I can\'t remember how to tell if a molecule is hydrophobic or hydrophilic just b

Question

I can't remember how to tell if a molecule is hydrophobic or hydrophilic just by looking at it! Please someone help explain it to me (yes I feel stupid). It's been too long since chemistry & I am doing biochem.

I am looking at the molecules: glucose, tryptophan, testosterone, sodium dodecyl sulfate, and also guanine.  Somebody please explain how to tell if each of these molecules are hydrophobic, or hydrophilic (polar)!  As far as I can remember it has something to do with charges...maybe...

Thanks!

Explanation / Answer

HYDROPHOBIC = NON-POLAR = "afraid" of water, example an oil seperates itself from water
HYDROPHILIC = POLAR = "likes" water, where as a juice, or salt/sugar will dissolve or mix into it.

small organic molecules (<5 carbons) with polar functional groups. (carboxylic adid, carboxylate, amine, ammonium, hydroxyl, sulfone, or sulfoxide groups) will be hydrophilic. Larger organic molecules (> 5 carbons), will require more of these groups to be hydrophilic. All the others would be hydrophobic, with a few notable exceptions, such as ethyl acetate.

hope this helps.

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