The elusive antiaromaticity of maleimides and maleic anhydride: Enthalpies of formation of n-methylmaleimide, n-methylsuccinimide, n-methylphthalimide, and n-benzoyl-n-methylbenzamide

In order to understand the antiaromaticity of maleimides, the enthalpies of formation and sublimation of N-methylmaleimide, N-methylsuccinimide, N-methylphthalimide, and N-benzoyl-N-methylbenzamide were measured. The numerical values of enthalpies of formation for these compounds in the solid state are -329.3 +/- 1.4, -469.8 +/- 1.6, -325.0 +/- 2.1, and -239.6 +/- 3.8 kJ mol(-1), respectively, while the corresponding values in the gaseous state are -256.0 +/- 1.5, -389.7 +/- 1.6, -233.9 +/- 2.2, and -119.5 +/- 3.8 kJ mol(-1), respectively. The values of enthalpies of sublimation for the same compounds are 73.3 +/- 0.5, 80.1 +/- 0.3, 91.1 +/- 0.5, and 120.1 +/- 0.4 kJ mol(-1), respectively. We find that the antiaromaticity of maleimides is only modest.