amine inversion and chiral molecules with no chiral centers we're combining two shorter lessons into one here uh we're getting towards the end of our chapter on isomers and stereochemistry and really the only thing left after this is a brief lesson on optical activity now if this is your first time joining me so my name is chad and welcome to chad's prep where my goal is to make science both understandable and maybe even enjoyable now this is my brand new organic chemistry playlist i'm releasing these lessons weekly throughout the 2020-21 school year so if you don't want to miss one subscribe to the channel click the bell notifications you'll be notified every time i release a new lesson all right so a couple of oddities in this chapter and the first one is what's called amine inversion so if you look at an sp3 hybridized nitrogen here so typically nitrogen has three bonds and a lone pair and it turns out that you know we talked about a a chiral center as being an atom with four different groups attached well it turns out a lone pair of electrons can actually be one of those groups just not typically with nitrogen so turns out amines undergo a process called inversion where the entire stereochemistry flips to the other side and the lone pair ends up on the other side of the nitrogen and it's just flipping back and forth it turns out that the activation energy for this process is super low and so at any kind of like even room temperature even quite a bit lower temperatures this just readily happens and so it turns out you can't have one of these enantiomers without the other one and so it turns out they they kind of went through and defined the world chiral a little bit different to take this into account they said chiral is when a molecule and its mirror image are non-superimposable and can't readily be interconverted between each other so in the solution or something like that so in this case these are not defined as chiral due to this process of amine inversion now a couple things you should know so if the nitrogens you know got bulkier groups attached well i've got fairly small ones here if they've got bulkier groups or part of a ring or something like that this probably is not going to happen and how bulky is bulky i don't really have a great answer for that there's you know there's not like a let's cut it off right there so however if you do see a nitrogen's part of a ring this is generally not going to be able to happen and stuff like that so but just something to be aware of here and if you kind of drew the analogous structure with say sulfur here just to kind of give it some contrast here so we got an ethyl a methyl a hydrogen and a sulfur with a lone pair and that's going to give him a positive formal charge so it turns out this kind of inversion here doesn't happen with sulfur and that lone pair again can be one of the four things and so if i said are there any chiral centers in this structure you should totally say yep the sulfur is a chiral center and this molecule is chiral and so once again with amine inversion we would not define these molecules as chiral because they readily inconvert you actually can't have one without the other at any kind of you know normal temperature but not not true with sulfur with sulfur you could totally have one enantiomer without having the other they don't readily interconvert the way amines do all right so now we're going to take a look at a couple of examples of molecules that can be chiral even though they don't have any chiral center and the first comes up with what's called an allene and an allen simply has this pattern of a carbon-carbon double bond with another carbon-carbon double bond butted up right next to each other something interesting kind of happens here so if we kind of take a look at the process here so if i put a carbon next to a carbon next to a carbon we'll put that single bond in there but now i actually want to draw in the orbitals that are overlapping for that double bond and so let's say i've got kind of these p orbitals right here and we're getting a little sideways overlap between them so way it works is when the pi bond is in this plane so then your other bonds coming off are in the perpendicular plane so they're going to be in this kind of horizontal plane the way i've kind of drawn it here so but we've also got a double bond in this location right here as well and if this p orbital's already been used to make this pi bond then i can't use it again and so instead of using the vertical p orbital i'm gonna have to use the horizontal p orbital instead and here's where my artistry is going to really suck and i apologize so and we'll have some more sideways overlap here in this horizontal plane but now if the pi overlap is in this horizontal plane then the other bonds will be in this perpendicular plane here instead and so as a result here so this is an sp2 hybridized carbon sp hybridized carbon sp2 hybridized carbon and so none of these are sp3 so none of them can be a chiral center by definition however if you look at this though so with these two groups being in this kind of vertical plane the plane of the board and these two being in the horizontal plane if you come just cover up the middle for a second it kind of resembles the tetrahedral shape and as a result that's what actually allows it to potentially be chiral and so if you look at this molecule and its mirror image if you kind of rotate this around to kind of try and see if they'll match up in any way shape or form in fact if i just took and rotated this exactly 180 degrees out of the plane of the board you'd find out that this methyl and this hydrogen would match up perfect right with this one but having turned it around this methyl group now would be in the back and that hydrogen would now be in the front and they wouldn't match here these are non-superimposable mirror images and so allen's have a chance of being chiral and it's not just allens it turns out as long you know you can get a big long chain you know of carbons that are all double bonded to each other and as long as the number of double bonds is an even number this is going to happen where you know you've got two groups in one plane and the other two on the other end in the other plane so again if you had an odd number of pi bonds well then they all be in the same plane this is not going to happen so we talk about allen but there's some more complex systems that aren't technically alien but you see the similar thing and again it just has to have an even number of pi bonds in a row like this all right one other thing you should note is that not all allenes are chiral like this so and you can kind of see this so let's say i just take and replace this hydrogen right here with a methyl group instead so i'll put it back in a second so but let's just say i did this well all of a sudden now what you'd find is that right down the middle of this molecule because these two are on the horizontal plane but right down the middle here the only thing that would be reflected if i try to make this a mirror plane right down the middle are these two methyl groups because everything else lies on this horizontal line and these are the perfect reflection of each other and so we would call this a sigma plane and so this molecule having an internal mirror plane of sigma plane has to be a chiral and so as a result this guy would not have an enantiomer he would be identical to his mirror image and again all i have to do is replace this one with a methyl group as well and now you could totally superimpose these you could flip this over and then you'd have to rotate it around but you could totally superimpose all the groups and so the idea then is for you to have an allene that ends up being chiral so what your requirement is is that the two groups on one side of the system have to be different from each other and the two groups on the other side have to be different otherwise there's going to be an internal mirror plane if i make both of these methyls or both of these hydrogens you'd have an internal mirror plane running right through the board instead and so as long as these two are different from each other and these two are different from each other even if these two are the same as these two but different on the same side here so that's going to be chiral and it will be different than its mirror image like we see here now the allen is one example of this we'll see one other with the biphenyl so the next example we'll see of a chiral molecule that does not have any chiral centers is part of what we call the biphenyl system and biphenyl just means you have two benzene rings directly bonded to each other so in this case we call the position right next to where they're bonded to each other the ortho position so there's four of these ortho positions two on each ring and the key is this this single bond is not part of the ring and it's just free to spin these are just free to rotate around in circles relative to each other unless you do one thing if you these if these four ortho positions are just little tiny hydrogens great no big deal but the moment you start putting bigger groups in these four places they're going to start bumping into each other and so it causes two things one it caused them to just not want to spin around so they'd kind of like act as little paddles and they'd hit each other and then you'd go back 90 degrees that way and hit another one and and so it'd keep them from spinning around like we'll see in this case here so but it does something else as well in addition not spinning around entirely it makes them not even want to be in the same plane it makes one want to be kind of in the vertical plane and the other one kind of rotate around to being in the horizontal plane and i've kind of tried to represent that here so in fact i should have made this a little bit of wedged looking as well so and so this one's in the horizontal plane on both sides and then these are in the vertical plane so and because of the ortho positions all having large groups they just kind of want to sit there they'll rotate a little bit but they can't rotate 360 for sure and they'd prefer to be orthogonal to each other these rings 90 degrees apart all right so a couple things need to happen for this to be chiral so it turns out these two are mirror images of each other and they're non-superimposable and there's no chiral centers all the atoms in the benzene rings these all these carbons are sp2 hybridized and being that they're not sp3 they can't be chiral centers and so without chiral centers these are still chiral molecules because they're not identical to their mirror images if you built these models out you'd find out that you can't line everything up perfectly if you took this and flipped it around to match things up you'd find out that this methyl and this methyl would line up perfectly this bromine and this bromide to line it perfectly but once you flip this around then the bromine instead of being out towards the front and the methyl in the back it would have the bromine in the back and the methyl out toward the front and they would not be the same molecule at all so these are enantiomers these molecules are chiral in this case and again two requirements you got to have four things in these ortho positions at least three out of the four that are bigger than hydrogen so not actually all four but three out of four bigger than hydrogen and the two things on each ring have to be different from each other so similar to what we saw at one end of allen the two things had to be different that were attached to that sp2 carbon same thing here the two things on a benzene ring have to be different from each other because the moment you put like a methyl here and a methyl here you'd have a mirror plane right down the horizontal again just like we saw with allen and any molecule that's got that internal mirror plane that sigma plane is a chiral it would be identical to its mirror image so as long as this rings two groups and the ortho positions are different and these ones are different from each other even if it's the same two on this ring in this ring as long as they're different on each individual ring it will be chiral and it will be different than its mirror image cool so these are enantiomers so and again this comes up in both contexts sometimes they'll give you this and and totally expect you to identify this molecule as chiral but sometimes they'll go through and just kind of get a little bit tricky on you and they'll give you this molecule and a lot of students will see this be like oh yeah we studied something special with the biphenyls yeah that's one of those exceptions he's going to be chiral well in this case because the ring's got two identical groups it wouldn't be chiral be a chiral so just keep in mind they can ask it to you either way and now it's back to being chiral now if this looks completely unfamiliar there's about a 50 50 chance half of undergraduates will see these examples and half won't so if it looks completely unfamiliar just double check your curriculum and see did we even cover this in my class so because maybe you didn't so but if you've benefited from this lesson please consider giving me a like and a share one of the best things you can do to support the channel and if you're looking for practice materials or the study guides that go with this check out my premium course on chatsprep.com