 Hello everyone. Welcome to the module on stereochemistry. I am Chidambar Kulkarni at the Department of Chemistry IIT Bombay. In this module we are going to look at how do atoms arrange in space and what are the consequences of it and how do we go about understanding this. The contents we are going to discuss are as follows. First we would look at the three-dimensional representation of organic structures or molecules in space. Why is that important? How did that come about? Followed by we shall look at what are called as structural isomers and stereo isomers. What do I mean by this and what are the different properties, examples and the significance of this. Following this we will go a bit deeper and look at configuration symmetry and chirality and what is the consequences of these aspects and then we further dwell into what are called as enantiomers and diastereomers. And what I am going to cover in this module would mostly be found in either the organic chemistry by this, Paula Erkanas Bruce 8th edition or an online resource called as Master Organic Chemistry Maintained by Dr. James Ashenmahast. With this preliminaries now let us get into looking at three-dimensional structure of organic molecules. So I am sure by the when I say three-dimensional structure of organic molecules you would start imagining something like this, tetrahedral carbon with each of these balls representing a different substituents, correct? You would have sort of heard of this or learnt in your 12 standards. But what we are going to do today is to actually look at a bit of the origin of this because this is something which is already taught to you or you know a bit about this but let us first see how did this come about. So to do that let us transpose ourselves back into the mid 19th century that is around 1850s to 1870s. Back then the concept of chemical bonding that is something which we show by this solid line of the solid stick here was actually not very completely understood it was just beginning to form. And in and around that time what happened is that a chemist or a person called Yokova's Henrikus van Thoff from Utrecht in Netherlands came up with this idea of la chemie dance-lay space meaning or translated into English as the arrangement of atoms in space. So what he did was he looked at various different kinds of molecules which were being studied at that time that is tartaric acids, lactic acids and other kinds of systems and these were mainly extracted from natural products like either grapes or from meat or from some other natural sources. And what people were trying to do in the 1850s was to actually take extract compounds out of these and look at their various properties both physical as well as chemical. Interestingly people found that they had compounds which looked very similar or had very similar properties except one or two but at that time they did not know how to rationalize or how to put that into perspective. So having this knowledge in the literature or having this knowledge around what van Thoff did was he went out and studied this and he came up with this proposition so this is not an experimentally proof okay. So that is what I would like you to appreciate that he came up with this proposition in a pamphlet so please note that this was not even a journal article or a very authenticated source it was in pamphlet in 1874 when van Thoff was just 22 years old he came up with the idea that carbon has a tetrahedral geometry that is the one which I'm currently holding now he came up with this idea when he was 22 years old and this was just an idea or an unproven concept. So as you can imagine today if I come up with a very revolutionary idea which is unproven you're bound to get some slack for it and that is exactly what followed later. So a very noted and established German chemist by the name of Herman Kolbe he was in Germany and he made this remark which I will read out to you and try and explain to you this a bit a Dr. J. H. van Thoff of the veterinary school at Utrecht has no liking apparently for exact chemical investigation by exact chemical investigation he means to do a proper scientific investigation. He further says he has considered it more convenient to mount Pegasus which is an horse in the Greek mythology and to proclaim in his La Shemidas space how in his bold flight to the top of the chemical Paranasus which is a very sacred mountain mythical mountain the atoms appeared to him to be arranged in cosmic space. So what he's trying to do is he's basically criticizing van Thoff of just being a more philosophical and taking a philosophical view on the tetrahedral carbon rather than coming up with exact proofs of this. This is a very well established chemist called Herman Kolbe criticizing the idea of van Thoff in 1877 this wasn't three years later. So what followed later was people actually looked into this idea and many people bought this idea that the carbon could be tetrahedral and that is one of the ways to account for or to rationalize the optical activity observed in systems. So following this van Thoff even van Thoff went on to win the Nobel Prize in chemistry in 1901 along with the French person called a label who also came up with the idea simultaneously and independently. So what this short story tells us is that first of all that even if you're young and still naive you can actually come up with a bold but well thought of ideas which might shake the current paradigms. So that is what is something to keep in mind as many of you young people would come up with very let's say strong arguments or ideas and second is not always the proof comes first. Sometimes the ideas or the propositions come first and then they get sort of goes around and people discuss these ideas then at a later time the proof comes. So these are the two points which I want you to take away from this. So having looked at the story behind the tetrahedral carbon and the discovery by the van Thoff now let's see how are the different ways in which representing a 3D structure right and broadly classified there are four different ways in which one can represent a three dimensional structure. The first one is called as the wedge and the hash projection which is the most popular and the second one is called as the Newman projection which is also quite popular and used and third is called as a saw horse projection and the final one is called as a fissure projection which was developed by Amel Fischer. So we're going to go into each one of this and look at how do people represent 3D structures in each of them. Let's begin by looking at a wedge and a hash projection. So to do that we are going to take a example here say I have a central carbon atom which is drawn here and then I have a central carbon atom which is at this then I have substitutes A, B, C and D. So if you now look at it I have represented it by two kind three kinds of lines one are these two lines which are called as the in-plane plane atoms okay and I have also drawn something which is a bit a solid line which actually is protruding out and this is what is called a wedge or a wedge projection and this is by convention taken to be coming out of the plane towards us. So this is a wedge and this comes out of the plane and this dashed hash line or dashed line this is what is called as a hash and this actually goes into the plane of the world. So just to illustrate this if we take a let's say if I take this particular tetrahedral carbon which I am showing I can hold any two of these in one plane that is let's say I am having these two atoms are in one particular plane that is the green as well as the white are in one plane then you have the orange which is product coming out of the plane towards you this is the wedge which is protruding out that's a C substitution and then on the backhand side I have another one which is the cyan color which you can see is goes back. So if you have a tetrahedral carbon like this with the central carbon being the black one what you see is that you can hold any two of them in the plane of the board or in one plane and then you have two atoms which are either pointing away or behind the plane of the paper. So this is what is called as a wedge and hash representation and in this particular representation people typically write structures like if I take a tetrahedral carbon and I put a hydrogen bromine chlorine iodine what this tells me is that I have the chlorine atom which is actually coming out of the plane of the board or of the screen and iodine atom actually goes behind the screen and what you would typically notice is that these two the ones which are in the plane that is these two are typically adjacent to one another and these two which are either behind or in front of the plane are also adjacent to each other. So this is the most correct way of doing it and there are many permutations and combinations one can do that is you can have different way is this you can have the iodine you have the chlorine the bromide and the hydrogen. So in this kind of a representation what you all you are trying to show is that the solid lines are in the plane and the wedges are coming out of the plane and the hash lines are actually going behind the plane and another important aspect is that if I take a simple alkane like let us say if I am trying to write a COH group here alcohol then I have hydrogen. So a structure like this is usually represented by something like this in most of the organic chemistry that is this is called as a line diagram or a line representation of the structure and here what is assumed is that the ones the if I do not show anything that means there is already a carbon here in this particular atom there is one carbon and this is also assumed that there is a carbon and then you have three hydrogens here which are attached. Similarly if you want to look at another structure a larger structure you would do something like this let us say I have an alcohol here in this case what is assumed is that you have two hydrogens which are attached to each of these corners which is a carbon atom and this is what is usually found in most organic chemistry textbooks and this hash and the wedge which I told you a second ago this is what is usually used to represent the stereochemistry or orientation of molecules in space. All right so now let us look at another important projection that is called as a Neumann projection so far what we looked at was a arrangement around a single carbon atom that is if I have a carbon atom and four different substituents like the one I showed you here correct. So now what happens if I go to more than one so for example if I go to an ethane molecule so here you see a molecule of ethane where I am trying to show where the black ball is the carbon and the white ones are the hydrogens which are attached to it. So how would one go about writing the conformation or the projections of this. So let us first write down the structure of the ethane so I have a cc and then I have three hydrogens attached to it and if I want to look at it then there are many different ways one can look about it but a simple way is that if you now take this particular molecule and since there is a single bond around the between these two carbon atoms this is actually free to move around I can actually if I hold it like this I can twist the second carbon atom and I can get different conformations of this right. So to represent this what one can do is if you are looking from your angle you will only see this particular carbon atom and three hydrogens you would not see the back carbon atom right because that is being masked by the front carbon atom. So if you look at it along the cc angle or the cc bond bond what you would see is that you would see the front carbon as the following where this is the front carbon and you have the hydrogens this is the first front carbon is what I am trying to draw currently and if you look at the back you actually do not see the back carbon but you only see the hydrogens that is if I put it in this conformation you will only see the three hydrogens and that is represented by drawing the circle and these bonds to show that there is an atom which is connected to it so this is the front carbon and this is the back carbon all right and this is when I am weaving along the carbon-carbon bond and if you want to now get a complete picture of this what we would do is we would just combine these two and then you will have something like this the front carbon with a dot here and the three hydrogens attached to it and the circle represents the back carbon and then again that is attached to three hydrogens so this is what is called as a Newman projection in which you are actually trying to look at the trying to look at how do two molecules which are when you look along the cc bond how you can represent the molecules in a two-dimensional picture and this sort of pictures are typically used to understand the confirmations or the confirmation analysis of alkanes or other molecules and so just to show you this let us say if I there are two different possibilities of this one is called as a staggered and another is called as eclipsed I am going to draw the same picture here at the circle and so I am writing the front carbon again and with the attached to three hydrogens and now the back carbons I am writing at about 60 degrees from each other and this is called as staggered staggered conformation and if you want to understand this if you actually now look at this front carbon you see that these three are here and the other three hydrogens attached to the carbon behind are actually at 60 degrees to between them so what I can do is I can actually go on flipping this go on turning this then I would end up in a conformation where you will actually not see from the back carbon as well as hydrogens that is there exactly behind the front carbon and this conformation is what is called as a eclipsed conformation so and so again I am drawing the circle for the back carbon the front carbon with the three hydrogens attached to it and now what I am going to do is I am going to draw just behind it to show that they are actually eclipsed so this is one of the ways in which one can use these Newman projections or to look at the conformation of simple organic molecules and this is actually a widely used technique to see how to how let us say staggered or eclipsed or gosh or different kind of conformations are possible in alkanes. Now we will move on and look at another related projection which is called as a sawhorse projection so till now what we did was in the Newman projection we took this ethane and we looked along this bond right along the CC bond just write down the CC bond now what we will do is we will actually take a slightly angled view of the CC bond and see how does it look at so that is what I am going to try and draw now so I will again use the same example but remember now I am trying to look at it from a side not along the CC bond so in this case what can happen is this is one carbon atom and this is another carbon atom I am drawing an exaggerated bond here and then so this is what you would look as a sawhorse projection or when you look at it from the side that is if you actually look at it from the side you would see something like this let us say in this kind of conformation you would see both the carbons and you will also see that one of them is up that is the back carbon here which is this particular carbon and then you have the front carbon in which the hydrogen is down that is what you see on the screen and these two are actually up but now the only difference between the Newman and the sawhorses I am now looking at it a slightly angled position so that I see both the front as well as the back carbon correct and this conformation is also used in looking at the conformation analysis and again here one can draw both eclipsed and staggered conformations as you might have already noted this is a staggered conformation because you see that this particular hydrogen is now exactly at 60 degree between these two hydrogens on the front carbon and if you want to now look at the eclipsed so I am going to go ahead and draw again a cc these two carbons and I will put this at the same two hydrogens and here I am going to put the same because I am looking at a staggered eclipsed conformation so this is the eclipsed conformation of ethane so I hope you can see that this is now you can see the eclipsed much more easily in this particular sawhors conformation compared to looking at it in a Newman projection because there both of them are completely one behind each other so you cannot really see what is happening but if you now look at it in this particular fashion you see that these are exactly identical both in the way they are arranged and that is much more evident if you just look at it a slight angle and this is what sawhors projections are useful for they actually tell you a bit more clearly how do atoms are arranged in along the cc bond all right and whatever I told you both the Newman as well as the sawhors projections are extensively used in looking at the conformations of molecules that is if I want to look at the barrier between this that is if I rotate this particular bond what is the conformational energy of the penalty I have to pay to understand this these models are excellent to understand them all right so now what we will do is we will go ahead and look at one last conformation that is called as a fissure conformation or the fissure projection so here I am going to draw similar tetrahedral structure which I showed you previously and then I have placed four different substituents A, B, C, D which is what we looked at as a wedge hash representation correct however what people have done is it is more mostly comes from a mill fissure where one can actually draw them as this that is you can draw them as a cross line and in this cross line what you do see is that you have both the substituents which are actually coming out at you that is you have A, B, you have A, B and then C, D which are at the back okay and this was done with a purpose because they wanted to study the arrangement of sugars or mostly carbohydrates and these are ideally suited to study the projections of carbohydrates rather than usual organic molecules so here what you actually do is you take the same tetrahedral carbon and here what I am going to do is I will not cut through any of the any of the atoms I will put pass a plane which is actually going like this between along this direction so then what you see is that I have two substituents at the back and two substituents in the front so the ones which are actually coming in front to me are labeled as A, B in this particular diagram and the ones which are actually going back are labeled as C, D that is in the vertical direction so whatever is actually coming towards you that is A, B is shown as a horizontal line and the substituents which is actually going back is shown as a dash as a vertical line so just to show you the utility of this here is the structure of S glycerol yet a monosaccharide and this is the usual way in which it is represented the one on the fissure projection which is shown on the left and if you really want to make it more clear you can actually put the hash and the wedge that will make it clear the way in which it is actually represented although these are actually nice representations fissure projections but these are actually mostly suited for carbohydrates and they are typically not used for most organic molecules so they have a very limited utility when it comes to organic or bio biological molecules so with this we stop here and in the next lecture we will actually look at what are the different kinds of stereoisomers and how do we study them thank you