A series of bromometric titrations of solution of iso-9-nitrofluorene in ethanol, dioxane and benzene was run.
The iso-form is thermodynamically more stable in the crystalline form than is the normal form; equilibria are established in solution in which the content of the normal form is greater with lower polarity of the solvent. The normal form of 9-nitrofluorene cannot be isolated since it disproportionates forming 1,2-dinitro1,2-dibiphenylene-ethane and fluorenone oxime. This reaction occurs at a lower rate than the reaction of formation of the normal form from the iso-form. Consequently, temporarily large concentrations of the normal form exist in solution and these were established by a chemical route.
The greater stability of the iso-form of 9-nitrofluorene is determined by the tendency of the cyclopentadiene ring to form an aromatic sextet by attracting electron pairs from the nitro group.
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Nenitzescu, K.D., Dinulescu, I.G. The equilibrium between the iso and the normal forms of 9-nitrofluorene. Russ Chem Bull 7, 1185–1189 (1958). https://doi.org/10.1007/BF00914948
- Normal Form