Hydrogen Bonding

Topic: Liquid Phase Intermolecular Forces

A hydrogen bond is a strong intermolecular force between a hydrogen atom bonded to an electronegative atom, and a lone pair of electrons on another electronegative atom, namely oxygen, nitrogen or fluorine.

A hydrogen bond forms when a hydrogen atom bonded to a relatively electronegative atom acts as a hydrogen bond donor. This happens when the electronegative atom (usually N, O, or F) attracts the electron cloud from around the hydrogen nucleus, leaving the hydrogen atom with a partial positive charge (δ+). This partial positive charge attracts a lone pair of electrons on a nearby electronegative atom, thereby acting as the hydrogen bond donor. The lone pair of electrons (δ-) on the nearby electronegative atom which it attracts, is known as the hydrogen bond acceptor.

Taking water as an example, the positive hydrogen atoms (H-bond donors) and negative lone pair of electrons on O (H-bond acceptor) of an adjacent water molecule attract each other in a strong electrostatic intermolecular interaction. The greater the electronegativity of the hydrogen bond acceptor, the stronger the hydrogen bond.

Hydrogen bonds are not only found when hydrogen is bonded to N, O, or F. For example, in chloroform (CHCl3), the carbon atom is bonded to electronegative atoms of chlorine, leaving the hydrogen with a partial positive charge which can attract the lone pairs on adjacent chlorine atoms.

Intermolecular (and intramolecular) hydrogen bonds occur in inorganic molecules, such as water, and organic molecules, such as DNA (between base pairs) and proteins (contributing to secondary, tertiary, and quaternary structures). Hydrogen bonding in water contributes to its unique properties, including its high boiling point (100 °C) and surface tension.

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Key Points

• Hydrogen bonds are strong intermolecular forces created when a hydrogen atom bonded to an electronegative atom (commonly O, N, or F) forms an electrostatic attraction to a lone pair on an adjacent molecule’s electronegative atom.

Key Terms

Hydrogen bond: The attraction between a partially positively charged hydrogen atom attached to a highly electronegative atom (such as nitrogen, oxygen, or fluorine) and a lone pair of electrons on a nearby electronegative atom.

Hydrogen bond donor: The partially positive hydrogen atom (bound to an electronegative atom) that is attracted to another atom’s lone pair of electrons.

Hydrogen bond acceptor: The lone pair of electrons on an electronegative atom that is attracted by a hydrogen atom of another molecule.

Electronegativity: The tendency of an atom or molecule to draw electrons towards itself, form dipoles, and thus form bonds.

Intermolecular: A type of interaction between two different molecules (H-bonding between different water molecules).

Intramolecular: A type of interaction between two parts within one molecule (H-bonding within a protein’s secondary structure).

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