Transcript orbital. - Faperta UGM

BAB 2
Orbital dan perannya pada
Ikatan Kovalen
FROM LEWIS DIAGRAMS TO
MOLECULAR SHAPE
VSEPR THEORY
IKATAN KOVALEN
( Model tumpang tindih orbital)
.
H
atoms
move
closer
.H
Atom yang
terpisah
.
.
H H
Tumpang tindih
orbital
H H
Pembentukan
Ikatan
QUESTION
Can we predict the shapes of molecules simply by
combining the atomic orbitals available on each atom?
….. LET’S TRY IT FOR H2O
H
..
O:
H
OXYGEN
ORBITALS
oxygen = [He]2s22p4
z
2p
y
2p22p12p1
.
O
.
2p
x
.. 2s
2p Orbital saling
tegak lurus (90o)
[ cartoon ]
OXYGEN
ORBITALS
z
oxygen = [He]2s22p4
2p
y
unpaired
.
..
2p
O
.
2p
x
.. 2s
unpaired
Combining atomic
orbitals to form H2O.
2p
.
.
H
1s
hydrogen = 1s1
oxygen = [He]2s22p4
z
..
O
.
2p
.
H
1s
y
2p
x
.. 2s
Incorrectly
predicts a
90o angle.
EXPERIMENTAL RESULT
H
O
105o
H
The actual H-O-H angle
in water (measured by
electron diffraction)
is 105o
This is not very good agreement with
the atomic orbital model!
VSEPR Theory
Valence
Shell
Electron
Pair
Repulsion
 Based on the simple idea that
groups of electrons repel each other
 Predicts molecular shapes quite
well
A better result is predicted by VSEPR theory
nucleus
consider the completed
valence shell to be a
spherical volume around
the nucleus
electron pairs (4 pair)
repel each other
try to minimize repulsions
by maximizing the distance
between all pairs of
electrons
valence
shell
in the final solution, they
should all be equidistant
TETRAHEDRAL
Basic Shapes of Molecules
A
A
A
B
A
LINEAR
Bond angle = 180°
or
B
A
B
A
A
TRIGONAL PLANAR
Bond Angles = 120°
A
B
A
A
A
TETRAHEDRAL
Bond angles = 109° 28'
A
A
A
B
A
A
A
A
A
TRIGONAL BIPYRAMID
Bond angles = 120°, 90°
B
A
A
A
OCTAHEDRAL
Bond angles = 90°
A
VALENCE SHELL ELECTRON PAIR REPULSION
VSEPR THEORY
pairs
O
R
G
A
N
I
C
geometry
angles
hybridization
6 pair
octahedral
90o
d2sp3
5 pair
trigonal bipyramid
120o, 90o
dsp3
4 pair
tetrahedral
(pyrimidal,
angular )
109o28’
sp3
3 pair
trigonal planar
120o
sp2
2 pair
linear
180o
sp
For most molecules, these predictions are
correct to within a few degrees ( 5o).
Orbitals

The region of space around an atom in
which an electron is likely to be found
is an orbital.
 The shape and size of the orbital are
determined by a mathematical
equation called a wave function.
Orbitals

When atoms combine to form
molecules, they do so by combining
the wave functions for the individual
atomic orbitals.
 We say that the orbitals “overlap.”
 The region of space defined by this
combination of orbitals is the
molecular orbital.
Sigma Bonds
 Head-on
overlap of atomic
orbitals
 Electron density is a symmetrical
cylinder around the bond axis
Atomic orbital combinations that
give s bonds:
s s
s
p
p
p
Pi Bonds
 Side-on
overlap of atomic orbitals
 Electron density is above and below a
nodal plane on the internuclear axis
Atomic orbital combinations that give p
bonds:
p p
p
d
HOW ARE THE
OBSERVED BOND ANGLES ACHIEVED?
HYBRIDIZATION
“Vision is the art of seeing things invisible.”
Jonathan Swift
WHY DOESN’T THE ATOMIC ORBITAL APPROACH WORK ?
These orbitals are for the
atom - we can’t expect
that they are suitable for
the molecule.
2s
2px,2py,2pz
atomic
orbitals
During bonding ….
new orbitals form that
are more suitable for
making bonds.
After bonding (overlap)
we get a totally
new solution for the
new molecule.
sp,sp
2py,2pz
LCAO
hybrid atomic
orbitals
s, p, p ,n
overlap
molecular
orbitals
HYPOTHETICAL BONDING PROCESS
NOTE. Formally LCAO theory and Molecular Orbital theory are
two completely different approaches. You do not need to use
hybid orbitals to derive the molecular orbitals, combinations
of any type of function will do. Nevertheless, the abstraction
presented above is quite useful, as we will see quite soon.
FORMATION OF TETRAHEDRAL HYBRID ORBITALS
New orbitals point
to the corners of a
tetrahedron.
4 pair
sp3(1)
2p
109o28’
2s
O
FILLED VALENCE
SHELL
hybridization
occurs when orbitals
are full and have
finished bonding
sp3(3)
sp3(4)
sp3(2)
tetrahedral geometry
(cartoon)
(1)
(2)
(3)
(4)
sp3 hybrid orbitals
FORMATION OF
SP3 HYBRID ORBITALS
2pz
(1)
(2)
2p
(3)
2s
2s
X
2py
These orbital shapes are
cartoons - actual shapes
are shown on the next
slide.
2px
sp3 hybridized
unhybridized
atom
[animation]
(4)
SP3 HYBRID ORBITAL
… and its cartoon
( cross section )
The hybrid orbital
has more density
in the bonding lobe
than a p orbital and
forms stronger bonds
sp3
To avoid confusion the
back lobe is omitted
from the cartoons,
already shown, and the
front lobe is elongated
to show its direction.
The shape shown
is calculated from
quantum theory.
Courtesy of
Professor George Gerhold
omitted
ORIGIN OF THE SP3 DESIGNATION
add together, divide in four
hybridization
2s
2p
(1)
(2)
(3)
(4)
sp3 hybrid orbitals
each new orbital is
1/4 s + 3/4 p (25% s, 75% p)
S1P3 = SP3
( 1+3 ) = 4 parts total
ORIGIN OF THE SP3 ORBITAL SHAPE
2s orbital
sp3 hybrid
orbital
2p orbital
+
x
--
++
RECALL:
signs are mathematical
coordinates, not
electronic charge
HYBRIDIZATION
[animation]
Ikatan pada
Alkana sp3
 Alkena sp2
 Alkuna sp

.
.
C. .
Carbon has 4 valence electrons, 2s22p2
Carbon can form single, double or triple bonds
sp, sp2 and sp3 hybrid orbitals.
Let’s do sp3 first.
2p
hybridize
2s
sp3
H
H
H
H
H H
H C C H
H
H
H
H
H
H
H
H
Multiple Bonds and hybridization
Ethylene
H
H
C
H
C
H
Each carbon is hybridized sp2 . The hydrogens are 1s.
One of the double bonds is sp2 - sp2. The other one is
p - p.
2p
2p
hybridize
2s
sp2
C
C
Note that a double bond consists of a s and a p
type bond
H
H
C C
H
H
H
H
C
H
C
H
What about acetylene?
H C
C H
Each carbon atom is sp hybridized. The hydrogens are
unhybridized, 1s orbitals.
2p
2p
hybridize
sp
2s
Note that a triple bond consists of a s and 2p bonds. The
two p bonds use unhybridized p orbitals.
H C C H
H
C
C
H
COMPARISON OF SPx HYBRID ORBITALS
bigger
“tail”
more “p” character
sp3
sp2
sp
more “s” character
more electron
density in the
bonding lobe
BOND STRENGTHS - MULTIPLE BONDS
CC
bond
bond
type
bond
length
bond energy
per mole
Kcal
molecule
measured
(KJ)
C-C
sp3-sp3
1.54 Å
88
(368)
CH3- CH3
C=C
sp2-sp2
and p - p
1.34 Å
145
(607)
CH2=CH2
=
C=C
sp - sp
1.21 Å
and two p-p
increasing
s-character
198 (828)
=HC=CH
Bond Energy
Molecular Distortions:
VSEPR Revisited

Four situations:
1) electron pair repulsion
2) effect of electronegative atoms
3) double bond and electronegativity
4) steric repulsion
Electron Pair Repulsion
H
:
.. C ..
H : H
H
symmetrical molecule
all repulsions are equal
perfect tetrahedral
all angles 109o28’
larger
repulsion
..
angle
becomes
larger
.. N ..
H : H
H
repulsion
smaller
angle becomes
smaller
not all pairs are equivalent
the unshared pairs repel more
strongly than the bonded pairs
Effect of double bond and electronegativity
H
121.5o
C
C
H
H
Cl
114o
C O
H
122o
C O
116o
H
117o
H
H
116o
H
H 121.5o
117o
C CH2
H
123o
C
Cl
CH2
F
110o
125o
C
F
C O
111o
Cl
Cl
122o
124.5o
F
CH2
126o
C O
108o
F
MOLECULES WITH PI BONDS
Most pi bonds have
a bond energy of
50 - 60 Kcal / mole
( 210 - 250 Kj / mole )
When the total energy of a multiple bond is given,
you must subtract the energy of the pi bonds to
obtain the sigma bond energy.
C=C 145 Kcal/mole thus: C-C = 95 Kcal/mole
both
( 145 - 50 = 95 )
TOTAL BOND ENERGY
bonds
MOLECULES WITH UNSHARED PAIRS
Non Bonded Electrons
(unshared pairs)
do not significantly
change their energy
in going from an atom
to a bonded molecule
2s
H
2p
sp3
hybridization
sp3 hybrids
sp3
Metanol
H C O
H
H
H
H
H
H
2s
2p
s bonds
sp2
hybridization
sp2
used for
H
2p
2p
sp2
C O
H
used for
p bond
H
C
H
O
2p
sp
2p
sp
H C N
2s
hybridization
2p
sp hybrids
H
C
N
H
sp2
sp
sp2
C C C
H
H
H
H
allena
H C H
end
view
H
H
H
C
C
C
H
H
molecule has a twist in the center
ASSEMBLY METHOD
Start with the
Lewis Diagram
Determine the
geometry of
VSEPR
each atom
H
H
H
C
N:
H
H
C = 4 pair = tetrahedral
N = 4 pair = tetrahedral
C = sp3
Use the correct
hybid in each
case
Assemble the
molecule from
the hybrids.
N = sp3
C
N
H
H
C
H
H
N
..
H
Sample Problems

Predict the hybridization, geometry, and bond
angle for each atom in the following molecules:
 Caution! You must start with a good Lewis
structure!
– NH2NH2
– CH3-CC-CHO
O
CH3
C
_
CH2