Environmental Engineering Reference
In-Depth Information
the
π
-electrons in >C
=
C< depends on the other functional groups bonded to the
alkene.
Except ethylene, alkenes are categorized into six basic structures on the basis of
the number of H atoms and their positions, including
cis
and
trans
conformations
(i.e., >C
=
C< , H>C
=
C< , H
2
C
=
C<, H>C
=
C<H (
cis
), H>C
=
C<H (
trans
), and
H
2
C
=
C<H). If the base structure is symmetrically associated with the number and
position of hydrogen atom(s), the probability of HO
•
addition to two unsaturated
carbons is considered to be identical, whereas it is different for the asymmetrical
base structure. This may reflect the differences in the
A
resulting from regioselectiv-
ity. Accordingly, the group rate constant,
k
º
(structure)-
h
, and group contribution factor,
Y
R
l
, for HO
•
addition to one of the base structures can be written using Arrhenius
frequency factor,
A
º
(structure)-
h
, and group contribution parameter,
E
a,add-alkene
R
l
of
the functional group Rl
l
(
l
denotes the number of functional groups,
l
=
1-4):
k
o
(
structure
)
-
h
=
A
o
(
structure
)
-
h
e
−
[
E
o
a
,
(
structure
)
]
RT
(2.15)
Y
R
1
=
e
−
[
E
a
,
(
structure
)
R
1
]
RT
(2.16)
where (structure) represents six base structures,
E
º
a(structure)
denotes a base part of
E
a
for (structure), and
h
denotes a position for HO
•
to add i.e., 1 and 2 for the
addition to the left and right carbon, respectively. The rate constant for HO
•
addi-
tion to alkene,
k
add-alkene
, can be written as below (Eq.
2.17
)
gk
(
structure
-
h
)
Y
R
1
k
add-alkene
=
(2.17)
where
g
indicates the 1 or 2 that represents asymmetrical and symmetrical addi-
tion, respectively. The rate constant for tetrachloroethylene (Cl
2
C
=
CCl
2
) as a typi-
cal example that is shown below (Eq.
2.18
):
k
=
2
k
>
C
=
C
<
Y
−
Cl
Y
−
Cl
Y
−
Cl
Y
−
Cl
(2.18)
Few rate constants are reported for the conjugated and unconjugated dienes. It
is shown that the group contribution factors do not linearly correlate with the Taft
constant. Two reasons can be considered. First, the functional group contribution to
the
E
a
does not follow the general inductive effect (i.e., Taft constant). Second, the
experimental rate constants do not seem to follow the inductive effect (e.g., vinyl
chloride > ethylene > vinyl alcohol) because of experimental errors or the existence
of unknown reaction mechanisms. Considering new reaction mechanisms such as
the excitation of alkenes by radiation may pave the way for future studies in that
regard. Despite the observation of the nonlinear correlation between the group con-
tribution factors and the Taft constant, 79 % of the calibrated rate constants were
within the error goal, which might be acceptable for a rate constant estimator.
Rate constants for HO
•
Addition to Aromatic Compounds
(Minakata et al.
•
2009
): The HO
addition to the aromatic ring often occurs at rates close to dif-
fusion-control. The electron-donating and -withdrawing functional groups on the
aromatic ring can significantly affect the rate constants and the ratio of
ortho
-,
meta
-,
para
-, and
ipso
-addition. For the HO
•
addition to aromatic compounds,
