Parabens and salicylates were examined as disinfection
byproduct
(DBP) precursors to explore the possible influence of ipso substitution
(i.e., halogen exchange) on the yield and speciation of trihalomethanes
(THMs) formed during water chlorination. Substoichiometric conversion
of C–Br bonds into C–Cl bonds was confirmed for several
parabens and salicylates. The co-occurrence of (mono)brominated and
nonhalogenated precursors in the presence of free chlorine (but in
the absence of added Br–) generated polybrominated
THMs, implicating ipso substitution. The THM molar yield, bromine
incorporation, and bromine recovery from brominated and nonhalogenated
precursor mixtures were commensurate with those observed from equimolar
additions of NaBr, indicating efficient displacement of aromatic bromine
by free chlorine followed by reincorporation of liberated HOBr into
DBP precursors. The THM molar yield from brominated precursors was
enhanced by a factor of ≤20 relative to that from nonhalogenated
precursors. Trends in THM molar yields and bromine incorporation differed
between brominated parabens and brominated salicylates, suggesting
that the influence of ipso substitution on THM formation varies with
the structure of the organic precursor. Collectively, these results
provide new evidence of the often-overlooked role ipso substitution
can play in promoting halogen exchange and bromine enrichment among
DBPs in chlorinated waters.