A new advanced method for heterogeneous catalysed dechlorination of 1,2,3-, 1,2,4-, and 1,3,5-trichlorobenzenes in hydrocarbon solvent

Article abstract of DOI:10.1016/S0040-4039(02)02377-8

A new advanced method for dechlorination of 1,2,3-, 1,2,4-, and 1,3,5-trichlorobenzenes in organic solvent catalysed by palladium on carbon support and solid hydrazine hydrochloride yields benzene in short reaction times. The catalyst system can be efficiently reused for several cycles. Ultrasound radiation of the heterogeneous catalyst reaction increases remarkably the rate of dechlorination.

Full text of DOI:10.1016/S0040-4039(02)02377-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 9645–9647
A new advanced method for heterogeneous catalysed
dechlorination of 1,2,3-, 1,2,4-, and 1,3,5-trichlorobenzenes in
hydrocarbon solvent
J. Gonzalo Rodr´ıguez* and Antonio Lafuente
Departamento de Qu´ımica Orga´nica, Universidad Auto´noma, Cantoblanco, 28049 Madrid, Spain
Received 4 September 2002; accepted 22 October 2002
Abstract—A new advanced method for dechlorination of 1,2,3-, 1,2,4-, and 1,3,5-trichlorobenzenes in organic solvent catalysed by
palladium on carbon support and solid hydrazine hydrochloride yields benzene in short reaction times. The catalyst system can
be efficiently reused for several cycles. Ultrasound radiation of the heterogeneous catalyst reaction increases remarkably the rate
of dechlorination. © 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Results and discussion
Chlorinated organic molecules, especially aromatic
compounds, present a very important environmental
problem because they are extremely persistent due to
their slow biodegradation, their toxicity and carcino-
genic character have been widely proved.1 The combus-
tion of chlorinated compounds releases even more toxic
compounds such as polychlorinated dibenzodioxins and
polychlorinated dibenzofurans.2
The samples of trichlorobenzenes (TCBs, 0.05 mmol)
prepared for dechlorination were dissolved in hexane
(40 g) and were treated with hydrazine hydrochloride
(moderate toxic reagent7) (250 mg, 3.65 mmol) and
sodium carbonate (390 mg, 3.65 mmol) and palladium
on carbon (60 mg, Pd 10%, 0.056 mmol). A multi-fre-
quency apparatus and ultrasonic reactors have been
designed (with frequencies centered at 250, 475, 650 and
800 kHz) to improve the heterogeneous catalytic
dechlorination. The reaction was run in a bath with
stirring at variable temperature or in a reactor with
ultrasonic radiation at room temperature, finally yield-
ing benzene as the sole aromatic product (HPLC). The
dechlorination reaction at 60°C (external bath) shows
the highest kinetic. Moreover, ultrasonic irradiation
produces a remarkable efficiency of the reaction in all
of the frequencies experimented. However, the reaction
completely proceeds for some shortest times, for 487
kHz versus 250 and 650 kHz.
In recent years, catalytic dechlorination methods have
been developed including transfer hydrogenation by
means of hydrogen donors such as metal hydrides,3
formic acid and its salts4 and alcohols,5 which use water
or methanol as solvents. The practical application of
catalysts to the dechlorination of organic halides is
always accompanied by the problem of the deactivation
of the catalyst.6
We report here the first finding of a complete dechlori-
nation of trichlorobenzenes in solution in hexane
catalysed by palladium over carbon and hydrazine as
hydrogen donor, at 60°C or room temperature and
ultrasound source. Application of this methodology to
the dechlorination of PCBs is in preparation and will be
promptly presented.
The analysis of all the reaction products were carried
out by HPLC with UV–vis detector, with an error
<0.1%. Evaluated dichlorobenzenes (DCBs) and
monochlorobenzene (MCB) hexane solutions were used
as reference.
3. Dechlorination of the trichlorobenzenes
Keywords: dechlorination; trichlorobenzenes; palladium; hydrazine;
sonochemistry.
* Corresponding author. E-mail: gonzalo.rodriguez@uam.es
The results of the dechlorination of 1,2,3-; 1,2,4-; and
1,3,5-trichlorobenzene (1,2,3-; 1,2,4-; 1,3,5-TCB) under
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02377-8

9646
J. G. Rodr´ıguez, A. Lafuente / Tetrahedron Letters 43 (2002) 9645–9647
the above conditions are summarised in Table 1
(Scheme 1).
ture of the reaction (at rt or 40°C) does not modify the
dechlorination order sequence. (b) In the same way,
1,3,5-TCB was transformed only in 1,3-DCB and ben-
zene but MCB was not detected. (c) The 1,2,4-TCB
allows a mixture of DCB isomers and MCB as chlori-
nated intermediates, being the 1,3-DCB isomer propor-
tionally very abundant in the mixture. Hence, a
meta-rule, meaning that Cl-atoms in the meta position
would be removed preferentially, was not observed.8 (d)
Moreover, catalytic hydrogenation of the benzene ring
The reaction occasionally shows an initiation period
but, after 5–10 min, yields sequentially the DCBs, MCB
and benzene, in short reaction times. Ultrasonic irradia-
tion avoids the initiation period. Moreover, some
aspects of the reaction must be considered: (a) at 60°C
the relative order of dechlorination of the TCB isomers
was 1,2,3->1,3,5->1,2,4-TCB (Table 1). The tempera-
Table 1. Dechlorination of the TCBs
t (min)a
1,2,3-TCB
1,2+1,4-DCB
1,3-DCB
MCB
Benzene
15
5b
0
0
0
0
0
0
0
0
100
100
1,2,4-TCB
15
30
42
14
0
0
13
4
0
7
0
0
0
6
0
0
0
32
82
100
100
45
15b
0
1,3,5-TCB
15
0
0
0
–
–
–
26
0
0
0
0
0
74
100
100
30
10b
a At 60°C.
b Ultrasonic radiation (487 kHz).
Scheme 1.
Table 2. Dechlorination of the TCB with the recovered catalyst system
t (min)a
1,2,3-TCB
1,2+1,4-DCB
1,3-DCB
MCB
Benzene
15
30
46
29
0
0
18
22
0
0
0
0
0
8
12
0
28
37
100
100
45
30b
0
0
1,2,4-TCB
5
30
120
180
45b
48
20
0
0
19
14
9
0
0
10
8
0
0
0
4
8
0
0
0
19
51
91
100
100
0
1,3,5-TCB
15
30
45
60
30b
22
10
0
0
0
–
–
–
–
–
19
11
5
0
0
6
0
0
0
0
53
79
95
100
100
a At 60°C.
b Ultrasonic radiation (487 kHz).

J. G. Rodr´ıguez, A. Lafuente / Tetrahedron Letters 43 (2002) 9645–9647
9647
was not detected. (e) In all cases, optimal sonication for
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After the first catalytic cycle, the palladium, hydrazine,
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We are indebted to the CICYT of Spain for financial
support (project no. PB-97-0060).