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Hovedindhold
Aktuel tid:0:00Samlet varighed:14:13

Pi-bindinger og sp2-hybridiserede orbitaler

Video udskrift

in the last video I Don the idea of a sigma bond and that was a bond where let me draw two nucleuses and let me just draw one of the orbitals let's say this is an sp3 hybridized orbital and that's on this atom and this is kind of its big lobe right there and then this guy has an sp3 hybridized orbital as well that's the small lobe and then that's the big lobe like that a sigma bond is one where there's an overlap kind of in the direction in which the lobes are pointed and you might say well you know how can there be any other type of bond than that well the other type of bond so this right here let me make this clear this right here this right here is a sigma sigma bond you say well you know what what other kind of bond could there be where my two over my two orbitals overlap kind of in the direction that they're pointing and the other type of bond you could have you can imagine if you have two P orbitals so let me draw to the nucleus of two atoms and I'll just draw one of each of their P orbitals so let's say that that's the nucleus and I'll just draw their p orbital so p orbital is just that dumbbell shape let me draw it a little bit let me draw them a little bit closer together so p orbital is that dumbbell shape so let me draw this guy's one of his P orbitals a little bit bigger than that and you'll see why in a second so one of his P orbitals right there it comes out like that and then this guy over here also has a p orbital that is parallel to this p orbital so it goes like that so it goes like that well let me draw that other one a little bit straighter it goes I want it to overlap more so it goes like that I think you get the idea so here our 2p orbitals are parallel to each other this do you could imagine these are sp3 hybridized orbitals they're pointing at each other here they're parallel p-orbitals parallel to each other and you see that they overlap on this kind of top lobe here and in this bottom lobe here and this is a PI a PI bond so let me make this clear that is a and this is one PI bond so you could call it two pi literally with the Greek letter pi PI bond sometimes you'll see this as just written as PI bond and it's written it's called a PI bond because it's the Greek letter for essentially P and we're dealing with P orbitals overlapping now Sigma bonds which nor are what form when you have a single bond these are stronger than PI bonds pi bonds come into play once you start forming double or triple bonds on top of a sigma bond to kind of get a better visualization of how that might work let's think about ethene ethene so it's chemical its molecular structure looks like this let me so you have C double bonded to C and then you have each of those guys have two hydrogen's each of those guys have two hydrogen's so let me draw what it would look like or our best visual or our best ability to kind of conceptualize what these what the orbitals around the carbon might look like so let me draw let me draw so first I'll draw I'll draw the sp2 hybridized orbitals so let me just make it very clear what's going on here so when we when we were dealing with methane when we were dealing with methane so methane which is literally just a carbon bonded to four hydrogen's and if I actually wanted to draw it in a way that it kind of looks a little three-dimensional with a tetrahedral structure it might look like this this hydrogen is pointing out a little bit this hydrogen is kind of in the plane of the page and then maybe that hydrogen is behind it and then you have one hydrogen popping up that's methane and we saw that these were all sp3 hybridized orbitals around the carbon and then they each formed Sigma bonds with each of the hydrogen's we saw that in the last video and when we drew its electron configuration in order for this to happen carbons electron configuration in when bonding in methane needed to look like this it needed to look like one s2 and then having and then you have and then instead of having two s2 and in 2p to what you essentially have is let me write it this way actually if it's better let me write it this better in 1s you had two electrons and then in instead of 2's you had two electrons in each of the peas you had one the s is and the peas all got mixed up the s is and the peas all got mixed up and you had a - sp3 hybridized orbital another - sp3 hybridized orbital another - sp3 hybridized orbital and then another one sp3 normally when carbon sitting by itself you would expect an S 2's here and then you'd have a 2p in the x-direction a 2p in the y-direction and then a 2p in the z direction but we saw in the last video they all get mixed up and they all have a 25% s character and a 75% p character when carbon bonds and methane and then the electrons kind of separate out in that situation when you're dealing with the carbons in ethene when you're dealing with the carbons in ethane remember earth is for two carbons and ian's because we're dealing with an alkene we have a double bond here in this situation the carbons electron configuration when they bond in ethene looks more like this so you have your 1s and then you have your and the 1s orbital is still completely full it has two electrons in it but then in your in your two shell you have so let me just I'll just write let me do this in a different color so in our two shell I'll show you what I mean in the second I'm not writing the s or PS so far on purpose but we're gonna have we're going to have four electrons just like we had before we're still forming four bonds we're gonna have kind of these four unpaired electrons we're still forming one two three four bonds with each of the carbon so they're going to be separated out they are going to be separated out but in this situation instead of all of them being a mixture kind of one part s three parts p the S mixes with two of the P orbitals so what you have is 2 SP 2 2 SP 2 2 SP 2 orbitals so you can imagine that the S orbital mixes with two of the p orbital so now it's one part s2 parts p and then one of the P orbitals kind of stays by itself and we need this p orbital to stay by itself because it is going to form it is going to be what's responsible for the pi bond and we're going to see that the pi bond does something very interesting to the molecule it kind of makes it up unrotated around a bond axis and you'll see what I mean in a second so let me see if I can in three dimensions draw each of these carbons each of these carbons so you have so you have let me do it a different color you have this carbon right there so let's say that's the nucleus although I'll put a C there so you know which carbon we're dealing with and then I'll draw we you know you could assume that the 1s the 1s orbital it's really small right around the carbon and then you have these hybridized orbitals the two sp2 orbitals and they're all going to be planar kind of forming a triangle or I guess maybe a ap sign on some level but I'll try to draw it in three dimensions here so you have one this is kind of coming out a little bit then you have one that's going in a little bit and then you have and they have another you know lobe a little bit on the other side but I'm not going to draw a little complicated they're still have characteristics of P so they'll have two lobes but one is bigger than the other and then you have one that's maybe going in this side so you can imagine that this is kind of a a Mercedes sign on if you draw a circle around it on its side so that's this carbon right here and of course it has its hydrogen's so you have you have this hydrogen there and so this hydrogen might be sitting right here it just has one electron and it's 1s and it's 1s orbital you have this hydrogen up here it's sitting right over there and now let's draw this carbon now let's draw this carbon this carbon will be sitting I'm drawing pretty close together this carbon will be sitting right there he has his 1s orbital there they have the exact same electron configuration he has his 1s orbital right around him and then he has the exact same configuration and either of these guys we've so far only or in this first guy I've only drawn these first three I haven't drawn this this unhybridized p orbital so I'll do that in a second but let me draw his bonds so first of all he has this or you could imagine that bond right there which would be an sp2 hybridized bond let me do that in the same color as so he has this bond right here which would be an sp2 hybridized bond just like that and notice this is a sigma bond they overlap in kind of the direction that they're pointing in that's the best way I could think about it and then he's got these two hydrogen's so one he's got this guy in the back this guy in the back and then there's one in the front I'll draw it a little bigger so it's kind of pointing out at us all right and then we have this hydrogen is sitting right over here and these are also Sigma bonds just to be very clear about things this is a s orbital overlapping with an sp2 orbital but they're kind of overlapping in the direction that they're pointed or kind of along the direction of each other of the two atoms this is a sigma bond sigma bond and then we have this hydrogen in the back we have this hydrogen in the back which is also going to form a sigma bond so everything I've drawn so far is a sigma bond so that that maybe I don't want to maybe I don't want to make this picture too so I could just put Sigma bond there Sigma bond there Sigma bond there Sigma Sigma so so far I've drawn I have drawn this bond this bond this bond this bond and this bond all of those Sigma bonds so what happens to this last p orbital for each of these guys well that's going to be kind of sticking out of the plane of the Mercedes sign it's the best way I can I can describe it and let me see if I can do that in a color that I haven't done yet only this purple color so you could imagine a pure p orbital so a pure p orbital and draw it I need to draw it even bigger than that actually a pure p orbital it normally wouldn't be that big relative to things but I have to make them overlap so it's a pure p orbital a pure p orbital that's kind of going in maybe you can imagine the z-axis that the other orbitals are Marcedes sign and the xy-plane and now you have the z axis going straight up and down and those bottom two have two overlaps let me draw them bigger so it looks like that and it looks like that and they're going straight up and down and notice they are now overlapping so these this bond right here is this bond if you know I could have drawn them in either way but it's that second bond and so what's happening now to the structure so let me make it very clear this right here that is a PI bond and this right here is also it's the same PI bond it's this guy right here it's the second bond in the double bond but what's happening here well first of all by itself it would be a weaker bond but because we already have a sigma bond since we already have a sigma bond that's kind of a track that's forming making these molecules come closer together this PI bond will make them come even closer together so this distance right here is closer than if we were to just have a single Sigma bond there now on top of that the really interesting thing is if we just had a sigma bond here both of these molecules could kind of rotate around the bond axis they would be able to rotate around they would be able to rotate around the bond axis if you just had one Sigma bond there but since we have these PI bonds that are parallel to each other and they're kind of overlapping and they're kind of locked into that configuration you can no longer rotate if one of these molecules rotates the other one's going to rotate with it because these two guys are locked together so what this PI bond does in this situation is it makes this carbon-carbon double bond or it means that the double bonds are going to be rigid that you can't have one molecule you can't have one molecule kind of flipping swapping these two hydrogen's without the other one having to flip with it so you wouldn't be able to kind of swap configurations of the hydrogen's relative to the other side that's what it causes so hopefully that gives you a good understanding of the difference between Sigma and PI bond and in your if you're curious when you're dealing with when you're dealing with just to kind of make it clear if we were dealing with f-fine so that we just this is the example of ethene but ethene looks like this you have a triple bond and so you have each side bonding to one hydrogen in in this case you one of these so the first bonds you can imagine so these bonds are all Sigma bonds that actually SP hybridized you that your to s orbital only mixes with one of the peas so these are these are SP SP hybrid orbitals orbitals forming Sigma bonds so all of these right here and then both of these both of let me do this in a different color both of these are PI bonds and if you have to imagine it if you have to imagine it you could imagine another PI bond kind of coming out of the page and another one here coming out of the page and into the page out and into the page and they too are overlap and you just have one hydrogen pointing out in each direction maybe I'll make another video on that so hopefully I didn't confuse you too much