Mar 25, 2019 12:54 PM

Indexé : 2026-05-19

[00:00:00]so in video 5 we're going to be looking at geometric isomers so I have here a little video clip that I've made just to demonstrate what's going on with geometric isomers so let me just show you this so I've got here some of the Molly mods here and you can see at the moment I have got a single carbon-carbon bond between the two black atoms which are the carbons there and you'll notice that with a single covalent bond they can rotate so I can twist this molecule round and round because the single bonds allows for rotation so it doesn't really matter what I do the name of the molecule would never change which would be one to die chloro ethane it's never going to change so that's quite an important thing if I was to change that bond to a double bond which is what I'm doing now and I'll make the double bond just by joining up two of the bonds there so now I've got a double bond and the thing about a double bond is that it no longer allows for rotation I can't twist them round they're not free to move in any other direction so the double bond does not allow these carbons to rotate now that gives us quite an important difference between the structures because now if I have the arrangement like this I can see that I've got the chlorines attached diagonally across this molecule and I can't ship them I can't shift the position of them so they're diagonally across because the double bond doesn't allow for rotation if I was going to shift it back again so I can't twist them round but if I can change the atoms then I can see that I've got the chlorines on the same side of the double bond so that is something that makes a big difference to what we're going to do because look this is the similar situation that we had here so this on the left hand side was one to dichloro a theme right yeah okay and then the other one that I made when they were on the same side it is also one to dichloro ething hmm so they both have the same name but they have to be different in some way because I can't twist them round and this is what a geometric isomer is it allows us to think about the arrangement around the double bond and to show that these molecules are different from each other I put these things on so this is trans trans one to dichloro ethene because the chlorines are going across like a transatlantic flight going across the double bond and a zigzag arrangement here because the chlorines are on the same side I have got sis so sis indicates the same side I like to think of it is that it's kind of like a sideways C as well so I remember to get them the right way round so by definition and I'd write this down molecules that have different spatial arrangement in relation to the double bond so they appear to be the same molecule but because that double bond does not allow for rotation they're subtly different from one another case so for geometric isomer to occur it must have a double bond between two carbons and that double bond does not allow for rotation and each carbon of the double bond must be attached to different atoms or groups of atoms now let's look at this example here this the name of this is one one dichloro ethene so let's have an examination of it well it's got a double bond between the two carbons which does not allow for rotation so that's good thing but if I examine this carbon I can see it's got the same atoms attached it's two chlorines attached to this carbon and this carbon has got two hydrogens attached so because each carbon of the double bond is attached to the same atom they don't have to all be the same but it's just each carbon of the double bond attached to the same atoms it cannot exist as a geometric isomer so if we'd had a hydrogen here instead of the chlorine that could because the carbon was attached to different atoms but here it is not going to exist as a geometric isomer now this is the kind of question that you are likely to get there will be a geometric isomer question in your exam it's there every year so this is the kind of question that you would get and they would give you the molecule that you're dealing with in condensed structural form like up here and it asks you to draw the geometric cysts and trans isomers in the box now most students will look at that and just immediately get stuck they don't even know where to start and it's because you can't see it when you look at this structure you can't see how things are really arranged so my advice would be first step draw it out so you can see everything I apologize for my drawing her but anyway you can still see the bonds so I've drawn the whole thing out and now I can see the double bond is in place so I can start thinking about how this is gonna work but still I look at this and think how am I supposed to work out like trans and cysts from this so the next step is to redraw this structure your displayed structure out around what I fondly refer to as a spiky structure and that is around the double bond and let me show you again I apologize for the drawing okay so we have here the carbon-carbon double bond there it is and I can see from one carbon I've got the hydrogen and the chlorine and yes I have from one I've got the hydrogen in the chlorine and from the other carbon of the double bond I've got a hydrogen going up yes I do he and I've got a ch2 ch3 group going down and that's what I've got here okay so I call this a spiky structure because well it looks spiky and now I can tell whether the groups are going to be on the same side or different and look look here look the hydrogen's are on the same side of the double bond so this is the cysts form and now it becomes really obvious to do the transform and I just swapped things around and make the hydrogen's go zigzaggy across the double bond to form my trans so now we get to our excellence on question justify why this molecule exists is a geometric isomer now there are certain ways of tackling this the first thing is you want to get your basic points out you know what do you need for geometric isomers to occur and this is what you would start this is like your generic point so for geometric isomers to exist there must be a C double bond C bond this does not allow for rotation and the second point is the carbons of this double bond must each be attached to different atoms or groups of atoms so that's your generic statement and you need both parts of this and that would get you usually a merit level but to get to excellence you need to be specific they've given you a molecule talk about it so the molecule we have got here on kildee we would say yes it has the C double bond C bond one carbon is attached to the H and Cl and the other is attached to the H and the C two ch3 group so can exist as a geometric isomer so it's just being specific about the molecule you've got now a word of warning if they ask you to talk about three molecules you need to talk about all three molecules why they do exist and why they don't exist and that's probably where students go wrong they just only talk about one thing so if there was something that didn't exist as a geometric isomer you would have to give your reasons why for example yes it does have a double bond that doesn't allow for rotation but one carbon is attached to two hydrogens so it cannot exist you see what I mean so you've just got to be really specific