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This research focuses on the X-ray structure of 4,6-dichloro-5-nitrobenzofuroxan <b>1</b> and of some of its amino derivatives (<b>4a, 4e, 4g,</b> and <b>4l</b>) and on DFT calculations concerning the nucleophilic reactivity of <b>1</b>. We have found that by changing the solvent used for crystallization, it is possible to obtain 4,6-dichloro-5-nitrobenzofuroxan (<b>1</b>) in different polymorphic structures. Moreover, the different torsional angles observed for the nitro group in <b>1</b> and in its amino derivatives (<b>4a</b>, <b>4e</b>, <b>4g,</b> and <b>4l</b>) are strictly dependent on the steric hindrance of the substituent at C-4. DFT calculations on the course of the nucleophilic substitution confirm the role of the condensed furoxan ring in altering the aromaticity of the carbocyclic frame, while chlorine atoms strongly influence the dihedral angle and the rotational barrier of the nitro group. These results corroborate previous observations based on experimental kinetic data and give a deep picture of the reaction with amines, which proceeds via a “non-aromatic” nucleophilic substitution.