Dipole Moments in Molecules

Key Concepts

• Dipole Moment Definition: Product of charge and distance between charges.
• Electronegativity: Measure of an atom's ability to attract electrons in a bond.

Comparison of Molecules

Methyl Chloride (Chloromethane) vs. Carbon Tetrachloride

• Methyl Chloride (CH3Cl):

  • C-H Bond:
    • Carbon (C) Electronegativity: 2.5
    • Hydrogen (H) Electronegativity: 2.1
    • Polar Character: Nonpolar (difference < 0.5)
  • C-Cl Bond:
    • Chlorine (Cl) Electronegativity: 3.0
    • Polar Character: Polar (difference > 0.5)
  • Dipole Moment:
    • Net dipole moment points toward Cl.
    • Experimental dipole moment: 1.87 D.

• Carbon Tetrachloride (CCl4):

  • Structure: Tetrahedral with four Cl atoms.
  • C-Cl Bonds:
    • All arrows (dipole moments) point toward Cl.
  • Net Dipole Moment: Zero (bonds cancel out).
    • Overall: Nonpolar molecule.

Isomer Comparison

Trans Isomer vs. Cis Isomer

• Trans Isomer:

  • Dipole moments cancel out.
  • Net Dipole Moment: Zero.
  • Polarity: Nonpolar.

• Cis Isomer:

  • Dipole moments don’t completely cancel.
  • Net Dipole Moment: Positive in upward direction.
  • Polarity: Polar.

Percent Ionic Character Calculation

CO Bond Example

• Given:
  • Bond Length: 140 picometers.
  • Experimental Dipole Moment: 0.7 D.
• Calculation Steps:
  • Ionic Dipole Moment: Charge × Distance
    • Charge of proton: 1.6 × 10^-19 C.
    • Distance: 140 picometers = 140 × 10^-12 m.
  • Dipole Moment Calculation:
    • 2.24 × 10^-29 C·m.
    • Convert to Debye: 6.707 D (using 1 D = 3.34 × 10^-30 C·m).
• Percent Ionic Character:
  • Formula:
    [ \text{Percent Ionic Character} = \left( \frac{\text{Experimental Dipole Moment}}{\text{Ionic Dipole Moment}} \right) \times 100% ]
  • Calculation: 10.4% ionic character.

OH Bond Example

• Given:
  • Bond Length: 96 picometers.
  • Experimental Dipole Moment: 1.5 D.
• Calculation Steps:
  • Ionic Dipole Moment:
    • Charge: 1.6 × 10^-19 C.
    • Distance: 96 picometers = 96 × 10^-12 m.
    • Calculate: 1.536 × 10^-29 C·m.
    • Convert to Debye: 4.6 D.
• Percent Ionic Character:
  • Calculation: 32.6% ionic character.

Summary

• Methyl chloride is polar with a dipole moment of 1.87 D.
• Carbon tetrachloride is nonpolar with a dipole moment of 0.
• Cis isomer is polar, trans isomer is nonpolar.
• Calculated ionic character provides insight into bond polarity.