Proton Hopping

Proton Hopping

Core Concepts

In this article, you will be able to describe the combinations of atoms which are capable of hydrogen bonding, identify the two features of hydrogen bonds between molecules, apply basic terminology for hydrogen bonding, and explain the role of proton hopping.

Related Topics

Hydrogen Bonding

Hydrogen bonding is among the strongest of the intermolecular forces (IMFs). IMFs are electrostatic interactions between neighboring molecules. Hydrogen bonding requires two conditions to occur. First, a hydrogen atom must have a bond with one of three electronegative atoms: nitrogen, oxygen, or fluorine. Second, an electronegative atom must posses a lone pair capable of accepting a hydrogen bond.

The figure above shows which covalent bonds are required to form hydrogen bonds.

Electronegativity refers to the tendency of a given atom to attract electrons towards itself. An atom with a high electronegativity will distort the electron cloud, leading to an unequal sharing of electrons between that atom and its bonding partners. Atoms with the higher electronegativity will usually have a partial negative charge. Conversely, an atom with a lower electronegativity will likely have a partial positive charge. The lowercase Greek letter delta (δ) represents atoms with partial charges in a molecule.

When two opposite charges are separated in a molecule, they form a dipole moment. The dipole moment is represented visually through an arrow pointing from a region of positive charge to a region of negative charge with a horizontal mark at the positive end, reminiscent of a plus sign.

If hydrogen forms a bond with an electronegative atom like oxygen, it acquires a partial positive charge, while oxygen acquires a partial negative charge. Shown below are three water molecules with the partial charges and dipole moments labeled.

Hydrogen bonding
The figure above shows how water molecules orient themselves in solution. Opposite charges attract, while like charges repel.

A hydrogen atom bonded to an electronegative atom is known as a hydrogen bond donor. An electronegative atom with a free lone pair is called a hydrogen bond acceptor. Water is both a hydrogen bond donor and acceptor, but not all molecules can participate in hydrogen bonding. Even though methane has four hydrogens, it is not a hydrogen bond donor, since they are all bonded to carbon, which is not an electronegative atom.

Hydrogen bonding
The figure above shows how a hydrogen bond forms. A hydrogen bond is composed of a hydrogen bond donor and acceptor.

The Nature of Proton Hopping

Proton hopping refers to the apparent movement of a proton from one water molecule to another. A proton may hop from one neighboring water to the next, several times in a row. The basis of this “hopping” is the simultaneous deprotonation and protonation of water molecules in aqueous solutions. The formal term of proton hopping is the Grotthuss mechanism.

Proton hopping
The figure above shows the nature of proton hopping. There is a net movement of protons in a given direction.

Proton Hopping Practice Problems

Problem 1

Determine whether or not Ethylene (C2H4) is capable of forming hydrogen bonds with itself.

Problem 2

Formaldehyde (CH2O) is often used in the preservation of tissue samples. Determine whether or not this molecule is capable of forming hydrogen bonds with itself.

Proton Hopping Problem Solutions

Problem 1

Ethylene contains a carbon-carbon double bond and hydrogen-carbon single bonds. The electronegativity difference between hydrogen and carbon is quite small. The bond cannot form a dipole moment conducive to hydrogen bonding. Thus, ethylene cannot hydrogen bond with itself.

Problem 3

Formaldehyde contains a carbon-oxygen double bond and hydrogen-carbon single bonds. The electronegativity difference between carbon and oxygen is quite large, which polarizes the bond between them. The oxygen atom has a partial negative charge and thus can act as an acceptor. However, the hydrogen-carbon single bond cannot act as a donor. Therefore, it cannot form hydrogen bonds with itself. However, formaldehyde can form hydrogen bonds with another hydrogen bond donor.