Experimental and theoretical studies of the halogen bond and the electrophilic bromination reaction

The work presented in this thesis is an investigation into the halogen bond using both experimental and theoretical techniques. These studies have contributed toward the understanding of the interaction during a period when the definition of this interaction was being debated.1 The interactions investigated have a range of strengths; varying from the very weak interactions with rare gas atoms to the strong interactions with halonium ions acting as halogen-bond donors. The competition and cooperation between halogen and hydrogen bonds have been investigated including a situation where halogen bonding can be favoured over hydrogen bonding. Small-molecule analogues of orthogonal halogen and hydrogen bonding observed in biological systems have also been produced. The similarity between the two interactions has been highlighted by the fact that the Steiner-Limbach equation can model the bonding in both cases. New halogen-bonded liquid crystals between dihalogens and alkoxystilbazoles and alkoxyphenylpyridines have been synthesised and the complexes between elemental iodine and alkoxystilbazoles unexpectedly showed SmC phases with high stability. Attempts to synthesise equivalent liquid crystals with elemental bromine were unsuccessful, an electrophilic bromination reaction followed by elimination of HBr taking place instead. In order to understand this reaction further, the intermediates of the electrophilic bromination reaction for different substituted stilbenes was investigated computationally and the results showed that a carbocation intermediate is favoured if an electron-donating substituent is present. In contrast, stilbenes with two electron-withdrawing substituents favoured symmetric bromonium ion intermediates. Such halonium ion intermediates feature a halogen atom that carries a positive charge, which can interact with Lewis bases in a novel category of halogen bonding, which has properties similar to halogen bonding with traditional halogen-bond donors.