[Big-Daddy]

First I'd like to ask about the order of solubility in water among organic compounds. I know, as a general rule of thumb, that organic compounds (particularly looking at the larger ones) will not be soluble in water. However, several will if they have a dipole moment. From most to least soluble, let's classify alcohols, thiols, ketones, esters, aldehydes, carboxylic acids, acyl halides, acid anhydrides, nitriles, primary amines, amides and imines. Any fairly soluble, common organic functional group I missed out would be appreciated if you mention it.

So far I'm looking at, probably (most to least soluble), acyl halides > carboxylic acids > amides > acid anhydrides > alcohols > nitriles > halogenoalkanes > imines > primary amines > thiols > aldehydes > esters > ketones. Am I right? If not, what is the right order - or, if I'm trying to be too general, can you propose an order that I can, on the whole, rely on? My halogenoalkane placing was a massive guess so it could be completely wrong - with the others I'm hoping they're in the right ballpark at least ...

[Corribus]

I think it will be hard to generalize.  What the functional group is attached to will make a big difference.  So how do you normalize for that?

[Big-Daddy]

Well, let's assume a straight alkyl chain of the same length in all cases.

Generally I can distinguish mentally but issues like ester vs. aldehyde/ketone confuse me. Similarly for distinguishing between the acid carbonyl compounds (between carboxylic acid, amides, and acyl halides), though I assume they are all more soluble than alcohols if the rest of the molecule is the same. Roughly where are the halogenoalkanes?

These generalizations are made all the time; I get questions like "what is more soluble, an ester or a ketone" as fodder for board exams.

[Corribus]

Well some of those by definition have other functional groups.  For instance, how do you compare an ester (which has two alkyl chains) to an aldehyde, which only has one?  You could compare propanal to propanol, I agree, but which ester do you compare it to?

[Big-Daddy]

I would suggest number of C atoms in the entire molecule is the factor we would keep constant across our generalizations.

But I'm surprised it makes a difference, given how common a question this is. Do you mean to say that there is a realistic probability there will be a difference in which compound is more soluble if one test involves an ester with 2 ethyl chains, and another involves an ester with 2 propyl chains (let's say we compare it to a ketone with 5 C atoms, exactly as many as the first ester).

[Corribus]

You don't think the chain length will make a difference? 

[Big-Daddy]

I'm sure it will, but assuming small lengths, and not too big a difference in chain length, hopefully not?