Electrophilic and Nucleophilic Substitution in Aromatic Systems •and the proportion of them in the product should be independent of
the nature of the original halogen; this, too, has been confirmed
experimentally. Support for the intervention of benzyne intermediates
is also provided by the fact that aryl halides having no hydrogen in the
o-positions, and so unable to eliminate H - Hal, are extremely resistant
to amination. But, perhaps most conclusive of all, it has proved
possible to 'trap' benzyne intermediates by reacting them with
dienes to produce recognisable addition products in the Diels-Alder
reaction (p. 151).
It has been shown using isotopically-labelled chlorobenzene that
its conversion to phenol proceeds both via a benzyne intermediate
and by an S^2 (aromatic) reaction simultaneously. A strong base is
always required for a replacement to proceed via a benzyne inter
mediate for the initial removal of a proton from the aromatic nucleus
is far from easy: