Hydrogen bonds

Section 2 of 4

A water molecule is about as heavy as a methane molecule. Methane boils at −162 °C, and water boils at 100 °C. Water's molecules hold on to each other with a much stronger pull, called a hydrogen bond.

Remember that oxygen pulls the shared electrons of an O–⁠H bond toward itself. The oxygen ends up with a partial negative charge, marked δ−, and the hydrogen with a partial positive charge, δ+. Nitrogen does the same in an N–⁠H bond.

A hydrogen atom has just one electron. With that electron pulled away toward the oxygen, the hydrogen's nucleus sits almost bare at the edge of the molecule. It is small and strongly δ+, so it can get very close to a lone pair on an oxygen or nitrogen atom of another molecule, and the two attract. That attraction is a hydrogen bond.

Hydrogen bonds between water molecules A water molecule in the middle, its red oxygen marked delta minus and its two white hydrogens marked delta plus. Each of its hydrogens points at a lone pair on the oxygen of a neighbouring water molecule, and each of its oxygen's two lone pairs, drawn as dots, holds a hydrogen of another neighbour. The four hydrogen bonds are drawn as dashed lines. δ− δ+ δ+
Hydrogen bonds (dashed) between water molecules. Each δ+ hydrogen points at a lone pair on a neighbour's δ− oxygen, so one water molecule can hold four neighbours.

Each water molecule has two δ+ hydrogens and two lone pairs, so it can make up to four hydrogen bonds. To boil water, heat has to break enough of them to set each molecule free, and that takes a temperature of 100 °C. A hydrogen bond is still about 20 times weaker than the covalent O–⁠H bond inside the molecule, which is why boiling water stays H₂O.

Same size, different boiling points

Propane and ethanol molecules are about the same size and weigh almost the same. Propane boils at −42 °C and ethanol at 78 °C. Ethanol's -⁠OH group, the familiar alcohol group, gives its molecules a δ+ hydrogen on an oxygen, so they make hydrogen bonds with each other.

Propane, C₃H₈Three carbon atoms in a chain joined by single bonds, with eight hydrogen atoms around them.HHHHCCCHHHH
Propane, C₃H₈: boils at −42 °C.
Dimethyl ether (methoxymethane), CH₃OCH₃An isomer of ethanol. Two carbon atoms joined through an oxygen atom in the middle, with three hydrogen atoms on each carbon.HHHCOCHHH
Methoxymethane, CH₃OCH₃: boils at −24 °C.
Ethanol, C₂H₅OHTwo carbon atoms in a chain; the second carbon is bonded to an oxygen atom that carries a hydrogen atom, an -OH group.HHHCCOHHH
Ethanol, C₂H₅OH: boils at 78 °C.

Methoxymethane is an isomer of ethanol: the same atoms, C₂H₆O, joined in a different order. Its oxygen sits in the middle of the chain, and all six of its hydrogens are on carbon, so its molecules make no hydrogen bonds with each other. Its polar C–⁠O bonds, with a δ− oxygen and δ+ carbons, give it a slightly stronger pull than propane, and it boils at −24 °C, about 100 degrees below ethanol.

Quick check

In which of these liquids do the molecules make hydrogen bonds with each other?