hydrogen bonding

Meaning: A hydrogen atom covalently bonded to N, O, or F (the donor) carries a strong partial positive charge and is attracted to a lone pair on another N, O, or F atom (the acceptor). It acts between molecules or within one large molecule, as in protein secondary structure and DNA base pairing. It is far weaker than a covalent bond but usually stronger than other dipole–dipole forces and London dispersion forces between small molecules.

On the MCAT: Rank boiling points for compounds of similar molar mass: carboxylic acids > alcohols > aldehydes and ketones > alkanes. Carboxylic acids rank highest because they form hydrogen-bonded dimers. Predict that small alcohols, amines and acids dissolve in water. Explain why ice is less dense than liquid water, or why G–C pairs (three hydrogen bonds) are harder to separate than A–T pairs (two). In IR spectra, hydrogen-bonded O–H gives a broad stretch.

Common confusion: Students count any H as a donor. The H must be bonded to N, O, or F, and C–H bonds do not count. A ketone or ether can accept hydrogen bonds but has no O–H or N–H to donate, so a pure sample has none, yet it can hydrogen bond with water. Boiling breaks intermolecular hydrogen bonds, not covalent bonds.