Environmental Engineering Reference
In-Depth Information
P-atom-containing functional groups bonded to a given compound in the aque-
ous phase forming a 2
σ
/1
σ
*
two-center-three-electron (2c-3e) adduct (Bonifa ˇi ˇ
1999
). These functional groups also affect the H-atom abstraction reaction by
donating or withdrawing electrons on the C-H bonds. The group rate constant,
k
R4
(Eq.
2.13
) represents the reaction of HO
•
with S-, N-, or P-atom-containing
compounds. The influence of neighboring functional groups is considered as neg-
ligible. The rate constant,
k
, for HO
•
addition to iminodiacetic acid (HOOC-CH
2
-
NH-CH
2
-COOH) as a typical example is expressed below (Eq.
2.23
):
k
=
2
×
2
k
o
sec
X
−
COOH
X
−
NH
−
+
k
−
NH
−
+
2
k
−
COOH
(2.23)
It is shown that the group rate constants
k
-CN
and
k
-NH2
can be compared with
the rate constants for compounds that react with HO
•
via only interaction such
as cyanogen and thiourea, respectively. The rate constant for thiourea (which
has two -NH
2
groups) is approximately twice
k
-NH2
, because the electron posi-
tive -CS- functional group does not significantly affect the electron density of
the N atom. The reaction of HO
•
with urea is presumably different because the
two amine functional groups of urea are bonded to the electron-negative func-
tional group, -CO-. Thus, another group rate constant
k
-N-CO-N-
is considered
for methylurea, tetramethyl urea, and 1,3-dimethylurea. The magnitude of most
group rate constants for the S-containing compounds is of the same order as for
the amine-containing ones, but approximately 1 order of magnitude larger than
for the amide-containing compounds. This might be caused by the electron-
egative -CO- functional group of the amide. The S-, N-, or P-atom-containing
group contribution factors apparently play the same role as the functional groups
for H-atom abstraction, i.e.,
X
R
i
=
e
−
(
Ea
, absR
i
)/
RT
. However, it is anticipated
that S-, N-, or P-atom-containing functional groups may have different effects
on H-atom abstraction. The group contribution factors for -S, -S-S-, and -SH,
and -NH
2
, -NH-, and -N<, respectively, are assumed to be identical due to the
following reasons: (1) limited data availability for single functional group com-
pounds, (2) similar electron inductive ability, and (3) application for the gaseous
phase. In addition, the same data sets for the S- and N-atom-containing com-
pounds are used to calibrate the group rate constants,
k
-S-
,
k
-S-S-
, and
k
-SH
, and
k
-NH2
,
k
-NH-
, and
k
-N<
, respectively. These group rate constants are not assumed
to be identical because the interaction of HO
•
with each functional group might
be more significant than the electron donating effects of the functional groups.
For similar electron inductive ability, the Taft constant indicates similar val-
ues among the S- and N-atom-containing functional groups. For example, the
Taft constants for SCH
3
, SC
2
H
5
, and SH are 1.66, 1.44, and 1.52, respectively
(Karelson
2000
), and those for NH
2
, NHCH
3
, N(CH
3
)
2
, NH(CH
2
)
3
CH
3
, and
N(C
2
H
5
)
2
are 0.62, 0.94, 1.02, 1.08, and 1.00, respectively (Karelson
2000
).
These values are well distinguished from 3.61 of NH
3
+
, 4.66 of NO
2
, 4.16 of
N
+
(CH
3
)
3
, and 3.64 of CN. Finally, it is assumed that the group contributed fac-
tors for -S-,-S-S-, and -SH, and for -NH
2
, -NH-, -N< , -NNO, and -NNO
2
are identical, which successfully predicted the gas-phase HO
•
rate constants
(Atkinson
1986
,
1987
; Kwok and Atkinson
1995
). A linear correlation between
