stable and non-reactive. However, bonds between carbon and elements with higher electronegativity that occupy the right side of the periodic table are polar because these elements, for example oxygen and nitrogen, strongly attract the valence electrons of carbon. The special ability of carbon to form polar and non-polar structures is important in our understanding of the cellulose molecule and therefore will be discussed in greater detail.

Organic chemistry is dominated by the so-called "functional group approach", whereby organic molecules are thought to be constructed from an inert (non-polar) hydrocarbon skeleton onto which functional groups are attached. A functional group is an atom or a group of atoms that exhibits similar chemical and physical properties whenever it occurs in different compounds (Table 1.6). A key role in the context of paper-water interactions must be attributed to hydroxyl or alcohol groups (R-OH) which define the characteristic physical and chemical

functional group name compounds possible reactions
R*-OH hydroxyl alcohols
sugars
cellulose
starch
phenol
lignin
polyethylene glycol (PEG)		 oxidation to
aldehydes, ketones (1. step)
carboxylic groups (acid) (2. step)
Aldehyde structure: R-C-H with double bond to O aldehyde formaldehyde
glucose
reducing end group of cellulose chains in open form		 oxidation to carboxyl groups (acid)
may be reduced to
hydroxyl-, alcohol groups
Carbonyl structure: R-C-R with double bond to O carbonyl acetone oxidation to carboxylic groups (acid)
by oxidation agents
may be reduced to hydroxyl-,
alcohol groups by reducing agents
Hemi-acetal cyclic structure hemi-acetal cyclic forms of glucose
reducing end group of cellulose (99.74%) in equilibrium with the open aldehyde form (0.26%)		 in the open form acts as
reducing agent
is oxidsised by Fehling's reagent
Carboxyl structure: R-C-OH with double bond to O carboxyl acetic acid
amino acids
'internal acidity' of cellulose
if attached to cellulose chain		 acid, donates (H+) in aqueous environment
formation of insoluble salts with alkaline earth ions, such as Ca2+ or Mg2+
Amine structure: R-N-R bonded to H amine amines basic, accepts protons in an aqueous environment
forms ammonium salts with acids
Amide structure: R-C-N-R with double bond to O and H on N amide proteins crosslinks with aldehydes (i.e. tanning)

* R stands for any "organic residue", e.g. CH3-, CH3-CH2- etc. attached to a functional group.

Table 1.6 Some important functional groups and their potential reactivities.