ib chemistry on delocalization and resonance
DESCRIPTION
IB Chemistry on Delocalization and ResonanceTRANSCRIPT
Delocalization of 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 structures carbonate ion
2
3CO
resonance structure 1 resonance structure 2 resonance structure 3
Resonance hybrid
• All bonds CO32- 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 resonance structure ✓
Delocalization of electrons
Resonance structures nitrate 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
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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 resonance structure ✓
Delocalization of electrons
Resonance structures nitrite 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 resonance structure ✓
Delocalization of electrons
Resonance structures sulfur 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
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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 resonance structure ✓
Delocalization of electrons
Resonance structures sulfur 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
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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 resonance structure ✓
Delocalization of electrons
Resonance structures methanoate
resonance structure 1 resonance structure 2
• All CO bonds are identical in length and strength • Hybrid of 2 resonance structures • 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 double bond • 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
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resonance hybrid
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Resonance structures ethanoate
Ethanoate ion COOCH3
resonance structure 1 resonance structure 2
resonance hybrid
H H
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 ✓
Delocalization of electrons
Resonance structures ozone
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
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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 • Beneficial prevent UV B/C from reaching Earth surface • 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 resonance structure ✓
Delocalization of 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 structures benzene
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 • Delocalization electrons above below plane • sp2 hybridization on carbon center
1.5
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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 ✓
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Kekule
Resonance/Delocalization Energy
ΔH cyclohexene = -120kJmol-1
ΔH cyclohexa 1,3 diene = -240kJmol-1
ΔH cyclohexa 1,3,5 triene = -360kJmol-1
ΔH Benzene = -208kJmol-1
Enthalpy change hydrogenation
✓
✓ 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
3 Evidence for Benzene structure 1
2
Click here evidence against 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
FORMAL CHARGE (FC)
Tool/Model for comparing which Lewis structures is 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 • Electronegative atom has negative while least electronegative atom has positive formal charge.
Formula formal charge
Click here video formal charges Click here video formal charges
V - valence electrons of atom
L – Lone pair electron
B - electrons shared in covalent bonds in the molecule
✓ ✓
All resonance structure contribute to electronic structure. Real structure is combination of them. Lowest formal charge (stable), contribute more than less stable structure. Sum of formal charges must be zero for neutral or equal to charge on ion.
L +
Formal charge concept