Cubic Mesophase in an Unsymmetrical Alkyl Ammonium Salt. Synthesis and Structural Model

N,N,N‐ butylethylpentylpropylammonium iodide 4 and related molecules have been selectively synthesised from commercially available aldehydes, amines and alkyl iodides using a reductive alkylation procedure. The crystalline texture of 4 obtained on cooling is optically isotropic between crossed polarisers, indicating a cubic structure. Differential scanning calorimetry (DSC, +10 K min −1 ) reveals a glass phase transition at −59 °C and a melting point at 192 °C. The melting entropy (23.9 J mol −1  K −1 ) indicates a first‐order transition between a highly disordered mesophase and the isotropic liquid. Powder X‐ray diffraction patterns were indexed in the cubic system ( a =14.08Å; Pm $\bar 3$ n space group). In this cell, the molecular packing with Z =6 corresponds to a rather low compactness of 65 %. Iodine and tetraalkylammonium ions occupy positions with a $\bar 4$ m 2 site symmetry. These highly symmetrical states may be generated by stepwise rotation of the ammonium cation. The same structural model for orientationally disordered crystal (ODIC) phases can be applied to a series of tetraalkylammonium bromides and iodides.