hydrophilic groups

Meaning: Hydrophilic groups are polar or charged functional groups that interact favorably with water. Polar groups such as −OH, −NH₂, −COOH and C=O form hydrogen bonds with water. Charged groups such as −COO⁻, −NH₃⁺ and phosphate form ion–dipole attractions, which are stronger. Adding such groups generally raises solubility in water. Hydrophobic groups, such as alkyl chains and benzene rings, are nonpolar and do neither.

On the MCAT: Expect questions that rank solubility in water, such as alcohols of increasing chain length, where solubility falls as the nonpolar part grows. Amphipathic molecules like soaps and phospholipids point hydrophilic heads toward water and tails inward in micelles and bilayers. In folded proteins, polar and charged side chains usually sit on the surface. In extractions, deprotonating a carboxylic acid to a carboxylate moves it into the aqueous layer.

Common confusion: A hydrophilic group does not make every molecule water soluble. Solubility depends on the balance of polar and nonpolar parts, so one −OH on a long hydrocarbon chain still leaves a poorly soluble molecule. Also, nonpolar groups are not truly repelled by water. Water prefers hydrogen bonding with itself, and ordering water around a nonpolar surface is entropically unfavorable.