March 6, 2015

Homework Help: chemistry

Posted by argh on Tuesday, October 31, 2006 at 7:55pm.

i am supposed to explain the order of boiling points from intramolecular forces. the order i ended up coming up with was this:

diethyl ether 34.6 degrees C
hexane 69
ethanol 78.3
pyridine 115.2
acetic acid 117.9
octane 126

I know all of them have
-London/Van der Waal forces,
and that
-diethyl ether has a dipole moment,
-ethanol and acetic acid have H-bond forces,
and that
-pyridine is in a nice "stackable" planar ring.
According to intramolecular forces alone, shouldn't acetic acid and ethanol (because they have H-bond AND London forces) have the highest boiling points?
And according to geometry shouldn't pyridine? I'm so confused!

i realized just now that acetic acid also has a dipole moment! why does octane (which looks so similar to hexane) have a higher boiling point?

Acetic acid has a dipole moment AND it has H bonding. Diethyl ether has no dipole moment. Look at the molecule:

Octane has a higher boiling point than hexane because the molar mass of octane is higher. You must factor in the molar mass in addition to van der Waals forces and H bonding and dipole moment.

thank you and happy halloween!

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