Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 126

The final type of interaction is hydrogen bonding, which is the strongest type of intermolecular force and has about one‐fifth the strength of a typical covalent bond. Hydrogen bonding is actually a special type of dipole–dipole interaction. Hydrogen bonds are formed between molecules that contain a hydrogen atom bonded to a small, strongly electronegative element such as nitrogen, oxygen, or fluorine.

Hydrogen bonding can occur when a hydrogen atom in a molecule is bonded to a strongly electronegative element such as nitrogen, oxygen, or fluorine. A hydrogen bond is formed between the H atom of one molecule and the N, O, or F atom of a neighbouring molecule.

The hydrogen atom is very small and has only one electron in its 1s orbital. When bonded to an electronegative element such as nitrogen, N, oxygen, O, or fluorine, F, a dipole exists, and the hydrogen atom adopts a partial positive charge (δ+) while the other atom becomes slightly negatively charged (δ−). The partial positive charge is therefore concentrated over a very small volume and makes the hydrogen atom strongly polarising. This allows the hydrogen atom from one molecule to attract electron density from a small electronegative atom (such as nitrogen, oxygen, or fluorine) in a neighbouring molecule, and the hydrogen atom becomes sandwiched between the two more electronegative atoms. This can be seen in Figure 2.32, which depicts the formation of a hydrogen bond between two molecules of water. The highly polarising hydrogen atom in one water molecule (δ+) is able to attract electron density in the form of a lone pair of electrons from a neighbouring oxygen atom (δ−). The lone pair is now ‘shared’ between the hydrogen atom and the oxygen atom of a neighbouring molecule, and a hydrogen bond is formed. Because hydrogen bonding requires both the bonded and the neighbouring atom to be highly electronegative and small, hydrogen bonding generally only takes place between hydrogen and nitrogen, oxygen, and fluorine.

Figure 2.32 (a) Formation of a hydrogen bond between two molecules of water where molecules are in constant motion; (b) formation of hydrogen bonds in ice, where molecules are in a fixed position.

In liquid water, hydrogen bonds are constantly forming and breaking, as in Figure 2.32a, whereas in ice (Figure 2.32b), the molecules are held together in fixed positions by hydrogen bonds. Each oxygen atom can form two hydrogen bonds to neighbouring molecules as it possesses two lone pairs of electrons.

Hydrogen bonds are also formed between different types of molecules. The group of organic molecules known as alcohols contains an —OH bond. Ethanol, C₂H₅OH, is a well‐known alcohol. Hydrogen bonding can occur between molecules of ethanol (Figure 2.33a). Hydrogen bonding can also occur between molecules of ethanol and molecules of water, as shown in Figure 2.33b.

Figure 2.33 (a) Hydrogen bonding between two ethanol, C₂H₅OH, molecules; (b) hydrogen bonding between ethanol, C₂H₅OH, and water molecules.