Safeners are used
extensively in commercial herbicide formulations.
Although safeners are regulated as inert ingredients, some of their
transformation products have enhanced biological activity. Here, to
fill gaps in our understanding of safener environmental fate, we determined
rate constants and transformation products associated with the acid-
and base-mediated hydrolysis of dichloroacetamide safeners AD-67,
benoxacor, dichlormid, and furilazole. Second-order rate constants
for acid- (HCl) and base-mediated (NaOH) dichloroacetamide hydrolysis
(2.8 × 10
–3
to 0.46 and 0.3–500 M
–1
h
–1
, respectively) were, in many
cases (5 of 8), greater than those reported for their chloroacetamide
herbicide co-formulants. In particular, the rate constant for base-mediated
hydrolysis of benoxacor was 2 orders of magnitude greater than that
of its active ingredient co-formulant,
S
-metolachlor.
At circumneutral pH, only benoxacor underwent appreciable hydrolysis
(5.3 × 10
–4
h
–1
), and under
high-pH conditions representative of lime-soda softening, benoxacor’s
half-life was 13 h—a timescale consistent with partial transformation
during water treatment. Based on Orbitrap LC–MS/MS analysis
of dichloroacetamide hydrolysis product mixtures, we propose structures
for major products and three distinct mechanistic pathways that depend
on the system pH and compound structure. These include base-mediated
amide cleavage, acid-mediated amide cleavage, and acid-mediated oxazolidine
ring opening. Collectively, this work will help to identify systems
in which hydrolysis contributes to the transformation of dichloroacetamides,
while also highlighting important differences in the reactivity of
dichloroacetamides and their active chloroacetamide co-formulants.