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Boiling point

Why is the boiling point of butan-1-ol higher than that of butan-2-ol?

Is it because there are more neighbouring carbon atoms to the one linked to oxygen, hence distributing the charge, leading to a weaker polarity?
Reply 1
Avatar for firestone
firestone
9
Stronger intermolecular (van der Waals) forces exist between butan-1-ol molecules due to them being more linear in shape. There is more surface area exposed for a greater number of van der Waals forces to form between molecules, and also they could pack together more closely, so that collectively they are stronger, needing more energy to overcome when it boils. Butan-2-ol molecules on the other hand are only able to come close enough at fewer points.
Reply 2
Avatar for Eau
Eau
OP
15
firestone
Stronger intermolecular (van der Waals) forces exist between butan-1-ol molecules due to them being more linear in shape. There is more surface area exposed for a greater number of van der Waals forces to form between molecules, and also they could pack together more closely, so that collectively they are stronger, needing more energy to overcome when it boils. Butan-2-ol molecules on the other hand are only able to come close enough at fewer points.


Then why does butanal have a higher b.p. than butanone? Surely butanal has a more open structure.
Reply 3
Avatar for firestone
firestone
9
No...the boiling point of butanone is 352.7K, that of butanal is 348.8K...

Both similarly have van der Waals forces, hydrogen bonding and dipole-dipole interactions.
There is no hydrogen bonding in alkanals and alkanones... hydrogen bonding can only occur when hydrogen atoms are directly attached to electronegative fluorine, oxygen or nitrogen atoms. Clearly with fluorine this can only be HF so the other cases are more important.
Reply 5
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cpchem
14
Charco is right on this one... although alkanals and alkanones are more commonly known as aldehydes and ketones.
Reply 6
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firestone
9
charco
There is no hydrogen bonding in alkanals and alkanones... hydrogen bonding can only occur when hydrogen atoms are directly attached to electronegative fluorine, oxygen or nitrogen atoms. Clearly with fluorine this can only be HF so the other cases are more important.

Although the carbonyl group of ketones and aldehydes do not have a hydrogen atom directly bonded to a small, strongly electronegative atom, they are however able to be on the receiving end of a hydrogen bond. The hydrogen atom of water is strongly postively polarised being directly bonded to the oxygen; the carbonyl group has an oxygen atom with a free lone pair of electrons therefore hydrogen bonding is possible despite the delta + hydrogen atom not being provided by the ketone or aldehyde. This is why they have broadened troughs on the infrared (is it?) spectra.

EDIT: Whoops sorry, for some reason I thought that they were being mixed with water. :redface: Either that, or I was thinking about carboxylic acids and the broadened C=O bond in its infrared spectrum. Sorry! So yeah, no hydrogen bonding in aldehydes and ketones.

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