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I found on the internet that propanol, propanone, propanoic acid and methyl ethanoate where all completely soluble in water at room temperature. (23 degrees Celsius) But I found that propanal was, comparatively, was not completely soluble in water (Wikipedia said it was 20g/100ml). I was wondering why it was not completely soluble like all the others as small aldehydes can interact with hydrogen bonds and dipole-dipole forces with water (like all the others). Is there some other intermolecular force that influences solubility that I'm missing?

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They're all on the verge of miscibility, methyl ethanoate (methyl acetate) is in fact not miscible according to wikipedia (soluble to 25%).

The acid is the most polar of all the C3 compounds you name. Butyric acid (C4) is also miscible with water, but valeric acid (c5) already not.

n-butanol is already rather badly soluble (8g in 100ml) in water. Same with butanone, etc.

Compared to the ketone, the aldehyde has a longer unpolar part, that is overall less influenced by the polar carbonyl bond. Both are clearly, i think, the least polar of the group, and so the aldehyde just drops beneath the line.

No mystery here. ;-)

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