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Main content
Current time:0:00Total duration:11:00
AP.Chem:
SAP‑4 (EU)
,
SAP‑4.C (LO)
,
SAP‑4.C.3 (EK)
,
SAP‑4.C.4 (EK)

Video transcript

the video on SP 3 hybridisation we saw carbon was bonded to four atoms in the video on sp2 hybridization we saw that carbon was bonded to three atoms and in this video we're going to look at the type of hybridization that's present when carbon is bonded to two atoms so if I look at this carbon right here and the ethane or the acetylene molecule this carbon is bonded to a hydrogen and it's also bonded to another carbon so we have carbon bonded to only two atoms and the shape of the acetylene molecule has been determined to be linear so we have a linear geometry we also have a bond angle here this bond angle is 180 degrees and so we must have a different hybridization for this carbon we have a different geometry a different bond angle and a different number of atoms that this carbon is bonded to and so to find our new type of hybridization we look at our electron configuration already in the excited state all right so we have carbons four valence electrons represented here in the excited state so one two three and four and we're looking for we're looking for two hybrid orbitals since carbon is bonded to two atoms so we're going to take an S orbital so we're going to promote an S orbital in terms of energy and we're going to demote a p orbital only one p orbital this time so we have an S orbital with one electron a p orbital with one electron that's going to leave behind two P orbitals each one of those P orbitals has one electron in it so we have carbons four valence electrons but this is no longer an S orbital because we're going to hybridize it with the p orbital to make an SP hybrid orbital this is no longer a P orrible because we're going to hybridize it to form our SP hybrid orbital and so this is called SP hybridization so this is SP hybridization because our new hybrid orbitals came from one s orbital and one p orbital like that so this carbon right here is SP hybridized since it's bonded to two atoms and this carbon right here is also SP hybridized let's think about the shape of our new SP hybrid orbitals so let's get a little bit of room down here so once again we know an S orbital shaped like a fear right so we took one s orbital and we took one P or a roll which shaped like a dumbbell and we hybridized these two orbitals together to give us two new hybrid orbitals two SP hybrid orbitals and go ahead and draw on an SP hybrid orbital here and once again we're going to ignore the small lobe right so we're going to ignore the small lobe and we draw our picture only think about this bigger frontal lobe here and when I think about the percentage of s character alright so we use one s orbital and one p orbital so that means it's 50% s character and 50% P character and this is more s character than in the previous videos right so in the video on sp3 hybridization we're talking about 25% s character the video on sp2 hybridization we talked about 33% s character and then for these hybrid orbitals we have even more s character up to 50% and since the electron density for an S orbital right is increased electron density closer to the nucleus for an S orbital than for AP orbital that means that this lobe here right has increased electron density closer to the nucleus which is one way to think about why these bonds get shorter as you increase in s character so in general as you increase in s character you're going to get shorter bonds because you have smaller hybrid orbitals here all right so let's go back up here to this picture of acetylene let's see if we can draw it and we now know that both of these carbons in acetylene are SP hybridized so let's go back down here and I'm going to redraw the dot structure really fast all right so we have acetylene here so we have carbon triple bonded to another carbon we know each of those carbons is SP hybridized alright so if each of those carbons is SP hybridized each carbon has an SP has two SP hybrid orbitals and go ahead and draw in one SP hybrid orbital again I'm ignoring the smaller back lobe and here's our other SP hybrid orbital on this carbon so let me go back and look at our diagram again alright so each each carbon let me use a user edit or this and so each SP hybridized carbon has an SP orbital with one valence electron in it put that in here and then there's another one right so there's another SP hybrid orbital with one valence electron in it so I'm going to go ahead and put the other electron over here in this hybrid orbital alright and also notice right if you're dealing with an SP hybridized carbon you also have two P orbitals two unhybridized p orbitals each p orbital with a valence electron so let me go back down here and I'm going to draw in here's one p orbital all right there's one p orbital with one valence electron and then here's another p orbital right here with another one of those valence electrons alright so now we have our picture of an SP hybridized carbon so let's say that was this carbon over here on the left and so now let's go ahead and draw on this carbon on the right this carbon the right is also SP hybridized therefore this carbon on the right has an SP hybrid orbital right with one valence electron in here and then another SP hybrid orbital with one valence electron here this carbon since it's SP hybridized right so once again go back up here to this diagram this carbon is also going to have a p orbital with the valence electron and another p orbital with another electron in it so go back down to here and we draw in those P orbitals so here's one p orbital all right here's one p orbital with one valence electron and then here's another p orbital with a with an electron in here like that alright finally we have to add in hydrogen all right so we have a hydrogen on either side here so we know that hydrogen has one valence electron in an unhybridized s orbital so over here we have a hydrogen with one valence electron an unhybridized s orbital and now we can finally analyze the bonding that's present alright so we know that if we have head-on overlap of orbitals I could write in here that's a sigma bond so there's one sigma bond here's a head-on overlap of orbitals between our two carbons so that's a sigma bond then finally we have a head-on overlap of orbitals orbitals here so that's another sigma bonds we have a total of three sigma bonds in the acetylene molecule in the video on sp2 hybrid we saw how to make a pie bond right so we had this side-by-side overlap of orbitals so here we have a here we have one PI bonds right here we have interaction above and below so that's one PI bond and then we have another PI bond here so we have side-by-side overlap of orbitals here as well and so we have two pi bonds present all right so we have two pi bonds present in the acetylene molecule so let's uh let's look at our dot structure again right so we saw the bond between this this carbon and this hydrogen was a sigma bond and we saw there was one Sigma bond between our two carbons so I'm just going to pick the one in the middle here say that's a sigma bond and then this bond over here we said was a sigma bond so there's our three sigma bonds and then we had a triple bond present and so there were two pi bonds also present so two of these are PI bonds here so a total of two PI bonds and three sigma bonds for the acetylene molecule here remember pi bonds prevent free rotation right so we can't rotate about the Sigma bond between the two carbons because of the PI bond so there's no free rotation no free rotation for our triple bond alright so we have a linear shape so let me go ahead and draw that line in here so you see this linear geometry for this molecule like that alright also in terms of in terms of bond length so the distance between these two carbons that is between this carbon and this carbon let me go and circle them the distance between these two carbons turns out to be approximately one point two zero on strim so an even shorter bond length than in our previous video so once again that's due to the increased s character right so increase s character gives you these gives you these up these smaller orbitals and so that's one way to think about the shorter bond distance and a triple bond compared to a double bond or a single bond all right so that's that's a lot that we've covered here let's uh let's go ahead and draw the dot structure one more time and analyze it using steric numbers so we have our triple bonds hmm and if we're doing steric number to find out the hybridization state we know it do steric number you take the number of Sigma bonds so let's say our goal was to figure out the steric number for this carbon the number of Sigma bonds I know this is a sigma bond I know in a triple bond I have one Sigma bond and two PI bonds all right so there are two sigma bonds here and zero lone pairs of electrons two plus zero gives me two so I need two hybrid orbitals all right which you make from one s orbital and one p orbital so if you get a steric number of two you think SP hybridization all right so this carbon is SP hybridized and so is this carbon as well and so that's how to think about it using using steric numbers so once again a linear geometry with a bond angles of 180 degrees let's let's do one more example using steric number to analyze the molecule let's do carbon dioxide so if we wanted to figure out the the hybridization of the carbon there let's go ahead and do that so using steric number so the hybridization of this carbon alright so the steric number is equal to the number of Sigma bonds so if I focus in on the double bond between one of these oxygens in this carbon I know that I know that one of these bonds is a sigma bond right from our previous video so I have one Sigma bond here and then for this other double bonds on the right I know that one of them is a sigma bond alright so I have I have two sigma bonds here and zero lone pairs of electrons around the carbon so two plus zero gives me a steric number of two so I need two hybrid orbitals for that carbon and of course that must mean this carbon is SP hybridized so this carbon here is SP hybridized as well and therefore we know that this is a linear molecule with a bond angle of 180 degrees and so once again steric number is just a nice way of analyzing the hybridization and also the geometry of the molecules and so the next video will look at a couple of examples of organic molecules in different hybridization States