Question

What is hydrogen bonding? How can we predict the presence of hydrogen bonding in a compound?

Short answer

Hydrogen bonding is an intermolecular force involving a hydrogen atom bonded to N, O, or F that interacts with an electronegative atom. It is predicted by identifying hydrogen atoms bonded to these electronegative atoms and looking for lone pairs that can act as acceptors for the hydrogen bonds.

Step-by-step solution

Define Hydrogen Bonding

Hydrogen bonding is a type of intermolecular force that occurs when a hydrogen atom that is bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine) interacts with an electronegative atom in another molecule or a different part of the same molecule. This results in a relatively strong type of dipole-dipole attraction.

Predicting Hydrogen Bonding

To predict the presence of hydrogen bonding in a compound, look for the following conditions: 1. The presence of hydrogen atoms bonded to highly electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F).2. The presence of lone pairs on electronegative atoms that can serve as hydrogen bond acceptors.If both a hydrogen bond donor (the H attached to N, O, or F) and a hydrogen bond acceptor (usually the lone pair of electrons on N, O, or F) are present, hydrogen bonding can occur.

Key concepts

Intermolecular Forces

When learning about the intricate world of chemistry, an essential concept to grasp is that of intermolecular forces. These are the forces that mediate interaction between molecules, influencing properties like boiling points, melting points, and solubilities. One prominent type of such forces is hydrogen bonding, but it's crucial to first understand the broader category they fall under.

Introducing three main types:

• London dispersion forces – occurring in all molecular substances regardless of their polarity.
• Dipole-dipole attractions – found between polar molecules where partial charges attract each other.
• Hydrogen bonds – a strong type of dipole-dipole interaction where a hydrogen atom is involved.

These forces may seem weak individually, but collectively, they can hold substances together, keep water liquid at room temperature, and enable DNA strands to hold their shape.

Electronegativity

Delving into the atomic level, electronegativity is a cornerstone concept linked to hydrogen bonding. It’s a measure of an atom's attraction to electrons when it forms a chemical bond. Atoms like oxygen, nitrogen, and fluorine are highly electronegative and hog the electrons when bonded to other atoms.

Why electronegativity matters:

In a bond between two atoms, if one atom is significantly more electronegative than the other, it will draw the bonding electrons towards itself more strongly, creating an uneven distribution of charge – known as a dipole. This dipole is what allows certain types of intermolecular forces to exist in the first place. Hydrogen must be bonded to one of these electronegative atoms (N, O, or F) for hydrogen bonding to arise, a condition derived from the substantial difference in electronegativity.

Dipole-Dipole Attraction

As we narrow down from the general to the specific, dipole-dipole attraction gains prominence as a precursor to hydrogen bonding. It's an attractive force between the positive end of one polar molecule and the negative end of another. When molecules are polar, meaning they have an uneven distribution of electrons, they tend to align so that the positive region of one is close to the negative region of another.

Understanding the link to hydrogen bonds:

Hydrogen bonds are a supercharged version of dipole-dipole attraction. The highly electronegative atoms (N, O, or F) in one molecule pull on hydrogen atoms in another, creating a significant dipole. This dipole is strong enough to form a bond-like interaction, termed a 'hydrogen bond,' which is stronger than other dipole-dipole interactions due to the unique properties of hydrogen when it's attached to small, highly electronegative atoms.