The acid catalysed condensation of phenol-4-sulphonic acid with formaldehyde

The reaction products formed when phenol-4-sulphonic acid reacts with formaldehyde have been investigated using ¹H and ¹³C nuclear magnetic resonance. It is shown that the reaction does not proceed readily at high phenol-4-sulphonic acid: formaldehyde ratios. As the ratio is decreased evidence of reaction is observed and further decreases lead to crosslinking. The crosslinking reaction is explained in terms of desulphonation and electrophilic substitution of the sulphonic acid group by a hydroxymethylene carbonium ion.