Chlorination of Barium Vanillyl Sulfonate Related to Mechanism of Pulp Bleaching

Abstract

In order to make clear the behavior of lignin during the bleaching process of pulp, the barium salt of vanillyl sulfonic acid as a model compound of lignosulfonic acid was treated with an aqueous chlorine solution.Part IThe results obtained when about 100% of chlorine (by weight against the weight of the barium-sulfonate; 3.84 mol. Cl2/CH3O) was brought into the reaction with vanillyl sulfonic acid are shown in Fig. 1, where curve I represents the amount of chlorine added and curve II chlorine consumed, the reaction being very fast in the beginning. After having been consumed nearly 80% of the added chlorine, the reaction becomes very slow and after 3 hr. 97.3% of the added chlorine consumed. Curve III represents the amount of the carbonyl group, about 0.5 carbonyl group per methoxyl group being rapidly formed and then remaining costant at this level. The amount of the weakly acidic group estimated conductometrically is represented by curve IV, which remains almost constant at 1.1/CH3O level. Since the vanillyl sulfonic acid has one phenolic hydroxyl group which can be titrated conductometrically, the increase of the conductometrically estimated weakly acidic group is only 0.1 per methoxyl group.The results obtained when about 130% of chlorine (by weight 5.25 mol. Cl2/CH3O) was brought into the reaction are shown in Fig. 2. Chlorine is absorbed very rapidly at first and the absorbed amount is somewhat higher than in the case of 100% of chlorine. The amount of the carbonyl group is, however, the same with the former case i.e. about 0.5/CH3O and the amount of the weakly acidic group increases up to 1.35 per methoxyl group after 3hr.Fig. 3, showing the amount of carbonyl group in Fig. 1 and 2 (Curve II) and weakly acidic group in Fig. 1 and 2 (Curve I) against the amount of chlorine reacted per methoxyl group, indicates that somewhat abrupt increase of the latter occurs soon after the consumption of about 4 mol. Cl2 per methoxyl group, where the weight of the consumed chlorine becomes nearly the same with that of the barium vanillyl sulfonate used and the weakly acidic group per methoxyl group are about 1.5 at the vicinity of Cl2/CH3O = 5.0.If no carboxyl group was formed, the amount of conductometrically estimated weakly acidic group would not exceed one per methoxyl group.Then, it may be suggested that the excess weakly acidic groups of about 0.5/OCH3 are carboxyl group.The ultraviolet absorption curves of the aqueous solution after reduction with sodium borohydride at various stages of the chlorination are shown in Fig. 4, the maximum absorption of the vanillyl sulfonic acid at 280 mμ moves to 285 mμ after the chlorination and the minimum at 255 mμ rises up so much and shifts apparently to 265-270 mμ region. It is remarkable that the reduction with sodium borohydride influences the absorption of the solution so much, the shape of the absorption curves become very clear after this treatment.The fact that when 2.96 to 4.50 mol. of chlorine are consumed the optical density of 285 mμ decreases to half as much as original density may suggest that the half of benzene nuclears are destroyed by chlorination.Part IIAn aqueous solution of barium vanillyl sulfonate was treated with chlorine solution (molar ratio 1 : 2.7) for about 1/2hr at 0°C, and the reaction mixture was extracted with chloroform followed by ether.From these fractions the following compounds were isolated and identified.(1) 2.5 dichloroprotocatechuic aldehyde : White needles, m.p. 163-166°C, identified as 2.5 dichloroveratrum aldehyde (pale yellow needles. m.p. 47-50°C) (2) 5.6 dichloroprotocatechuic aldehyde : identified as 5.6 dichloroveratrum aldehyde (white needles. m.p. 120-120.5°C.)