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Hybridization
The Ins and Outs
Do you remember..?
! Can you answer the following questions? If you can, then you are good to go…
! Can you explain why atoms hybridize s and p orbitals to form sp^3 , sp^2 or sp orbitals?
! Can you identify the hybridization of an atom in a molecule?
! Do you recall the geometries and bond angles for the hybrid orbitals?
! Can you compare bonds lengths and strengths?
So - ARE you good to go?
! If you think you are comfortable with the topic of hybridization, skip to the end (Slide 36) and see if you can answer the questions.
! If you think its been too long, just start clicking through… Hopefully this will get you up to speed…
Why atoms hybridize…
! There are several reasons why hybridization makes sense…
! First, atoms in their elemental state aren’t prepared to make the necessary bonds. In order to make covalent bonds (the sharing of a pair of electrons), the atoms must have unpaired electrons to share to form covalent bonds.
! Consider carbon – its electronic configuration is 1s^2 2s^2 2p^2. With four valence electrons, how many unpaired electrons does it have?
! Only 2…
Why atoms hybridize…
! Looking at the valence electrons of carbon in the second shell, it has four total electrons but only two unpaired electrons:
! If you need to make four covalent bonds, you don’t have enough unpaired electrons!
2s
2p
Why atoms hybridize…
! Clearly having enough unpaired electrons to make bonds with is important…
! Now for the second reason for hybridization. The orbitals that contain those valence electrons in the atoms’ elemental states are not in what we would call the “best spatial orientation”.
! What does that mean? “Best spatial orientation”? It means that in the elemental state, the s and p orbitals are putting those negatively charged electrons too close to each other. And we know that like charges repel like charges… The farther away the electrons can get, the more stable the system.
Why atoms hybridize…
! See if you can picture what the elemental Carbon atom would look like if we used the one s and three p orbitals. The s orbital in the center and three p orbitals, each placed 90º apart, like the axes on a graph:
! Put an electron in each…
P (^) x
P (^) y
P (^) z
s
Why atoms hybridize…
! Ground State: Valence = 2s^2 2p^2
! The pz orbital is empty. The pair of electrons in the s orbital must then un-pair to make four covalent bonds, placing one electron in the pz orbital.
P (^) x
P (^) y
P (^) z
90º
90º
Why atoms hybridize…
! With a single electron in each orbital:2s^1 2px^1 2py^1 2pz^1 :
! Notice how the electrons are pretty much all clustered together 90º apart with the s electron close to the center. A lot of electron repulsions would occur in this arrangement. High energy!
P (^) x
P (^) y
P (^) z
90º
90º
Why atoms hybridize…
! Spatial orientation is a major driving factor for hybridization to occur. Those electrons want to get as far away from each other as possible.
! The third and final reason to hybridize – you will make stronger bonds using hybrid orbitals.
! How you ask? (Okay – so maybe you didn’t ask, but let me tell you…)
! Hybrid orbitals overlap much better head-to-head to form stronger sigma bonds.
Why atoms hybridize…
! Consider a normal p orbital simply overlapping with an s orbital:
! There’s a small bit of overlap to make a bond and so a sigma bond can certainly form. This picture doesn’t really do it justice: the p orbital is VERY narrow, relative to the hybrids we will see.
Why atoms hybridize…
! Now consider a hybrid orbital overlapping with an s orbital and remember that hybrid orbitals are asymmetrical and have one larger lobe for overlapping to form bonds:
! There’s a a lot better overlap to make a bond with the larger lobe and a stronger sigma bond will form. Stronger bonds means more stable and lower energy system!
sp
3
Hybrid Orbitals
! Q: When is sp^3 hybridization going to occur?
! A: Any time an atom has four groups of electrons around it.
! Q: What are “groups”?
! A: Groups are pairs of electrons found in a sigma bond or a lone pair of electrons.
! Q: Where do you find sigma bonds?
! A: There is one sigma bond inside every single, double or triple bond.
sp
3
Hybrid Orbitals
! Each of the following compounds all contain atoms (highlighted in red) that are sp^3 hybridized:
! Make sure you can clearly see the FOUR GROUPS - the sigma bonds (in single bonds for these) and lone pairs of electrons.
C
H
H
H H H O H N
H
H H
sp
2
Hybrid Orbitals
! How do sp^2 hybrid orbitals form? Sp^2 hybrid orbitals form from the combination of one s orbital and two p orbitals resulting in three sp^2 orbitals. You will typically find sp^2 orbitals with atoms that are part of a double bond.
! A double bond is comprised of one sigma bond and one pi bond. When we combine orbitals, we need to set aside one p orbital to form a pi bond with and combine the remaining s and two p orbitals to form the new hybrids.
sp
2
Hybrid Orbitals
! As before, in any hybridization process, the elemental carbon atom much first un-pair the pair of electrons in the s orbital so you have four unpaired electrons to make four covalent bonds. This does take some energy…
2s
2p
2s
2p
sp
2
Hybrid Orbitals
! Now we can combine the s orbital and two of the p orbitals orbitals to make three new hybrid orbitals, leaving a p orbital available on the atom to form a pi bond.
2s
2p
2sp^2
2p
sp
2
Hybrid Orbitals
! The sp^2 orbitals are all equal and the same. Geometrically, they are all contained on the same plane (called trigonal planar or even just “planar”).
! The remaining p orbital sits perpendicular to the plane of the sp^2 hybrid orbitals. Only the larger bonding lobes of the sp^2 orbitals (outlined in red) are shown above, while both lobes of the p orbital (gray and white) are represented.
side view:
sp 2 sp 2
sp 2
p orbital
sp
2
Hybrid Orbitals
! The bond angle for an sp^2 hybridized atom is 120º as you can see from the top view (only larger bonding lobes shown):
! The orbitals are 67% p and 33% s. The larger bonding orbital is wider than a p or sp^3 orbital and even shorter due to the increased s character of the orbital. Even better to bond with!
top view
sp
2
Hybrid Orbitals
! Keeping everything relative, remember again that the s orbital is the lowest in energy (electrons are closest to the positively charged nucleus) and p is the highest in energy (electrons are furthest from the nucleus).
! The sp^3 hybrid orbital is about 25% more stable than a p orbital.
! The sp^2 hybrid orbital is about 33% more stable than a p orbital. More stable, lower in energy. Shorter, rounder orbital forming stronger bonds…
sp
2
Hybrid Orbitals
! Q: When is sp^2 hybridization going to occur?
! A: Any time an atom has three groups of electrons around it. Remember you are only counting the sigma bonds and lone pairs of electrons.
! The molecule shown below has two atoms (shown in red) that are sp^2 hybridized:
H
C
H
N
H
sp Hybrid Orbitals
! How do sp hybrid orbitals form? Sp hybrid orbitals form from the combination of one s orbital and one p orbital resulting in two sp orbitals. You will typically find sp orbitals with atoms that are part of a triple bond.
! A triple bond is comprised of one sigma bond and two pi bonds. When we combine orbitals, we need to set aside two p orbitals to form two pi bonds with and combine the remaining s and p orbitals to form the new hybrid sp orbitals.
sp Hybrid Orbitals
! As before, in any hybridization process, the elemental carbon atom much first un-pair the pair of electrons in the s orbital so you have four unpaired electrons to make four covalent bonds. This does take some energy…
2s
2p
2s
2p
sp Hybrid Orbitals
! Now we can combine the s orbital and one of the p orbitals orbitals to make two new hybrid orbitals, leaving two p orbitals available on the atom to form two pi bonds.
2s
2p
2sp
2p
Try a Few Questions…
! Determine the hybridization of the highlighted atoms shown below:
H C C 1 C 2 N
H
H
H
H
O 1
C
H
O 2
H
H
C 3
H
C 4
H
C 5 H
Hybridizations?
! How did you do?
! C 1 is sp^3 , O 1 is sp^2 , N is sp^3 , O 2 is sp^3 and C 5 is sp hybridized.
H C C 1 C 2 N
H
H
H
H
O (^1)
C
H
O (^2)
H
H
C (^3)
H
C (^4)
H
C 5 H
Orbitals?
! Can you recognize the hybrid orbitals that make up specific sigma bonds?
! Determine what orbitals comprise the following sigma bonds: C 1 -C 2 , N-H, C 3 –C 4 and C 5 -H
H C C 1 C 2 N
H
H
H
H
O (^1)
C
H
O (^2)
H
H
C (^3)
H
C (^4)
H
C 5 H
Orbitals…
! How did you do?
! C 1 -C 2 is comprised of sp^3 and sp^2 orbitals, respectively ! N-H is comprised of sp^3 and s orbitals, ! C 3 –C 4 is comprised of sp^3 and sp orbitals and ! C 5 -H is formed from sp and s orbitals. ! Remember that pi bonds are always made of p orbitals, should that question arise!
H C C 1 C 2 N
H
H
H
H
O 1
C
H
O 2
H
H
C 3
H
C 4
H
C 5 H
Bond Angles…
! This time, try some bond angles…
! Determine the bond angles for C 1 -C 2 -N, H-C-O 2 and C 4 -C 5 - H.
! In order to do so, you must identify the hybridization of the central atom
H C C 1 C 2 N
H
H
H
H
O 1
C
H
O 2
H
H
C 3
H
C 4
H
C 5 H
Bond Angles…
! Remember that when determining bond angles, it is the central atom in the sequence that decides the bond angle.
! The bond angle for C 1 -C 2 -N is 120º because C 2 is sp^2.
! The bond angle for H-C-O 2 is 109.5º because C is sp^3.
! The bond angle for C 4 -C 5 -H is 180º because C 5 is sp.
H C C 1 C 2 N
H
H
H
H
O 1
C
H
O 2
H
H
C 3
H
C 4
H
C 5 H
Lone Pairs…
! The lone pairs of most atoms will be contained in the hybrid orbitals of the atom itself.
! What hybrid orbital contains the lone pair on O 1 and O 2?
H C C 1 C 2 N
H
H
H
H
O 1
C
H
O 2
H
H
C 3
H
C 4
H
C 5 H
Lone Pairs…
! How did you do?
! O 1 is sp^2 hybridized therefore the lone pair on O 1 is in an sp 2 hybrid orbital and
! O 2 is sp^3 hybridized therefore the lone pair on O 2 is in an sp 3 hybrid orbital.
H C C 1 C 2 N
H
H
H
H
O 1
C
H
O 2
H
H
C 3
H
C 4
H
C 5 H
Bond Lengths… and
Strengths…
! Compare bond lengths and strengths.
! Determine which sigma bond is longer: C 1 -C 2 or C 3 -C 4?
! Determine which sigma bond is stronger? O 1 -C 2 or O 2 -C 3?
H C C 1 C 2 N
H
H
H
H
O (^1)
C
H
O (^2)
H
H
C (^3)
H
C (^4)
H
C 5 H
Lengths and Strengths…
! Remember that longer orbitals form longer, weaker bonds but shorter, rounder orbitals make shorter, stronger bonds because of better overlap.
! #1: Which bond is longer: C 1 -C 2 or C 3 -C 4? C 1 -C 2 is formed from sp^3 and sp^2 orbitals and C 3 -C 4 is formed from sp^3 and sp orbitals. Since the difference is the comparison of sp^2 for one and sp for the other, the sp^2 orbital is longer than the sp orbital, C 1 -C 2 is the longer bond.
H C C 1 C 2 N
H
H
H
H
O 1
C
H
O 2
H
H
C 3
H
C 4
H
C 5 H
Lengths and Strengths…
! And finally:
! #2: The sigma bond of O 1 -C 2 is formed from sp2-sp^2 overlap while sigma bond of O 2 -C 3 is formed from sp 3 -sp^3 overlap. The sp^2 orbitals are shorter and rounder and will overlap better to form stronger bonds.
H C C 1 C 2 N
H
H
H
H
O (^1)
C
H
O (^2)
H
H
C (^3)
H
C (^4)
H
C 5 H
And that’s about it…
! That’s all you need to know… if you can answer those basic questions, then you are good to go for what you need to be able to do in Orgo I. The shape and structure of a molecule often determines its reactivity so you’ll be identifying hybrid orbitals throughout the course.
