Hybrid orbitals
sp hybrid orbitals
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- Here is the dot structure of ethyne molecule, also called acetylene molecule.
- and we can see there is triple bond present now.
- and each carbon is only bonded to two atoms this time.
- So look at this carbon on the left and
- it is bonded to two other atoms.
- So I can see that the fact that there is triple bond here
- and the fact that is bonded to two other atoms.
- It would make me think of different type of hybridization, right?
- This can't be sp3 or sp2 hybridization.
- Let's look at the orbital notation.
- This way we can figure out the type of hybridization here.
- So once again, this shows the excited states
- for the valence electrons of carbon, right?
- So the four valence electrons.
- One of the electrons on the 2s orbital has already been promoted to 2p orbital.
- For the example of acetylenes,
- its carbons are bonded to two other atoms.
- We need to form two, new hybridized orbitals.
- and we are going to form those hybridized orbitals
- by promoting, the s-orbital.
- So, this is the s-orbital with one valence electron.
- and demoting this time only one of the p-orbitals.
- We are good to demote one of the p-orbitals, right?
- with one valence electron.
- That leaves two p-orbitals untouched.
- So, we are going to leave these two p-orbitals unhybridized up here.
- So, we are going to take one s-orbital and one p-orbital
- and we are going to hybridize them together.
- to create two sp-hybrid orbitals.
- so there is s-orbital on the left,
- what used to be s-orbital is now sp-hybrid orbital.
- This p-orbital on the right is no longer p-orbital,
- it is now sp-hybrid oribital.
- Since we formed our new orbitals from one s-orbital and one p-orbital, right?
- this is called sp-hybridization.
- so sp hybri-di-zation, like that.
- and when you see a triple bond present, right?
- you know that this carbon on the left is sp-hybridized
- and the carbon atom on the right is also sp-hybridized.
- so we have our new sp-hybrid orbitals
- which must therefore have 50% s-character right?
- and 50% p-character.
- so the shape of sp-hybrid orbital is going to look a little bit more like
- like an s orbital than our previous examples.
- right? so it's still going to have two lobes.
- The front lobe and the back lobe.
- Just like we saw in sp3 and sp2.
- The difference is we increased the amount of s-character even more.
- Now we are up to 50% s-character.
- right? and since s-orbitals are spherely shaped and smaller than p-orbitals.
- we can expect these lobes to be a little bit smaller as well.
- so we can expect the lobe to be a little bit smaller.
- once again we are going to ignore the back lobe which we are drawing so that it doesn't confuse us.
- All right. So 50 s-character and 50% p-character.
- If i want to draw a picture showing the acetylene molecule and showing with its sp-hybridized orbitals right?
- So, here is our acetylene molecule.
- Now we know that each carbon has sp-hybridized.
- which means that each carbon is going to have two sp-hybrid orbitals around it.
- So, these maybe two sp-hybrid oribitals around it.
- Each sp-hybrid orbital contains one valence electron.
- Let's go ahead and draw that.
- S,o we are going to draw this acetylene molecule showing sp-hybrid orbitals.
- Here is one of the carbons of acetylene.
- and each carbon has two sp-hybrid orbitals
- so here is one of the sp-hybrid orbitals.
- so once again I'm ignoring the back lobe here.
- and here is the other sp-hybrid orbital.
- Each orbital has one valence electron.
- So, we are going to put our valence electrons in there.
- For the acetylene molecule, there is another carbon.
- It also is sp-hybridized.
- So I'm going to draw this carbon with its sp-hybridized
- meaning, there is one sp-hybrid orbital.
- There is another sp-hybrid orbital.
- I'm ignoring the back lobe, so it's not confusing.
- Each sp-hybrid orbital has one valence electron.
- So, I'm going ahead and put one valence electron in each.
- Alright. For the acetylene molecule, I have two hydrogens.
- One hydrogen is on the carbon on the left.
- and one hydrogen is on the carbon on the right.
- so I know that hydrogen has one valence electron.
- and that valence electron is in s-orbital.
- s-orbitals are shaped like spheres.
- so here is the s-orbital for the hydrogen atom.
- and then here is the valence electron for the hydrogen atom.
- So, go ahead and put in hydrogen atom over here, like that.
- Let's go back up here to our orbital diagram, right?
- so we can see that each sp-hybridized carbon has
- two un-hybridized p-orbitals.
- Each of those un-hybridized p-orbitals
- has a valence electron in it.
- Let's go ahead and draw that down here.
- This carbon on the left is sp-hybridized.
- Therefore, I know it has two un-hybridized p-orbitals.
- Each p-orbital shapes like a dumbbell.
- So, here is one p-orbital.
- and I know it has one valence electron in it, like that.
- and so I need to draw another un-hybridized p-orbital, right?
- So, here is another un-hybridized p-orbital.
- and there is this one valence electron.
- I do the same thing for the carbon on the right.
- So, the carbon on the right is also sp-hybridized,
- which means it has two un-hybridized p-orbitals.
- So, here is one un-hybridized p-orbital with a valence electron in it.
- and then I need one more.
- So here is the other p-orbital,
- and here is the valence electron.
- Let's analyze our diagram
- and figure out what types of bonds we have.
- So, I can see that between my two carbon atoms
- there is an overlap of this sp-hybrid orbitals.
- Side, uhm, head-on i should say, so that, of course, is a sigma-bond, right?
- The sigma bond comes from the head-on overlapped orbitals.
- I can see this is also a sigma-bond
- between the sp-hybridized orbital of the carbon,
- and s-orbital of the hydrogen.
- and there is a sigma-bond over here as well.
- So, I can immediately see
- there are three sigma bonds in the acetylene molecule.
- When I look at the p-orbitals, right?
- so these p-orbitals here can never overlap head-on
- but if they were big enough,
- and they got close enough together in space,
- you can see there be side-by-side overlap of those p-orbitals.
- So, we call that pi-bonds, right?
- So, there is one pi-bond
- resulting from the side-by-side overlap
- of those two p-orbitals.
- and if you look over here there is also an opportunity
- for these p-orbitals to overlap side-by-side.
- So, there is another pi-bond in the molecule.
- So, there is a total of two pi-bonds.
- There are three sigma-bonds,
- and two pi-bonds in the molecule.
- Let's go back to the sigma-bond really quickly.
- So, look at this sigma-bond
- one that is between my two carbon atoms, right?
- This sigma-bond are formed from two sp-hybrid orbitals,
- which we know are smaller than sp3 or sp2.
- And, that's the reason why
- you are going to see a triple bond
- being shorter than a double bond or a single bond
- that has to do with the sp-hybrid orbitals.
- Go ahead and sketch out the dot structure really fast, right?
- We can see the triple bond there between the carbon atoms.
- And, if we analyze these bonds here,
- Let's do that really quickly.
- So, if I look at the dot structure,
- and I'm trying to figure out how many sigma or pi-bonds I have,
- all I have to do is to count of the single bonds,
- and now that will tell me how many sigma-bonds I have.
- So, here is the single bond, that is the sigma-bond.
- Here is the single bond, that is a sigma bond.
- and here is the triple bond between my carbons.
- I know that one of those bonds is a sigma-bond.
- This doesn't really matter which one you say it is
- for the dot structure.
- So, I'm just going to say this one, right?
- So, that gives me my three sigma-bonds.
- What I'm trying to figure out is pi-bonds,
- I know that the triple bond, there is one sigma-bond and two pi-bonds,
- so these must represent my pi-bonds.
- So, those are my two pi-bonds, like that.
- and so once again, for a triple bond, right?
- this is going to be the shortest bond like
- due to the fact that the sigma-bond comes from the overlap,
- the head-on overlap of two sp hybrid orbitals.
- When I think about the bond angles,
- When I think about the bond angle
- between this hydrogen and this carbon, and this carbon here.
- I can see that it's going to be, right?
- this is a straight line over here in my diagram
- so, let me see if i can draw this really fast, right?
- so this is a straight line, running through
- all of these atoms, right here.
- So, therefore, the bond I go must be 180 degrees.
- So, I have 180 degrees here.
- and you can see that that straight line runs through all my sp-hybrid orbitals.
- So, it's those hybrid orbitals
- that are going to determine the geometry.
- The pi-bonds will help to stabilize that
- and prevent any free rotation, right?
- So, the pi-bonds are going to make sure
- that the triple bond cannot undergo any kind of free rotation.
- Let's do one example
- to sum up everything we have learned about
- sp3-hybridization, sp2 and sp.
- So, if I start with just drawing
- of a molecule here.
- Let's go ahead and put in some carbons
- right? and some hydrogens like that.
- So, here we have a molecule.
- and let's try to answer a few questions about this molecule.
- The first question would be to identify the hybridization state
- of each carbon, right?
- So, if I look at this carbon, right?
- I can see that there is a double-bond present,
- so I can immediately see it's sp2-hybridized.
- The same thing for this carbon, right?
- It also has a double-bond, so it's sp2-hybridized.
- This carbon over here
- This one only has a single bond around it.
- So, we saw in a video of sp3-hybridization that
- this carbon must be sp3-hybridized.
- and we've just seen in this video
- how if you have a triple bond present, those carbons must be sp-hybridized.
- All right, next.
- Let's think about the shape and also the bond angle.
- So, we think about
- Let's start with sp3-hybridized carbon
- we saw in sp3 in the video of sp3-hybridization
- that the atoms around it want to try to adopt the tetrahedral bond angle,
- and try to get as close as possible to 109.5 degrees.
- So, that takes care of sp3-hybridization.
- sp2-hybridization, right?
- We saw that that's going to give us a trigonal planar geometry
- for the atoms around those carbons
- with the bond angle approximately 120 degrees.
- For sp-hybridization, we saw in this video,
- that's going to be linear, which is why I've drawn the dot structure in linear
- and giving us the bond angle of 180 degrees.
- Finally, let's count up the number of sigma- and pi-bonds in this molecule.
- So, I'm going to start with the number of sigma-bonds.
- So, all I have to do is to count up my single bonds.
- so this is a, this is one, two, three,
- for a double-bond, I know one of those is a sigma-bond from my video on sp2-hybridization.
- so four, five, six, seven, eight
- nine, and then,
- in this video, we saw that for triple bond, one of those is sigma-bond, right?
- so one of those is a sigma-bond there,
- so there is a total of ten, sigma-bonds.
- so we just count 10 sigma-bonds.
- What about pi-bonds?
- Well, those will be the ones that are left, right?
- So, for the double-bond, you have one sigma-bond and one pi-bond.
- So, this should be a pi-bond.
- For triple bond, we have one sigma-bond and two pi-bonds.
- So, there are total of three pi-bonds for this molecule.
- So, this is the type of question that you are usually asked
- in a test for organic chemistry.
- You should be able to analyze an organic molecule in this way
- it's an important skill for later in the course.
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