IB Chemistry on Delocalization and Resonance

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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 3 CO resonance structure 1 resonance structure 2 resonance structure 3 Resonance hybrid All bonds CO 3 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 1 1 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

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IB Chemistry on Delocalization and Resonance

Transcript of IB Chemistry on Delocalization and Resonance

Page 1: IB Chemistry on Delocalization and Resonance

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

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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 ✓

Page 2: IB Chemistry on Delocalization and Resonance

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

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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 ✓

Page 3: IB Chemistry on Delocalization and Resonance

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 ✓

Page 4: IB Chemistry on Delocalization and Resonance

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 ✓

Page 5: IB Chemistry on Delocalization and Resonance

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

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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 ✓

Page 6: IB Chemistry on Delocalization and Resonance

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 ✓

Page 7: IB Chemistry on Delocalization and Resonance

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

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 • 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 ✓

Page 8: IB Chemistry on Delocalization and Resonance

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

Page 9: IB Chemistry on Delocalization and Resonance

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

Page 10: IB Chemistry on Delocalization and Resonance

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