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What are Hydrogen Bonds?

Core Concepts

Hydrogen bonds are an essential intermolecular force that are responsible for many molecular actions specifically in biological systems. In this tutorial, we will learn the definition of hydrogen bonds, examples, and why they are so important in chemistry and biology.

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Vocabulary

  • Intermolecular force: forces between interacting molecules within the same substance. These are different than intramolecular forces, which are forces that exist between atoms within a molecule.
  • Ionic bond: bond that forms due to electrostatic attraction between two oppositely charged ions. Ex: Na+ and Cl form an ionic bond to create NaCl
  • Covalent Bond: bond that forms due to two atoms sharing electrons. Ex: CO
  • Dipole: A partial charge on an atom due to the natural movement of electrons that results in partial positive and negative charges on an otherwise neutral molecule.

What are hydrogen bonds?

Hydrogen bonds are a type of intermolecular force that occurs between hydrogen in one molecule and an electronegative atom of another molecule. Specifically, they only occur between compounds that contain hydrogen and one or more of the following atoms: oxygen, fluorine, or nitrogen. A clever way to remember that hydrogen bonds only occur with these molecules is, “chemistry is FON(fun).” Hydrogen bonds are the strongest of the three types of intermolecular forces.

Are hydrogen bonds the same as covalent bonds?

No. Hydrogen bonds are an intermolecular force, while covalent bonds are an intramolecular force. These bonds are often present in many covalent compounds but are very different than covalent bonds. Though these bonds are strong, they are not nearly as strong as covalent bonds and are often broken and reformed constantly.

How do hydrogen bonds occur?

These bonds occur when hydrogen forms a positive dipole in one molecule and fluorine, oxygen, or nitrogen form a negative dipole in another molecule. The positive dipole of hydrogen is attracted to the negative dipole on the electronegative atom, creating an attraction between the two molecules.

Examples of hydrogen bonds

Understanding hydrogen bonds becomes simpler when looking at common examples of hydrogen bonding.

Hydrogen bonding in water

Hydrogen bonding in water is often the most common example of these bonds due to their importance in many properties of water. In water, the hydrogen bond occurs between the oxygen of one molecule and the hydrogen of another.

what is a hydrogen bond

These bonds result in important properties in water such as cohesion (the ability for water to stick to itself), adhesion (the ability for water to stick to other molecules), surface tension, high specific heat, and its solvent properties.

Hydrogen bonding in DNA

Hydrogen bonds are extremely important in biology, as they are the reason for the structure of DNA and its properties. These bonds are responsible for the connections between the two strands of DNA between the nucleotide base pairs. Adenine and thymine form two hydrogen bonds between them and cytosine and guanine form three. These bonds keep DNA in its double helix formation. Additionally, these bonds are important for the replication of DNA, as the strength of the bonds keep the strands together, but they are weak enough to be broken with some force, allowing for the strands to be separated when it is time for replication.

These bonds are also the reason that DNA and many proteins have an optimum pH and temperature. If pH is low, this means that there is a high concentration of H+ ions. Since these ions are reactive, they can break the hydrogen bonds and replace them by bonding to the electronegative atom. The same can occur when pH is high, meaning there is a high concentration of OH ions. The negative oxygen is attractive to hydrogens, which can result in the bond breaking and a new bond forming between oxygen in the hydroxide and the hydrogen in the DNA base pair. Read more about pH here.

These bonds can be broken by temperature as well. As the system heats up, the molecules begin to move faster and obtain enough energy to break the hydrogen bond. Since these bonds are essential to the structure of DNA and many proteins, they can result in detrimental changes in biological systems.

Hydrogen bonding in drugs

Hydrogen bonds are essential when considering the composition of a drug and how it will interact in the body. Specifically, if a drug is acting on a specific target molecule, it must bind to the molecule in some way in order to change its function. In order for this binding to occur, hydrogen bonds are essential as they allow the drug to tightly bind to an active site on a molecule. This can also be seen in enzyme-substrate interactions, where hydrogen bonds allow for tight yet reversible binding to occur.

Hydrogen bonding and boiling point

As discussed previously, hydrogen bonds are the strongest intermolecular force. As a result, when there is an increase in the pressure or temperature of a substance, these bonds are challenged. Since these bonds are strong, it takes more energy to break these bonds than it would to break Van der Waal or dipole-dipole forces.

For example, this table depicts the dramatic change in the boiling point that occurs when hydrogen bonds exist in a compound. In this case, the boiling point triples with only a slight change in the molecule’s structure, allowing for the introduction of these bonds. They are still weaker than covalent or ionic bonds as shown above, but it is clear that their strength allows for added stability that results in a dramatic change in boiling point.

Further reading

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