Читайте также:
|
The degradation of wood carbohydrates can be divided into three basic reactions.
End-wise peeling (A, Scheme 4.13) slowly lowers the DP from the reducing end,
whereas oxidative peeling (B) cleaves the polymer chain randomly and alkaline
hydrolysis (C), occurring at higher temperatures, is also referred to as secondary
peeling. All of these processes (cf. Scheme 4.13) may occur simultaneously, their
rate being highly dependent on the chemical structure and the prevailing conditions
(type of monomer, branching, state of oxidation, concentration of hydroxyl
ions, temperature).
oxidized unit
reducing end unit
anhydrosugar unit
acid or lacton
B: oxidative peeling A: peeling
+ + +
considerable DP loss
C: alkaline
hydrolysis
+
Scheme 4.13 Schematic model of cellulose/hemicellulose
degradation under alkaline conditions.
Peeling
Peeling removes the terminal anhydro-sugar unit, generating a new reducing end
group until a competitive stopping reaction sets in, forming a stable saccharinic
acid end group. The peeling reaction starts with the well-known Lobry de Bruyn–
Alberda van Ekenstein rearrangement [61], an isomerization reaction of carbohydrates
under alkaline catalysis with the intermediate formation of an enediol
anion species (28) [62,63].
The elimination of the cellulose chain in b-position to the anionic intermediate
leads to a dicarbonyl structure (33) in the leaving unit, which is extremely
4.2 Kraft Pulping Processes 175
unstable under alkaline conditions and undergoes various degradation reactions,
such as benzilic acid rearrangements or Cannizzaro reactions, eventually yielding
isosaccharinic acid (34) or 2,5-dihydroxypentanoic acid, respectively, as main degradation
products of cellulose (Scheme 4.14). Enolization of the starting carbohydrate
as the rate-determining step is accelerated by increased OH– concentrations.
Depending on the reaction conditions, about 50–60 glucose units are peeled off
before a competitive stopping reaction sets in (Scheme 4.15) [64]. Here, reducing
RO
HO
HOH2C
OH
OH
O
H
H
-OH
O
H H
-OH
H
RO
HO
HOH2C OH
O
H
O
H
RO
HO
HOH2C OH
OH
O
RO
HO
HOH2C OH
OH
OH
RO
HO
HOH2C OH
OH
O
-OH
-ROHO
HOH2C OH
OH
O
O
HOH2C OH
OH
O CH2OH OH
O
HO OH
HO
RO
HO
HOH2C OH
OH
H
O
27 28 29
30 31
Endiol
BAR
R = cellulose chain
-
Scheme 4.14 Mechanism of the peeling reaction. BAR = benzilic acid rearrangement.
RO
HOH2C OH
O
H
OH
RO
HO
HOH2C OH
OH
O
RO
O
HOH2C
OH
OH
OH
RO
O
OH
OH
- H2O
O
O
OR
O
HOH2C
H
RO OH
O
OH
RO
HO
HOH2C
HO
OH
O
H
H
RO
HOH2C OH
O
H
O
RO
HOH2C OH
O
OH
OH
27 29
R = cellulose chain
-H2O
+
BAR
Дата добавления: 2015-10-21; просмотров: 97 | Нарушение авторских прав
| <== предыдущая страница | | | следующая страница ==> |
| Reactions of Carbohydrates | | | Specific Reaction of Xylans |