Phthalic Anhydride, Phthalan, and Phthalimide

Consider now phthalic anhydride, (X = Z = CO, Y = O) (XVIII). This species has eight 7t electrons. Is it antiaromatic HiickeTs 4n + 2 rule would suggest this although we acknowledge that this rule strictly applies only to monocyclic species. The relevant reaction is  

This reaction is exothermic by 14.4 kJ mol 1. This value is some 7 kJ moT more exothermic than that of its carbocyclic counterpart of 1,3-indanedione. Is this to be ascribed to destabilization of phthalic anhydride and plausible antiaromaticity associated with its eight n electrons  

Before doing so, consider the corresponding reaction for phthalan (X = Z = CH2, Y = O) (XIX), where no additional aromaticity is expected beyond that found in its benzene ring. Using the related reaction and the enthalpy of formation of phthalan [34]  

What about the nitrogen or sulfur analog of phthalic anhydride Acknowledging earlier disparate results for solid phthalimide (X = Z = CO, Y = NH) (VIII), the enthalpy of formation of gaseous phthalimide has recently been measured as -211.1 kJ mol-1 [35], Accepting this value, using the equation  

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