chapter 1 dipoles, molecular polarity. ways to represent polarity in molecules h f electron rich...
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Chapter 1
Dipoles, molecular polarity
Ways to Represent Polarity in Molecules
H F
electron richregion
electron poorregion
Dipole Moment ()
H F
electron richregion
electron poorregion
= q x r
magnitude of charge = = | |
bond length moment in Debyes (D)1 D = 3.36 x 10-30 Coulomb.meter
Explain the Trend
EN drops
More Complex Molecules? Consider
molecular shape (bond orientation) bond polarity lone pairs (and sometimes) atom size
What is symmetry of e- distribution?
Consider NH3
Lewis Structure Need # valence e- from all atoms Need bond arrangement (connectivity)
Consider NH3
Lewis Structure # valence e- = 5 (N) + 1 (3H) = 8e- Three N-H bonds
H - N - H | H
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Consider NH3
From the Lewis structure Deduce e- pair geometry at central atom THEN find molecular geometry/shape
Any predictions?H - N - H
| H
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Consider NH3molecular shapeindividual bond dipoleslone 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 shapeindividual bond dipoleslone pair “dipoles”
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Which molecule is more polar?
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Which molecule is more polar?
Attractions Between Dipoles
Coulombic Attraction
n Ecoulomb αq1q2/d
n Ecoulombαqn Ecoulombα1/dn Ecoulomb=- when q1 and q2 have opposite charge
d
q1 q2•Ecoulomb α q
•Ecoulomb α 1/d
Dipole-Dipole Interactions
Dipoles align, + of one dipole to of another
Magnitude of attraction Ecoulomb α (+)()/d
Increases with dipole moment ~5-25 kJ/mol
…... …... d d
NOF
H ..… NOF
Hydrogen Bonding: Special Type of Dipole-Dipole Attraction
From another molecule
From one molecule
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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H bond
Why Are Hydrogen Bonds So Strong?
EN for H and N,O, F = very large, so N-H, O-H, F-H bonds are very polar (+-are
large) H atoms are small, so
N, O, F of one molecule can closely approach H of another molecule (d is small)
NOF
H…NOF
Ecoulomb α (+)()/d
H bond
Hydrogen Bonds in Water
Large Molecules Can Have Intramolecular Hydrogen Bonds
Hydrogen Bonds in Chitin