Indoles carbon-hydrogen bond functionalizations

Like the amides (see below), the pyrrole and indole bases deviate badly from the activity coefficient behavior of the Hammett indicators (185,357). This is perhaps not too surprisii since they protonate on carbon to produce a delocalized carboniunr-ammonium ion. As a result, the pyrrol seem to require their own separate acidity function intermediate between those for Hammett bases and aiylolefins. It is significant that both the a- and protonated conjugate acids appear to follow this acidity scale over a wide range (357). This is further evidence that the acidity fimctions differ more as a result of hydrogen-bonding factors than variations in delocalization (82,85). 

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