Br⋯Br and van der Waals interactions along a homologous series: Crystal packing of 1,2-dibromo-4,5-dialkoxybenzenes

Abstract

The crystalline structures of four homologues of the 1,2-dibromo-4,5-dialkoxybenzene series [Br2C6H2(OCn H2n+1)2 for n = 2, 12, 14 and 18] have been solved by means of single-crystal crystallography. Comparison along the series, including the previously reported n = 10 and n = 16 derivatives, shows a clear metric trend (b and c essentially fixed along the series and a growing linearly with n), in spite of some subtle differences in space groups and/or packing modes. A uniform packing pattern for the aliphatic chains has been found for the n = 12 to 18 homologues, which slightly differs from that of the n = 10 derivative. The crystalline structures of all the higher homologues (n = 10-18) seem to arise from van der Waals interchain interactions and, to a lesser extent, type II Br⋯Br interactions. The dominant role of interchain interactions provides direct structural support for the usual interpretation of melting point trends like that found along this series. Atoms in Molecules (AIM) analysis allows a comparison of the relative magnitude of the interchain and Br⋯Br interactions, an analysis validated by the measured melting enthalpies.