Experimental work began with a systematic study of a range of monosubstituted aryl systems bearing Lewis basic directing groups of varying strengths, and how the molar fraction of the d0-, d1-, and d2-isotopologues varies with time in the corresponding HIE reactions. Literature data are available to quantify the Lewis basicity of some of the functional groups employed; These data are available for a subset of the specific substrates considered here and for their parent directing groups (Table in the ESI); † these illustrate the Lewis basicity range of the substrates, however, none of these metrics correlate directly to our recently established relative directing ability of the corresponding substrates. This was the first hint that the behaviour of these systems was more complicated than we had initially anticipated.