Question

What are the requirements for molecules to form hydrogen bonds? (What atoms must be present and involved in such bonds?

Short answer

Hydrogen bonds require hydrogen atoms bonded to N, O, or F, with lone pairs on N, O, or F.

Step-by-step solution

Identify the Hydrogen Bond Donor

A hydrogen bond donor must have a hydrogen atom directly bonded to a highly electronegative atom. The highly electronegative atoms commonly involved are nitrogen (N), oxygen (O), and fluorine (F).

Determine the Electronegative Atom

The hydrogen bond acceptor must also be a highly electronegative atom with a lone pair of electrons. This means it must be either nitrogen (N), oxygen (O), or fluorine (F).

Ensure Presence of Lone Pairs

The electronegative atom that accepts the hydrogen bond must have at least one lone pair of electrons available to interact with the hydrogen atom.

Key concepts

Understanding the Hydrogen Bond Donor

A key part of forming hydrogen bonds in molecules is identifying the hydrogen bond donor. This involves a molecule that has a hydrogen atom directly bonded to a highly electronegative atom, such as nitrogen (N), oxygen (O), or fluorine (F). These atoms pull the electron density away from the hydrogen atom, making it slightly positive.
This positive character allows the hydrogen atom to interact attractively with other electronegative atoms in nearby molecules or within the same molecule to form a hydrogen bond.
Remember:

• The hydrogen bond donor must have a hydrogen directly attached to N, O, or F.
• This setup is crucial because it enables the hydrogen to reach out and form a bond with another electronegative atom.

Role of Electronegative Atoms in Hydrogen Bonding

Electronegative atoms are necessary for both donating and accepting hydrogen bonds. These atoms tend to attract electrons towards themselves, creating partial charges in the molecule. Nitrogen (N), oxygen (O), and fluorine (F) are common examples of highly electronegative atoms.
When one of these atoms is bonded to a hydrogen atom, it essentially 'pulls' some of the electron density towards itself, leaving the hydrogen slightly positive. This partial positive charge on the hydrogen is crucial for forming hydrogen bonds because it allows the hydrogen to interact with lone pairs of electrons on another electronegative atom.
To recap:

• Electronegative atoms like N, O, and F are essential for hydrogen bonding.
• They create the conditions necessary for hydrogen to develop a partial positive charge, enabling bond formation.

The Importance of Lone Pairs of Electrons

For a hydrogen bond to form, the electronegative atom that acts as the hydrogen bond acceptor must have at least one lone pair of electrons. Lone pairs are pairs of valence electrons that are not shared with another atom and are crucial in the bonding process.
When a hydrogen atom, attached to a highly electronegative atom, comes close to another electronegative atom with lone pairs, these lone pairs can interact with the partially positive hydrogen.
This interaction completes the hydrogen bond, forming a relatively strong intermolecular or intramolecular force that significantly influences the properties of substances, such as water.
Key points on lone pairs:

• Lone pairs are necessary for accepting hydrogen bonds.
• They provide a spot for the slightly positive hydrogen to 'attach' to during bond formation.