so we know that straight chain alkanes adapt a zigzag confirmation because it's the confirmation that's the most stable and it's the most stable for two reasons the first reason is that we have the least amount of steric strain in this molecule and so that puts the kind of bulky groups as far apart as they can go and the second reason is because we're in the staggered confirmation and the staggered confirmation is going going to be the confirmation where you have those electrons being able to delocalize to an extent being able to spread out into an empty orbital and that stabilizes the molecule but what can we do about planer cyclic alkanes or cyclic alkanes in general what can we say about their bond angles well we're going to go through some first off thoughts theories behind them and then we'll look at like actual case scenarios so we know that ideally an sp3 hybridized carbon has a bond angle of 109.5° and in 1885 there was a German chemist named Adolf Von and what he said was he said well all cyclic compounds are planer okay that means all cyclic compounds have their carbons in a plane in a single plane and so therefore what he said was that since this is clearly planer since these molecules are clearly planer we can predict their stabilities by looking at the angles within those bonds so for example in a triangle those angles are 60° in a square they're 90° in a pentagon they're around 108° and in a hexagon they're 120° so what he said was that if they're planer then the most stable of those molecules would be the one that has the bond angles that are closest to the ideal at 109.5° so according to his thoughts the most stable cyclic compound should be cyclopentane because in a planer State the bond angles are closest to what we would expect them to be for an sp3 hybridized carbon at 109.5° okay but let's think about let's say cyclopropane that has its angles in a planer confirmation about 60° right there would be a pretty big deviation from 109.5° which would decrease the stability of the molecule is what he said said because the orbitals would not have a strong overlap anymore so thinking about two sp3 hybridized orbitals overlapping right we have good overlap of the orbitals and that would form a strong bond that's what we would potentially see in a tetrahedral Bond angle of 109.5° but if we think about like cyclopropane or something where those atoms are going to have poor overlap we will create a weak Bond and the bond angle is not going to be the ideal Bond angle of 109.5° so the more overlap you have the stronger the bond is going to be and since in cyclopropane or something like that those overlapping orbitals can't Point directly at each other they're not in a line so therefore there's less of this orbital overlap than there is in a normal carbon carbon Bond and so if you decrease the amount of orbital overlap you decrease and weaken that carbon carbon Bond and this is something called angle strain angle strain results from poor orbital orbital overlap because we now don't have the ideal overlap of orbitals right we have a deviation from that 109.5 degree Bond angle for the sp3 hybridized orbitals and we have this angle strain so in addition to the angle strain of the carbon carbon bonds all of the adjacent carbon hydrogen bonds in cyane propane are eclipsed rather than staggered which makes it even more unstable so again not only do we have potential angle strain in propane but we have hydrogen's eclipsed when it's in a planer confirmation so when all the carbon atoms are in the same plane all of the hydrogens in that molecule would also be in the same plane and therefore if they're in the same plane they would be eclipsed and we know that they prefer not to be in the eclipse State because they can't spread out those electrons um they don't have hyper conjugation there and so that is going to destabilize cyclopropane even more so in addition to angle strain making cyclopropane unstable eclipsed hydrogens also make cyclopropane unstable now let's talk about cyob butane so planer cyclobutane would theoretically have less angle strain than cyclopropane right because in a square those angles in a planer Square those angles Les would be about 90° Which is less than our ideal at 109.5° but it's not awful right not as bad as cyclopropane but it would have eight pairs of eclipsed hydrogens if this was in a planer confirmation so cyclop butane then would be this unstable molecule as well because of the eclipsed hydrogens and the angle strain so ultimately what happens is that Adolf Von buer was wrong CYO alkanes don't tend to stay in a planer confirmation these molecules tend to twist out of a planer Arrangement and the reason why is they want to minimize the amount of angle strain that's present and also the number of eclipsed hydrogens because remember they'd prefer to be staggered so by twisting we are getting a little bit more angle strain but because we're decreasing the amount of eclipsed hydrogens that's going to increase the stability even of the molecule even more than increasing the angle strain does okay so we get more stability out by decreasing the amount of of eclipsed hydrogens then we do by the increase in angle strain so cyclobutane twists it twists a little bit as much as it can to minimize the amount of eclipsed hydrogens that are present we see the same thing for cyclopentane even though cyclopentane angles are close to the ideal sp3 hybridized angles it still does twist to stagger some of those hydrogens which causes some angle strain so if cyclopentane were planer like like Von berer predicted there would be 10 pairs of eclipsed hydrogens in cyclopentane but because eclipsed hydrogens are not as stable as not as favorable we have that cyclopentane twisting and taking on some angle strain to minimize those eclipsed hydrogens and so we see cyclopentane kind of twisting a little bit to get some of the hydrogens in more of a staggered confirmation um and so what we actually end up seeing is that cyclopentane even though Von berer predicted cyclopentane would be the most stable it's not the most stable even though it has Bond angles close to the ideal Bond angles if it were planer it's not planer okay cyclopentane is not a planer molecule because it twists so it still does have angle strain even though it has close to Ideal Bond angles in the planer state so what this means is that while cyclopentane is still fairly stable and we do see compounds that exist with cyclopentane rings cyclohexane is actually the most stable cyclic compound it's even more stable than cyclopentane because these cyc alanes tend to bend and twist to minimize the amount of eclipsed hydrogens and and also minimize their angle strain their ring strain so we're going to focus this next part of the lecture on cyclohexane the kind of Ideal stable CYO alane structure cyclohexane and understanding its structure is so important because there are so many organic molecules that are composed of cyclohexane rings so we need to understand it's new nuances and the properties of these cyclohexane rings and understand what makes them so stable and also the different confirmations of a cyclohexane ring that can exist so first let's start off just by looking at the structure and considering the amount of eclipsed hydrogens and angle strain so thinking about this there is a confirmation of cyclohexane that's actually actually completely free of any strain angle strain and the strain of eclipsed hydrogens and the reason is because of that bending and twisting of cyll alkanes so cyclohexane is not pler but the chair conformer of cyclohexane is completely free of strain because all of the bond angles are close to Ideal at 111° and here's the kicker all all of the bond angles that are adjacent to each other are staggered so there are no eclipsed hydrogens when we look at the chair confirmation of cyclohexane and this is one way to represent it that we're going to see in the next slide what we see is that looking at these carbon carbon bonds we see the hydrogens being staggered from each other there's like a little window that each of these hydrogens next to each other can fit into and so this makes the chair conformer of cyclohexane very very extremely stable I'll try to remember to have a model for you guys to pick up and look at and kind of see how all of these different Bond angles are staggered and if you have a model kit please feel free to make six carbons in a line in or in a cycle and you can twist it in such a way that you can see all of those molecules all those atoms being staggered it's called the chair confirmation because it kind of looks like a lounge chair right we've got the back here the seat and the kind of base for a footrest so the chair confirmation of cyclohexane is so important that we need to know how to draw it because we're going to be drawing structures that are specifically or oriented in the chair confirmation um and so we need to show specific Bonds in this particular molecule so the first thing we need to do is we need to draw two parallel lines and those two parallel lines are then connected with a V at each of the ends so draw two parallel lines and then just make a v on both ends and notice that that chair confirmation has three sets of parallel lines meaning that three bonds within this molecule are going to be parallel to each other so these two bonds are parallel these two bonds are parallel and then finally these two bonds are parallel as well so once we have the base of that cyclohexane drawn we need to add the bonds and in this cyclohexane chair confirmation each carbon is going to have what's called an ax and an equatorial bond an axial bond is pointed up and down so they're represented with the red lines and the red triangles and you can see that they're vertical they alternate above and below the ring so draw these axial bonds start with one up and then put the next one down and then put the next one up and then down and then up and then down right so these axial bonds alternate on either side of the Ring after you draw the axial bonds there are also in cyclohexane Equatorial bonds and these equatorial bonds which are represented as the red lines with the blue balls here are going to point outward from the ring just like the equator goes around the earth these equatorial bonds go around kind of the outside of the molecule and so the bond angles are greater than 90° remember those Bond angles are close to 109.5° and because of this we see these equatorial bonds being on a slant right so if we have one axial pointed up we will have one equatorial pointed kind of down and away because they're greater than 90° so after you draw the axial bonds draw the equatorial bonds pointed greater than 90° and out from that cyc loxane ring so if the axial bond is pointed up point the equatorial Bond on a downward slant if the axial bond is pointed down point the equatorial Bond on an upward slant so when we're looking at cyclohexane here remember this is the front of the molecule we're viewing it kind of face on this is the front of the molecule and this is the back of the molecule and when we think about also the these equatorial bonds we can see that each equatorial bond is actually parallel to two of the Ring bonds so you see these ring bonds represented here in red and this equatorial bond is going to be parallel to those and we see that for um various equatorial bonds within that molecule so we've now got this structure of CYO um hexane in this chair confirmation and the chair confirmation is the most stable confirmation of the molecule but what we're going to see is that there's actually two chair con conformers for cyclohexane so cyclohexane very very rapidly interconverts between these two stable chair conformers because carbon carbon bonds are easier are fairly easy to rotate and this is what's called a ring flip okay so we see the back of the chair here and the footrest of the chair here and the seats kind of tilted down what we can do is we can put this headrest down and make it a footrest and put this footrest up and make it a headrest we still have all of the high hydrogens of cyclohexane being staggered everything is still in the staggered confirmation so it's just as stable as this original confirmation but what's different about it is that when these two chair conformers ENT converts when they undergo this ring flip the bonds that are equatorial in one share conformer become axial in the other chair conformer okay and the bonds that are axial in one chair conformer become equatorial in the other chair conformer so this property of cyclohexane to be able to alternate between the various chair conformers is very important because what it allows cyclohexane to do is to have two stable options to stable chair conformers and what we're going to see eventually not this not in this this um next few slides but in uh future slides is that when you have something like a substituent on the ring and it creates this like bulky thing for the ring to have to deal with that would normally decrease stability what's going to happen is that it's going to put that bulky substituent in a place that's pointed away from the interior of the ring so bulky substituents tend to be in Equatorial positions and so in one chair conformer of the Ring these bulky substituents are in an equatorial position but when it under goes a ring flip now it would be in the axial position and these are not as stable when you put substituents in the axial positions and so we're going to see that for cyclic compounds for cyclohexane specifically and the cherod formers if you start adding substituents the chair conformers don't necessarily have equal stabilities anymore for cyclohexane they do because everything's a hydrogen but we're going to be working towards being able to predict which chair conformer will be the most stable when we add substituents to the ring okay so the chair conformer is the most stable conformer of cyclohexane because all of the carbon hydrogen bonds are going to be staggered rather than eclipsed and also there's minimal angle strain those Bond angles are closer to 111° they're closer to the ideal Bond angles than we've seen for other cyclic compounds and this is because it doesn't exist in a planer state it bends and twists to give it the most stable confirmation there are other noteworthy confirmations of cyclohexane the other confirmation of cyclohexane that's noteworthy is called the boat conformer for obvious reasons right it looks like a boat and just like the chair conformer the boat conformer is also free of angle strin so there is about 111 degree Bond angles that have pretty good overlaps of orbitals however the boat conformer of cyclohexane is not as stable as the chair conformer because in the boat conformer we have some carb carbon hydrogen bonds that actually are eclipsed and you can see that represented here in the Newman projection okay while there are some that are staggered there are also carbon hydrogen bonds that are eclipsed which decreases the stability of that molecule and also the boat conformer is also destabilized by the presence of what's called flagpole hydrogens those flagpole hydrogens you can think about them being at the bow and the stern of the boats and those cause these increased amounts of steric strain in that molecule because it puts their electron clouds um closer together so you can think of the boat conformer of cyclohexane as just a least a less stable Arrangement a less stable orientation of those bonds so again you can convert the chair conformer to the other chair conformer and to the boat conformer and anywhere in between just by rotating those bonds within a molecule so when we think about the stability of these various conformers of cyclohexane we see that the two chair conformers those two extremes there are going to be the most stable and when you kind of pull down the seat and twist it a little bit it becomes what's called a half chair this is going to have le the least amount of stability because it has kind of the most amount of eclipsed hydrogens that it can have if you then kind of twist it a bit to minimize the amount of eclipsed hydrogens we get to a confirmation called a Twist boo confirmation which is a little bit more stable than the boat because we're kind of twisting those hydrogens out of the eclipse State okay and then the boat which has eclipsed hydrogens and some some staggered but a lot of eclipsed so it tends to not be as stable and then we kind of go to the other chair confirmation that has the most stability so chairs are the most stable boats are kind of medium stability and then the intermediate States these half chairs are the least stable confirmations of cyclohexane so again we're talking about one molecule here one cyclohexane molecule that can bend and twist into these different confirmations so it's not like these are different molecules we're looking at these are the same molecule but because carbon carbon bonds can rotate we can see these different Arrangements of atoms within that molecule some arrangements of atoms are more stable like we see with the chair conformers and some arrangements of atoms are less stable like we see with the half chair and the boat conformers so in class we're only going to do a couple of problems but I do want you guys to make models of cyclohexene so if you have a model kit bring it and with these models what we're going to look at are practicing making the chair conformer and the boat conformer and understanding why in the chair conformer we are going to have no eclipsed hydrogens and in the boat conformer we will have certain amounts of eclipsed hydrogens and kind of just understanding the confirmations and stabilities of cyclohex