Course on Stereochemistry Professor Amit Basak Department of Chemistry Indian Institute of Technology Kharagpur Module No 01 Lecture 01: Constitution and Configuration (V2-note-version) Welcome to this course on Stereochemistry which is a basic subject branch of chemistry. The course has been designed to primarily focus on the fundamental aspects.. Now stereochemistry like any other branch of sub-disciplines has emerged from a historical perspective. The subject area is fundamentally important and is applicable to not only to chemistry, but also to various other branches of sciences like biology and medicine, in particular. Why is the subject so important? That is because of many reasons. the primary reason is that the world that we are living in today, is a chiral world. What does it mean? That means it is made up of systems which have a handedness. Handedness lies in the nature of the two hands. If I see the two hands, they look almost the same but they are not same. This is because you cannot superimpose one hand on top of each other. (remember! Exactly similar objects (clones of a particular object) are always superimposable). The reason for the two hands not the same lies in their 3D-geometry that is the way the fingers are aligned. Similar things happen in the molecular world. I have already said, we live in a chiral world and that means that all the living systems that are there, they are made up of molecules which exhibit this type of handedness. The stereochemistry is a subject which deals with the property of the molecule that are controlled by the attached functionalities in three-dimensional space. Chemistry as a subject developed on the basis of function, reactivity and structure of molecules but purely looking them at a two-dimensional platform. That means the chemistry what was known earlier was two-dimensional chemistry. Basically we looked at molecules from a two- dimensional aspect. (Refer Slide Time: 2:47) Since this is the 1st class of stereochemistry so I thought that I will introduce the course instructor which is myself, Professor Amit Basak and my teaching assistants, you can see the names, Arundhati Mandal, Eshani Das, and Monisha Singha. They will help in answering your queries and they will also upload the questions and the other teaching materials.They will also try to dispel any doubt that you have while going through this course using the forum available for this course. The problem of stereochemistry lies with the visualization of molecules in three-dimensional space because you do not have the luxury of having a molecular model in your hand when you see them in an examination hall. What you have to do if you have a problem? You will need to visualize these in three-dimension and then try to figure out that what would be the perfect geometry, how these interact with other systems. This is the most difficult part of stereochemistry; that is the visualization of molecules just by thought process in three-dimensional space. Now, let me make little clearer. What is the problem between this two-dimensional chemistry I was talking about. (Refer Slide Time: 4:14) Before I do that let me show you what are the different modules that are there in this course. These are already put in the way when I think you can go through these 8 modules that are orchestrated in such a fashion that slowly you would learn how to draw the three-dimensional structure and then try to analyze their interrelationship and then finally we will come to the reactivity of these molecules and the reactions involving with these molecules. So basically this is a very fundamental undergraduate course at the BSc level and I do hope that initially there may be some difficulties in visualization and conceptualization of this 3-D molecules but later on, I am confident that if you go through this course, you will also feel confident like me in conceptualizing 3D structures, the reactivity of molecules which can exhibit three-dimensional geometry. (Refer Slide Time: 5:32) Now, I again come back to the definition of stereochemistry. It is a function of, molecule in three-dimensional space. I can clarify it little bit. If you take benzyle ehyde vs ortho methoxy benzyledehyde, you know that the carbonyl group in benzaldehyde is more susceptible to nucleophilic addition because of greater electrophilicity of the carbonyl carbon. Both these two aldehydes react with a nucleophile, like X- but their reactivity is little bit different; their rate of reaction will be different because of the electronic effect of this methoxy group vs the hydrogen. What are these electronic effects? That is the - I and + R. So here the reactivity difference comes from the electronic effects.. I am saying is not that the methoxy is pointing in this direction and that is the cause of difference in the activity of this carbonyl vs the other carbonyl. While if you take the completely reduced system of these 2 molecules especially the methoxy compound, If you reduce it, all the double bonds are reduced. you get what is called a cyclohexyl system. And when you reduce, you have a a very interesting situation and that this the aldehyde may be aligned on the same face as the methoxy group or the aldehyde may be aligned in the opposite face of the methoxy group. And because of this relationship, which is a geometrical relationship, the reactivity of these carbonyls will be different. And this difference now comes as a result of the different geometrical relationship between the methoxy and the aldehyde groups. In benzaldehyde case, again I repeat, the methoxy is exerting electronic effects as compared to the hydrogen and that is the reason for their difference in reactivity. So the activity of the reduced aldehyde, now is dependent on the steric disposition, that means the three-dimensional disposition of the methoxy group in space, its relation to the aldehyde. So that is a very simplified way of describing what is stereochemistry. Now this subject did not evolve from a very rational approach like many other subjects. Take for example, the discovery of medicines, Many of the medicines have discovered by chance and then people know how these medicines worker and then developed newer medicines. Similarly stereochemistry also developed by chance by trial and error, by serendipitous discovery. And then people finally come out and explain what was happening. what was the serendipitous observation, why was it happening. (Refer Slide Time: 8:52) I will tell you the historical perspective of the development of the subject. Now it all started with the property of light which we know that it is electromagnetic it is an electromagnetic wave. So there are 2 vectors, electrical vector, electrical waves and a magnetic vector, a magnetic wave and both are perpendicular to each other. Now if you consider only the electric vector, so what happens, in normal light the vibrations, the electrical wave vibrations, are taking place in all directions, in all directions that is possible around a point. That means if I take this, this is the light passing from here to there. Then the wave actually is taking place in every possible place in every possible direction. So basically it encompasses the whole 360 degree around this axis of migration of the light. That is the normal light. Now in the 17th century, the Dutch astronomer, Huygens, had discovered what is called plane polarised light. This means what I was telling earlier that the wave was vibrating in all possible directions. That is the normal light. But when it passes through a special type of glass which is made up of some crystals, something else happens. It allows only the vibrations which are occurring in a particular plane and then the light that comes out, all the vibrations are taking place in a particular plane and that is what is called plane polarised light. In this case because I am showing it is in the horizontal direction, so that will be called horizontally polarised. But actually these are what are called either linearly polarised light or for you as a beginner, you may just call it as plane polarised light. And the plane in which the vibrations are taking place is called the plane of polarization. This happens when light passes through a transparent plate made up of a special kind of a crystal. For example, crystal of calcium carbonate which is known as calcite, they have this special property. Not all crystals will do this. Now if you allow this plane polarised light to travel through another crystal which can also produce plane polarised light thent what happens? If you now have these crystals or this plate aligned in such a direction thatl it will have the plane of polarisation perpendicular to the plane of polarisation that the 2nd crystal is is generating. In that case, the question is what is the outcome? This will completely block the propagation of the light on the right site. On the other hand if you rotate this and ultimately by 90 degree, then what happens? The axis of polarisation of both the plates will become aligned and then the light will come out. So using these two plates, you can actually analyse whether if you put a compound in between the plates, the light going from here to there or from left to right, then what happens? Suppose the plane of polarisation is rotated while passing through the compound. Then the light which comes out of the 2nd plate will be blocked. You will have to rotate the 2nd plate in order to align with the plane of polarization . Then light will come out with full intensity as before. . (Refer Slide Time: 13:08) And so it all started with the plane polarised light and its interaction with some special types of molecules. People became interested to know how these molecules behave when plane polarised light passes through these in solution or in liquid state. And very interestingly, they found the following: . is the first crystalline plate by the way is called polariser. So it will produce the plane polarised light and suppose the vibrations are taking place in a vertical direction. When it goes through the solution, if the solution does not change the plane of polarisation, the plane of polarisation remains vertical. Now you put the other plate to view the light coming out of the solution The 2nd plate is called an analyser (so called because you are analysing whether there is any perturbation in the plane of polarisation of this light). So using this analyser, you can now check whether there is any rotation in the plane of polarization while passing through the solution of the sample. People started doing this sort of experiments using different types of molecules. And what they found that there are some molecules which rotate this plane of polarisation either clockwise or in anticlockwise fashion and there are some molecules which do not. So if it rotates, suppose this is an example where the plane of polarisation is getting rotated in a clockwise direction and how do we know it? You rotate the analyser so that the axis becomes aligned to the rotated plane of polarisation. Now you can see the light coming out from the analyser. So by the amount of rotation that is needed to see the light coming out, that will be the amount of rotation that has occurred while light is going through the solution . (Refer Slide Time: 15:28) Biot, a French scientist who first discovered this phenomenon that there are some molecules where if you pass the plane polarised light that undergoes rotation that means the plane of polarization undergoes rotation and then you can find the amount of rotation and that is characteristic of the sample. But as I told you, this does not happen with all compounds. There are some particular compounds which have this typical property and he described the phenomenona by saying that the molecules are showing optical activity and these molecules he called optically active molecules. The reason for this rotation of plane of polarisation was not known at that time. Then what happened? (Refer Slide Time: 16:29) After this event, Louis Pasteur, the famous French chemist came into the picture. So what he did? At that time, known as tartaric acid was known. It was obtained from various sources like it can be obtained from fermentation of grape juice. He isolated this tartaric acid, and then did a crystallization experiment. This tartaric acid obtained from grape juice fermentation was called Racemic acid. And this Racemic acid, he crystallised as the sodium ammonium salt because it is a dicarboxylic acid. So one is sodium, one hydrogen is replaced by sodium and the other one by ammonium. So it is a sodium ammonium salt. And then he crystallised it and what he noticed that there are two kinds of crystals that were obtained. And using using a microscope, he could separate these two kinds of crystals. And then he could find that this one set of crystals is the perfect mirror image of the other crystal. And then with the separated crystals, he analysed them. That means he wanted to know what is the effect on the plane polarized light when it passes through these crystals but the crystals are in solution now. So basically Racemic tartaric acid was divided into two sets of crystals, these crystals were mirror images of each other and then when light passes through these crystals, he found that one set of crystals is rotating the light in a clockwise direction, the other set of crystals which are in solution, they are rotating the light in the other, that is in the anticlockwise direction. So the overall observation is that basically you have the same molecule, tartaric acid. In two- dimensional chemistry, this looked to be only one compound but because they have different rotating power,, they cannot be the same molecule. So the same, the molecule which looks the same in two-dimensional geometry, Pasteur has shown that actually they can be consisting of two different sets of molecules. So that actually is the beginning of the three-dimensional chemistry which is now called stereochemistry. Thus Pasteur laid the foundation of stereochemistry. (Refer Slide Time: 19:28) But the question is why? How can you really explain this type of phenomena that where you have the same molecular formula, same type of connectivity but you can have two systems generated out of that? But before that, Pasteur actually did another experiment. See, apart from Racemic acid which can be separated into two sets of crystals, he also studied another form of tartaric acid. He found another form of tartaric acid, the same molecule formula, same constitution. That means, same type of atoms attached to similar type of atoms. And then he found that this tartaric acid which was called mesotartaric acid, which he failed to separate it into two isomers unlike racemic acid and then he found that this does not rotate the plane of plane polarised light. So this is again different from the earlier Racemic acid that he obtained from the grapefruit juice.