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)

..…

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