On the Mechanism of Cellulose Acetylation

INTRODUCTION The inability of acetic anhydride to acetylate pulp efficiently in the absence of a catalyst has received much attention from different research workers. In fact the acetylation with acetic anhydride is unsuccessful if pre-activation has not been carried out. It can be assumed that the energy of interaction between acetic anhydride and cellulose in the absence of a catalyst is insufficient to weaken the intermolecular action in cellulose. Theoretical considerations as regards the process of acetylation indicate that the following procedures can be followed for effective acetylation. 1. The pulp is treated with reagents which cause a rupture or weakening of intermolecular bonds. In such a case, agents which give rise to swelling, or dissolution of the pulp, serve as "catalysts". 2. Substances which are capable of reacting with cellulose are added to the acetylation bath. The resulting intermediates have groups which are substituted for acetate groups during the reaction with acetic anhydride. The energy of at least one of the two reactions must be high enough to weaken the intermolecular action. 3. Substances which react with acetic anhydride are added to the acetylation bath. These form intermediates capable of reacting with cellulose. The energy of interaction of these intermediates with cellulose has to be of a magnitude such that the intermolecular action is weakened. A large number of substances have been proposed as catalysts for acetylation. Of these, sulphuric and perchloric acids have found the most extensive application. The mechanism of the catalytic actions of sulphuric and perchloric acids in cellulose acetylation was studied by measuring the conductivity of acetylating mixtures with and without cellulose. A special device was used for recording the conductivity which changed with time. This device consisted of a sound generator, a registering potentiometer and a special arrangement.