Chemistry, Third edition

80 5 · MORE ABOUT BONDING

3. Cellulose

Cellulose is found in the cell wall of plant cells and helps to give plants their structure.

It is found in large amounts in trees and cotton fibres (which are nearly pure cellu-

lose). Cellulose has many uses, including the making of paper. Cellulose molecules

have many O–H bonds and the strength of wood is due, in part, to hydrogen bonding

between nearby molecules.

Strength of bonds: an overview

The last two chapters have discussed the different ways in which atoms bond

together and the various attractions between molecules. Table 5.3 gives an idea of

the relative strengths of some of these interactions.

Fig. 5.20The double helix of DNA.

(a) Typical hydrogen bonding interaction

between strands. (b) Two strands of DNA

• the horizontal lines represent hydrogen
  bonds.

Table 5.3Examples of the approximate relative strengths of interparticle

attractions

Interacting Description Typical energies associated

species with the attractions (k J mol^1 )

Intramolecular bonding

Ions e.g. Naand Cl Ionic bonding – attraction between 500–4000

oppositely charged ions

Atoms sharing electron A covalent bond – two nuclei 200–1100

pairs e.g. Cl—Cl share an electron pair

Metal atoms e.g. Cu Metallic bonding – metal cations 100–1000

surrounded by an electron cloud

Intermolecular bonding

Polar molecules Dipole–dipole – attraction 5–25

e.g. H—Br · · · H—Br between dipoles

Non-polar and polar London forces between molecules 0–40

molecules and/or atoms and/or atoms caused by temporary

e.g. I 2 · · · Ar dipoles in electron clouds

Covalently bonded H and A hydrogen bond – a ‘special’ 10–50

covalently bonded O, F or dipole–dipole attraction

N e.g. O—H · · · F—H