in this video we're going to talk about the confirmation of alkanes so let's get started so if you look at this structure this is the structure for ethane uh you can see the there are two carbon atoms and six hydrogen's atoms and if you look at very closely you can see the hydrogen's carbon hydrogens the bond angle here is 90 degree but this all carbons here these two carbons are sp3 carbons so theoretically their angle should be 109.5 degree so if you draw the 3d structure of these compounds ethane it will look like this so if you right the first carbon is this carbon the second carbon at the last carbon so then uh you can convert uh this 2d structure into 3d structure in these two different structural form how usually when you when you are going to get this to structure we wrote we actually rotate about a carbon-carbon bonds we know carbon carbon bonds are single bonds here so those are sigma bonds and sigma bonds are usually formed when sp3 orbitals of this carbon do and hidden overlap with another sp2 sp3 carbon of this carbon sp3 hybridized of these carbons so again probably just to say again the sp3 orbitals of this carbon and is with three orbitals of this carbon they are usually do head on overlap and then form these carbon-carbon single bonds when when you have a heading overlap or end on uh and an overlap so then you can rotate that bonds and if you rotate that bonds you usually end up with this type of structures this type of structure this is usually called special arrangement of atoms a different special arrangement of atoms usually results from the rotation about carbon-carbon single bond as i was saying and those we are called conformers so special arrangement of atoms that are usually results from carbon-carbon rotation we call conformations and these two here we call conformational isomers this process we can confirmations and that these two we can convert conformers and the representation here is shown we call horse projections or sawhorse projections there's another way to describe the conformational isomer structure is to look at the molecule along the axis of the bond of interest and that positions we can niemann projections is a graphical representations to view another way to view the conformational isomers the way that we usually draw the demon projections if you have sars projections let's say you are looking this along this axis this carbon to these carbons and this is your first carbon so if you look straight along this axis this is your first carbon this is your back carbon so the way that you should look at here this is your front carbon so we have to draw a circle and then from at the center the center carbon is the first carbon that we call front carbon now the front carbon has the chlorine substituents on the top and then on the left hand side it has hydrogen and on the right hand side you have another hydrogen and the big circle here that this circle represent here is the back carbon so that back carbon it has the methyl group on the top again it is pretty much aligning with the chlorine then it has hydrogens on the left and another heart disease on the right so these overall representations are formed when you are looking along these axis if you're looking along this axis then you can have these projections we call neumann projections if you look at this structure here this conformers this is your first carbon this is the first carbon and chlorine on the top hydrogen on the left and this hydrogens on the right and when you have uh the back carbon which is the the circle back down there and this back carbon has the methyl group at the bottom and the hydrogens on the left and this hydrogen at the left so these two are we call neiman projections and this name neiman projections and between these two this is the least stable newman projections and this is the most stable niemann projections and we will talk about that why this particular representation is less stable and this particular representation is most stable later on when butane molecule rotates about the carbon-carbon bond there are two extreme conformers can be generated so if we want to draw the neumann projection of these two uh the eyes conformers of this butane again butane is one two three four four carbonyl can you call butane and we are kind of uh interested here c two and c three bonds if you rotate that bond here then there are two extreme conformers can be generated and we call staggered confirmations another is we call eclipsed confirmations so if you think about this structure like this form so this is your front carbon this probably c2 and this is c3 so this is c2 and this is c3 c2 has a methyl group here so this is the black circle represent the methyl group and the rate circle here represent uh the hydrogen's group so this is c2 and this is c3 carbon we like to rotate about this c2 and c3 bonds then you will end up with um this is the newman projection so of these source conformations we have two methyl group here on the front carbon c2 and c3 carbon has a methyl group so this is our front carbon so methyl group on the top hydrogens on the left under the hydrogen the right the back carbon method on the top hydrogens at the left and the hydrogens on the right so this is the conformations where we can see the methyl group methyl group they are same directions the hydrogens are next to each other same directions in general sometimes in organic chemistry we call four bond four bond angle called dihedral angle so you have this methyl carbon then c three c two and these the dihydrogen angle is zero that means this methyl group these methyl groups they're in the same direction so when the dihedral angle is zero um then we usually call the eclipse conformations so eclipse conformations were about c2 c3 bonds in butane where we see the dihedral angle between these methyl groups c3 c2 and this methyl group is zero another representation of these compounds here butane this is where you can see the methyl group and this methyl group on carbon number two and three they're anti to each other they're in opposite direction if you draw the neiman projection so this is your first carbon here that's the first carbon you see the methyl group on the top one hydrogens on the left another hydrogens on the right and this is the back carbon the back carbon methyl group at the bottom uh down and two hydrogens one one is on the left another is on the right so here what you can see the angle between these two methyl this methyl group c2 c3 and this method of the angle is 180 degree so the dihedral angle for this particular molecule is 180 degree and these conformations we call staggered conformations this is staggered confirmations and there is another type of confirmation is possible for particularly uh butane case as you see we call gauss confirmations where uh the the diagonal angle here in the first case eclipse confirmation was zero here is 180 degrees they're completely anti to each other so this method grammated to this method group this hydrogen is anti to these hydrogens is hydrogen tied to these hydrogens but there is another type that's called gauss conformations and i usually the methyl group and this methyl we can see the diagonal angle between this methyl group c2 c3 and this methyl group is 60 degree when you have that type of conformation we call cause conformation and they are not completely eclipsed neither they are completely staggered confirmation they are in between confirmation we can gauss conformations now that take us to our next lecture problem here draw the neiman projection that represent the most stable conformations of butane view along the c1 c2 bonds so this is our c1 and this is c2 bonds and then we want to draw the neumann projections if you look at this first if you consider this is your first carbon this is the second carbon then you can see the first carbon has three hydrogens one hydrogen at the top two hydrogens at the bottom and one over two hydrogen one on the left another on the right so as you see here when you showing this way that this one this hydrogen this carbon decided this is like 120 degree angle that's how you have to think of and the the back carbon which is the circle here this ethyl group is on the top and then one hydrogen the left another hydrogen on the right again the angle is always think about like 120 degree uh that way it will make a circle and here is another uh confirmers of this one so this is eclipse and this is the staggered confirmation so you just kind of keep the first carbons fixed and then rotate um the back carbon by 180 degree then that will give you your staggered confirmation so here is your front carbon three hydrogens one on the top one hydrogens on the left another hydrogens on the right and the back carbon as i said you keep the first carbons fixed when you are going from eclipse to staggered conformation the usual rule is you just keep the one carbon either back carbon right front carbon um fixed and then rotate the other carbon so we are fixing here the first carbon and the rotating the back carbon 180 it was on the top now it's on the bottom so this is because staggered conformations and staggered confirmations are usually the most stable confirmations here is another problem draw the neumann projection that represents the most stable confirmation of pentane view along the c2c3 bonds again if you want to look at here so this is one two three four five so this is five carbon pentane and we're looking from carbon number two and carbon number three so this is your front carbon this is your back carbon so front carbon methyl carbon on the top and then hydrogen's one hydrogen the left another hydrogen on the right again this ch3 this carbon discovery is 120 degree angle that's how it form 360 degree the back carbon here if you look at here the back carbon has a ethyl group on the top and two hydrogens at the bottom one or two one horizontal left another hydrogen on the right that gives you this confirmation we call eclipse confirmations of pentane about the c2c3 bonds now you can draw a staggered confirmation from here so again the rule is the first fix the first carbon or even the back carbon effects so we are here fixing the first carbon in place uh here is the the carbon with the methyl group on the top and then hydrogens of this hydrogen now we're rotating the back carbon 2 down here 180 degree that's how it keeps this uh ch2ch3 here the hydrogens this hydrogen moves down there this head is moved down there so this and these and that's how you integrate the staggered conformations of this pentane structure and of course this is gonna be the most stable this is less stable so usually as i say the rotation about of carbon-carbon bonds they sh they are not usually completely free uh so if you think here for example the simplest molecule is the ethane our two carbon is bonded with six hydrogens and this molecule this carbon-carbon bonds usually the sigma bonds they are rotating now all the time it can be rotated um but this is not completely free so it's not you can you cannot think of like this carbon copper bonds always rotating although theoretically it should but what is actually happening what what is actually preventing that rotation is that the c electrons of ch bond the repels another ch bond if they get too close together so as you see here in this particular structure if this ch bond wants to get closer to this h bond so that this bonding electron will repel this bonding electron preventing the rotations about that carbon-carbon bonds so and therefore when the molecules try to be in eclipse conform confirmation form then this electrons electron repulsions will prevent that molecule to to adopt till eclipse electron um eclipse conformational form and this is why we always say the eclipse conformations is not stable um as the staggered confirmation because in english confirmation the ch bond electrons always repel the other bonds therefore they prevent those bonds to come closer if they don't come closer that means they don't form eclipse conformations and the molecule will remain in staggered conformations where you can see the bonds this bond is anti to this bond this bond anti to that bond that bond is anti to that bond they don't have that electron electron repulsions as type of strain and we usually call this type of electro electro repulsion in organic chemistry we call torsional strain so staggered conformational staggered conformers are torsional strain free molecules but eclipse conformation has high degree of torsional strain and because of that they are not very stable so therefore one can actually draw the neumann projections of ethane molecule at different angles if you draw the neiman projections i had like zero degree eclipse conformations then at 60 degree this is kind of staggered conformation again away from going here to there is like if you kind of fix the first carbon and rotate the back carbon by 60 degree on in the right hand directions like in clockwise then you'll have this one then you again rotate the another 60 degree that will give you 120 degree you rotate another 60 degree then it will give you 180 degree 240 degree 300 degree and final 360 degree and if you draw if you kind of uh put it on the energy diagram so if you see the english confirmations are high energetic because they have uh torsional strains involved there so their energy is higher than the staggered confirmation then again eclipse confirmation then staggered confirmation eclipse confirmation staggered confirmations ellipse confirmation so this is how you can see the zero degree 120 degree and 240 degree and 360 degree their energy is higher whereas 60 degree 180 degree and 300 degrees there um energy is lower so those are staggered conformations and these are eclipse confirmations on the potential energy crab again you can practice how you can go from zero degree to 60 degree 60 degree to 80 degree again the rule is you just keep one of the carbs from the carbon with the front carbon or back carbon in uh fixed in place and then rotate the second carbon uh to the clockwise or anti-clockwise directions by 60 degree or by 120 degree all right you can do the same thing for butane so butane again here is a 0 degree 60 degree 120 degree 180 degree and if you do that zero daily we can see the methyl metal they're eclipsed to each other if you do it is 60 degrees this ch through this yeast either 60 degree apart we can cause conformations and here again eclipse iglesia is 120 and you can see the hydrogens and methyl group they're pretty much next to each other so that's the eclipse conformation and anti-means are completely opposite this methyl group completely opposite to this methyl group this hydrogen completely opposite this hydrogen this is called anti so remembers gauss and ate it anti is a little bit different gauzes like methyl group methyl group they're 60 degrees or not 180 degrees but in this case they're 180 degrees for in in case of anti now one can also draw the similar potential energy diagram and see how they are locating on the diagrams and in order to do that one can also calculate how much the strain energy involved in each particular controller stress so to do that to to calculate how much the strain energy involved for each of these conformers then you need to know the certain um the energy values for each of them see if the hydrogen hydrogen eclipse to each other the torsional strain is four kilojoule per mole and is one kilo cal per per mole hydrogen methyl eclipse group mostly torsional strain and if you have ch3ch3 eclipse they are torsional stereoplastic strain that's 11 kilojoule per mole and if you have ch3ch3 gauss that's a star extent 3.8 kilojoules per mole so determine the total steel energy for the following eclipse conformation so by using this table help one can calculate the total strain energy for this particular conformance or this particular conformance all right so again so when we talk about let's say hydrogen hydrogen eclipse so if we have hydrogen hydrogen eclipse two of them methyl methyl eclipse we have methyl methyl actually that's 11 kilojoules per mole so if you want you can pause the video and try to answer these questions on your own and then i'm going to show you the answer in a second so you can check your answer uh with your check my answer with your answer so let's take a look how much energy involved per for each of these conformers here so if you have uh the first molecule we see the metal and metal that the interactions the that's the eclipsing metal metal eclipse conformational strain is 11 kilojoule per mole according to that table as a stirring interaction with torsional string that involves 11 kilojoule hydrogen hydrogens four kilojoules hydrogen hydrogens four kilojoules so if we add those eleven plus four plus four kilojoules that will add up to 19 kilojoules so the energy the total strain energy for this conformer is 19 kilojoule per mole whereas the second conformer here we have methyl hydrogens which is six kilojoules and then hydrogen hydrogen this is four kilojoules we have methyl hydrogen that is six kilojoule so the total is six plus four plus six that's 16 kilojoule per mole so yeah this is how one can actually calculate the total strain energy for each of these conformers and can actually potentially write their energy on on the potential energy diagrams but another interesting story i would like to tell you why what we see here metal metal group here they are giving the highest strain energy here 11 kilojoule per mole if you look at this structure here you can see the methyl this carbon-carbon sigma bond this carbon carbon sigma point they are repelling each other so that we call torsional strain in addition to the torsional strain that we learned in the previous slide this particular molecules you can see these hydrogens is very close to these hydrogens when the atoms they are getting close to each other as you look at here this atom getting closer to the disk atom then their van der waals uh interaction increased so they like so so this will repel this uh atom and as a result of that i started that studying in um that interactions are hindrance because static hindrance means when atom get closer to close to each other they rebuild each other um and that interaction with the strutting interactions and because of that when two methyl group that eclipsing to each other they have torsional skin plus 30 interactions because they're too close to each other they will repel each other and that's how they end up with such a high value 11 kilojoule per mole so again when you have the butane and you're rotating about c2 and c3 bond this is zero degree dietal angle this is 60 degree 120 degree 180 degree and 240 degree 300 360 degree and in calculate the energy that's if 21 kilojoule 3.8 15 15 33.8 21 kilojoules and if you'll put it in an energy diagram so here is your potential energy this is uh zero degree should be the highest as you see two kilojoules and you can see two methyl groups they are eclipsing to each other that's 11 kilojoule right there so um that's the highest energy and then when you have a 60 degree gauss conformation the way they did it here they keep the fur back carbon fixed and rotate the front carbon this method grip to the clockwise direction by 60 degree so when they have this caused conformations with this metal metal uh interactions here but it's not completely like these so this is around 3.6 kilojoules and the energy at 60 degrees around 3.8 kilojoules and this is around here then again you rotate another system 120 degree then it will be right here 180 degree right here this is a stain free molecule there's no energy involved here strain energy involved this is most stable then you have 240 degree then you have 300 degree and finally 360 degree as you see here 60 degree and 300 their energy is equal to 3.8 kilojoules and they are the same level 180 degrees the most stable one their energy is pretty much low here and then we have 240 and 120 degree those two are eclipse and they're they're up higher energy but they're smaller than zero degree or 360 degree so that is the potential energy diagram for butane and that's all for this video