Chapter 2: Structure and Properties of Organic Molecules, continued

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Chapter 2 Structure andProperties of Organic
Molecules,continued

• Molecular polarity
• Intermolecular forces and physical properties
• Isomerism
• Functional groups

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Bond Dipole Moments

• Due to differences in electronegativity

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Molecular Dipole Moment

• Overall electron distribution within a molecule
• Depends on bond polarity and bond angles
• Vector sum of the bond dipole moments
• Lone pairs of electrons contribute to the dipole
  moment
• Consider both magnitude and direction of
  individual bond dipole moments
• Symmetrical molecules with polar bonds nonpolar

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Intermolecular Forces

• Strength of attractions between molecules
• Influence physical properties (bp, solubility)
• Classification depends on structure
• Molecular polarity
• Dipole-dipole interactions
• Hydrogen bonding
• London dispersions (van der Waals)

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Dipole-Dipole Forces

• Between polar molecules
• Positive end of one molecule aligns with negative
  end of another molecule
• Lower energy than repulsions
• Larger dipoles cause higher boiling points

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Hydrogen Bonding

• Strongest dipole-dipole attraction
• H-bonded molecules have higher boiling points
• Organic molecule must have N-H or O-H
• The hydrogen from one molecule is strongly
  attracted to a lone pair of electrons on the
  other molecule

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London Dispersion Forces

• van der Waals forces
• Between all molecules
• important with nonpolar compounds
• Temporary dipole-dipole interactions
• Molecules with more surface area have stronger
  dispersion forces and higher boiling points
• Larger molecules
• Unbranched molecules

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Boiling Points and Intermolecular Forces

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Solubility and Intermolecular Forces

• Like dissolves like
• Polar solutes dissolve in polar solvents
• Nonpolar solutes dissolve in nonpolar solvents
• Molecules with similar intermolecular forces will
  mix freely

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Ionic Solute with Polar Solvent
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Ionic Solute withNonpolar Solvent
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Nonpolar Solute withNonpolar Solvent
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Nonpolar Solute with Polar Solvent
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Isomers

• Molecules with the same molecular formula, but
  different arrangements of atoms
• Constitutional (structural) isomers
• Differ in their connectivity
• Stereoisomers
• Differ only in the spatial arrangement/orientation
  of their atoms

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Structural Isomers
dimethyl ether ethanol
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Stereoisomers

• Cis-trans isomers are also called geometric
  isomers
• There must be two different groups on the sp2
  carbon

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Classes of Compounds

• Classification based on functional group
• Collection of atoms at a site within a molecule
  with a common bonding pattern
• Reacts in a typical way, generally independent of
  the rest of the molecule
• Three broad classes
• Hydrocarbons
• Compounds containing oxygen
• Compounds containing nitrogen

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Hydrocarbons

• Alkane single bonds, sp3 carbons
• Cycloalkane carbons form a ring
• Alkene double bond, sp2 carbons
• Cycloalkene double bond in ring
• Alkyne triple bond, sp carbons
• Aromatic contains a benzene ring

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Compounds Containing Oxygen

• Alcohol R-OH
• Ether R-O-R'
• Aldehyde RCHO
• Ketone RCOR'

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Carboxylic Acids and Their Derivatives

• Carboxylic Acid RCOOH
• Acid Chloride RCOCl
• Ester RCOOR'
• Amide RCONH2

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Compounds Containing Nitrogen

• Amines RNH2, RNHR', or R3N
• Amides RCONH2, RCONHR, RCONR2
• Nitrile RCN

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End of Chapter 2