Question

how can you tell if a functional group is hydrophilic

Answer 1

Hydrophilic functional groups include hydroxyl groups (resulting in alcohols though also found in sugars, etc.), carbonyl groups (giving rise to aldehydes and ketones), carboxyl groups (resulting in carboxylic acids), amino groups (i.e., as found in amino acids), sulfhydryl groups (giving rise to thiols, i.e., as found in the amino acid cysteine), phosphate groups (as found in nucleic acids and phospholipids), etc. In addition are various hydrophylic linkages such as ethers (i.e., C-O-C), esters linkages (as found holding together fats, i.e., triglycerides), phosphodiester linkages (nucleic acids), glycolytic linkages (disaccharides and polysaccharides), and peptide bonds (polypeptides/proteins).

Answer 2

And an alternative answer would be if the functional group looks like water or not? For instance; an alcohol, OH, resembles water, and the electrical field of OH resembles water (think of OH as 2/3 of water). Remember how water is bi-, sorry di-polar, and how water molecules bind to each other (O in molecule one binds to H in molecule 2 via electro-static forces). So the more your functional group (and electrical field) resembles water, the easier it binds to water and is hydrophilic.

Answer 3

@shrutipande9 is right. Think about the electronegativity within the functional groups and the greater the difference within the bonds the more polar the side group will be (and the more polar the more hydrophilic). If you don't remember what electronegativity is: the more electronegative (pulls in electrons more) elements appear at the top right (Flourine has a eneg of 4.0) of the periodic table and the least electronegative appear at the bottom left. Hope this helps!