Not the whole story
http://www.flickr.com/photos/fenris117/4525319652/
Chemical Bonding 8
Delocalization and Resonance Structures
Reference: Higher Level Chemistry, p. 139-144
1
Delocalized = ???
In The Free Dictionary
delocalize, delocalise [diːˈləʊkəˌlaɪz]
vb (tr)
1. to remove from the usual locality
2. to free from local influences
delocalization , delocalisation n
http://www.thefreedictionary.com/delocalized
2
Delocalized = ???
In Chemistry (1)
In metals, valence electrons are “delocalized”.
The valence electrons of the metal atoms move freely
from one cation to another.
3
Delocalized = ???
In Chemistry (2)
In covalent substances, a localized electron pair in a bond is held
in a specific position in the molecule - usually between 2 atoms.
O
H
H
H
N
H
H
This e pair has nowhere else to go.
It is localized between the O and H atoms.
There is no other place to put all the
electron pairs in this structure.
All the electron pairs are localized.
4
Delocalized = ???
In Chemistry (2) continued
In some covalent substances, an electron pair in a bond
might be placed in different positions when you draw the structure.
O
S
O
O
S
O
Neither of these structures is accurate.
The electron pair in the pi bond (second
bond of the double bond) actually spreads
out over THREE atoms instead of two.
O
S
O
When the electrons spread out, the structure is more stable.
http://crmafia.files.wordpress.com/2011/08/gasp.jpg
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Delocalized = ???
Delocalization of electrons within a covalent moleculs happens when:
• bonding electrons are in pi bonds - they can spread out over
three atoms (or more) within the same molecule
• there is more than one possible position for a double
bond in a molecule or polyatomic ion
Which structure contains delocalized electrons?
1-
O
N
O
Cl Be Cl
O
O
C O
The electrons in the pi bond spread out over all the atoms.
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Resonance Structures
When electrons are delocalized over several atoms within a
covalent molecule/ion, several Lewis structures are drawn:
1-
O
N
O
O
1-
O
N
O
1-
O
O
N
O
O
These are called resonance structures.
The double headed arrow is used to indicate that
the “true” structure is a blend of all three structures.
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Resonance Structures
Sometimes this representation is used when delocalized
electrons spread out:
1-
O
N
O
O
This suggests that ALL the N-O bonds are the same
in terms of bond length and bond energy.
The N-O bonds are all an “intermediate” type of bond
between a single N-O bond and a double N=O bond.
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Resonance Structures
Evidence for Resonance Structures - Ozone
O
O
O
O
O
O
bond length (nm)
single O-O bond
0.148
double O=O bond
0.121
bonds in O3
0.132
Bond length measurements in O3 show both oxygen-oxygen bonds are the same.
There are NOT two different bond lengths, as suggested by the Lewis structures.
Instead, the oxygen-oxygen bond in O3 is intermediate
between a single and a double bond.
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Resonance Structures
Draw the resonance structures for:
carbonate ions (CO32-)
methanoate ions (HCOO–)
Describe where the delocalized electrons are found in each ion.
Comment on the carbon-oxygen bond lengths in each ion.
All carbon oxygen bonds have the same length - intermediate between single and double bonds.
10
Resonance Structures
HCOO- vs HCOOH
HCOO– (methanoate ion) forms resonance structures.
Will HCOOH (methanoic acid) also form resonance structures?
methanoic acid
no resonance structure
(C=O cannot move)
methanoate ion
Comment on the carbon-oxygen bond lengths in HCOOH vs HCOO–.
2 different carbon-oxygen bond lengths:
C-O is longer and C=O is shorter
only 1 carbon-oxygen bond length:
both C-O bonds are intermediate between the
single and double C-O bonds due to resonance
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Resonance Structures
Benzene (C6H6) - An Interesting Molecule!
(ask your TOK teacher about “Kekule”!)
resonance structure
trigonal planar around each
C atom (120º bond angle)
sp2 hybridisation
all C-C bonds are the same length ...
intermediate between single C-C and
double C=C bonds
flat hexagonal ring of
carbon atoms
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Resonance Structures
Benzene (C6H6) continued
http://ibchem.com/IB/ibnotes/full/bon_htm/14.3.htm
p orbitals on each carbon
before overlapping to form
pi bonds
electrons in pi bonds are
delocalized and spread out
over all 6 carbons within the
molecule
http://www.isis.stfc.ac.uk/science/biological-and-bio-medical-science/breakthrough-in-aromatic-molecule-research10853.html
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Resonance Structures
Benzene (C6H6) continued
the delocalized pi bond electrons DO
move around within the C6H6 molecule
Is benzene an electrical conductor?
Consider a collection of benzene molecules.
X
No.
The delocalized pi bond electrons only
move around within one molecule, and do
NOT jump from one molecule to another.
This is true for any covalent molecular substance with
“delocalized” electrons
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Graphite
http://chemistryatomicstructure.blogspot.com/2009/05/lesson-7-atomic-structures-of-diamond.html
Graphite is composed of sheets of carbon hexagons with
alternating single and double bonds.
The electrons in the pi bonds spread out over the entire ring (like in benzene).
Since the rings are all bonded to one another, the pi bond electrons are
delocalized over the entire plane. These electrons can move from one end
to another in an electric field - so graphite is an electrical conductor!
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Properties of Substances with Delocalized
e
1. intermediate bond lengths and bond strengths
(as compared to single or double bond lengths between the same atoms)
2. greater stability when electrons are delocalized
(e-e repulsion is less)
3. electrical conductivity in metals and graphite
(but not molecular covalent substances)
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