3.12 : Hydrogen Bonds

A hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) is attracted to another electronegative atom from another polar molecule, such as water (H₂O), hydrogen fluoride (HF), or ammonia (NH₃). The huge electronegativity difference between the H atom (2.1) and the atom to which it is bonded (4.0 for an F atom, 3.5 for an O atom, or 3.0 for an N atom), combined with the very small size of an H atom results in unequal distribution of charges between the hydrogen atom and the electronegative atom. This causes the hydrogen atom to become partially positive and the electronegative atom to become partially negative. Thus, molecules with F-H, O-H, or N-H moieties are strongly attracted by similar moieties of nearby molecules and form a particularly strong dipole-dipole attraction called the hydrogen bond.

Hydrogen bonds have a pronounced effect on the properties of liquids and solids. For example, the melting point and boiling point for methylamine (CH₃NH₂) are greater than ethane (CH₃CH₃), although both are similar in size and mass. Unlike ethane, methylamine possesses an −NH group that enables it to form hydrogen bonds and increase the intermolecular forces between molecules, eventually raising its melting and boiling points.

Hydrogen bonding is common in the natural world, such as the DNA that contains genetic information and is found in every organism. Each base pair of DNA is formed by hydrogen bonding via three hydrogen bonds. This complementary base pairing contributes to the shape and stability of the DNA double helical structure.

This text is adapted from Openstax, Anatomy and Physiology 2e, Section 2.2: Chemical Bonds and Openstax, Chemistry 2e, Section 10.1: Intermolecular Forces