Question

What is hydrogen bonding? How can you tell whether a compound has hydrogen bonding?

Short answer

Hydrogen bonding occurs when a hydrogen atom covalently bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine is attracted to a lone pair of electrons on another electronegative atom. To determine if a compound has hydrogen bonding, look for H atoms bonded to N, O, or F and neighboring N, O, or F atoms with lone pairs that can act as acceptors of hydrogen bonds.

Step-by-step solution

Understanding Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular attraction between the hydrogen atom of one molecule, which is covalently bonded to a highly electronegative atom (like nitrogen, oxygen, or fluorine), and a lone pair of electrons on a highly electronegative atom of another molecule. This type of bond is weaker than an ionic or covalent bond but stronger than van der Waals forces.

Identifying Hydrogen Bond Donors

To determine whether a compound can participate in hydrogen bonding, look for hydrogen atoms that are directly attached to nitrogen, oxygen, or fluorine atoms within the molecule. These are necessary for hydrogen bonds to occur since these are the electronegative atoms that create the partial charges needed for hydrogen bonding.

Identifying Hydrogen Bond Acceptors

Next, identify if there are any nitrogen, oxygen, or fluorine atoms with lone pairs of electrons that can act as hydrogen bond acceptors. These atoms should have available electron pairs that could interact with hydrogen atoms covalently bonded to other electronegative atoms.

Combining Donors and Acceptors

If a molecule has both hydrogen bond donors (hydrogens attached to N, O, or F) and acceptors (lone pairs on N, O, or F), then it is likely to form hydrogen bonds with itself or other molecules. The presence of both donors and acceptors within the same molecule or in different molecules can lead to the formation of a network of hydrogen bonds.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring particles (atoms, molecules, or ions). These forces are crucial because they determine the physical properties of substances like boiling and melting points, viscosity, and solubility. Among the various types of intermolecular forces, hydrogen bonds are among the strongest.

Aside from hydrogen bonding, there are other types of intermolecular forces. Van der Waals forces are the weakest, including London dispersion forces which occur in all molecules, whether polar or nonpolar. Dipole-dipole attractions are stronger than London forces and occur between polar molecules. A special case of these are ion-dipole interactions, important in solutions of ions. Hydrogen bonds can be viewed as an extremely strong form of dipole-dipole interaction but only occur when hydrogen is bound to a highly electronegative atom.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons within a chemical bond. The Pauling scale is commonly used to measure electronegativity, with fluorine being the most electronegative element. Understanding electronegativity is key to predicting the type and strength of chemical bonding a molecule can exhibit, including hydrogen bonding.

The difference in electronegativity between atoms in a molecule contributes to the polarity of the molecule. When the difference is large, the bond is considered ionic, with one atom effectively 'donating' its electron to the other. As the difference becomes smaller, the bond is increasingly seen as covalent, where electrons are more equally shared. But, when hydrogen is covalently bonded to an atom with high electronegativity, like nitrogen, oxygen, or fluorine, this creates a large enough dipole for hydrogen bonding to occur.

Chemical Bonding

Chemical bonding is the process of attraction that holds atoms together to form substances with distinct properties. There are three primary types of chemical bonds: Ionic, covalent, and metallic. Ionic bonds form between metals and nonmetals with a significant difference in electronegativity, resulting in the transfer of electrons. Covalent bonds, on the other hand, involve the sharing of electrons and typically occur between two nonmetals.

Metallic bonding is found in metal elements, where electrons are free to move around, giving rise to properties like conductivity. Hydrogen bonding is often taught as a type of chemical bonding, but it is technically an intermolecular force. However, it is stronger than most other intermolecular forces and has a significant impact on the properties of the substances it occurs in, such as water's high boiling point relative to its molecular mass.