social pharmacy d-pharm 1st year by Pragati K. Mahajan
IB Chemistry on Delocalization and Resonance
1. Delocalizationof electrons
Resonance
• Describing delocalization of electrons within a molecule/polyatomic ion
where bonding cannot be express by ONE single Lewis structure
• Delocalization of π bond – π electrons spread over more than 2 nuclei
• π electrons are shared
• π electrons spread – more stable
Resonance structurescarbonateion
2
3CO
resonance structure 1 resonance structure 2 resonance structure 3
Resonance hybrid
• All bonds CO3
2-
are identical in length and strength
• Hybrid of 3 resonance structures
• Negative charge equally distributed over all oxygen
• No C-O (single) or C=O (double) bonds found
• Only C ----- O bond
• Intermediate in character bet single and double bond
• Bond Order = 3
11
Carbonate Ion
• charge -2 delocalized into -2/3
• lower charge – more stable
Click here on video carbonate
C
Resonance Hybrid more stable than any of the resonancestructure ✓
2. Delocalizationof electrons
Resonance structuresnitrate ion
3NO
resonance structure 1 resonance structure 2 resonance structure 3
resonance hybrid
• All bonds NO3
-
are identical in length and strength
• Hybrid of 3 resonance structures
• Negative charge equally distributed over all oxygen
• No N-O (single) or N=O (double) bonds found
• Only N ----- O bond
• Intermediate in character bet single and double bond
• Bond Order = 3
11
Nitrate Ion
• charge of -1 delocalized into -1/3
• lower charge – more stable
Click here to view video
Resonance
• Describing delocalization of electrons within a molecule/polyatomic ion
where bonding cannot be express by ONE single Lewis structure
• Delocalization of π bond – π electrons spread over more than 2 nuclei
• π electrons are shared
• π electrons spread – more stable
3
1
3
1
3
1
Resonance Hybrid more stable than any of the resonancestructure ✓
3. Delocalizationof electrons
Resonance structuresnitrite ion
2NO
resonance structure 1 resonance structure 2
resonance hybrid
• All bonds NO2
-
are identical in length and strength
• Hybrid of 2 resonance structures
• Negative charge equally distributed over all oxygen
• NO N-O (single) or N=O (double) bonds found
• Only N ------ O bond
• Intermediate in character bet single and double bond
• Bond Order =
Nitrite Ion
• charge of -1 delocalized into -1/2
• lower charge – more stable
Resonance
• Describing delocalization of electrons within a molecule/polyatomic ion
where bonding cannot be express by ONE single Lewis structure
• Delocalization of π bond – π electrons spread over more than 2 nuclei
• π electrons are shared
• π electrons spread – more stable
Click here video nitrite
2
1
2
1
1.5
Resonance Hybrid more stable than any of the resonancestructure ✓
4. Delocalizationof electrons
Resonance structuressulfur dioxide
2SO
resonance structure 1 resonance structure 2
• All SO2 bonds are identical in length and strength
• Hybrid of 2 resonance structures
• NO S-O (single) or S=O (double) bonds found
• Only S ------O bond
• Intermediate in character bet single and double bond
• Bond Order =
Sulfur Dioxide
Click here to view
S
Resonance
• Describing delocalization of electrons within a molecule/polyatomic ion
where bonding cannot be express by ONE single Lewis structure
• Delocalization of π bond – π electrons spread over more than 2 nuclei
• π electrons are shared
• π electrons spread – more stable
resonance structure 3
How about structure
3?
resonance hybrid
1.5
Resonance Hybrid more stable than any of the resonancestructure ✓
5. Delocalizationof electrons
Resonance structuressulfur trioxide
resonance structure 1 resonance structure 2
• All SO3 bonds are identical in length and strength
• Hybrid of 3 resonance structures
• NO S-O (single) or S=O (double) bonds found
• Only S ----- O bond
• Intermediate in character bet single and double bond
• Bond Order = 3
11
Sulfur Trioxide
3SO
resonance structure 3
S 120
Click here to view video
Resonance
• Describing delocalization of electrons within a molecule/polyatomic ion
where bonding cannot be express by ONE single Lewis structure
•Delocalization of π bond – π electrons spread over more than 2 nuclei
•π electrons are shared
•π electrons spread – more stable
resonance structure 4
How about structure
4 ?
resonance hybrid
Resonance Hybrid more stable than any of the resonancestructure ✓
6. Delocalizationof electrons
Resonance structuresmethanoate
resonance structure 1 resonance structure 2
• All CO bonds are identical in length and strength
• Hybrid of 2 resonancestructures
• Negative charge equally distributed over oxygen atom
• NO C-O (single) or C=O (double) bonds found
• Only C ----- O bond
• Intermediate character bet single and doublebond
• Bond Order =
Methanoate ion
HCOO
Resonance
• Describing delocalization of electrons within a molecule/polyatomic ion
where bonding cannot be express by ONE single Lewis structure
• Delocalization of π bond – π electrons spread over more than 2 nuclei
• π electrons are shared
• π electrons spread – more stable
Click here to view
resonance hybrid
Click here to view
Resonance structuresethanoate
Ethanoate ion
COOCH3
resonance structure 1 resonance structure 2
resonance hybrid
HH
CH3
2
1
2
1
2
1
2
1
• charge of -1 delocalized into -1/2
• lower charge – more stable
1.5
Resonance Hybrid more stable than any of resonance structure ✓
7. Delocalizationof electrons
Resonance structuresozone
resonance structure 1 resonance structure 2
resonance hybrid
• All bonds O-O are identical in length and strength
• Hybrid of 2 resonance structures
• NO O-O (single) or O=O (double) bonds found
• Only O ----- O bond
• Intermediate in character bet single and double bond
• Bond Order =
Ozone
3O
Click here on video ozone
Resonance
• Describing delocalization of electrons within a molecule/polyatomic ion
where bonding cannot be express by ONE single Lewis structure
• Delocalization of π bond – π electrons spread over more than 2 nuclei
• π electrons are shared
• π electrons spread – more stable
• Pale blue gas, polar, dimagnetic
• Oxidizing agent
• Potent respiratory hazard and pollutant at ground level
• Beneficialprevent UV B/C from reaching Earthsurface
• Highest ozone level in stratosphere,(10 km and 50 km)
UV radiation
Ozone at
stratosphere
strongest radiation
3O
O-O
Single
bond
O=O
Double
bond
O=O=O
Inter
mediate
Bond length/pm 148 121 127
Bond enthalpy/kJ
mol-1
144 498 364
Bond order 1 2 1.5
✓
1.5
Resonance Hybrid more stable than resonancestructure ✓
8. Delocalizationof electrons
Resonance
• Describing delocalization of electrons within a molecule/polyatomic ion
where bonding cannot be express by ONE single Lewis structure
• Delocalization of π bond – π electrons spread over more than 2 nuclei
• π electrons are shared
• π electrons spread – more stable
Resonance structuresbenzene
Benzene
6HC6
resonance structure 1 resonance structure 2
Resonance hybrid
• All bonds C6H6 are identical in length/strength
• Hybrid of 2 resonance structures
• No C-C (single) or C=C (double) bonds found
• Only C ----- C bond
• Intermediate character bet single/double bond
• Bond Order =
• Unhybridised p orbital
• Delocalizationelectronsabove below plane
• sp2 hybridization on carbon center
1.5
Click here to view
Delocalized electrons
Kekulé structure
Cyclohexa- 1,3,5 triene
χ ✓
double/single bonds bet them
Benzene
Hexagonal, planar
Resonance Hybrid more stable than resonance structure ✓
Click here to view
Kekule
9. Resonance/DelocalizationEnergy
ΔH cyclohexene = -120kJmol-1
ΔH cyclohexa 1,3 diene = -240kJmol-1
ΔH cyclohexa 1,3,5 triene = -360kJmol-1
ΔH Benzene = -208kJmol-1
Enthalpychangehydrogenation
✓
✓
χ
……
• Benzene lower in energy by 150kJ
• More stable due to delocalization
of π electrons
150kJ
-150
C-C
Single bond
C=C
Double bond
C=C
Benzene
Bond length/pm 154 134 140
Bond
enthalpy/kJmol-1
346 614 507
3Evidencefor Benzenestructure
1
2
Click here evidenceagainst Kekule
• X ray hit benzene crystal
• Interact with electron (electron density map)
• X ray diffraction produced
• Bond length measured
X ray crystallography
NO single/double bond detected ✓
✓
3 Addition reaction for unsaturated C=C
✓Addition reaction
Substitution reaction
NO double bond
10. FORMAL CHARGE (FC)
Tool/Modelfor comparingwhich Lewis structuresis more acceptable
Lewis structure SO2
Which is
acceptable?
Lewis structure SO3
Formal Charge
• Treats covalent bond with equal electron distribution no EN diff bet atom
• Electronegativeatom has negative while least electronegative atom
has positive formal charge.
Formula formal charge
Click here video formal chargesClick here video formal charges
V - valence electrons of atom
L – Lone pair electron
B - electrons shared in covalent bonds in the molecule
✓ ✓
All resonancestructure contributeto electronic structure.
Real structure is combinationof them.
Lowest formalcharge (stable), contribute more than less stable structure.
Sum of formalcharges must be zero for neutral or equal to charge on ion.
L +
Formal charge
concept