KimJongIan
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#1
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Why wouldn't ketone have a lower boiling point than aldehydes (they have a higher boiling point).

I thought that because of the greater positive inductive effect in ketones, the carbonyl bond is less polar and thus the dipoles are weaker. Aldehydes have only one positive inductive effect so will have a more polar carbonyl bond.

Can anyone tell me why the statement above isn't true and why ketones actually have higher melting points than aldehydes (of the same carbon chain length).

Many thanks
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Gerry-Atricks
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this is because of two electron donating alkyl groups around the C=O group which makes them more polar, so higher electron density and so stronger bonds which need more energy to break
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KimJongIan
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(Original post by glad-he-ate-her)
this is because of two electron donating alkyl groups around the C=O group which makes them more polar, so higher electron density and so stronger bonds which need more energy to break

I thought that the electron donating alkyl groups would reduce the delta positive charge on the carbon and make the overall bond less polar
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Plantagenet Crown
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Ketones will generally have a higher boiling point relative to its corresponding aldehyde because the ketone will have a higher molecule weight, therefore more electrons and this greater intermolecular forces.

Remember though, bonds aren't actually broken when a substance melts or boils glad-he-ate-her
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Gerry-Atricks
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(Original post by Plantagenet Crown)
Ketones will generally have a higher boiling point relative to its corresponding aldehyde because the ketone will have a higher molecule weight, therefore more electrons and this greater intermolecular forces.

Remember though, bonds aren't actually broken when a substance melts or boils glad-he-ate-her
Good point, that is true
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Gerry-Atricks
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(Original post by Plantagenet Crown)
Ketones will generally have a higher boiling point relative to its corresponding aldehyde because the ketone will have a higher molecule weight, therefore more electrons and this greater intermolecular forces.

Remember though, bonds aren't actually broken when a substance melts or boils glad-he-ate-her
I googled it and there seem to be two theories
"This is an interesting question... there are two possible explanations that I can think of, but I'm interested to see what others have to say...

I'm making some assumptions -- like no weird(er) functional groups like benzene and stuff (for simplicity's sake)

1) The higher boiling point could simply be due to a increased molecular weight in the ketone, with the added functional group.

2) Polarity -- the strong dipole is formed in the carbonyl group of both compounds. However, in the aldehyde, the (now-slightly-positively-charged) carbon is still slightly more electronegative than it's attached hydrogen. Thus, perhaps it's able to borrow from the hydrogen to alleviate it's positive charge.

In the ketone, the central carbon has only other C-C bonds -- no electronegativity difference -- so the actual polarity ends up being more than the corresponding the aldehyde, which could explain a higher melting/boiling point."
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