Hydrogen Bonding (College Board AP® Chemistry): Revision Note

Hydrogen bonding

• This is a special type of dipole-dipole interaction found in covalent molecules in which hydrogen is directly bonded to either a nitrogen, oxygen or fluorine atom

  • These elements are very electronegative

  • Therefore, they form polar bonds with hydrogen

Polar H—F bond

A polar covalent bond in hydrogen fluoride is described by the presence of δ+ on the hydrogen and δ- charges on the electronegative fluorine atom

• The hydrogen atom is significantly less electronegative than a nitrogen, oxygen or fluorine atom

  • Therefore, the hydrogen atom has a partial positive (δ+) charge

  • The very electronegative element has a partial negative charge (δ-)

• Hydrogen bonding is the attraction between the δ+ hydrogen atom in this polar bond and a lone pair on an N, O or F atom in a different molecule, or in a different part of the same molecule

  • A single hydrogen bond is stronger than a typical dipole-dipole attraction, but hydrogen bonds act in addition to London dispersion forces and are not always the strongest force overall: large molecules with many electrons can have stronger total dispersion forces

Hydrogen bonds between molecules

Hydrogen bonding between molecules of ammonia, water and hydrogen fluoride

• The strength of the hydrogen bond is highest in molecules where the hydrogen atom is bonded to a fluorine atom

  • This is because fluorine has the highest electronegativity

• Generally, the strength of hydrogen bonds increases in the order:

  • H—N < H—O < H—F

• The total strength of hydrogen bonding in a substance also depends on how many hydrogen bonds each molecule can form

• For example, acetic acid, CH₃COOH, and 1-propanol, CH₃CH₂CH₂OH:

  • They both have the same molecular mass, 60 amu

  • They are both capable of forming hydrogen bonds

• However, a pair of acetic acid molecules can form two hydrogen bonds, whereas a pair of 1-propanol molecules can form only one

  • Hence, the boiling point of acetic acid is greater than propanol

Multiple hydrogen bonds

Hydrogen bonding in acetic acid and propanol. Each acetic acid can form two hydrogen bonds while propanol can form only one hydrogen bond.

• Similarly, the total hydrogen bonding in water is stronger than in hydrogen fluoride, even though a single hydrogen bond to F is stronger

  • Each water molecule can form up to four hydrogen bonds — two through its δ+ H atoms and two through the lone pairs on O — an average of two per molecule

  • Each HF molecule has only one δ+ H atom, so it averages only one hydrogen bond per molecule, even though F has three lone pairs

  • Hence water has a higher boiling point than hydrogen fluoride 

Hydrogen bonding in H₂O and HF

Each water molecule forms four hydrogen bonds (two per molecule on average), while each HF molecule averages one