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Chapter 2 Structure and Properties of Organic Molecules

Organic Chemistry , L. G. Wade, Jr. Chapter 2 Structure and Properties of Organic Molecules. CH2:4 Hybridization and Molecular Shape. Molecular Shapes. Bond angles cannot be explained with simple s and p orbitals. Use VSEPR theory.

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Chapter 2 Structure and Properties of Organic Molecules

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  1. Organic Chemistry, L. G. Wade, Jr. Chapter 2Structure and Propertiesof Organic Molecules

  2. CH2:4 Hybridization and Molecular Shape Molecular Shapes • Bond angles cannot be explained with simple s and p orbitals. Use VSEPR theory. • Hybridized orbitals are lower in energy because electron pairs are farther apart. • Hybridization is LCAO within one atom, just prior to bonding. Chapter 2

  3. sp Hybrid Orbitals • 2-VSEPR pair -1 S and 1 P orbital – 2 sp hybridized orbitals • 180° bond angle Chapter 2

  4. sp2 Hybrid Orbitals • 3 VSEPR pairs – 1 s and 2 p orbitals • Trigonal planar e- pair geometry • 120° bond angle Chapter 2

  5. sp3 Hybrid Orbitals • 4 VSEPR pairs – 1 s and 3 p orbitals • Tetrahedral e- pair geometry • 109.5° bond angle Chapter 2

  6. Sample Problems • Predict the hybridization, geometry,and bond angle for each atom in the following molecules: • Caution! You must start with a good Lewis structure! • NH2NH2 • CH3-CC-CHO Chapter 2

  7. Rotation around Bonds • Single bonds freely rotate. • Double bonds cannot rotate unless the bond is broken. Chapter 2

  8. Isomerism • Same molecular formula, but different arrangement of atoms: isomers. • Constitutional (or structural) isomers differ in their bonding sequence. • Stereoisomers differ only in the arrangement of the atoms in space. Chapter 2

  9. Structural Isomers Chapter 2

  10. Trans - across Cis - same side No cis-trans isomers possible Stereoisomers Cis-trans isomers are also called geometric isomers. There must be two different groups on the sp2 carbon. Chapter 2

  11. are due to differences in electronegativity. depend on the amount of charge and distance of separation. In debyes,  = 4.8 x (electron charge) x d(angstroms) Bond Dipole Moments Chapter 2

  12. Molecular Dipole Moments • Depend on bond polarity and bond angles. • Vector sum of the bond dipole moments. Chapter 2

  13. Effect of Lone Pairs Lone pairs of electrons contribute to the dipole moment. Chapter 2

  14. Intermolecular Forces • Strength of attractions between molecules influence m.p., b.p., and solubility, esp. for solids and liquids. • Classification depends on structure. • Dipole-dipole interactions • London dispersions • Hydrogen bonding Chapter 2

  15. Dipole-Dipole Forces • Between polar molecules. • Positive end of one molecule aligns with negative end of another molecule. • Lower energy than repulsions, so net force is attractive. • Larger dipoles cause higher boiling points and higher heats of vaporization. Chapter 2

  16. Dipole-Dipole Chapter 2

  17. London Dispersions • Between nonpolar molecules • Temporary dipole-dipole interactions • Larger atoms are more polarizable. • Branching lowers b.p. because of decreased surface contact between molecules. Chapter 2

  18. Dispersions Chapter 2

  19. Hydrogen Bonding • Strong dipole-dipole attraction. • 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. • O-H more polar than N-H, so stronger hydrogen bonding. Chapter 2

  20. H Bonds Chapter 2

  21. ethanol, b.p. = 78° C ethyl amine, b.p. = 17 ° C Boiling Points and Intermolecular Forces Chapter 2

  22. Solubility • Like dissolves like. • Polar solutes dissolve in polar solvents. • Nonpolar solutes dissolve in nonpolar solvents. • Molecules with similar intermolecular forces will mix freely. Chapter 2

  23. Ionic Solute with Polar Solvent Hydration releases energy. Entropy increases. Chapter 2

  24. Ionic Solute with Nonpolar Solvent Chapter 2

  25. Nonpolar Solute withNonpolar Solvent Chapter 2

  26. Nonpolar Solute with Polar Solvent Chapter 2

  27. Classes of Compounds • Classification based on functional group. • Three broad classes • Hydrocarbons • Compounds containing oxygen • Compounds containing nitrogen. Chapter 2

  28. 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 Chapter 2

  29. Compounds Containing Oxygen • Alcohol: R-OH • Ether: R-O-R' • Aldehyde: RCHO • Ketone: RCOR' Chapter 2

  30. => Carboxylic Acids and Their Derivatives • Carboxylic Acid: RCOOH • Acid Chloride: RCOCl • Ester: RCOOR' • Amide: RCONH2 Chapter 2

  31. Compounds Containing Nitrogen • Amines: RNH2, RNHR', or R3N • Amides: RCONH2, RCONHR, RCONR2 • Nitrile: RCN Chapter 2

  32. End of Chapter 2 Chapter 2

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