Aromatic isomerization reactions. Part 5. Isomerization of some biphenyldisulphonic acids in aqueous sulphuric acid at elevated temperatures

Abstract

The isomerization of six biphenyldisulphonic acids, containing one sulpho-group per phenyl ring, in 75 wt. % sulphuric acid at 140 and 180 °C has been studied. At 140 °C biphenyl-2,2′- and -2,4′-disulphonic acid isomerize to biphenyl-4,4′-disulphonic acid within 96 h, whereas biphenyl-2,3′-disulphonic acid isomerizes to biphenyl-3,4′-disulphonic acid within 24 h. Biphenyl-3,3′-, -3,4′-, and -4,4′-disulphonic acid do not show any isomerization at 140 °C after 96 h. At 180 °C all biphenyldisulphonic acids are eventually converted into the 3,4′-isomer, which remained unchanged. The reactivity order for the isomerization of the biphenyldisulphonic acids is 2,3′ > 2,2′ > 2,4′ 4,4′ > 3,3′.