Dipoles, molecular polarity

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Chapter 1

• Dipoles, molecular polarity

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Ways to Represent Polarity in Molecules
electron rich region
electron poor region
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Dipole Moment (m)
electron rich region
electron poor region
m q x r
moment in Debyes (D) 1 D 3.36 x 10-30
Coulomb.meter
bond length
magnitude of charge d d-
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Explain the Trend
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More Complex Molecules?

• Consider
• molecular shape (bond orientation)
• bond polarity
• lone pairs
• (and sometimes) atom size
• What is symmetry of e- distribution?

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Consider NH3

• Lewis Structure
• Need valence e- from all atoms
• Need bond arrangement (connectivity)

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Consider NH3

• Lewis Structure
• valence e- 5 (N) 1 (3H) 8e-
• Three N-H bonds

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Consider NH3

• From the Lewis structure
• Deduce e- pair geometry at central atom
• THEN find molecular geometry/shape
• Any predictions?

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Consider NH3
molecular shape individual bond dipoles lone pair
dipoles

• 4e- pairs at N
• tetrahedral e- pr geometry
• Lone pair at N molecular geometry/shape is
  trigonal pyramidal

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Which molecule is more polar?
molecular shape individual bond dipoles lone pair
dipoles
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Which molecule is more polar?
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Which molecule is more polar?
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Attractions Between Dipoles
d d-
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• Ecoulomb a q
• Ecoulomb a 1/d

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Dipole-Dipole Interactions
d d-
d d-
d d-
d
d
...
...

• Dipoles align, d of one dipole to d- of another
• Magnitude of attraction
• Ecoulomb a (d)(d-)/d
• Increases with dipole moment
• 5-25 kJ/mol

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Hydrogen Bonding Special Type of Dipole-Dipole
Attraction
..
N O F
N O F
H
From another molecule
From one molecule
H bond
d- d
d- d
..

• Only applies to molecules with H bonded to N, O,
  or F
• 10-40 kJ/mol (larger than most dipole-dipole
  attractions)

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Why Are Hydrogen Bonds So Strong?
Ecoulomb a (d)(d-)/d
H bond

• DEN for H and N,O, F very large, so
• N-H, O-H, F-H bonds are very polar (d, d- are
  large)
• H atoms are small, so
• N, O, F of one molecule can closely approach H of
  another molecule (d is small)

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Hydrogen Bonds in Water
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Large Molecules Can Have Intramolecular Hydrogen
Bonds
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Hydrogen Bonds in Chitin