Question

Explain why hydrogen bonding is exhibited by some hydrogen-containing compounds and not by others.

Short answer

Hydrogen bonding occurs when hydrogen is bonded to a highly electronegative atom like O, N, or F, allowing attraction to other electronegative atoms with lone pairs.

Step-by-step solution

Understanding Hydrogen Bonding

Hydrogen bonding is a type of weak chemical bond that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom, typically oxygen, nitrogen, or fluorine. This bond occurs because the hydrogen atom becomes slightly positive due to the electron density shifting towards the more electronegative atom, allowing it to be attracted to another electronegative atom nearby.

Identify Electronegativity

For hydrogen bonding to occur, the hydrogen atom must be covalently bonded to a highly electronegative atom. The main elements that can cause hydrogen bonds when bonded with hydrogen are nitrogen, oxygen, and fluorine because of their high electronegativity compared to hydrogen.

Consider Molecular Structure

The molecular structure also impacts hydrogen bonding. The highly electronegative atom bonded to hydrogen should have lone pairs of electrons. This structure allows the slightly positive hydrogen to interact with a nearby electronegative atom with a lone pair, forming a hydrogen bond.

Examine Examples and Exceptions

Hydrogen bonds are commonly found in water (where hydrogen bonds with oxygen) and ammonia (where hydrogen bonds with nitrogen). However, hydrogen chloride (HCl) does not exhibit hydrogen bonding, even though it contains hydrogen and a highly electronegative atom (chlorine), because chlorine is not as electronegative as oxygen, nitrogen, or fluorine and HCl lacks the structure needed for hydrogen bonding.

Key concepts

Electronegativity

Electronegativity is a key concept when it comes to understanding hydrogen bonding and other chemical interactions. It describes an atom's ability to attract and hold onto electrons. - Highly electronegative atoms, such as nitrogen, oxygen, and fluorine, have a strong pull on electrons. - When hydrogen forms a bond with one of these elements, the electrons in the bond are drawn closer to the electronegative atom, making hydrogen slightly positive. This positive charge allows hydrogen to interact with nearby negative or slightly negative atoms. In hydrogen compounds, if the hydrogen is bonded to a highly electronegative atom, it can potentially form hydrogen bonds. Beyond hydrogen bonding, electronegativity also affects molecular shapes and reactions due to the way electrons are shared or transferred.

Molecular Structure

The structure of a molecule plays a crucial role in hydrogen bonding. To form hydrogen bonds, the molecular structure must support these interactions. A few points to consider about molecular structure include:

• The hydrogen atom must be directly bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine.
• The electronegative atom usually has lone pairs of electrons that can interact with the slightly positive hydrogen atom.

For example, in a water molecule (H₂O), each oxygen atom has two lone pairs, allowing it to form hydrogen bonds by connecting with hydrogen atoms from other water molecules. Conversely, if the molecule lacks a suitable structure for these interactions, even with a highly electronegative atom present, hydrogen bonding may not occur. This is often seen in compounds like hydrogen chloride (HCl), where the structure doesn't support hydrogen bonding despite the presence of chlorine.

Covalent Bond

Covalent bonds are fundamental in the formation of compounds and play a significant role in hydrogen bonding. These bonds occur when two atoms share one or more pairs of electrons. - In hydrogen bonding, the hydrogen atom forms a covalent bond with a highly electronegative element, resulting in an uneven distribution of electron density. - This sharing makes the bonding electronic cloud become distorted, leading to a partial positive charge on the hydrogen and a partial negative charge on the electronegative atom. The covalent bond is essential for stabilization and determines the reactivity and interactions of the molecule. For hydrogen bonds specifically, having a covalent bond with an electronegative atom is vital. This pairing is what allows the hydrogen to act as a bridge, facilitating the weak yet essential hydrogen bonds that affect the physical properties of the compound, such as boiling and melting points.