 Hello everyone, welcome to the module on stereochemistry. We will continue our discussion on stereochemistry and 3 dimensional arrangement of atoms in space. Before we do that, let us just do quick recap of what we looked at in the last lecture. In the last lecture, we first looked at what is the significance of 3 dimensional structural representation of organic molecules. Here we had seen the story of Vantov and how he came up with this brilliant idea of 3 dimensional arrangement of organic molecules and that was consequently proved by experiments. And we also looked at different ways of representing organic molecules in 3D by using either a wedge hash projection or Newman projection, and finally Fischer projections. Among this, we had concluded that the wedge hash is the most widely used method to represent organic molecules and we will take, build on this and look at what are called as isomers in this particular lecture. So, I am sure you would have come across this idea of isomers in organic compounds or in other branches of chemistry like the one shown here that is a cyclohexene and one hexene have both the same empirical formula that is C6H12. But the way in which the atoms are connected in 3 dimensional space is totally different between these two organic molecules. So, this is one particular example of isomers in organic compounds and another is a very simple example where if you take ethanol that is CH3CH2OH and that is again has a same empirical formula as that of dimethyl ether which is C2H6O. So, although these compounds have a similar empirical formula they could have different physical or chemical properties. The properties could be melting point, boiling point, dielectric constant, dipole movement and many other. And one more example of isomers in organic chemistry is this two methyl butanol which is shown here where the only difference is in the way which the carbon is arranged that is the CH3 group on the second carbon either comes out of the plane or goes behind the plane. This is what is also another example of isomers. So, before we go ahead and look at some of the classification of isomers you might be interested in asking this question of why to study these isomers because this looks very simple and very naive. Is there any consequence of this or is there any applications of this? Just to point you one there is a very important application that is if you take a, if you actually take a simple molecule such as this which is shown here which is the xylenes. So, there are three different possible isomers one is the paraxylene that is the CH3 you have. So, this is called the paraxylene. Similarly, you could have an ortho and metaxylene correct. So, this is metaxylene and finally one can also have an orthoxylene. These compounds are actually obtained from the cracking of crude oil and once you get these you get them as a mixture of all the three that is paraxylene, metaxylene and orthoxylene and it is actually very very difficult to separate them because they have very similar properties that is chemical and physical properties. They are different but they are not exactly same. So, the difference between the properties is minor. So, now what is the consequence of this? So, if you actually look at all the water bottles which we use for drinking that is the polyethylene turphthalate bottles they are typically made by, they are made by paraxylene or paraxylene is one of the starting materials to make these para PET water bottles. So, it is a very important commercial starting material to and it is very important to separate it from the other isomers like metaxylene and orthoxylene. So, this very simple idea of isomers can actually have a profound influence on the material properties. So, even today in 2021 there is actually research going on on how to efficiently separate paraxylene, metaxylene and orthoxylene in the crude mixture because that has a very important consequence for other materials which we use in our day to day life such as the polyethylene turphthalate bottles. So, I hope this at least gives you an idea or convinces you of the importance of these isomers and it is not just an academic interest it has relevance in our day to day life as well. So, with this let us get it now looking at isomers in a little more detail and so what you see is that isomers are compounds which we defined as having a similar empirical formula, but they could have a different chemical as well as physical properties. And these can be further subdivided into two categories first one is called as the constitutional or structural isomers that is if the atoms are connected in a different manner between the two molecules which are trying to compare then it is called as a constitutional or structural isomer and the second one is called as the stereo isomer that is the connectivity remains same, but the way they are arranged in space is different. So, the difference between these two may not be apparent at this moment, but we will delve into this in a in a minute to see what is the exact difference and the stereo isomers can further be classified into two more categories called as conformational isomers and configurational isomers. And this is a very subtle, but an important distinction between these two classes. Conformational isomers actually have do not involve any change in the do not any involve any bond making or bond breaking whereas to go from one isomer to another isomer of a configurational isomer we have to break a bond and arrange it in a different form to get another isomer. So, I will show you examples of that as well and further these configurational isomers can be again subdivided into two more categories which is called as a cis trans isomers and the other one is the isomers containing an asymmetric carbon and this is a very both of these are very important classes and they have a profound influence on our day to day life as well. So, we will delve into that in a bit. So, let us just first begin with the constitutional isomers. A few slides ago you saw that we had looked at cyclohexane and one hexane and we said that they have the same empirical formula that is a C6H12, but the if you now look at the chemical connectivity I hope you see that the chemical connectivity is completely different between cyclohexane and one hexane. This is a cyclic molecule cyclohexane whereas here you have an open chain system this is an open chain and whereas here you have a cyclic. As a consequence the properties of these systems will also be different that is melting point, boiling point, refractive index and many such properties and not just these two molecules you can also draw another isomer of this which is a three hexane which is a very simply moving the double bond from the first carbon from here the double bond to the middle carbon here that is one you would get a three hexane and you can also come up with more elegant structures that is three methyl one pentene that again has a same empirical formula that is C6H12 and finally you can also come up with a little more innovative ideas like one ethyl two methyl cyclopropane as shown here on the right hand side. So here what you are saying is that with a given molecular formula you can write many many different kinds of structures and all of these are having completely different connectivity that is in cyclohexane it is a closed structure like for example here as you can see in this it is a completely closed structure of a cyclohexane whereas if you now go to the one hexane it is an open chain structure and if you now go to the completely extreme you have a cyclopropane derivative as well. So just to make you understand a bit better if you can take the analogy of these cats and I hope you can see that the one on the left which is here is a normal cat with four legs and a tail and to show you the constitutional isomers what we have done or what is done here is that you take the same cat chop off one of the legs and put it back into its the position of the tail and take the tail back and put it in the position of the leg. So these are classic examples of constitutional isomers because to go from one form to the other you need to rearrange or you need to break some bonds and make some more bonds at different places. So I hope this gives you an idea of what do you mean by constitutional isomers. So now let us go ahead and look at stereo isomers in a bit more detail. So we told that in stereo isomers the arrangement the you do not have breaking of you do not have a significant change but their spatial arrangement is what differs. So in this there are two categories one is called as a confirmation another is called as a configuration. So the first one is what we are looking at is a confirmation here that is we have a methyl cyclohexane that is what I am trying to show here is the same thing here. So I have a cyclohexane which is in the chair confirmation as you can I hope you can all see this there is a cyclohexane in the chair confirmation and then I have put the methyl group that is the CH3 in the axial position here and I can actually twist this this this boat in if I twist it then I can actually get this axial back into the equatorial position which is shown on the right hand side and this can be done without breaking of the bonds and this equilibrium this let us say so this conversion between the between the two forms that is the axial axial CH3 and the equatorial CH3 is actually continuously taking place at room temperature. So you would typically when you take a bottle of methyl cyclohexane if you buy it from a commercial source what you would get is you will always have this two forms which are actually equilibrating with one another and you cannot actually differentiate or distinguish them unless you go to very very very low temperature something like minus 50 or even lower then you might start seeing the signatures of these two different forms that is axial and the equatorial. So this is what we call as confirmation that is the isomers which do not involve breaking of bond they actually undergo conversion between them rapidly without involving a breaking of a bond and the next one is what is called as a configurations that is if you look at this particular example here which is shown on the right hand side here what I have taken is a two methyl butanol is what I have taken and this particular carbon is a stereo center or a chiral center and to go from this to this form that is one enantiomer to another enantiomer I will come back to this term what do you mean by enantiomer in in either this class or the next class so one has to actually break the bond you cannot interconvert between these two forms that is between say form 1 and form 2 by just rearranging the atoms one has to actually break the bond and again form a different kind of a bond so that is what and these are actually isolatable you can one can actually isolate them and they have very different they have different properties mostly in terms of the way they rotate a plane polarized light and rest all of the properties of these two forms that is one and two remain the same and it is very important to get this distinction between conformations and configurations because both the words look very very similar so please do not get confused between conformations and configurations so to just to illustrate this further I will show you I will give you one more example of conformations which we looked at previously so if you take the example of the ethane so I had shown you in the Newman projection that I have a front carbon which I am now drawing here I have hydrogens and this is a back carbon correct so this is what we looked at the ethane staggered configuration this is the staggered configuration and now we had also looked at another configuration that is conformation that is the eclipsed this is a back carbon and you have similarly so this is the eclipsed okay so these two are actually conformations and not configurations because if I again go back to my model of ethane what I have is I have this let us say I will start with a staggered configuration I have if you look at it from this along this bond cc bond you see that this actually does not this hydrogen does not coincide with anything at the back you clearly see three in the front and three in the back I can just rotate the back carbon the back carbon along this cc bond to get to the eclipsed form where you actually do not see the back carbon along with the three hydrogens and this I can do without any breaking so I can just rotate freely along this bond along this cc bond and this is what is called as a conformation rather than a configuration if I have to let us say change something like for example if I have to go from a particular configuration say I have a in this particular example I have chosen let us say in this particular example I have one atom is in front another is back if I actually want to flip it if I want to really flip it and change it or to make another enantiomer I will have to break the bond without that it is not possible and this is an example of a configuration and not a conformation so please keep this distinction between a conformation and a configuration so now let us go at a bit in the in this configuration and look at two different kinds of configurations in the previous example here I hope I had shown you that this particular example on the right hand side is where the stereo center is different on the carbon which is marked with an asterisk right but there is also another kind of a configurational isomers and that is called as a cis trans isomers so in this particular case what happens is that if I if I take a so what all the while now we were looking at actually a cc single bond if I actually put a two bonds between the two carbon atoms then this rotation actually is now completely hindered because you have an orthogonal p orbital you have p orbital which is actually bonding between these two and that will not allow this carbon to rotate freely right so that is why you only have a free rotation along the cc single bond and not along the cc double bonds or even the triple bonds so now once you cannot rotate freely then you will actually get isomers based on how you substitute in ethene and that is what we are going to look at in terms of the cis and the trans isomers so if you now take the if you now take this particular ethene let us say for example which is shown here and it has been now substituted with the bromine chlorine and the two hydrogens this is one of the form and you can also do it in a slightly different form that is you can have the chlorine on the other position on the other side of the double bond and the bromine on this side so this would actually give rise to two different isomers so these are two different isomers if you want they will call them form 1 and form 2 and these are actually completely isolatable you can isolate them as two different products they have different melting points they have different boiling points based on whether it is a solid or liquid and they will have different dipole moments as well so I hope that is at least the dipole moment part is apparent because here you will have a this bond is now polarized along this direction and here you also have polarized along this direction because you have an electronegative chlorine and bromine so net you will have a vector addition of the dipole moment which would be somewhere along this direction right however so thus you will have a mu would be some number which is let us say I am putting x x d by however if I go to this form the form 2 I will have one vector along this direction and the other vector would be along this direction which is opposite and if I add these two vectors vectorially individual dipole moments vectorially then I would get a I will call this mu 1 and this is mu 2 I will get a number y d by because the two dipole moments are opposing you would have the mu 2 being less than mu 1 and this is clearly borne out based on how the atoms are arranged in the space that is either they are arranged on the same side or they are arranged on the opposite side of the double bond. So and the nomenclature for this is as follows if you have both of this on the same side then you call them as a cis that is there on the same side or UPAC nomenclature would be it is called as a Z isomer and this Z comes from the Zuzerman which in German means together that is you have both the bromine as well as the chlorine together that means they are on the same side of the double bond. So that is why it is called as a Z isomer and if you now go to the structure 2 it is called as what is called as a trans or they are there on the opposite side or it is also called as in the IUPAC E isomer. Here E stands for the antigen which in German means opposite. So I hope this gives an idea of how by just going from a single bond to a double bond you can first completely do away the free rotation, now your restricted rotation and that leads to the isomers called as cis trans isomers and the way we get to them I will tell you in a minute. So let us say if I have a double bond now like this with substituents say Bromo and H I am going to just take the same example here Chloro and H. So to do this what is typically done is on each of these carbon atoms that is this and this you need to look at the priority of the atoms. So here and that is done by looking at the atoms with the highest atomic number will have the higher priority. So obviously in this structure Bromine has a higher atomic number so this would have a higher priority. So I am just going to write it like this higher priority and if I come to this again here Chlorine has a higher priority. So I am going to write it as a higher priority. So if I do this now I see that both the groups on the carbon atoms having a higher priority or on the same side and then I would name this as a cis or a G isomer. You can do the same exercise for the second form and you would get the transform correct. So this looks obvious because you have hydrogens and Chlorines which are very easy to distinguish. So now let us look at one example where it is a little more trickier. So here I have shown you two forms that is let us call this A and B. I am going to call this form as A and this form as B okay. So now let us go ahead and try to look at the priority first because based on priority we are going to assign which is the highest priority. So let us take at the structure A and let us start from this particular carbon here. So I have a CH carbon here and I have again a carbon here so their priority is the same. So now I need to move to the next carbon or the next atom. So here I go to the next atom. Here the next atom is again carbon whereas if I move to the next atom this is chlorine. So obviously this has a higher atomic number so this would get a higher priority. Higher priority lower priority I am putting it as LP. Now I am come on this carbon atom which is this. If I go to the first attachment it is again a carbon here similarly carbon here so not allowed to distinguish. So now I need to move to the next atom and if I move next here I have again a carbon or a carbon whereas on this substitute if I move ahead I have an oxygen atom which has a higher atomic number compared to carbon right. So now this would be the highest priority and this would be the lowest priority group. So in this case now what happens is you see that both the groups which have the highest priority are on the opposite side that is then this would be a trans or an E or E isomer right. So similarly if you do the same exercise here what you would get is you would get the highest priority is on this carbon here and the lowest priority is this based on the same arguments as on the structure A and if you come to the other carbon atom here you will get the highest priority on the highest priority group is now actually lower or it is on the same side as that of the other carbon and now you have the lowest priority group this side. So in this case what you see is that both the groups which have the highest priority are on the same side that means this structure is Isis or you want to call it as a Z Isomer. I hope this gives you at least some idea of what do we call it as a Cistans Isomer. So now again you must be wondering what use is this of or is this just purely for an academic purpose or just to study not really. So the answer is that these kind of Cistans Isomers are actually very very critical and in fact important for the vision which we are all blessed with. So the fact that we see different molecules of different colors and things around us is based on this continuous Cistans Cistans rotation in our eyes in a molecule called as Retinol and this particular molecule is the one which would Retinol is the molecule which is actually responsible for undergoing continuous Cistans Cistans Isomerizations in our eyes and this Isomerization actually is what is gives rise to the vision which we all observe. So I hope this gives you an idea that these Isomers which might look very simple with the structures which we have shown actually have a very profound influence both on the materials we come across as well as in the biological processes. With this we stop here and in the next lecture we will look at a bit more closely on Stereo Isomers and what are called as Enantiomers and Diastereomers and what role do they play in various kinds of materials. Thank you.