hydrogen bonds

Meaning: A hydrogen bond forms when a hydrogen covalently bonded to N, O, or F (the donor) is attracted to a lone pair on a nearby N, O, or F (the acceptor). The very electronegative atom pulls electron density away, leaving the small H with a strong partial positive charge, so it can approach the lone pair closely. It is much weaker than a covalent bond but stronger than ordinary dipole–dipole or London dispersion forces in molecules of similar size. It can act between molecules or within one large molecule.

On the MCAT: Expect boiling point rankings for compounds of similar molar mass. Carboxylic acids (which can form dimers) boil higher than alcohols. Alcohols boil higher than aldehydes, ketones, and ethers. These boil higher than alkanes. Primary and secondary amines can hydrogen bond to themselves, but tertiary amines cannot donate. Small alcohols, acids, and amines dissolve in water because they hydrogen bond with it. Solubility falls as the nonpolar carbon chain gets longer. The same idea appears in water's high boiling point, protein secondary structure, and DNA base pairing.

Common confusion: Students count any molecule containing O or N as a donor. The H must be directly attached to N, O, or F. An ether or ketone has an acceptor oxygen but no O–H, so a pure sample cannot hydrogen bond with itself. Also, boiling breaks hydrogen bonds between molecules, not covalent bonds.