Stereochemistry II Lecture 1

0:00 so we're going to start with the first part that is conformational analysis so okay so there is a difference between these two compounds and these two compounds we call these compounds as these are configurations now configurations are the different arrangements of atoms in the space but i cannot change this form into the other form by simply rotating the bonds because if i'll rotate it it won't change to the other form it will remain in the same configuration i can change this configuration into other configuration only when i can break these bonds i will break this

0:47 chlorine and allow it to attack on the other side iodine will leave and iodine will come on this side so this completely involves bro bond breaking and formation process you cannot convert one one form to the other form simply by means of the rotation but in the case of these two compounds we call them as the conformations in the case of conformations we have to remember we can change one form to the other form simply by rotation around the carbon carbon bond this is carbon-carbon single bond so this can be changed from one form to other form without affecting the bond strength that is very important so we call these structures as the conformations

1:34 now in the case of the stability of these two conformations we have studied the energy profile diagrams now in the case of energy profile diagram i will draw it for these two so let's say i'm drawing for the ethane so on the y-axis i will present the energy and this is presenting the dihedral angle now what is dado angle let's say i'm drawing for this newman projection this hydrogen and this hydrogen this is eclipsed form that's why we have

2:20 now the plane containing this hydrogen and the plane containing this hydrogen they're in same plane they coincide with each other and that's why the angle between this plane is 0 degree similarly if i'll draw it for the staggered form this is the front carbon and this is the backside carbon so the plane containing this hydrogen and the plane containing this hydrogen the angle between them is 60 degree okay so we call this particular

3:06 conformation as a staggered conformation so now what is happening in the energy profile diagram i will start from zero degree so in the zero degree conformation these two hydrogen they have what their specific van der waals radii and they try to come more closer than their some of their vanderbilt's radia that is going to create the repulsion between them and that's why at 0 degree the energy level of this conformation will be very high because it's having more repulsion but as it comes to this 60 degree at 60 degree these two hydrogens are away from each other and that's why there is a less repulsion more stability and that's why we are expecting this kind of

3:53 so that's why we are expecting this kind of curve now this will reach again at 120 this will come again at 180 so like this but in case of butane sorry in ethane remember this 60 degree and this 180 degree they are not different because we are finding or we are coming across the interaction between the hydrogen atoms only so 60 degree and 180 degree will have the same energy levels because they're going to experience the same kind of repulsion okay so now there is one word confirmations and the other word is conformer

4:38 there is a difference between these two words remember now when i talk about the confirmation confirmations there are numerous number of confirmation present in the compound because if you talk about 0 degree 60 120 there are various angles available you can take 0.1 degree 0.2 degree there are also different conformations so all these conformation in case of ether there are numerous number of conformations are available but conformers are only those which are the minima of the energy profile diagram i will only focus on this because this and this is same so this minima of the energy profile diagram is called as conformer so staggered is the conformer of the ethel okay so how many confomara are there for the ethane one how many confirmations

5:24 are there there are numerous confirmations all conformers are confirmations but all confirmations are not necessarily to be conformer okay now i will take one more example here that is of butane so in case of butane we'd write the structure at ch3 ch2 ch3 and we just observe the demand projection across this 2 and 3 bonds so when i talk about it i am drawing the newman projection okay so these are all different conformations of the butane now you can

6:12 see the dihedral angle between the first two methyl we are going to focus on these two methyls so here the dihedral angle is 0 degree here the angle is 60 degree the next angle is 120 degree next angle is 180 degree next is 240 and 300 and finally at 360 will come to the first structure again now in the case of the nomenclature of this conformations we'll be using two word one is the periplanar word the periplanar means the two groups are in same plane

6:58 and the second one is the clinal where we say they are in different plane also two more words one is anti when the two groups are on different sides or not different i will call it as opposite side okay and the second one is sin where they are on same side so using these words we're going to define each and every conformation so let us talk about conformation number one two methyl groups are on same side so i will call it a sin and they lie in one plane because the

7:44 diadel angle is zero here so this is sin periplanar structure the next one's second conformation where still the two methyl groups are on same site but they are not in same plane so this is synclinal now this is kind of actual equatorial position so the methyl groups are on opposite side axial is upwards equatorial is downwards so here they are anti but they are anticlinal fourth one is the most important structure where the two methyl groups are far away from each other so this is anti

8:30 but this is periplanar because they their two planes are coinciding then we have the fifth structure which is also anticlinal if you'll see the third and the fifth structure they are going to experience the same kind of repulsion and the last one sixth which is similar to the second structure which is your synclinal structure okay so when you draw the energy profile diagram for this remember in the case of energy profile diagram this is presenting the dietal angles 0 degree

9:16 60 120 180 240 300 and finally we can come across 360. so at 0 degree this structure is going to experience more repulsion because the two methyl groups are very close to each other so this is going to have the highest energy repulsion at 60 degree this energy will decrease but it won't be much like it is not a more stable structure still there is some kind of repulsion is there from first conformation we have come

10:01 across the second now we will talk about the third confirmation now in case of third confirmation again it is coming to the eclipse but this eclipsed and the first eclipse is different because in the case of first eclipse there is more repulsion between two methyl groups in this case there is a less repulsion and that's why in this case though the energy level is increasing but it will not increase to this level it is lesser than that the fourth structure is the most stable form and that's why the energy level will be very low which will again change to the fifth structure which will have the same energy level like your third so this is fifth structure

10:46 is again anticlinal fourth is the most stable form and then it will go to the again the energy level like second and finally it will reach to the again 0 at 360. so how many minima are there over here there are three minima in the case of butane and that's why there are three conformers how many conformations are there in the case of butane there are numerous number but how many conformers are there three out of this which is the most stable conformer that is anti-periplanar anti-periplanar unit is also called as the anti-butane unit remember this word

11:32 because this is we are going to use for the current syllables so this is anti-butane unit okay so this kind of analysis studying the relative stabilities of the compounds on the basis of internal forces and their properties how they are affected due to their structures these all studies we call it as the conformational analysis now while studying the conformational analysis we are going to come across the various terms so these terms i will explain it to you so this part we are going to talk about types of strain in the case of types of strain the first

12:18 strain that i will talk is the bare strain bare strain is also called as the angle strain now what is meaning of the angle strain this is something a comparative study of the carbon from its expected angle for example if i say the carbon is present in its respective structure carbon is present in its saturated ring form so i have a carbon in the saturated ring form so what is the angle here the angle is 109.5

13:04 degree but if i arrange the same carbon in the cyclic form this is also saturated sp3 hybridized but what is the angle here 60 degree so the angle is deviated from the normal expected angle and because of this deviation there is some strain in the ring how to calculate that strain in the ring for that we have a formula and the formula says the angle strain is equals to half into the reference angle i will call it as a reference angle because here we have considered the carbon is at in the sp3 hybridization we can have a different hybridization

13:50 and accordingly the angle we will select minus alpha alpha is the internal angle of the cyclic chain so this is half into 109.5 which is the expected angle but actual angle is 60 so this comes out to be around 24.75 degree but if i calculate this for cyclobutane ring over here the angle is 90. so what happens to the angle strain half into 109.5 minus 90 degree so when you calculate for this the angle comes out to be around 9.75 degree so if you'll see from

14:35 propane to butane the angle strain has decreased because as you go on increasing the carbon atom this carbon-carbon carbon chain goes on opening up so it they are free to have the more angle towards the 109.5 and that's why angle strain goes on decreasing okay so angle strain is something a deviation from the expected angle which creates the strain in the molecule that we calculate using this formula second type of strain that i'm going to talk is the van der waals strain or which is also called as the steric strain for this i will take the same example of

15:21 the ethane present in eclipse and the staggered form now over here the repulsion is more in the case of eclipse form and here it is the less so when these atoms crosses their van der waals radia they start repelling each other which creates the strain in the molecule that strain we call it as the van der waals strain or we call it a stearic strain this is existing between the atoms attached to the adjacent carbon these hydrogens are not directly attached to each other they are attached on the adjacent carbon and they are creating the string the third type of strain that we will discuss is the torsional strain or we also call it a pizza strain

16:08 now what is this torsional or the pizza string now when i talk about this uh eclipsed and the staggered form if i say they're free they're allowed to rotate freely in the compound then what will happen there will be random rotations and there will be more disorderness in the molecule this disorderness definitely is going to give us the higher entropy entropies disorderness in the structure so if these molecules are allowed to rotate freely they will create more randomness more disorderness and that's why there will be more entropy but actual entropy is found to be less than that that means they are not free to rotate

16:54 and since they are not free to rotate there is some energy barrier if you will see this energy barrier in the case of ethane is around 3.3 kilo calories per mole so that difference they have to overcome to change from one form to the other form and that's why these molecules are expected not to have a random rotation because of there is a strain in the molecule that's called as a torsional strain that energy difference okay and the last type of strain that we will talk is nothing but trans annular strain now in case of strong's annular strain i will take example of a

17:43 simple cyclohexane now cyclohexane we know it's present in the six member this is planar form which is not actual form of the cyclohexane it's stable in its chair form but there is one more confirmation for this which is possible is the boat form there are various confirmations but i'm taking this boat form specifically because if you'll remember in the boat form these flagpole atoms they come closer now if you look at these two hydrogens they are not attached to the same carbon they are not attached to the adjacent carbon they are actually across the rings they are the atoms attached to the across the ring but because of this non-planar structure

18:29 since these two carbon atoms come closer this hydrogen crosses their van der waals radii and that is going to create the strain in the molecule and that strain we call it as the trans annular string okay so these are the four types of strain bare strain steric strain torsional strain and trans annular strain so now the structures of uh the in case of the cyclo octane for that i will share chem sketch with you okay so first we will talk about the structure of the cyclo octane so when i talk about cyclooctane these

19:16 are the eight carbon arranged in the cyclic form we cannot arrange them in the planar form like this so the most preferred confirmation for this is the crown form so in the crown form the four hydrogen which i have highlighted here sorry carbon which i have highlighted here this one the alternate carbon they are above the plane and remaining four are the below the plane if you look at the structure from the top it may look like this but remember four carbons are above the plane and four carbons are below the plane so it looks symmetrical like this so previously it was expected that the structure is present in this uh symmetrical form in the highest

20:03 population but later on through the nmr spectroscopy even through the x-ray crystallography and some energy free energy level calculations it has been seen that it is found to be present in this bought chair form now this is kind of like a boat and this is kind of like a chair conformation of the cyclohexane type so this is having the board chair conformation now in this board chair conformation we'll see the first carbon having two hydrogens here now this hydrogen will be involved or in the closer distance with the hydrogen

20:49 atoms of the fourth and the sixth carbon atom so they will have a kind of a repulsion between them this strain we have already discussed is called as the trans annular strain so there is a strands annulus strain between the hydrogen atom of the first fourth and the sixth carbon similar kind of the interaction is also expected between the proton of fifth carbon seven carbon and the third carbon they will also have the trans annular interaction between them so this transaniler interaction is exhibiting between the carbon hydrogen atoms of the first four and six

21:36 carbon as well as of the third fifth and the seventh carbon the hydrogen atoms which are involved in this kind of strain are called as intra-annular hydrogens while the one which is present here which is not involved in any kind of interaction we call it as the peripheral hydrogen so in the cyclo octane the six are the tran intra annular hydrogens and the remaining are the peripheral hydrogens this kind of strain can be decreased by means of changing the hybridization of the carbon we can change it from sp3 to sp2 hybridization so we can replace it by the carbonyl group let's say

22:23 so if you replace it by carbonyl group this hydrogen won't be there to involve in the trans annular interaction so obviously the strain will be reduced in the model okay you can also replace this carbon atoms by some hetero atoms like oxygen sulfur or nitrogen so that will also reduce this trans annular strain now we will discuss the structure of the cyclo decane now cyclodekin also exist in the different uh confirmation form uh but the most populated one i have drawn a structure here this is kind of like a boat in between there is a chair form and

23:08 again this is an inverted board so we call this as boat chair board conformation now in this boat chair boat conformation let us concentrate on carbon number one now this carbon has two hydrogen one is here and one is here now the one on this side will be involved in the trans annular interaction with the carbon number four and carbon number eight okay so there is a trans-annular interaction between the hydrogens attached to the carbon number one carbon number four and carbon number eight okay so at first carbon we can see one is the intra annular hydrogen

23:55 and other is the peripheral hydrogen similarly over here the hydrogen atom attached to the third sixth and the ninth carbon they are also involved in the trans-annular interaction okay so now we will concentrate on this few carbons specifically if you look at this structure if you look at this first carbon and third carbon let us compare them now in the case of just i'll draw the normal bond here hydrogen now if you'll see here in this case this is an axial bond and this is an

24:42 equatorial bond so at the first carbon the axial bond is intra annular and the equatorial bond is the peripheral whereas at the third carbon atom the axial is a peripheral and equatorial is an intranuller so the first carbon and third carbon they are not seen they are having different environments as well as look at this second carbon at this case in in this particular case the two hydrogen are not involved in any kind of strain both are peripheral axial as well as equatorial so there are three types of carbon these ten carbons overall in the structure are divided into three types now what are

25:29 the three types the first type in which the axial proton is intra annular and equatorial proton is peripheral second type where both actual equatorial are peripheral third type where the equatorial is a intra annular and the axial proton is the peripheral so out of 14 hydrogen total there are 14 hydrogen in the structure so out of this 14 hydrogen six hydrogens are intra annular well remaining eight hydrogens are peripheral ten carbons are divided into three types okay the proper model of this structure if

26:16 you look compare it it will resemble with the uh diamond and let eyes because diamond is also made up of the cyclohexene rings in the chair conformation so somewhere it is resembling with this cyclo decane structure now in this case of cyclodexin this kind of strain can be avoided or it can be reduced as i have discussed in this a case of cyclooctane we can replace this carbon we can with the heteroatom like oxygen hydrogen sorry oxygen nitrogen or sulfur or we can just change the hybridization from sp3 to sp2 to reduce this string now the there is a conceptor called as the ring flipping or the studio rotation we

27:02 can avoid that pseudoradiation ring see if i compare it with the normal cyclohexane ring that undergoes the flipping process very easily but in this case if i have to reduce this uh the flipping process what simply you will do is you will replace these hydrogens by some substituents especially this kind of geminal substitutions will be comfortable because when you have this methyl groups over here this compound will try to achieve its stable position where there is a no steric crowding because of this methyl group and in that conformation this compound will remain in the rigid form it will not flip to the other form because as it will change to the other form this

27:48 methyl groups can come closer and they can create the steering effect that's why they will try to achieve only one conformation where the methyl groups are far away from each other and that's the most stable so in that stage it will not undergo any flipping or the pseudo-rotation process okay so this is about the structure of the cyclodecane and previously we revised about the cyclo octane i have discussed only structural part of the cyclo octane and decane in the next lecture we will talk about the unusual properties that the medium size rings have compared to the small size rings and the larger rings okay thank you