Transformations of dichloromethane radicals in alkaline water solutions

Article abstract of DOI:10.1023/A:1016526518030

Among transformation products of CHCl₂ and CH ₂Cl radicals generated by γ-irradiation of deaerated water solutions of dichloromethane oxygen-containing compounds (formic acid, formaldehyde, carbon monoxide) and chloride ions were found. The radicals CHCl₂ suffer nucleophilic substitution by hydroxyl ions affording anion-radicals HCOO^-2 that further transform into formic acid. At the growing concentration of alkali the frequency of chlorine substitution with hydroxyl ions in the dichloromethane radicals increases, and their transformation process becomes a chain reaction.

Full text of DOI:10.1023/A:1016526518030

Russian Journal of Organic Chemistry, Vol. 38, No. 4, 2002, pp. 475 479. Translated from Zhurnal Organicheskoi Khimii, Vol. 38, No. 4, 2002,
pp. 499 503.
Original Russian Text Copyright
2002 by Kosobutskii, Vrublevskii.
Transformations of Dichloromethane Radicals in Alkaline
Water Solutions
V. S. Kosobutskii and A. I. Vrublevskii
Belorussian State University, Minsk, 220050 Belarus
Received February 4, 2000
Abstract Among transformation products of CHCl2 and CH2Cl radicals generated by -irradiation of
deaerated water solutions of dichloromethane oxygen-containing compounds (formic acid, formaldehyde,
carbon monoxide) and chloride ions were found. The radicals CHCl2 suffer nucleophilic substitution by
2
hydroxyl ions affording anion-radicals HCOO that further transform into formic acid. At the growing
concentration of alkali the frequency of chlorine substitution with hydroxyl ions in the dichloromethane
radicals increases, and their transformation process becomes a chain reaction.
We formerly demonstrated that formaldehyde
arose in the alkaline water solutions of dichloro-
methane under -irradiation by nucleophilic substitu-
tion of chlorine in CH2Cl radicals with hydroxyl ion
[1]. Here we report on more detailed investigation of
reaction between dichloromethane radicals and
nucleophilic species resulting in the other products of
radicals transformation.
chemical yields G 2.8, 2.8, 0.55 and 0.7 molecules
per 100 eV respectively [2]. The radicals react further
with dichloromethane molecules along reactions (2 5).
H2O
HO , eaq, H , H2O2
(1)
(2)
CH2Cl2 + HO
H2O + CHCl2
Deaerated water solutions of dichloromethane
(0.1 mol l ) containing various quantities of alkali
I
1
were subjected to ionizing irradiation. We observed
increase in the radiation-chemical yield of chloride
ions and decreased yield of dichloroethane. Radiation-
chemical yields of transformation products originat-
ing from dichloromethane radicals are listed in the
table. Typical kinetic curves of accumulation of
identified radiolysis products are presented on
Figs. 1, 2.
CH2Cl2 + eaq
Cl
+
CHCl2
(3)
II
I
H2
+
(4)
(5)
CH2Cl2 +
H
HCl + II
Dichloroethane forms along reaction (6).
2 II CH2ClCH2Cl
At -irradiation of water [reaction (1)] arise active
species HO , eaq, H , and H2O2 with radiation-
(6)
Radiation-chemical yields G of products from radical transformations in water solutions of dichloromethane
[NaOH],
mol l
Yield, molecules per 100 eV
1
a
Cl
C2H4Cl2
HCOOH
CH2O
CO
Cl
0
6.0 1.0
9.2 1.2
12.4 1.3
13.6 1.5
1.20 0.21
1.05 0.14
0.35 0.10
0.10 0.02
1.37 0.48
1.58 0.79
4.33 1.27
4.91 1.43
0.21 0.04
0.56 0.13
0.72 0.21
0.73 0.22
0.12 0.02
44 5
101 17
0.1
0.3
0.5
110 20
a
1
Radiation-chemical yields of chloride ions obtained at absorbed dose power 0.18 1018 eV l 1 s .
1070-4280/01/3804-475$27.00 2002 MAIK Nauka/Interperiodica

476
KOSOBUTSKII, VRUBLEVSKII
H2O2+ H2O +
I
HO
+ HCOOH + 2HCl (7)
H2O2+ II
HO
+
H2CO + H2CO + HCl (8)
Taking into account reactions (3, 5, 7, 8) the
radiation-chemical yield of chloride ions amounts to
GCl
Ge-aq + GH + 2GH O
4.8 ion per 100 eV,
2
2
that is by 1.2 ion per 100 eV less than shows the
experiment. Therefore radicals I and II react also
with water molecules by reactions (9) and (10).
2H2O +
I
HC(OH)2
+
2HCl
(9)
III
H2O + II
CH2OH + HCl
(10)
IV
Radicals III and IV further can transform into
formic acid and formaldehyde by reactions (11, 12)
and to lesser extent by reactions (13, 14).
Fig. 1. Concentration of carbon monoxide (1) and formic
acid (2, 3) in dichloromethane water solutions as a function
of radiation dose. [NaOH], mol l : 0.5 (1), 0 (2, 3).
1
III + CH2Cl2
IV + CH2Cl2
III + H2O2
IV + H2O2
HCOOH + II + Cl
CH2O + II + Cl
(11)
(12)
(13)
(14)
HCOOH + HO
CH2O + HO
+
HO
HO
+
Reaction (9) also helps to understand the formation
of carbon monoxide [reactions (15) and (16)] that is
present in the products of dichloromethane radicals
transformation in neutral solution.
III
H2O + HCO
(15)
The rate constant of a similar dehydration process
for CH3C(OH)2 radical is k 3 104 s 1 [3].
V
CO +
H
(16)
The CO formation directly indicates the nucleo-
philic substitution reaction between radicals I and
water molecule to occur [reaction (9)].
Fig. 2. Concentration of dichloroethane (1, 4) and form-
aldehyde (2, 3) as a function of radiation dose. [NaOH],
mol l : 0 (1, 3), 0.5 (2, 4).
1
On addition of sodium hydroxide to the dichloro-
methane solution the yield of chloride ions grows up
to 13.6 ions per 100 eV. Under these conditions the
yield of formic acid and formaldehyde also increases.
The radiation-chemical yield of dichloroethane de-
creased with growing alkali concentration (see table).
The yield of CO decreased to G < 0.001 molecule
The radiation-chemical yield of chloride ions (see
table) suggests that chloride ions form not only in
reactions (3) and (5). They might arise in reaction of
hydrogen peroxide that forms by water radiolysis
with radicals I and II along reactions (7) and (8).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 4 2002

TRANSFORMATIONS OF DICHLOROMETHANE RADICALS
per 100 eV (sensitivity level of the analytical proce-
477
dure) already at alkali concentration of 0.1 M. These
data show that hydoxyls more efficiently react with
radicals I, II [reactions (17, 18)] than water
molecules.
I + 2HO
II + HO
III + 2Cl
IV + Cl
(17)
(18)
Radicals III, IV in alkaline medium turn to
radical-anions [reactions (19, 20)] and transforming
along reactions (21, 22) end up as formate anion and
formaldehyde.
IV + HO
CH2O
IV
+
+
+
H2O
(19)
III + HO
HCOO
VII
2H2O
(20)
VII + CH2Cl2
VI + CH2Cl2
HCOO
II + Cl
(21)
(22)
CH2O + II + Cl
The sharp decrease in CO yield in alkaline solution
may be rationalized by reaction (20). The low yield of
dichloroethane that forms by second-order reaction
(6) is due to diminishing of radicals II stationary con-
centration for in alkaline solutions they are involved
in reaction (18). The described reactions in total
provide a possibility of chain process of chlorine
substitution in the dichloromethane radicals whose
efficiency would be greater at lower power of the
absorbed radiation dose. Under conditions of lower
dose power the concentration of dichloromethane
radicals and the frequency of reactions of second-
order chain termination would be considerably less.
We measured radiation-chemical yields of chloride
ions at 10-fold reduced dose power in the solution
containing various alkali concentrations (see table).
The yield of chloride ions at NaOH concentration
0.5 M amounted to 110 ions per 100 eV and sig-
nificantly exceeded the overall yield of radical species
in water radiolysis equal to 6 species per 100 eV.
This fact confirms the chain character of chlorine ions
formation process.
Fig. 3. ESR spectrum of dichloromethane water solution
1
irradiated at 196 C (1) with a dose of 2.5 1021 eV l , and
changes thereof at temperature risen to the following values:
80(3), 63(4), 20(5), 0 C(6); ge 2.0055; [NaOH] 0.5 M.
The short-lived intermediate products of dichloro-
methane radicals transformation were studied by
means of ESR.
right-hand part of the spectrum sharply decreased,
and that of three bands in the left part increased.
Their maximum intensity is reached at 63 C. The
final pattern of the spectrum is also attained at 63 C.
As seen from Fig. 3, at rising temperature the ESR
spectrum of the dichloromethane water solution gets
simpler. The intensity of the absorption band in the
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 4 2002

478
KOSOBUTSKII, VRUBLEVSKII
Fig. 5. ESR spectrum of water solution containing 0.1 M
Fig. 4. ESR spectrum of pure dichloromethane irradiated
1
with a dose 3.2 1021 eV l at following temperatures: 196
HCOOH and 0.6 M NaOH after irradiation with a dose
1
11.4 1021 eV l at 63 (1), 20 (2), and 0 C (3).
(1), 102 (2), 86 C (3).
that the ESR signal originates from radical-anions VII
that form by reactions (17) and (20). The large
negative charge of these radicals prevents their
recombination.
Further heating does not change the qualitative
pattern of the spectrum up to 0 C (without phase
transition).
The loss of intensity of the right spectrum band is
due to disappearance of HO radicals [4]. Therefore
the spectrum in Fig. 3 corresponding to 80 C
belongs to radicals arising from dichloromethane as a
result of its reactions with HO , H and eaq species,
and also probably to radical products formed at
replacement of chlorine with HO ions. In the pure
dichloromethane all radical species (also radicals I
and II) are terminated by recombination already at
86 C. It is clearly shown on Fig. 4 where is
presented the ESR spectrum obtained at irradiation
of pure dichloromethane.
At 63 C the ESR spectrum of water solution
containing formic acid in 0.1 M concentration and
NaOH is 0.6 M concentration obtained after irradia-
1
tion with a dose 11.4 1021 eV l (Fig. 5) is similar
to the spectrum from the irradiated alkaline dichloro-
methane solutions (Fig. 3). Both spectra in the left-
hand part contain three bands of unequal intensity that
remain till 0 C. The irradiation of the alkaline solu-
tion of formic acid may give rise to radical-anions
VII under the action of hydrated electrons. The rate
1
constant of this reaction is 1 104 l mol 1 s ) [5].
Therefore it is possible to state that three absorption
bands in the left part of the ESR spectra of the
irradiated alkaline dichloromethane solutions (Fig. 3)
originate from radical-anion VII. The band in the
right part of the spectrum belongs presumably to
radical-anion VI that forms by reactions (18, 19). The
charge hampers its disappearance by the second-order
processes. On the other hand, the destruction of
Consequently, in the ESR spectrum of the alkaline
solution of dichloromethane (Fig. 3) at 63 C all the
absorption bands originate from species formed in
reaction of radicals I and II with hydroxyls. The
existence of ESR spectrum at 20 C and 0 C shows
that the radical species corresponding to this spectrum
difficultly undergo recombination. This fact suggests
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 4 2002

TRANSFORMATIONS OF DICHLOROMETHANE RADICALS
479
radical-anions VII provides radical-anions VI [by
reaction sequence (21), (18), (19)].
carrier gas helium. Chloride ions were analyzed by
potentiometric titration with silver nitrate solution
calibrated against a standard sodium chloride solu-
tion. The samples before titration were neutralized
and acidified with nitric acid.
The formation of radical-anion VII [by reactions
(17) and (20)] already at 80 C evidences that activa-
tion energy of reaction (17) is very small as follows
also from results of calculation performed in [1].
The ESR spectra of radical species were recorded
on spectrometer ESR220 at 196 C. The magnetic
field was calibrated with the use of magnetometer
MJ-110R by the hyperfine structure lines of a
standard sample of Mn2+ /MgO. Variable temperature
measurements were performed with the use of
standard cooling baths from liquid nitrogen with
methanol, methyl ethyl ketone, ethyl acetate, chloro-
form, and tetrachloromethane [10]. The sample
before measurement was maintained at a given
temperature for 10 min.
EXPERIMENTAL
In the studies was used dichloromethane of pure
grade after distillation. Before irradiation the solu-
tions were deaerated by successive dilution procedure
[6] with the use of argon of high purity. The NaOH
used was of chemically pure grade. The irradiation
was carried out in sealed ampules on an installation
LMB- -1M. The power of the absorbed dose as
measured with ferrosulfate dosimeter was 2.12
1
1018 eV l 1 s , the range of doses used was
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reference served the initial nonirradiated solution.
The molar extinction factor determined with calibrat-
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104 lmol 1 cm ). The formic acid was determined by
procedure from [8]. To the samples under study was
added the appropriate amount of fine magnesium
turnings and hydrochloric acid, and after reduction
of the acid to formaldehyde its content was measured
by spectrophotometric procedure. As reference here
was used the initial solution that was not subjected to
reduction. The molar extinction factor for the formic
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1
obtained was (1.15 0.3) 104 l mol 1 cm ). Di-
chloroethane analysis was carried out by GLC on
chromatograph LKhM-8MD equipped with flame-
ionization detector, column 2 m long, stationary
phase PEG-600 (10%) on Chromosorb W, carrier gas
argon. Carbon monoxide was analyzed on chromato-
graph LKhM-8MD equipped with katharometer,
column 1 m long packed with activated carbon [9],
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 4 2002