STUDIES ON LIGNIN AND RELATED COMPOUNDS: VI. THE MECHANISM OF AQUEOUS HALOGENATION

The rates of addition of hypochlorous and hypobromous acids to several unsaturated compounds, under the influence of various catalysts, have been studied.The addition of hypochlorous acid to allyl alcohol and dipropenyl glycol is catalysed by both hydrogen and chlorine ions, the catalytic effects of these ions, mole for mole, being equal. The effect of hydrogen and chlorine ions, when added together as hydrochloric acid, is not the sum of their separate effects but is proportional to their product, indicating undissociated hydrochloric acid as the real catalyst. The catalytic effect of varying amounts of added hydrochloric acid is approximately proportional to the square of the amount added, again indicating undissociated hydrochloric acid as the real catalyst.The addition of hypobromous acid to allyl alcohol is catalysed by hydrochloric and hydrobromic acids to about the same degree. This catalysis of hypobromous acid addition by hydrochloric acid provides further support for the hypothesis of undissociated hydrochloric acid as the real catalyst in the addition of hypochlorous acid.This catalytic effect of undissociated halogen acid on the addition of hypohalous acids is put forward as an explanation of the greater rates of addition of hypochlorous and hypobromous acids from chlorine and bromine water as compared with solutions of hypochlorous and hypobromous acids.The addition of hypobromous acid to fumaric acid is not catalysed by hydrobromic acid but, on the other hand, is considerably decreased. It is catalysed, however, by hydrochloric acid, although not nearly to the same degree as the addition to allyl alcohol. This difference in behavior between fumaric acid and allyl alcohol shows the necessity for studying the effect of hydrochloric and hydrobromic acids on the rate of addition of hypobromous acid to a wide variety of unsaturated compounds, before any general theory of aqueous halogenation applicable to all types of unsaturated compounds can be put forward. The possible influence of conjugation has, however, been pointed out.An attempt has been made to apply the theory of hypohalous acid addition catalysed by undissociated halogen acid to the aqueous halogenation of phenols. The reaction between hypobromous acid and m-nitrophenol has been shown to be catalysed by hydrochloric acid although not to the same extent as by hydrobromic acid. This indicates hypobromous acid addition, catalysed by undissociated hydrobromic acid, as one of the mechanisms involved in the aqueous bromination of phenols.It has been pointed out that a similar mechanism cannot be the fastest one in the aqueous chlorination of phenols, since it is in disagreement with kinetic data of Soper and Smith (21), showing chlorine to react faster than hypochlorous acid with phenols.It has been shown that resorcinol and phloroglucinol do not fall under the general classification proposed by Soper and Smith for phenols. Even here, however, the main reaction between chlorine water and the phenol seems to involve chlorine rather than hypochlorous acid.It has been suggested that several parallel mechanisms may be involved in the aqueous halogenation of phenols and the necessity for further work in this field has been emphasized.