Investigations on intermediate acetal formation during polarographic estimation of certain benzaldehyde derivatives

Terephthalaldehydic acid methyl ester (TAE) and 4-carboxybenzaldehyde (CBA) which are identified as critical impurities in the manufacturing of dimethyl terephthalate (DMT) and purified terephthalic acid (PTA), respectively, are compounds under investigation. During their polarographic estimation, it is seen that TAE had good stability in alkaline water-methanol whereas CBA was converted to an electroinactive species. Nuclear magnetic resonance, infrared, ultraviolet and mass spectrometric techniques were employed to identify the electroinactive product as an acetal. Neutralization of the acidic group in CBA is seen to prevent acetal formation. A method has been developed for the quantification of TAE and CBA incorporating a preconcentration step, and applied to their analysis in DMT and PTA.