Hydroxyl Free Radical

As mentioned earlier, this species is extremely reactive [136]. It will react with any

molecule it encounters, and does so immediately. It can abstract a H atom, leaving

another free radical. The anionic form, the oxyl anion radical (see Scheme 7.4),

displays properties that are distinctly different from those of the hydroxyl radical.

In contrast to the latter, the oxyl radical reacts predominantly by hydrogen abstraction

and is therefore probably less selective than the hydroxyl radical [145].

Note: The terms hydroxyl free radical and hydroxyl radical are used synonymously.

Care must be taken using the term hydroxyl ion, which is the synonym

for the hydroxide ion (OH–).

7.3.2.3.9 Electrophilic–Nucleophilic Reactions

As noted, delignification during bleaching is initiated by electrophilic reactions,

which may be followed by nucleophilic processes [6–9]. The reactive oxygen species

(ROS) are listed in Tab. 7.11, according to their electrophilic–nucleophilic

character. Under the conditions of oxygen- alkali bleaching, the hydroperoxyl radical

is deprotonated to produce the superoxide anion radical. About half of the hydroxyl

radical is present as its base the oxyl anion radical (Tab. 7.11; see also

Scheme 7.4), and about half of the hydroperoxy anion is present as hydrogen peroxide

(Scheme 7.4).

7.3 Oxygen Delignification 647

Tab. 7.11 Reactive oxygen species (ROS) listed according to

their electrophilic – nucleophilic character.

Electrophiles

Triplet dioxygen 3O2

Hydroperoxyl radical HOO_

____

p K a_4_8

_O_2 Superoxide anion radical

Hydroxyl radical HOO_

____

p K a_11_9

_O_2 Oxyl anion radical

Nucleophiles

Hydroperoxy anion HOO–

Singlet dioxygen 1O2

The sites of electrophilic and nucleophilic attacks in lignins are shown in

Fig. 7.26. The p-system of the aromatic ring can be overlapped by the lone electron

pairs on the oxygen atom in para -hydroxy and the para -alkoxy groups, creating

centers of high electron density (Fig. 7.26), that can be attacked by electrophiles.

High electron density (d-) also appears at the Cb atom of aliphatic double bonds

O-

R2 OCH3

HC R1

arylalkane unit

R1 = OH, OAr or OAlk

arylpropene unit

O

R2 OCH3

CH2

CH

ä- ä-

ä-

H2C R1

ä-

O

R2 OCH3

C

C

H2C R1

R ä-

O-

R3

- ä-

á-carbonyl group containing

R = OAr, Ar or Alk

O

R2 OCH3

CH

ä+ ä+

ä+

arylalkane unit

R1 = OH, OAr or OAlk

arylpropene unit

quinone-methide intermediate

O

R2 OCH3

CH

HC

CH2

O

R2 OCH3

C

C

CH2

R

O

R3

á-carbonyl group containing

R = OAr, Ar or Alk

ä+

ä+ ä+

ä+

ä+

ä+

ä+

ä+

C C C C O

ä+ ä+

C

C O-

C

C O

-

ä-

ä-

ELECTROPHILIC

NUCLEOPHILIC

O

R2 OCH3

CH R1

ä- - ä-

ä-

-

O-

R2 OCH3

CH

CH

H2C R1

Câ

Cá

Fig. 7.26 Sites of electrophilic (d-) and nucleophilic (d+)

attacks in lignin (adapted from Ref. [2]).

648 7Pulp Bleaching

conjugated to the aromatic ring. By elimination of an a– (see Section 4.2.4, Chemistry

of kraft pulping, Scheme 3) or, in conjugated structures, a c-substituent, a

quinone-methide intermediate is formed from the arylalkane unit (Fig. 7.26),

which involves the loss of two electrons, resulting in the generation of centers of

low electron density (d+) that constitute the sites of attack by nucleophiles [2].

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