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