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Topic: Boiling points  (Read 2246 times)

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Offline CrazyAssasin

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Boiling points
« on: February 27, 2013, 10:58:33 AM »
According to wikipedia, the boiling point of diethyl ether is 307,8 K, while the boiling point of pentane is 309,4 K. The difference between them is quite small, but I can't understand the fact that the boiling point of diethyl ether is lower one. While speaking about intermolecular forces, pentane molecules interact with each other using London forces only, while diethyl ether has oxygen atom which makes stronger London interaction because of greater electron number and ,although dipole moment might cancel out because of symmetry, oxygen atom still makes for molecules to form instantaneous dipole moments. I would like you to confirm whether my thinking is good or not? If not what could be the reason for this?

Offline ramboacid

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Re: Boiling points
« Reply #1 on: February 27, 2013, 12:12:12 PM »
Just a thought, could the electronegativity of the oxygen atom restrict electron movement, thus hindering chance polarization by london dispersion forces?
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Offline sankalpmittal

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Re: Boiling points
« Reply #2 on: February 28, 2013, 11:11:49 AM »
Just a thought, could the electronegativity of the oxygen atom restrict electron movement, thus hindering chance polarization by london dispersion forces?

Yes.. This is the reason only. And it does not hinder just london forces, but debeye and keesom forces as well. In other words this factor dominates over molecular mass which could have increased vander waals forces.

Offline Enthalpy

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Re: Boiling points
« Reply #3 on: March 01, 2013, 11:25:27 AM »
The electric quadripole is repulsive at most positions of two molecules; attractive only when a hydrogen from one molecule is near the oxygen of an other molecule. Could the net result be less attractive than pentane?

I've just got confirmed from Mpbpvp that the same happens with these:
triethylamine CCN(CC)CC bp=+89,3°C (meas)
3-ethylpentane CCC(CC)CC bp=+93,5°C (meas)

and with:
CCCOCCC and CCCCCCC, CCCN(CCC)CCC and CCCC(CCC)CCC

But these behave as expected:
COC and CCC, CN(C)C and CC(C)C

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