Uncatalyzed Hydrodechlorination of Dichlorobiphenyls

Article abstract of DOI:10.1134/S1070428019070121

Mono-, di-, and trichlorobiphenyls showed different reactivities toward alkali in 2-aminoethanol under reflux: 3-chlorobiphenyl remained unchanged, 2,4,5- and 2,4′,5-trichlorobiphenyls were completely converted to hydroxy derivatives, whereas 3,4-dichlorobiphenyl and a mixture of 2,4′-, 3,4′-, and 4,4′-dichlorobiphenyls gave rise to chlorobiphenyls in addition to hydroxybiphenyls.

Full text of DOI:10.1134/S1070428019070121

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2019, Vol. 55, No. 7, pp. 988–990. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Zhurnal Organicheskoi Khimii, 2019, Vol. 55, No. 7, pp. 1089–1092.
Uncatalyzed Hydrodechlorination of Dichlorobiphenyls
a
a
a
a
T. I. Gorbunova ,* M. G. Pervova , V. I. Saloutin , and O. N. Chupakhin
a
Postovskii Institute of Organic Synthesis, Ural Branch, Russian Academy of Sciences, Yekaterinburg, Russia
e-mail: gorbunova@ios.uran.ru
*
Received January 16, 2019; revised March 4, 2019; accepted March 15, 2019
Abstract—Mono-, di-, and trichlorobiphenyls showed different reactivities toward alkali in 2-aminoethanol
under reflux: 3-chlorobiphenyl remained unchanged, 2,4,5- and 2,4′,5-trichlorobiphenyls were completely con-
verted to hydroxy derivatives, whereas 3,4-dichlorobiphenyl and a mixture of 2,4′-, 3,4′-, and 4,4′-dichloro-
biphenyls gave rise to chlorobiphenyls in addition to hydroxybiphenyls.
Keywords: polychlorobiphenyls, congeners, nucleophilic substitution, hydrodechlorination, hydroxy
derivatives.
DOI: 10.1134/S1070428019070121
The interdisciplinary approach to the disposal of
polychlorinated biphenyls (PCBs), which is developed
in recent years, implies initial chemical functionaliza-
tion of these compounds and subsequent complete
mineralization of the new PCB derivatives by the
action of bacteria [1, 2]. The microbiological stage
involves a problem related to the transportation of PCB
derivatives to aquatic habitat of bacteria. Taking into
account that PCB congeners are hydrophobic, the first
stage of the above approach should produce hydro-
philic derivatives. Undoubtedly, introduction of hy-
droxy groups into PCB molecules should enhance their
hydrophilicity. However, conventional alkaline hydrol-
biphenyl (PCB 29, 1c), 2,4′,5-trichlorobiphenyl
(PCB 31, 1d), and a mixture of 2,4′-, 3,4′-, and 4,4′-di-
chlorobiphenyls (PCB 8, PCB 13, PCB 15; 1e–1g)
which were prepared by the Gomberg–Bachmann–Hey
coupling from the corresponding polychlorobenzenes
and polychloroanilines in the presence of isopentyl
nitrite [4]. The reactions of all congeners 1a–1g with
potassium hydroxide in 2-aminoethanol (2AE) were
carried out under similar conditions: PCB–KOH–2AE
ratio 1:6:40, temperature ~170°C (reflux), reaction
time 17 h (Scheme 1). The reaction mixture was then
acidified with dilute aqueous HCl to pH ≤7 and
extracted with toluene, and the extract was analyzed
by GC/MS. The components were quantitated by the
internal normalization method based on peak areas
(Table 1).
ysis of PCBs (KOH, DMSO, H O) does not ensure
2
their complete conversion [3], so that the hydro-
philicity of the hydrolysis products is insufficient.
The present work was aimed at studying nucleo-
philic substitution of chlorine in mono-, di-, and tri-
chlorobiphenyls by the action of alkali in 2-amino-
ethanol.
It was found that compound 1a did not react with
KOH in 2AE (no any new derivative was detected in
the reaction mixture). Unlike 1a, the conversion of tri-
chlorobiphenyls 1c and 1d under the same conditions
was complete: compound 1c was converted to three
isomeric dichloro(hydroxy)biphenyls 2b, and com-
The substrates were 3-chlorobiphenyl (PCB 2, 1a),
3
,4-dichlorobiphenyl (PCB 12, 1b), 2,4,5-trichloro-
Scheme 1.
KOH, H N(CH ) OH
reflux, 17 h
2
2 2
Cln–1
(OH)2
Cln
+
+
OH
2a, 2b
Cl
Cl
1
a–1g
3
4
1
, X = 3-Cl (a), 3,4-Cl
2
(b), 2,4,5-Cl
3
(c), 2,5,4′-Cl
3
(d), 2,4′-Cl
2
(e), 3,4′-Cl
2
2
(f), 4,4′-Cl (g); 2, n = 2 (a); 3 (b).
9
88

UNCATALYZED HYDRODECHLORINATION OF DICHLOROBIPHENYLS
989
pound 1d gave rise to a mixture of four isomeric
hydroxydichlorobiphenyls 2b and two isomeric chloro
Table 1. Reactions of compounds 1b–1g with potassium
hydroxide in 2-aminoethanol
(
dihydroxy) biphenyls 3.
Initial
comp. no.
Product
no.
Number Concentra- Conver-
The mass spectra of all dichloro(hydroxy)biphenyl
of isomers tion, %
sion, %
isomers 2b obtained from 1c and 1d were similar: the
2
2
2
4
3
4
2
17.7
82.3
1
b
4
100
+·
base peak was that with m/z 238 [M] with an isotope
peak ratio corresponding to the number of chlorine
atoms in the molecule; also, fragment ion peaks with
2
4
a
1
e–1g
05.0
053
+
·
+·
m/z 202 [M – HCl] , 168 [M – 2Cl] , and 139 [M –
2
2a
2b
2b
3
47.9
+
Cl – HCO] (relative intensity 20–40%). The mass
1
1
c
1000.0
95.7
100
100
spectra of chloro(dihydroxy)biphenyls 3 showed
a similar pattern. The molecular ion peak with m/z 220
d
+
·
04.3
[
M] had the maximum intensity, and fragment ion
+
+·
peaks with m/z 191 [M – HCO] , 184 [M – HCl] , 157
M – Cl – CO] , and 128 [M – HCl – 2 CO] with
+
+
[
observed. The mass spectra of 2a were generally
consistent with the mass spectra of structurally related
compounds included in the NIST2014 database.
intensities of 10 to 15% were present. The mass
spectra of 2b and 3 coincided with those given in
NIST2014 database.
Obviously, monochlorobiphenyls 4 are formed as
a result of uncatalyzed hydrodechlorination of 1b and
1e–1g. On the basis of analytical and chromatographic
data [6], compounds 4 were identified as 3-chlorobi-
phenyl (1a, PCB 2) and 4-chlorobiphenyl (PCB 3),
which confirmed the low reactivity of 1e (PCB 8).
The conversion of 1d with the chlorine atoms
located in both aromatic rings was higher than the
conversion of 1c where all chlorine atoms are located
in the same ring. Presumably, nucleophilic substitution
of chlorine in 1d by the action of KOH in 2AE follows
two mechanisms, classical S Ar (ipso-substitution)
and aryne (cine-substitution). In the case of compound
N
As a rule, high conversion in the hydrodechlorina-
tion of chloroarenes is attained in the presence of metal
catalysts, palladium ones being the most efficient
1
c, similar conclusions cannot be regarded as reliable
since only three isomers 2b were isolated. Analogous
results were obtained previously in the reactions of
[7−9]. The source of hydrogen for the reductive
dehalogenation may be both external (gas cylinder or
generator) and internal (generated in situ). Organic
compounds belonging to different classes, such as
alkanes, alcohols, hydrazines, etc., can act as an in-
ternal hydrogen source. Uncatalyzed hydrodechlorina-
tion of chloroarenes in the presence of an internal
hydrogen source requires harsh conditions, e.g., super-
critical fluids [10].
2
,4-dichlorobiphenyl (PCB 7) and 2,4,4′-trichloro-
biphenyl (PCB 28) with sodium methoxide [5]. Due to
the lack of authentic samples, we failed to unambigu-
ously determine the position of substituents in new
compounds 2 and 3.
The results of reactions of 1b and mixture 1e–1g
with KOH in 2AE were more surprising. The conver-
sion of 1b (both chlorine atoms are located in the same
benzene ring) was complete, whereas congeners 1e–1g
Catalytic hydrodechlorination of chloroarenes by
the action of alkalies in propan-2-ol has been studied
in sufficient detail [11−14]. In these studies, the sub-
strates were polychlorobenzenes, PCBs, and poly-
chlorinated dibenzodioxins and dibenzofurans. It was
found that in the hydrodechlorination of chloroarenes
over Pd/C, Rh/C, or Rh–Pt/C as catalyst, the source of
hydrogen (more precisely hydride ion), is the solvent
(α-hydrogen atoms in i-PrOH). This was confirmed by
using deuterium-labeled alcohols in the reductive
dechlorination [12]. Initially, an alcohol molecule is
coordinated on the solid catalyst surface through lone
electron pairs on the hydroxy oxygen atom, which
increases the lability of α-hydrogen atoms in i-PrOH
and enhances their intermolecular bonds with chlorine
(
chlorine atoms are located in different rings) were
converted by 53%. The initial ratio 1e/1f/1g was
62:28:10, whereas the product ratio was 79:16:5;
~
i.e., the conversion of 1g (PCB 15) was the highest,
and the conversion of 1e (PCB 8, nonplanar ortho-
substituted biphenyl) was the lowest.
Both 1b and 1e–1g were converted mainly to
chloro(hydroxy)biphenyls 2a, but in both cases
relatively large amounts of chlorobiphenyls 4 were
detected. In the mass spectra of 2a formed from 1b and
+
·
1
e–1g, the base peaks were those with m/z 204 [M] ,
and low-intense (Irel+5–10%) fragment ion peaks with
·
+
m/z 168 [M – HCl] and 141 [M – Cl – CO] were
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 55 No. 7 2019

9
90
GORBUNOVA et al.
atoms of chloroarenes. Next follow cleavage of the
CONFLICT OF INTERESTS
Carom−Cl bond in the chloroarene and of the C –H
bond in the alcohol and exchange of Cl for H .
3
sp
–
–
The authors declare the absence of conflict of
interests.
In our case, the hydrodechlorination of 1b and
e–1g occurred in the absence of a catalyst which
1
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 55 No. 7 2019