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Topic: Boiling Points  (Read 13033 times)

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Offline Nemesisof99

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Boiling Points
« on: October 11, 2009, 09:29:56 AM »
Why is the boiling point of Butan-2-ol less than that of Butan-1-ol?

Why is boiling point of Pentanal less than that of pentan-1ol?
In pentanal, there is a highly polar C-O bond which makes it easier for hydrogen bonding..is that correct?

Offline Arctic-Nation

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Re: Boiling Points
« Reply #1 on: October 11, 2009, 09:44:43 AM »
Pentanal does have a highly polar C=O bond, making it a good hydrogen bond acceptor, but as it doesn't have any acidic hydrogens, no actual hydrogen bonding can take place. 1-Pentanol, on the other hand, is both a hydrogen bond donor and a hydrogen bond acceptor, which allows for the increase of intermolecular interactions and thus an increase in boiling point.

The difference in boiling points of alcohol isomers, however, is a bit more difficult (read: I don't know how to explain it correctly), so I'll leave that to someone else. ;)

Offline Darwin

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Re: Boiling Points
« Reply #2 on: October 11, 2009, 02:12:20 PM »
1-butanol is a rather straight molecule where the molecules can closely interact each other, which means larger van der Waals forces. 2-butanol, however, is a bulky structure where the molecules cannot interact with each other as close as in 1-butanol's case.

Offline Nemesisof99

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Re: Boiling Points
« Reply #3 on: October 12, 2009, 09:32:36 AM »
What I don't understand is how branching actually decreases the surface area and lowers van der waals attraction?  ???

Online Borek

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Re: Boiling Points
« Reply #4 on: October 12, 2009, 09:54:38 AM »
Take a piece of rope.

Hold it straight - that's unbranched. It has whole surface ready to interact with other molecules.

Roll it - that's effectviely the same as making it heavily branched. Do you see that external surface is now smaller?

In general, sphere has the smallest ratio of surface to volume. The more branched the molecule, the more spherical it is.
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