This edition of conjugated or not we’re going to talk about whether these compounds are conjugated or not hey or b so we have five compounds here we just learned about what conjugation is so we should be able to tell if these molecules have conjugation work they don’t and there is one of these that is a little bit trixie so pause the video decide for yourself and
Then we’ll check the answers after you’re done oop paused emboss’d okay um this one it is conjugated overlap of those two pete those two pylons this one is not it’s got a ch2 here ch2 here both of those are sp3 hybridized they break up the conjugation this one is conjugated this bond is overlapping with this bond this one is conjugated this pi bond is overlapping
One of these pi bonds and lastly the one that’s trixie little hobbit and it’s not be okay so these ones were relatively straightforward hopefully you had no problem working through those on your own this one is a little bit different and i added an h then that was not on the know packet so now it’s fixed um okay so why is this one not conjugated it looks like we
Have a pi bond and a pi bond pi bonds are in p orbitals why are they not overlapping and to answer that we have to think about what p orbitals are and when we learned about orbitals in the first place we said there’s an s orbital and three p orbitals and what else did we say about this p orbitals we said those p orbitals were on axes right this would be maybe a
A px py and a pz um so let’s think about this central carbon atom here this central carbon atom has two molecular groups around it so it’s going to hold them as far apart as possible which is 180 degrees so this is going to be sp hybridized so to become sp hybridized that central carbon would have combined its s orbital with this p orbital and in its hybrid form
It would have had to as p orbitals make them separate to different sp orbitals and then we’ll say a p y orbital and the pz orbital okay so what does that look like well if we draw this one out a little bit more we know that that center carbon atom is going to have two bonds to the carbons on the other sides and these sigma bonds are made from a carbon sp orbital
And then these carbons here are technically sp2 hybridized so it’s a carbon sp to a carbon s t 2 because this is sp2 hybridized this is sp2 hybridized but what p orbitals are making these bonds well we’ll just say for no specific reason that this carbon is bonded to this carbon and the p y orbital so if it’s a py orbital on a graph where this is the x axis this
Is the y axis the z axis comes out of the out of the board and goes into it the y orbitals are going to be up and down so the electron density is above and below this carbon-carbon bond what about the other carbon-carbon bond well in this case we’ve already used our py orbital we only have our pz orbital left so what that means is that the wurmple would have to
Come out toward you and go into the board because that’s where that pz axis is so we’re going to draw that like this so there’s an overlap that orbital and overlap of that orbital so essentially what we have here is that central carbon atom has two p orbitals that are like this they’re criss-crossed and because of that they mean if they were to be overlap they
Would need to be in the same axis so they would need to be in the same axis if they were overlapping but because they are opposite axes they are not overlapping so this is a non conjugated system so that’s the molecular orbital reasoning behind why this is not conjugated what was the other thing that we could do to check ourselves to say is this system conjugated
Or not and what we said we could do is our resonance structures and see if they are balanced so for instance on this one we said we could draw a resonance structure like this which we move the electrons up there so it’s negatively charged we move them away from that once that’s positively charged let’s try and draw a resonance structure for this molecule well if
We proved that like this and let’s try and overlap this cc pi bond onto that area like we did here where we moved it between two other bonds if we do that we just follow the arrows so you might want to pause the video again and see if you can draw what we just kind of did up here what you end up with is a positive charge on that carbon atom this carbon atom here
Would still be double bonded over here um which uh which would uh what would make it also positively charged because it will it had four bonds that has three months and then this carbon on the end would have to have four bonds and a lone pair which makes it to minus charged um so first off all these charges to many charges right that’s crazy we would never have
Them all like that to negative on one atom we’ve never seen that ever but also what’s wrong with this structure how many electrons is that carbon has that carbon would have two four six eight ten electrons on it again this is a texas carbon too many electrons so we can’t draw that structure so this is the molecular orbital view of why it’s not conjugated and then
We can’t use our resonance pneumonic to show overlap either so this must be a non conjugated system so i hope you enjoyed this edition of conjugated or not
Transcribed from video
Conjugated or NOT By UWRF O’Reilly Organic Chemistry