1. THE SHAPES OF
MOLECULES
ELECTRON PAIR REPULSION
THEORY.
•Molecular shape depends on electron
pairs around a central atom.
•What are the rules that determine the
shapes of simple molecules ?
3. Electron charge clouds
• Repel each other – why ?
• Shape of the charge clouds affect how much
• So the greatest angles are between lone
pairs of electrons…
4. A BETTER THEORYA BETTER THEORY
V alence
S hell
E lectron
P air
R epulsion
5. The new orbitals point to
the corners of a tetrahedron.
Bond angles are 109o
28’’
NEW BONDING ORBITALSNEW BONDING ORBITALS
Each bonding situation
can be treated in a
similar fashion ……..
6. VALENCE SHELL ELECTRON PAIR REPULSION
VSEPR THEORY
4 pair tetrahedral 109o
28’ sp3
( pyrimidal,
angular )
3 pair trigonal planar 120o
sp2
2 pair linear 180o
sp
pairs geometry angles hybridization
For most molecules, these predictions are
correct to within a few degrees (± 5o
).
6 pair octahedral 90o
d2
sp3
5 pair trigonal bipyramid 120o
, 90o
dsp3
O
R
G
A
N
I
C
7. INTRODUCTION
A) Lewis structures do not indicate the three
dimensional shape of a molecule. They do not
show the arrangement space of the atoms,
what we call the molecular geometry or
molecular structure.
B) Molecules have definite shapes and the
shape of a molecule controls some of its
chemical and physical properties.
8. II. Valence Shell Electron Pair Repulsion
Theory - VSEPR - predicts the shapes of a
number of molecules and polyatomic ions.
A) Assumptions of VSEPR Theory
1) Electron pairs in the valence shell of an
atom tend to orient themselves so that the
total energy is minimized. This means that:
the electrons will approach the nucleus so
that we can predict the shapes according to
the order of repulsion:
2) bbbbbbbbbbbbbbpositions as far away
from each other as possible to minimize
_______________ .
9. ORDER OF REPULSION BETWEEN
VARIOUS ELECTRON PAIRS
• bp –bp LESS THAN bp-lp LESS THAN lp-lp
WHERE ‘bp’ MEANS BONDING OR
BOND PAIR AND ‘lp’MEANS LONE
PAIR
10. 2) Because lone pairs of electrons are spread
out more broadly than bond pairs, repulsions
are greatest between two lone pairs,
intermediate between a lone pair and a bond
pair, and weakest between two bonding pairs
of electrons.
3) Repulsive forces decrease rapidly with
increasing interpair angle - greatest at 90o
,
much weaker at 120o
, and very weak at 180o
.
B) What are the ideal arrangements of
electron pairs to minimize repulsions?
11. 1) We need to identify the number of regions
of high electron density, i.e. the number of
stereoactive sets of electrons. One
stereoactive set can be a single, double or
triple bond.
2) If only single bonds, the number of
stereoactive sets is equal to the bond pairs
plus the lone pairs of electrons around the
central atom.
13. VARIATIONS ON TETRAHEDRALVARIATIONS ON TETRAHEDRAL
TETRAHEDRAL
PYRIMIDAL
ANGULAR
C
N
O
..
..
..
H
H
H
H
H
H
H
H
H
Distinctions are often
made in general chemistry.
Although the molecules
have different shapes,
the orbitals use the same
tetrahedral arrangement
(includes pairs).
Usually we won’t bother.
14. H
O
H
H
C
H
H
H
H
N
H
H
4 pair = tetrahedral ( 109o
28’)
109o
28’
107o
105o
H
C
F
H
F
CH3
N
CH3
CH3
Cl
C
H
H
H
tetrahedral
pyrimidal
bent
108o
112o
108o
110o
108o
H
C
CH3
H
CH3
112o
106o
H
S
H
92o
H
P
H
H
93o
Period Three
Perfect Symmetry
EXPERIMENTAL RESULTS
15. N
H H:
....
H
..
EXPERIMENTAL MEASUREMENT OF ANGLESEXPERIMENTAL MEASUREMENT OF ANGLES
Atoms have dense nuclei,
the H-N-H angle is easy to
measure.
Electron pairs are diffuse.
It is difficult to measure the
angle between an electron
and an atom or another pair.
107o
?
Most experimental methods
(X-ray and electron diffraction)
rely on locating the atoms
which have massive nuclei
and are easy to find.
Other angles are
generally calculated.
17. C
H
H H
H
:
:
....
symmetrical molecule
all repulsions are equal
perfect tetrahedral
all angles 109o
28”
N
H H:
....
H
.. angle
becomes
larger
larger
repulsion
smaller
repulsion angle becomes
smaller
not all pairs are equivalent
the unshared pairs repel more
strongly than the bonded pairs
UNEQUAL REPULSIONS BETWEEN PAIRSUNEQUAL REPULSIONS BETWEEN PAIRS
107o
H-N-H angle reduced to 107o
109o
28”
19. PERIOD THREE ELEMENTS ARE NOTPERIOD THREE ELEMENTS ARE NOT
AS LIKELY TO BECOME TETRAHEDRALAS LIKELY TO BECOME TETRAHEDRAL
N
P
..
..
..
..
PERIOD 2
PERIOD 3
larger
repulsions
smaller
repulsions
Not as much is gained by adjusting to ideal angles.
The unshared pair on P does take a lot more space.
Electrons are more diffuse
and further from the nucleus.
H
P
H
H
93o
H
N
H
H 107o
21. 3 pair trigonal planar ( 120o
)
F B
F
F
HC
H
H
+
120o120o
( incomplete octet )
carbocation
all repulsions equal
3 equivalent bonds
not a stable molecule,
but an “intermediate”
ion - it reacts quickly
EXPERIMENTAL RESULTS
22. 3 pair trigonal planar ( 120o
)
( pi bonded )
C C
HH
H H
117o
121.5o
C CH2
H
H
121.5o
117o
C CH2
Cl
Cl
123o
114o
C CH2
F
F
125o
110o
C O
H
H
116o
122o
C O
H
H
116o
122o
C O
Cl
Cl
124.5o
111o
C O
F
F
126o
108o
double bonds count
as only one pair
EXPERIMENTAL RESULTS
24. C ....
..
..
larger repulsion
one pair : two pairs
smaller repulsion
one pair : one pair
> 120o
< 120o
In alkenes the C=C H angle is typically larger-
than the H C H angle.- -
UNEQUAL REPULSIONSUNEQUAL REPULSIONS
C
H
H
25. UNEQUAL REPULSIONSUNEQUAL REPULSIONS
more
repulsion
less
repulsion
Electrons in C-H bonds
are shared nearly equally.
Electrons in a C-Cl
bond are closer to
chlorine and further
from each other.
POLAR BOND
The electronegative Cl
draws electrons closer
to that end of the bond.
NONPOLAR BOND
Electrons in C-H bonds
are closer to carbon
(near center of bond)
than in the case below.
C ....
..
..
H
H
r
.
.
C ....
.
.Cl
Cl
polar
R
Fluorines cause even smaller angles.
C CH2
H
H
121.5o
117o
C CH2
Cl
Cl
123o
114o
C CH2
F
F
125o
110o
27. 2 pair linear ( 180o
)
CH3 Mg CH3
H C N
H C C H
CH3 C C CH3
CH2 C CH2
CH2 C O
both 180o
incomplete octet pi bonded
all 180o
O C O
H Be H
triple bonds count
as only one pair
EXPERIMENTAL RESULTS
There are no
deviations from
the ideal 180o
when observing
linear molecules!
29. SOME MOLECULESSOME MOLECULES
PREDICT ANGLES ( assume no distortions occur )
amide
azo
compound
Remember this
functional group?
Not a functional
group you have to
know right now.
C C
O
N
H
H
H H
H
..
..
:
N N CC
H
H
H H
H
H
.. ..
