a pathbreaking discovery in terms of the proposal that a saturated carbon should have a geometry of this kind and not a geometry of this kind where you have square planer. Now, what is the difference between these two structures? This is a two-dimensional structure. It is confined to a plane. Whereas this is

00:17:25 a three-dimensional structure. If you want to understand this particular structure, this hydrogen carbon and this hydrogen they lie on the plane of the blackboard. Whereas this carbon hydrogen bond is protruding inside the plane of the blackboard. Whereas this carbon hydrogen is projecting outside the plane of the blackboard. The same thing can be represented by means of a tetrahedral arrangement of carbon in this particular fashion. again indicating for example this carbon hydrogen bond to be inside and this

00:18:02 carbon hydrogen bond to be outside the plane of the blackboard. This is forming a tetrahedrin if you can see clearly here. So the in the tetrahedrin this carbon is at the center of the tetrahedral arrangement and the four hydrogen's are occupying the four vertices of the tetrahedral structure because all the four hydrogens's are equal. This is a symmetrical tetrahedral structure as opposed to a distorted tetrahedrin. If one of the hydrogen were to be replaced by a chlorine for example, all the carbon hydrogen bonds are equal

00:18:54 and all the carbon hydrogen hydrogen angles are also equal. This angle if you look at this will be 109° 54 minutes. Similarly, this will be 109° 54 minutes. This will be 109 54 minutes. Similarly, this also 109° 54 minutes in the three-dimensional aspect. On the other hand, if you look at this structure, this will be only 90. Each of this will be only 90 because this is a planer structure. That would mean that this carbon hydrogen bond and this carbon hydrogen bond will be closer together in comparison to the carbon

00:19:39 hydrogen bonds in this particular structure. And this is precisely for the reason that this structure can be discarded because if a carbon hydrogen bonds can be having a larger angle if they can be further away the bonding electron electron repulsion can be minimized in this particular structure in comparison to this particular structure here. Now this is the structure of methane which is tetrahedral in nature. We can explain the formation of the tetrahedral carbon or the trional carbon or an sp kind of a

00:20:15 carbon using the principle of hybridization. What is hybridization? It is a fairly simple concept. You take a set of atomic orbitals, mix them together and redistribute them in certain orientation. This is called hybridization. So essentially one can write this as mixing of atomic orbitals and redistribution of the orbitals. in specific orientations. This is called the hybridization process. Now, in order to do the hybridization, there are certain rules that one needs to follow. Conditions for hybridization

00:21:25 are one can only hybridize orbitals in valency cell. In other words, only the outermost electrons which are capable of undergoing bonding interaction with other atoms are capable of undergoing hybridization. orbitals undergoing hybridization should be close in energy. When you say it is a valence shell, electrons are the orbitals which can undergo hybridization. The orbitals has to be close in energy. For example, you cannot take a 1s electron and a 2p electron and hybridize. On the other hand, you can take a 2s electron and a

00:22:35 2p electron and do a hybridization because they are closer in energy. These are widely separated in energy whereas these are closer in energy. So this can lead to hybridization like the sp hybridization, sp2 hybridization and sp3 hybridization. Whereas this cannot undergo hybridization because they are vastly different in their energy in terms of the relative energies. Now for the hybridization to occur one need not promote an electron from one orbital to another orbital. This I will explain it in a minute.

00:23:23 motion of electron from one orbital to another orbital is not an essential condition for hybridization. both filled orbital and half- filled orbital can undergo hybridization. So these are essential conditions for hybridization to take place. Once the hybridization takes place, what is the outcome of the hybridized molecular orbital? The first one is number of atomic orbital that is hybridized. This will be equal to the number of hybridized orbital obtained after hybridization. In other words, you start with

00:24:54 three molecular three atomic orbitals. You will end up with three hybridized orbital. That's what it essentially means. All hybridized orbitals will have same size, shape and energy. In other words, if you have taken four atomic orbital and hybridized to produce four hybridized orbital, all the four hybridized orbitals will have the same shape as well as energy and size in terms of the redistribution. Hybridized orbital point or orient in specific direction depending on the In other words, the hybridized orbital

00:26:30 has very specific orientation in space. That orientation essentially dictates the shape of the molecule that is formed by certain hybridization. For example, if you say sp3 hybridization, it is a tetrahedral geometry. Sp2 hybridization, it is a trional geometry. and sp hybridization it will be a linear geometry and so on. So these are some things one need to remember. The conditions for hybridization is essentially very simple. Only orbitals in the valance shell can undergo hybridization. Orbital supposed to be

00:27:05 close in energy. You cannot take for example 1s orbital with the 2p or 3p orbitals and hybridize them together because they are vastly different in energy. You can take 2 ss and 2p orbitals which are close in energy in order to do the hybridization. If the orbital is a filled up orbital, it is not necessary to promote the electron to an empty orbital in order to do hybridization. So one can do with both filled orbital as well as half filled orbital can undergo hybridization to produce hybridized orbitals. For

00:27:38 example, the outcome of the hybridization essentially is that if you take n number of atomic orbital and hybridize them, you'll get exactly the same number of hybridized orbitals also. All the hybridized orbitals will have same shape and identical energy. In other words, this is called degenerate orbitals. Degenerate orbitals essentially means they have identical energy in terms of the energy of the orbital is concerned. Hybridized orbitals actually orient in a very specific direction in space.

00:28:17 Depending upon the hybridization the orientations will be different and that is what gives the definitive shapes for organic molecules. Having said this now let us look into the concept of hybridization in a little more detailed manner. To understand hybridization, one needs to understand carbon's electronic configuration. Carbon has an electronic configuration of 1 s2, 2 s2 and 2 p2. In other words, in the valence shell, carbon has four electrons. Two of them in the s orbital and two of them in

00:29:01 the p orbital. And that is what gives the four valency of carbon. tetra valency carbon. If you want to draw the box diagram for the electrons this is according to the horns rule of maximum multiplicity they should be parallel with respect to each other. So this is 1 s orbital, 2s orbital and 2p x y orbitals. Now if you take the s orbital and the p orbital, you can combine them together and hybridize them to give sp3 hybridized orbital. In other words, the 2s orbital of carbon and the 2px, 2py and 2pz orbitals of

00:30:10 carbon are mixed together and hybridized to give a hybridization what is known as sp3 hybridization indicating that the hybridization involves one of the s orbitals namely the 2s orbital and three of the p orbitals which are the px, py and pz orbitals. So you have taken four orbitals atomic orbitals. So that should end up with four hybridized orbitals. This is one of the rules of the hybridization. If you start with n number of atomic orbitals, you will end up with n number of hybridized orbital.

00:30:55 Now the orientation of the sp3 hybridized orbital is important. First of all let us look at the shape of the sp3 hybridized orbital. So if you were to draw the cartician coordinates of the XYZ axis for example the 1s electron is spherical. So one can draw the 1s electron in this particular fashion this is x y and z. The p orbital has a dumbbell shaped. In other words, the p orbital has a shape like this one for example. This is what is known as the dumbbell shape of p orbital. So the px orbital will be

00:31:51 having a orientation in this particular manner. Similarly, the P Y will have a orientation along the Y or Y Y axis of the cartition coordinate. And finally, the isert will also have orientation along the Z axis of this particular shape. So if you combine all of these things, you get the sp3 hybridization. You start with four atomic orbital, one 2s orbital and a three p orbital namely px yz orbital. The resulting sp3 hybridized carbon has orientation in such a manner. The initially the hybridized orbitals have different

00:32:44 shape. Once the hybridization is over, the hybridized orbital has a shape which is this like this only one lobe with a small lobe that is present at the end of the narrow end of the lobe that we have here. So this will be the shape of the sp3 hybridized orbital for example. So there are four sp3 hybridized orbital. Let us consider the orientation of the four sp3 hybridized orbital in this particular fashion where you have one sp3 hybridized orbital and another sp3 hybridized orbital for example of a carbon pointing

00:33:24 in the same plane of the which is a plane of the blackboard for example. The other hand the third sp3 hybridized orbital is inside the plane of the blackboard. The fourth sp3 hybridized orbital is in the projecting outside the plane of the blackboard forming a tetrahedral arrangement of. So if you want to draw this this is a carbon one of the orbital it's like this. The other orbital is again on the plane of the board only. The third orbital is behind the plane of the board and the fourth orbital is projecting in

00:34:10 front of the plane of the board which is drawn with a thick line like this. So this is a tetrahedral geometry is what we are referring to. In other words, if you want to draw it in another fashion, this is a hydrogen here. Hydrogen here, hydrogen here. The carbon is at the center of the tetrahedrin regular tetrahedrin. So the orbital will be essentially overlapping in this particular manner. This will be going inside here and the fourth one will be projecting outside here. Let me draw it properly.

00:35:13 a tetrahedral carbon. Let me give a color coding so that you understand the coding properly. The blue one essentially goes inside the plane of the blackboard and the red one or the magenta one projecting outside the plane of the blackboard. So these two white orbitals are on the plane along with the carbon they are on the plane of the blackboard. This goes behind the plane of the blackboard the blue one and the magenta one essentially projects in front of the plane of the blackboard. One can also confine the tetrahedrron

00:36:01 inside a cube. Let me draw it over here. You draw a cube. In the cube, the carbon is at the center of the cube. And now you connect diagonally opposite corners of the cube. that will be pointing at the positions. Let us say these two positions are connected to the carbon and these two positions are also connected to the carbon. So this you can see here this will be projected this way. This will be projected like this. For example in this fashion only it will be projected as a tetrahedral carbon.

00:37:10 So the tetrahedral loes will be essentially pointing towards the another way of representing a tetrahedral carbon inside confined inside a cubical structure of this kind. So it is important to understand the nature of the shape of the sp3 hybridized carbon. So an sp3 hybridized carbon essentially has four orbitals hybridized orbital in this particular fashion. Now how is methane formed? Once the sp3 hybridized orbital is formed, the 1s electron of hydrogen which is spherical in nature essentially overlaps with the

00:38:08 sp3 hybridized orbital. In other words, this would be a carbon hydrogen bond. For example, this will be the 1s orbital of the hydrogen and one of the sp3 hybridized orbital of the carbon. they overlap with respect to each other. This again is a 1s hydrogen orbital and the sp3 hybridized orbital. Finally, so the tetrahedral arrangement is what is responsible. The sp3 hybridization is what is responsible for the tetrahedral shape of of a saturated carbon. So methane is formed like this. How is ethane formed? Ethane is formed by

00:39:18 overlap of one of the sp3 hybridized carbon of the one carbon so that will form a sigma bond. Sigma bonds are formed when orbitals overlap along the axis. So this is the carbon carbon sigma bond for example. Then you have the 1s orbital of hydrogen essentially forms the will be essentially the structure of the ethine molecule. The same structure can be written in this fashion. indicating that one of the carbon hydrogen bond in this carbon and this carbon are projecting in front of the plane of the blackboard. These two

00:40:21 carbon hydrogen bonds are projecting inside the plane of the blackboard. These two carbon hydrogen bonds are on the plane of the blackboard for example. So that will form two tetrahedral arrangement to be overlapping in this particular fashion. So this is one tetrahedrin. This is another tetrahedrin. They combine together form the carbonarbon bond in a ethane molecule. So this is a simple way of explaining the shape of methane. And the reason methane is tetrahedral is not a square plan or molecule is because of the fact the

00:41:02 square plan or molecule you have the bond angles closer which is 90° compared to a tetrahedral arrangement where you have the bond angle 109° 54 minutes or All the carbon hydrogen hydrogen bond angles are equal in a tetrahydral arrangement and the bond lengths are also equal. It's roughly 1.543 angstrongs or so. 154 pometer is what the length of the carbon hydrogen bond in a molecule like ethane that we have sorry it is a carbonarbon bond of the sp3 sp3 carbonarbon bond is about 1.543 angstrongs. The carbon hydrogen bonds

00:42:20 are about 1 05 or something angstrongs. For example, the carbon hydrogen bonds are much shorter than the carbon carbon bonds in a methane kind of a molecule. If the hybridization is carried out only with two of the p orbitals then you will have sp2 hybridization. In other words, one s orbital and two p orbitals are hybridized together. So totally three orbitals are hybridized together with the three electrons. So the three orbitals are oriented in this particular fashion in a trional fashion like this.

00:43:08 In other words, it is the three orbitals are all in the plane of the blackboard only. It is a two-dimensional structure. One of the orbital is oriented along this direction. The other orbital is oriented along this direction. The third orbital is oriented along this direction for example. So each of the bond angle between the sp2 hybridized orbitals will be 120. This is called trional geometry. Only three electrons are used up in the trional geometry. In other words, if you draw the trional geometry on a plane

00:43:58 perpendicular to the plane of the blackboard, it will be oriented in this particular manner. The fourth orbital, one of the p orbital with one electron will be perpendicular to that. This will be dumbbell shaped. This is original p orbital, pz orbital. of ethylene sorry a paid orbital of the carbon. So in a sp2 hybridized orbital there are three orbitals pointing in a trional fashion like this in a triangular manner like this. This is all in one plane which is on the plane of the blackboard. If you just

00:44:44 tilt it like this and look at it, this is how it is going to look like with the fourth orbital which is the unhybridized p orbital which is going to be perpendicular to the plane of the paper that is shown here for example. So if the hybridized molecular orbitals were to overlap with each other as in the case of ethylene they will overlap in this particular fashion. This is along the axis of the hybridized orbital. So it is a sigma bond is going to be formed between two carbons and then you have a 1s electron

00:45:27 forming the CH bonds which are also sigma bonds. For example, remember we are now trying to describe the geometry of ethylene. Ethylene has this particular geometry where you have a trional hybridization here and a trional hybridization here. For example, this is one of the trional hybridized sp3 sp2 system. This is other trional hybridized sp2 system. For example, in addition to that, there is also a single p orbital which is in this particular oriented in this particular manner. And the lateral overlap of this p

00:46:15 orbital is what is going to give you the pi bond. If one were to draw the orbital, p orbital in this diagram, let me use a different chalk here. This would be the p orbital of the carbon. This will be the other p orbital of the next carbon. And the lateral overlap of the p orbital in this particular fashion gives a pi bond. Just to clarify at this point, if you have a hybridized orbital which is overlapping in this particular fashion along the axis, then it will form the sigma bond. Sigma bonds are always formed when the

00:46:58 orbitals overlap along their axis. On the other hand, the atomic orbital namely the p orbital pz orbital in this particular case. This is the this would be the axis of the orbital. They don't overlap along the axis. Instead, they overlap sideways in this particular manner. That will give you the pi bond. So it is important to understand the concept of formation of sigma bond and pi bond how they are formed in the organic molecule. So this diagram essentially explains the formation of ethylene by initially two sp2 carbons

00:47:36 overlapping with each other to form a carbonarbon bond and the remaining two sp2 hybridized orbitals are overlapping with 1s electron orbital of the hydrogen forming the carbon hydrogen carbon hydrogen bond. Here again carbon hydrogen and carbon hydrogen bond. Finally the fourth atomic orbital which is the pz orbital which is perpendicular to the plane of the blackboard. If this is ethylene is in this plane then this orbitals are projecting outside the plane of the blackboard. Or if you consider ethylene to be

00:48:06 perpendicular to the plane then this will be on the plane of the blackboard. In other words if you are to draw the structure of ethylene on a piece of paper which is this sheet here. This is the orientation of the sp2 hybridized orbital. Then perpendicular to that will be the orientation of the p orbital in this particular manner. So this lateral overlap is what gives the pi bond of the ethylene in this particular case. Lastly, now the angles are very clear. This is 120°. This is 120°. So, sp3 hybridized orbital have 120° and

00:48:54 the carbonarbon bond for example has a length of about 1.4 5 or so angstrongs. A carbonarbon double bond in this particular case. Finally, you can take one S orbital and one P orbital, combine them together to form only an SP hybridization. So you have SP hybridization that is being formed by combination of S orbital and only one P orbital. The remaining py and the pz orbitals are intact on the carbon. Such a hybridization is called sp hybridization. Since the two atomic orbitals combine to form the hybridized orbital, you get two

00:49:51 hybridized orbital. In this particular, the two hybridized orbitals are along the line. So this is linear geometry. with a angle of 180° between the hybridized orbitals. Now if you have a carbon with an sp3 sorry sp hybridized orbital another carbon with another sp hybridized orbital like this they can combine together form the sigma bond. This would be the overlap that forms a sigma bond. So this is a carbonarbon bond that is being formed. Then the other molecular other hybridized orbital combines with

00:50:46 let's say for example 1s electron of a hydrogen. So this would be hydrogen and this would be hydrogen. So you have basically explained the formation of the sigma bonds in acetylene. This is a sigma. This is sigma and this is also a sigma bond. But then acetylene is an unsaturated compound. It has pi bonds. So one of the pi bond is formed by the pz orbital overlapping with another pz orbital of the other carbon. So this lateral overlap essentially will give you the pi bond one of the pi bond. Then you

00:51:28 have one more. Remember you have taken only one s and one p. So the remaining two py and pz are remaining in this one. What you have drawn is a pz. Let us draw the py in this particular fashion. So overlap between the lateral overlap between the px sorry py and pz orbitals essentially give lateral overlap of p y and pz atomic orbitals. gives two pi bonds. So the pi bonds of acetylene are essentially formed by the overlap of the py and the pz orbitals in a lateral fashion to give the particular geometry

00:52:39 which is a linear geometry. The carbonarbon bond length here is roughly 1.28 2A tank strongs or so it's much shorter than the so if you compare the structures of ethylene acetylene and ethane the geometry the bond length and the bond angles are easily explained on the basis of invoking the concept of hybridization. This has 109° 54 minutes angle. This is 120 and this is 180. This is a linear geometry. This is a trional geometry. And this is a tetrahedral geometry for example. So all the geometries of the organic molecules can

00:53:31 be explained on the basis of invoking the hybridization namely sp hybridization, sp2 hybridization and sp3 hybridization. However complex the molecule can be, the concept of hybridization helps us to understand the kind of geometry and shape the organic molecules have in organic structures. Organic molecules can be classified and the classification is what I'm going to discuss in the next few minutes. organic compounds. Broadly they can be classified into open chain or cyclic. These are called asyclic

00:54:45 and these are called cyclic organic compounds. The cyclic organic compounds you can have carboylic or homocyclic or you can have for example heterrocyclic. A open chain compound would be simply ethane. The simplest example is ethane for example or butadene is a open chain compound. A cyclic compound will be cycllohexane or cycllohexene for example. This is also a open chain compound or if you want to go carocyclic this would be hexine hexane and so on. Heterrocyclic compound you just need to have one hetroat atom present in the

00:55:46 system. It could be oxygen, it could be sulfur, any one of the hetroatom can be present in the system. So these are examples of heteroscyclic and homocyclic compound. You can have aromatic or non-aromatic compounds. Benzene would be a typical example of a aromatic compound. Hexadine would be an example of a non-aromatic compound. Similarly, you can have aromatic non-aromatic compounds. Non-aromatic compound would be pipidine which is this particular structure. The same thing if you want to make it

00:56:46 aromatic you just put the pi bonds. Pyodine for example is an aromatic compound. In the aromatic system you can have benzenoid aromatic non-benzenoide aromatic compound. Benzinide aromatic compounds are benzene, napylene, anthraine. All of them which have benzene rings fused together they are benzenoid compound. On the other hand if you have a aromatic compound which is this is called aulene. This is also an aromatic compound but it is not a benzinide aromatic compound. You don't see benzene ring. This is a

00:57:42 seven membered ring. This is a five membered ring fused together for example. Or you can have a seven membered ring like this which has a cationic structure here. This is aromatic. This is called tropelium cation. This is also aromatic in nature. This would be a non-benzenide compound. So in a broad manner the compounds can be classified into openchain compound or a closed chain compound. In the closed chain compound you can have carocyclic or heteroscyclic. Hetraycleic means it just has an atom other than carbon and

00:58:21 hydrogen in the ring. And the homocyclic compound can be aromatic or non-aromatic in nature. Examples are benzene and hexadene. Here again you can have aromatic or non-aromatic. Non-aromatic would be this one which has no pi bonds. Aromatic is the one which has pi bonds conjugated to each other. The benzinide in the aromatic compound you can have benzenide or non-benzenide. These are examples of benzenoide compounds. These are examples of non-benzenide compound. So in this lecture we took a short tour

00:58:55 of the history of organic chemistry starting from Bcilius theory of vital force theory then moved on to OER synthesis of ura. Then we moved on to the concept of hybridization to explain the shape and geometries of organic molecules. Finally classified the organic compounds in various category in this particular lecture. Thank you for your attention.