DEET) as a product of the anodic Fenton treatment of DEET which
involved hydroxyl radical oxidation process. However, no hydrox-
ylated DEET was observed in our study. In this study, ozone was
introduced continuously into the DEET solution. Presumably, the
instability of hydroxylated DEET under such strong oxidation con-
ditions may be the main reason why these compounds were not ob-
served. However, we postulated that these hydroxylated DEET to be
present as an intermediate in the formation of V (Fig. 4b).
The initial attack of ozone on DEET through electrophilic substi-
tution on the aromatic ring led to the formation of monohydroxy-
lated DEET (M). M then reacted with the hydroxyl radical to form
dihydroxylated (N) and trihydroxylated (O) DEET which further re-
acted with a hydroxyl radical to form the radical P. Intra-molecular
rearrangement of radical P led to the formation of aromatic ring-
opening by-product through the cleavage of C1–C6 bond to form
the radical R which then rearranged into the radical T. Radical T re-
acted with a hydroxyl radical to form the initial ring-opening by-
product U, an enol with the hydroxyl groups attached to a carbon
with C–C double bond. According to Hornback (2005), the enol
group is an unstable species which tautomerised rapidly to its keto
form. The aromatic ring-opened by-product detected in this study
is suggested to be V, instead of U. The presence of W and X, a break-
down product of V, further conﬁrmed the aromatic ring-opening
reaction during ozonation. Presumably by-products from ozona-
tion can further react with the highly reactive hydroxyl radical
and ozone present in the reaction mixture to form other products
that are not identiﬁed in this study.
This research was ﬁnancially supported by the Malaysia Toray
Science Foundation (MTSF) and University of Malaya (UM-RU
Grant SF025-2007A). We thank Emeritus Professor Bernd R.T. Sim-
oneit (Oregon State University) for his constructive comments on
the proposed reaction pathways.
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