Reaction Path B

Elimination Reactions

Starting from the quinone methide, the c-hydroxymethyl group can be eliminated

as formaldehyde, resulting in an enol ether structure. This reaction is prevalent in

soda pulping, causing a lower selectivity [5], but also observed at the start of the

bulk delignification phase during kraft pulping, despite a high initial charge of

sodium sulfide [50]. The amount of enol-ether structures approaches a maximum

at about the time when the maximum cooking temperature is reached, and is

believed to have a decisive influence on the amount of residual lignin [14]. However,

the total amount of enol-ether is rather small, especially in residual lignins.

Interestingly, a large quantity of enol-ether units was detected in dissolved soda

lignin, as expected, but not in the corresponding residual soda lignin [10].

The b-hydrogen can also be eliminated in another base-induced reaction. The

formaldehyde, which is highly reactive, can further condense with carbon-centered

mesomers of phenoxide anions leading to stable diarylmethane compounds

10 (Scheme 4.7) that have been suggested to be present in residual lignin [15,16].

The amount of these diarylmethane structures have been roughly estimated by a

combination of nitrobenzene oxidation and the nucleus exchange method [15–19],

though this technique has been proven to be erroneous.

The b-aroxy styrene-type structures (9, 11) show a high resistance towards alkali,

and may even survive the kraft cook (cf. structure of residual lignin)[20,21].

O

OAr

OMe

CH2OH

O

OAr

OMe

O

OMe

O

MeO

O

OAr

OMe

CH2OH

OH

-

OH

-

-HCHO

HCHO

9 10

Scheme 4.7 Formation of enol ethers and elimination of

formaldehyde with subsequent condensation to diarylmethane-

type structures.

The action of the strong nucleophile HS– also causes a partial demethylation

reaction at the methoxyl groups of lignin [22]. The formed methyl mercaptane is

itself a strong nucleophile, and reacts further with another methoxyl group to

yield dimethyl mercaptane (Scheme 4.8), an extremely volatile (but nontoxic) compound

which is responsible for the typical odor of kraft mills. Methyl sulfide can

4.2 Kraft Pulping Processes 169

be further oxidized to dimethyl disulfide upon exposure to air. In total, about 5%

of the methoxyl groups of lignin are cleaved.

OR

O CH3

-

SR

OR

O

CH3SH CH3SSCH3

+ CH3SCH3

ox.

R=H or Me

Scheme 4.8 Demethylation reaction at lignin methoxyl groups and formation of mercaptanes.