Lecture Notes: Polarity and Boiling Points

Introduction

• Discussion on ranking compounds based on boiling points.
• Importance of understanding boiling points for exams.
• Focus on alcohols having higher boiling points due to hydrogen bonding.

Boiling Point Ranking

• Alcohols (e.g., ethanol) have higher boiling points due to hydrogen bonds.
• Compounds with carbonyl groups have lower boiling points than alcohols.
• Non-polar hydrocarbons have the lowest boiling points due to van der Waals forces.
• Dipole-dipole interactions are weaker than hydrogen bonds.

Polarity and Molecular Structures

• Polar Bonds: Difference in electronegativity between atoms (e.g., HCl with partial charges due to electronegativity differences).
• Electronegativity Trends: Increases across a period and decreases down a group.
• Dipole Moments: Vector representation of polarity; more electronegative atoms pull electron density towards themselves.
• Example: HCl has a dipole moment due to electronegativity difference.

Analyzing Polar Bonds

• Recognition: Polar bonds occur in molecules with heteroatoms (e.g., O, N, halogens).
• Examples: Alcohols, carbonyls, amines, et cetera.
• Charges: Partial negative charges on more electronegative atoms.

Nucleophiles and Electrophiles

• Nucleophiles: Electron-rich; donate electrons.
• Electrophiles: Electron-poor; accept electrons.
• Examples: Nucleophiles often have lone pairs or pi bonds.

Molecular Polarity

• Dipole Moment of Molecules: Vector sum of individual bond dipoles.
• Example Molecules:
  • Methanol: Polar due to the O-H bond.
  • CO2: Non-polar overall due to canceling dipoles.
  • CH2Cl2: Polar due to non-canceling dipoles.
• Tetrahedral Geometry: Leads to specific dipole interactions (e.g., CCl4 being non-polar due to symmetrical dipole cancellation).

Trends in Polarity and Boiling Points

• Boiling Points: Depend on the type and strength of intermolecular forces.
  • Hydrogen bonds > Dipole-dipole interactions > Van der Waals forces.
• Practical Example: Fentanyl has polar bonds influencing its bioactivity.

Polarity in Chemical Reactions

• Role in Biological Systems: Understanding polarity is crucial in predicting reactions (e.g., adrenaline synthesis).

Conclusion

• Recognition of polar bonds is crucial in organic chemistry.
• Vector addition of dipoles helps determine molecular polarity.
• Boiling point trends can be influenced by molecular polarity and structure.