Transcript PowerPoint on Hybridization

VSEPR & HYBRIDIZATION
Two Theories Which Seem To Reinforce Each Other
• VSEPR is essentially the theory which helps to explain molecular geometry
(shape, and bond angle), because it accounts for the repulsive interaction(s)
between the bonded and/or un-bonded electrons of the valence shell.
• Hybridization is the theory which replaces G.N. Lewis’s Valence Bond Theory.
Hybridization explains the experimental data, indicating equal bond
energies and bond lengths involving a central atom, and the common atoms
to which it is bonded, despite the fact that the bonding electrons are of
different energies (because they are in different sublevels).
• Hybridization supports, and is supported by, VSEPR, even though they are
two separate (different) theories.
sp3 hybridization with carbon, and 4 equal bonds
made from two different sublevels of different
energies!
2px
2py
2pz
sp3
E
sp3
sp3
Notice the change in energy. As
the s and p orbitals merge, the
energy of the new hybridized
orbitals is averaged, and equalized.
2s
1s
sp3
These electrons are not used in
bonding and remain unchanged
1s
The volume, shape and energy of the bonding atomic
orbitals are changed with sp3 hybridization.
Atomic (unhybridized orbitals)
Hybridized orbitals. The shaded area is
volume where the overlap of the covalent
bond occurs. Notice the spatial orientation.
Notice that only the 2s and 2p
sublevels are diagramed, as the 1s
sublevel of C, is not hybridized.
http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter__1
%3A_Chapter_1%3A_Introduction_to_organic_structure_and_bonding_I/Section_1.5%3A_Valence_bond_theor
y%3A_sp,_sp2,_and_sp3_hybrid_orbitals
sp3 hybrid orbitals point towards the corners of a
tetrahedron, thus supporting VSEPR Theory
http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter__1%3A_Chapter_1%3A_
Introduction_to_organic_structure_and_bonding_I/Section_1.5%3A_Valence_bond_theory%3A_sp,_sp2,_and_sp3_hybrid_orbitals
Now, study the aldehyde, methanal, CH2O as an example of sp2 hybridization
We only need enough hybrid orbitals to accommodate the groups around the central carbon atom
(That would be 3 in this case, the 2 H and the O) Groups can be atoms or un-bonded electrons.
There is no need to have
this 2pz orbital hybridized.
2px
2py
Notice that one s and 2 p orbitals
are blended, creating 3 bonding
orbitals or sp2 hybridization
2p
2pz
E
sp2
2s
1s
Again, these electrons are still not used in
bonding and they remain unchanged
1s
sp2
sp2
This 2p orbital is
used to create a
double bond (a
pi bond)
between the C
and the O, thus
creating the
carbonyl
functional group
O
//
H-C-H
sp2 hybridization helps to explain the double bond (pi bond)
The carbon-carbon double bond in ethene consists of one sigma bond,
formed by the overlap of two sp2 orbitals, and a second bond, [a π (pi)
bond]. Pi bonds are formed from the overlap of parallel un-hybridized p
orbitals on adjacent atoms. In this case, they are formed by the side-byside overlap of the two un-hybridized 2pz orbitals from each carbon. Because
they are the result of side-by-side overlap (rather then end-to-end overlap
like a sigma bond), pi bonds are not free to rotate. Rotation would break
the pi bond. The presence of the pi bond thus ‘locks’ the six atoms of
ethene into the same plane. In larger alkenes, this phenomenon leads to
the production of structural isomers.
http://butane.chem.uiuc.edu/pshapley/GenChem2/A6/3.html
When there are only 2 groups surrounding the carbon, as in CO2 sp
hybridization occurs … Again, notice that the number of hybridized bonds
depends upon the number of bonded atoms
Notice that one s and p orbital are blended, creating
2 bonding orbitals or sp hybridization
There is no need to
Hybridize these 2 orbitals
2px
2py
2p
2pz
E
sp
2s
1s
Again, these electrons are still not used in
bonding and they remain unchanged
sp
2p
These 2p orbitals
are used to
create the pi
double bonds
between the C
and the O atoms
O=C=O
1s
So, In Summary ….
sp3 orbitals are
• the result of blending one s and 3 p orbitals
making the sp3 75% p and 25% s
• used to explain the equality of bond length, and
strength in molecules such as CH4 (with 4 bonded
groups) and leads to tetrahedral molecular
shapes, and free rotation around C to C single
(sigma) covalent bonds.
http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter__1%3A_Chapter_1%3A_I
ntroduction_to_organic_structure_and_bonding_I/Section_1.5%3A_Valence_bond_theory%3A_sp,_sp2,_and_sp3_hybrid_orbitals
sp2 hyridized orbitals are
 used to explain the existence of the double bond,
for, a 2p orbital is left un-hybridized and used to
make a pi double bond
 used to explain three bonded groups and leads to
trigonal planar shapes
 used to explain the carbonyl group (with its one
double bond) in aldehyde and ketone compounds,
when carbon is the central bond.
sp hybridization
• explains the linear shape when only two groups
are bonded to the central carbon.
• leaves two un-hybridized p orbitals capable of
producing double bonds (pi bonds)
• is 50% s and 50% p
• is seen in molecules such as CO2, BeF2, C2H2
• explains multiple double bonds (pi bonds) when
carbon is the central atom.
To help understand sigma and pi bonds a bit better, try these
two videos …Come in with questions, tomorrow!
• Try this quirky 5 minute video with some very
nice points to make about sigma and pi
bonds, and hybridization.
• Check out the whole video, but take a special
look at around 3:58 for a nice little clay model
view of pi bond formation.
http://www.youtube.com/watch?v=ree49ge4VA4
• Per usual, Crash Course Chemistry keeps its
rapid pace dialogue and imagery going, but
each installment has some nice points to
make. You might want to stop and replay a
section now and then.
• This 11 minute video has some special points
for review and edification:
at 3:48 you will get an review at to why
water is a polar molecule
and at
7:50 there is a really nice visualization of
pi bond formation.
http://www.youtube.com/watch?v=cPDptc0wUYI
Citations
Based upon ideas / work found at/in:
1. http://www.chem.uiuc.edu/CLCtutorials/104/Hybridization/SeeIt.html
2. Morrison and Boyd: Organic Chemistry
3. Pauling, L: General Chemistry
4. http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/C
hapter__1%3A_Chapter_1%3A_Introduction_to_organic_structure_and_bonding_I/Section_1.5%3A_
Valence_bond_theory%3A_sp,_sp2,_and_sp3_hybrid_orbitals
5. http://butane.chem.uiuc.edu/pshapley/GenChem2/A6/3.html
6. https://en.wikipedia.org/wiki/Pi_bond