Fourier transform infrared (FT-IR) spectroscopy has been successfully employed to follow the formation of
phthalic anhydride and
1,8-naphthalic anhydride on heating their corresponding
acids. The effects of three heterocumulenes,
cyanamide,
dicyandiamide, and
sodium cyanate, on the temperature of formation of the
anhydrides were also investigated using this method. It was found that the
carbodiimides cyanamide and
dicyandiamide dramatically lowered the temperature at which thermal
dehydration of the
acid led to
anhydride formation. It was noted that
cyanamide had a stronger catalytic effect than
dicyandiamide, presumably due to the electron-withdrawing effect of the amidine group.
Sodium cyanate was also found to promote the thermal
dehydration of the
acids to form the corresponding
anhydrides. Under more severe conditions,
phthalic acid anhydride formed is seen to react further, leading to the formation of
phthalimide. The discrepancy between the products obtained with
cyanamide and
sodium cyanate leads to the conclusion that, unlike earlier claims,
imide formation is not due to the reaction of the
anhydride with the
urea formed but with
sodium cyanate itself. However, only the
phthalic anhydride five-membered ring system is sufficiently reactive towards the CNO- nucleophile to form the
imide; the six-membered
1,8-naphthalic anhydride system does not react in this way.