Most people are comfortable with the idea of ionic and covalent bonds, yet unsure about what hydrogen bonds are, how they form, and why they are important.
Key Takeaways: Hydrogen Bonds
- A hydrogen bond is an attraction between two atoms that already participate in other chemical bonds. One of the atoms is hydrogen, while the other may be any electronegative atom, such as oxygen, chlorine, or fluorine.
- Hydrogen bonds may form between atoms within a molecule or between two separate molecules.
- A hydrogen bond is weaker than an ionic bond or a covalent bond, but stronger than van der Waals forces.
- Hydrogen bonds play an important role in biochemistry and produce many of the unique properties of water.
Hydrogen Bond Definition
A hydrogen bond is a type of attractive (dipole-dipole) interaction between an electronegative atom and a hydrogen atom bonded to another electronegative atom. This bond always involves a hydrogen atom. Hydrogen bonds can occur between molecules or within parts of a single molecule.
A hydrogen bond tends to be stronger than van der Waals forces, but weaker than covalent bonds or ionic bonds. It is about 1/20th (5%) the strength of the covalent bond formed between O-H. However, even this weak bond is strong enough to withstand slight temperature fluctuation.
But the Atoms Are Already Bonded
How can hydrogen be attracted to another atom when it is already bonded? In a polar bond, one side of the bond still exerts a slight positive charge, while the other side has a slight negative electrical charge. Forming a bond doesn't neutralize the electrical nature of the participant atoms.
Examples of Hydrogen Bonds
Hydrogen bonds are found in nucleic acids between base pairs and between water molecules. This type of bond also forms between hydrogen and carbon atoms of different chloroform molecules, between hydrogen and nitrogen atoms of neighboring ammonia molecules, between repeating subunits in the polymer nylon, and between hydrogen and oxygen in acetylacetone. Many organic molecules are subject to hydrogen bonds. Hydrogen bond:
- Help bind transcription factors to DNA
- Aid antigen-antibody binding
- Organize polypeptides into secondary structures, such as alpha helix and beta sheet
- Hold together the two strands of DNA
- Bind transcription factors to each other
Hydrogen Bonding in Water
Although hydrogen bonds form between hydrogen and any other electronegative atom, the bonds within water are the most ubiquitous (and some would argue, the most important). Hydrogen bonds form between neighboring water molecules when the hydrogen of one atom comes between the oxygen atoms of its own molecule and that of its neighbor. This happens because the hydrogen atom is attracted to both its own oxygen and other oxygen atoms that come close enough. The oxygen nucleus has 8 "plus" charges, so it attracts electrons better than the hydrogen nucleus, with its single positive charge. So, neighbor oxygen molecules are capable of attracting hydrogen atoms from other molecules, forming the basis of hydrogen bond formation.
The total number of hydrogen bonds formed between water molecules is 4. Each water molecule can form 2 hydrogen bonds between oxygen and the two hydrogen atoms in the molecule. An additional two bonds can be formed between each hydrogen atom and nearby oxygen atoms.
A consequence of hydrogen bonding is that hydrogen bonds tend to arrange in a tetrahedron around each water molecule, leading to the well-known crystal structure of snowflakes. In liquid water, the distance between adjacent molecules is larger and the energy of the molecules is high enough that hydrogen bonds are often stretched and broken. However, even liquid water molecules average out to a tetrahedral arrangement. Because of hydrogen bonding, the structure of liquid water becomes ordered at lower temperature, far beyond that of other liquids. Hydrogen bonding holds water molecules about 15% closer than if the bonds weren't present. The bonds are the primary reason water displays interesting and unusual chemical properties.
- Hydrogen bonding reduces extreme temperature shifts near large bodies of water.
- Hydrogen bonding allows animals to cool themselves using perspiration because such a large amount of heat is needed to break hydrogen bonds between water molecules.
- Hydrogen bonding keeps water in its liquid state over a wider temperature range than for any other comparable-sized molecule.
- The bonding gives water an exceptionally high heat of vaporization, which means considerable thermal energy is needed to change liquid water into water vapor.
Hydrogen bonds within heavy water are even stronger than those within ordinary water made using normal hydrogen (protium). Hydrogen bonding in tritiated water is stronger still.