Class Summary and Notes from 8/31/00 and 9/5/00 - CHEM3312

Physical Properties

Related Reading:

Practice Problems:

17.3, 19.24*, 20.14, 20.15*, 21.34*

*  after drawing the requested compounds - identify the intermolecular forces that would exist in a pure sample of each

Summary:

• strong
  ionic: oppositely charged ions are attracted (this is electrostatic attraction)
• intermediate
  hydrogen bonding: a specific instance of a dipole-dipole interaction in which the positive end of the dipole involved is ALWAYS a hydrogen (must be attached to an electronegative atom in order to qualify as being partially positive)
  dipole-dipole: partial charges at the ends of polarized bonds will be attracted to opposite partially charged regions of other molecules (also electrostatic, but of a lesser degree)
• weak
  Van derWaals (dispersion): an electrostatic attraction that arises as the movement of electrons in molecules creates temporary partial charges (of even a lesser degree than those found at either end of a polarized bond). Temporary charges in one molecule will induce the temporary development of the opposite charge in a neighboring molecule.

Problems Discussed In Class

• 1-butanol: strongest intermolecular interaction is hydrogen bonding, gives relatively high boiling point (actual BP=117.2 deg. C)
  
  
• propanethiol: strongest intermolecular interaction is dipole-dipole of the hydrogen bonding type, but it will be a weak example of hydrogen bonding because the bonds to sulfur are not very polar, expected to have a lower boiling point (actual BP=67 deg. C)
  
  
• diethyl ether: strongest intermolecular interaction is dipole-dipole, expected to have intermediate boiling point (actual BP=34.5 deg. C - lower than expected due somewhat to molecular geometry, alkyl groups interfere with the ability of molecules to interact)
  
  
• butanal: strongest intermolecular interaction is dipole-dipole, expected to have intermediate boiling point (actual BP=76 deg. C)
  
  
• 2-butanone: strongest intermolecular interaction is dipole-dipole, expected to have intermediate boiling point (actual BP=80 deg. C)
  
  
• propanoic acid: strongest intermolecular interaction is hydrogen bonding, expected to have relatively high boiling point (actual BP=141 deg. C - higher than alcohols because two carboxylic acid molecules can align themselves to share two hydrogen bonds)
  
  
• methyl ethanoate: strongest intermolecular interaction is dipole-dipole, expected to have intermediate boiling point (actual BP=57 deg. C)
  
  
• propanamide: strongest intermolecular interaction is hydrogen bonding, expected to have relatively high boiling point (actual BP=213 deg. C - similar to carboxylic acid, amides can often share multiple hydrogen bonds, in this case, the amide has two hydrogens that can participate in hydrogen bonds, so the hydrogen-bonded dimer will be able to hydrogen bond with other hydrogen-bonded dimers)
  
  
• butanenitrile: strongest intermolecular interaction is dipole-dipole, expected to have intermediate boiling point (actual BP=118 deg. C - higher than expected due to geometry and hybridization, triple bond of nitrile is flat allowing good interaction of the dipole in one molecule with the dipole in the next molecule, there are also three polarized bonds between the carbon and nitrogen leading to a greater charge separation and thus a stronger dipole)
  
  
• ethanoyl chloride: strongest intermolecular interaction is dipole-dipole, expected to have intermediate boiling point (actual BP=51 deg. C)

Special Notes

Also, compounds that cannot participate in intermolecular hydrogen bonds in a pure sample may be able to participate in hydrogen bonds with a different type of molecule.  The examples from class include the nitrile, and the ether.  These molecules both have electronegative atoms (one-half of the requirement for hydrogen bonding) but do not have a hydrogen atom attached to an electronegative atom.  Thus nitriles and ethers both are able to hydrogen bond with substances like water (which will influence their solubility and their reactivity).

Physical properties (boiling point, melting point, solubility, etc.) all are measured WITHOUT changing the internal structure of individual molecules!  Therefore the strength of covalent bonds within a single molecule are irrelevant to discussions of the physical properties of the molecule.

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