Abstract
Compounds such as PhIF 2, Ph 3PF 2 and XeF 2, which have been used previously as unrelated fluorinating agents, are shown to be periodically related as isoelectronic molecules E 3AF 2 of trigonal-bipyramidal shape, where E represents a bonded or non-bonded electron pair and A a main Group V – VIII element. These compounds are arranged in order of halogenating ability by estimating the magnitude of reduction couples, approximated by δH o f(E 3AF 2-E 3A), or by noting the direction of redox reactions involving the couples. The A sequence deduced Kr>Xe⋍Cl>Br>I>S>Se>Te⋍As⋍Sb>P agrees with the limited experimental data available. Evidence for an ionic mechanism involving ‘onium’ monohalide ions is given for halogenations with these reagents when carried out under “Friedel-Craft” conditions although no stable salts containing these ions have as yet been isolated because of intramolecular halogenation. These ions act as sources of positive fluorine. The use of ring deactivated reagents to achieve halogenation is discussed.
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