Hybridization
The Ins and Outs
Are you good to go?
Can you answer the following questions? If you can, then
you ARE good to go…
Can you explain why atoms hybridize to form sp3, sp2 or sp
orbitals instead of just using s and p 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 good to go, skip to the end 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 bother with hybridization?
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.
Consider carbon – its electronic configuration is 1s22s22p2.
How many unpaired electrons does it have?
Only 2…
Why bother with hybridization?
Looking at the valence electrons in the second shell, it has
four total electrons but only two unpaired electrons:
2p
2s
If you need to make four covalent bonds, you don’t have
enough unpaired electrons!
Why bother with hybridization?
Clearly having enough unpaired electrons to make bonds
with is important…
Now for the second reason for hybridization. The orbitals
that contain those electrons in the atoms’ elemental states are
not in what we would call the “best spatial orientation”.
What does that mean? It means that the orbitals with the
electrons are putting those negatively charged electrons too
close to each other. And we know that like charges repel like
charges. The farther away, the better!
Why bother with hybridization?
See if you can picture what the elemental Carbon atom
would look like if we used an s and three p orbitals:
py orbital with one electron
90º
px orbital with one electron
Empty pz orbital, perpendicular
to the plane (z-axis)
lone pair of electrons in 2s orbital
Why bother with hybridization?
When we un-pair the 2s electrons, and promote one into the
higher energy 2pz orbital, the spatial orientation is not any
better:
p orbital with one electron
y
90º
px orbital with one electron
pz orbital, perpendicular to the
plane (z-axis), with one electron
One electron in 2s orbital
Why bother with hybridization?
Notice how you now have THREE electrons that are 90º
apart from each other. And one electron in an s orbital
desperately trying to get as far away as possible. That
electron will be on the far side of the atom, trying to avoid
the other electrons.
WAY TOO CLOSE!
Why bother with hybridization?
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
sigma bonds.
Why bother with hybridization?
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.
Why bother with hybridization?
Now consider a hybrid orbital overlapping with an s orbital
and remember that hybrid orbitals are asymmetrical and have
one larger lobe for overlapping:
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!
Why bother with hybridization?
So, now you know. You get the correct number of unpaired
electrons for the making of bonds and they will be better
oriented in space relative to each other. With the asymmetric
hybrid orbitals (that one big lobe for bonding), you will form
stronger bonds and more stable compounds. All excellent
reasons for atoms to hybridize away from their elemental
state.
Now… Let’s consider each possible type of hybridization…
When (and how) do they occur…?
Sp3 Hybrid Orbitals
First things first…. In any hybridization process, the
elemental carbon atom much first unpair the pair of electrons
in the s orbital so you have four unpaired electrons to make
four covalent bonds. This does take some energy…
2p
2p
2s
2s
Sp3 Hybrid Orbitals
Now we can combine the four orbitals to make four new
hybrid orbitals. Orbitals are only math equations. Add them
together and divide by four:
2p
2s
2sp3
Sp3 Hybrid Orbitals
These orbitals are all equal and the same. Geometrically, the
large end of each hybrid orbital points to a corner of a
tetrahedron.
Sp3 Hybrid Orbitals
The bond angle for an sp3 hybridized atom is 109.5º.
109.5º
The orbitals are 75% p and 25% s. The larger bonding orbital
is wider than a p orbital and a bit shorter. Better to bond
with!
Sp3 Hybrid Orbitals
The orbitals are 75% p and 25% s. The larger bonding orbital
is wider than a p orbital and a bit shorter. Better to bond
s
with!
p
1 s + 3 p =
subtractive additive
25% s, 75% p
Note 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 sp3 hybrid orbital is about 25% more stable (lower
energy!) than a p orbital.
Sp3 Hybrid Orbitals
Question: When is sp3 hybridization going to occur?
Answer: Any time an atom has four groups of electrons
around it.
What are “groups”?
Groups are pairs of electrons found in a sigma bond or a
lone pair of electrons.
Where do you find sigma bonds?
Every single, double or triple bond has one sigma bond.
Sp3 Hybrid Orbitals
The following compounds all contain atoms that are sp3
hybridized:
H
H
C
H
H
H
O
H
H
N
H
H
Make sure you can clearly see the FOUR GROUPS - the
sigma bonds (in single bonds for these) and lone pairs of
electrons.
Sp2 Hybrid Orbitals
How do sp2 hybrid orbitals form? Sp2 hybrid orbitals form
from the combination of one s orbital and two p orbitals
resulting in three sp2 orbitals. You will find sp2 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 hybrid.
Sp2 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…
2p
2p
2s
2s
Sp2 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.
2p
2p
2sp2
2s
Sp2 Hybrid Orbitals
The sp2 orbitals are all equal and the same. Geometrically,
they are all contained on the same plane (PLANAR or
trigonal planar)
side view with p orbital
The remaining p orbital sits perpendicular to the plane of the
sp2 hybrid orbitals.
Sp2 Hybrid Orbitals
The bond angle for an sp2 hybridized atom is 120º.
top view
120º
The orbitals are 67% p and 33% s. The larger bonding orbital
is wider than a p or sp3 orbital and even shorter due to the
increased s character of the orbital. Even better to bond
with!
Sp2 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 sp3 hybrid orbital is about 25% more stable than a p
orbital.
The sp2 hybrid orbital is about 33% more stable than a p
orbital. More stable, lower in energy. Rounder orbital
forming stronger bonds…
Sp2 Hybrid Orbitals
Question: When is sp2 hybridization going to occur?
Answer: 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 that are sp2
hybridized:
H
H
C
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 two 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…
2p
2p
2s
2s
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.
2p
2s
2p
2sp
Sp Hybrid Orbitals
The two sp orbitals are equal and the same. Geometrically,
they face away from each other in opposite directions
(LINEAR). Only the large lobes of the hybrids are shown
here (in blue).
perpendicular
p orbitals
sp hybrid orbitals
(shown in blue)
The remaining p orbitals sit perpendicular to each other
ready to form two pi bonds.
Sp Hybrid Orbitals
The bond angle for an sp hybridized atom is 180º.
180º
The orbitals are 50% p and 50% s. The larger bonding orbital
is wider than a p or sp3 or sp2 orbital and even shorter due to
the increased s character of the orbital. Even better to bond
with!
Sp 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 sp3 hybrid orbital is about 25% more stable than a p
orbital.
The sp2 hybrid orbital is about 33% more stable than a p
orbital.
The sp hybrid orbital is 50% more stable than a p orbital.
More stable, lower in energy. Rounder orbital forming an
even stronger bond.
Sp Hybrid Orbitals
Question: When is sp hybridization going to occur?
Answer: Any time an atom has two 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 that are sp
hybridized:
H
C
N
So… Now What?
We’ve reviewed all the possible hybridizations that we
commonly see for the atoms found in an organic molecule.
Hopefully you have a sense for WHY they are utilized and
where they come from (yes, I know – that whole orbital
recombination thing is a higher mathematical nightmare –
just remember that orbitals are math equations of where
you’ll most likely find an electron around an atom and math
equations can be added, subtracted, multiplied and divided.)
You need to be able to recognize hybridizations of atoms and
answer some basic questions.
So… Now What?
“What kinds of questions will be on the test?”, you ask. Let
me give you some examples:
The very basic “What is that atom’s hybridization?” is a
good one to start with.
You may also be asked to determine the geometry of an
atom, what orbitals make up a bond, what is the bond angle
for a given set of atoms or what bond is
stronger/weaker/longer/shorter. All of these questions
pertain to the hybridization of the atoms involved.
Now… Try a Few…
Determine the hybridization of the highlighted atoms shown
below:
H
H
H
O1
H
C
C1 C2 N
C
H
H
H
H
H
O
C3 C4 C5 H
H
Now… Try a Few…
How did you do?
H
H
H
O1
H
C
C1 C2 N
C
H
H
H
H
H
O
C3 C4 C5 H
H
C1 is sp3, N is sp3, O is sp3 and C5 is sp hybridized.
Now… Try a Few…
Can you recognize your orbitals?
H
H
H
O1
H
C
C1 C2 N
C
H
H
H
H
H
O
C3 C4 C5 H
H
Determine what orbitals comprise the following sigma
bonds: C1-C2, C2-N, C3–C4 and C5-H
Now… Try a Few…
How did you do?
H
H
H
O1
H
C
C1 C2 N
C
H
H
H
H
H
O
C3 C4 C5 H
H
C1-C2 is comprised of sp3 and sp2 orbitals, C2-N is comprised
of sp2 and sp3 orbitals, C3–C4 is comprised of sp3 and sp
orbitals and C5-H is formed from sp and s orbitals. Hydrogen
is the only atom that will not hybridize.
Remember that pi bonds are always made of p orbitals!
Now… Try a Few…
This time, try some bond angles…
H
H
H
O1
H
C
C1 C2 N
C
H
H
H
H
H
O
C3 C4 C5 H
H
Determine the bond angles for C1-C2-N, C-O-C3 and C3-C4C5.
Now… Try a Few…
Remember that when determining bond angles, its the
central atom in the sequence that decides the bond angle.
H
H
H
O1
H
C
C1 C2 N
C
H
H
H
H
H
O
C3 C4 C5 H
H
The bond angle for C1-C2-N is 120º, C-O-C3 is 109.5ºand C3C4-C5 is 180º.
Now… Try a Few…
Compare bond lengths and strengths.
H
H
H
O1
H
C
C1 C2 N
C
H
H
H
H
H
O
C3 C4 C5 H
H
Determine which bond is longer: C1-C2 or C3-C4?
Which bond is stronger? O1-C2 or O-C3?
Now… Try a Few…
Remember that longer orbitals form longer bonds but shorter
rounder orbitals make stronger bonds because of better
overlap.
H
H
H
O1
H
C
C1 C2 N
C
H
H
H
H
H
O
C3 C4 C5 H
H
Determine which bond is longer: C1-C2 or C3-C4? C1-C2 is
formed from sp3 and sp2 orbitals and C3-C4 is formed from
sp3 and sp orbitals. Since the sp2 orbital is longer than the sp
orbital, C1-C2 is the longer bond.
Which bond is stronger? O1-C2 or O-C3?
Now… Try a Few…
Remember that longer orbitals form longer bonds but shorter
rounder orbitals make stronger bonds because of better
overlap.
H
H
H
O1
H
C
C1 C2 N
C
H
H
H
H
H
O
C3 C4 C5 H
H
O1-C2 is formed from sp2-sp2 overlap while O-C3 is formed
from sp3-sp3 overlap. The sp2 orbitals are shorter and
rounder and will overlap better to form stronger bonds.
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.