Cleavage path way and the product distribution pattern during the electrochemical perfluorination of tripropylamine

Article abstract of DOI:10.1016/S0022-1139(02)00009-X

The electrochemical fluorination (ECF) of tripropylamine (TPA) was carried out in anhydrous hydrogen fluoride medium using a Simons type cell. Under optimum conditions, yield of perfluorinated product was about 51% and the selectivity of perfluorotripropy

Full text of DOI:10.1016/S0022-1139(02)00009-X

Journal of Fluorine Chemistry 115 (2002) 21–26
Cleavage path way and the product distribution pattern during
the electrochemical perfluorination of tripropylamine
D. Velayuthama,*, K. Jayaramana, M. Noela, S. Krishnamoorthya, P. Sartorib
aCentral Electrochemical Research Institute, CSIR, Karaikudi 630 006, India
bGerhard-Mercator University of Duisburg, FB-6-Inorganic Chemistry, Lothar Strasse-1, D-47057 Duisburg, Germany
Received 9 May 2001; accepted 23 December 2001
Abstract
The electrochemical fluorination (ECF) of tripropylamine (TPA) was carried out in anhydrous hydrogen fluoride medium using a Simons
type cell. Under optimum conditions, yield of perfluorinated product was about 51% and the selectivity of perfluorotripropylamine (PFTPA)
was about 87%. Even in this process involving the well-known starting material, apart from PFTPA, about nine perfluorinated by-products
were also identified. Perfluorinated products obtained were characterized by GC, 1H and 19F NMR spectra and were compared with reported
values. The perfluorinated products obtained support the fission and formation of C–C and C–N bonds. The nature of the minor products
formed also supports the involvement of cyclization and isomerization reactions during electrochemical fluorination. Suitable reaction
schemes for the formation of different perfluorinated by-products were suggested based on the identified products obtained from ECF of TPA
which clearly supports the free radical mechanistic pathway for this process. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Electrochemical fluorination; Tripropylamine; Perfluorotripropylamine
1. Introduction
2. Experimental details
Perfluorotripropylamine (PFTPA) has already found com-
mercial applications as an inert liquid in electronic industries
[1] and oxygen solubilising medium in bio-medical applica-
tions [2]. PFTPA is produced in fairly large quantities by
electrochemical perfluorination (ECPF) of tripropylamine
(TPA). However, there are only few reports on optimization,
product selectivity and mechanism of the process. Widely
different yields between 28 and 60% are reported [3–5].
Partially fluorinated starting materials have also been used
[6,7]. Fairly high cell voltages (upto 6.5 V) have also been
recommended [7]. Further work to understand some of the
issues cited is desirable.
Recently an effort was made to ascertain the pathway of
electrochemical fluorination (ECF) by identifying the pro-
duct distribution including the minor constituents obtained
from the ECF of N,N,N0,N0-tetramethylethylenediamine and
2-fluoropyridine [8,9]. A free radical pathway involving
highly reactive surface bound high valent nickel fluoride
has been suggested [9,10] by us and other workers [11–13].
In this work, an attempt is made to study the yield, product
selectivity and mechanistic pathway of ECF of TPA.
2.1. Electrochemical cell
A double walled 200 ml capacity stainless steel electro-
lysis cell with a pack of alternate nickel anodes and cathodes
was employed in the present study (effective anode
area ¼ 3:0 dm2). Temperature of the cell and the condenser
was maintained at 0 and À30 8C, respectively, using cryo-
stats. Volatile products were collected in a PTFE-FEP traps
keptat À78 8C using a dry ice–ethanol mixture at the outlet of
the cell. Liquid products from the cell were drained through a
ball valve at the bottom of the cell. A 1 l capacity (active
anode area ¼ 14 dm2) ECF cell was used in Exp. no. 7.
2.2. Electrochemical fluorination of tripropylamine
Pre-electrolysis was done at the start of each experiment in
order to dry anhydrous hydrogen fluoride (AHF) and activate
electrode surfaces. Pre-electrolysis was carried out for about
36–48 h till the initial current of 4 A reduces to 0.2 A. Cell
voltage was maintained between 5.0–5.5 V during this per-
iod. A required quantity of TPA and AHF mixture was
prepared separately before each addition of amine.
ECF was carried out under galvanostatic condition at
various current densities from 0.67 to 2.3 A dmÀ2. TPA
* Corresponding author.
0022-1139/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 0 2 ) 0 0 0 0 9 - X

22
D. Velayutham et al. / Journal of Fluorine Chemistry 115 (2002) 21–26
concentration of 6% (w/v) was taken initially and the
electrolysis was carried out in a Pre-electrolysed AHF
medium till the concentration of TPA reaches 1.5%. This
was done by passing the theoretical current of electricity
required to fluorinate 4.5% (9.0 g) of TPA. Required con-
centrations of TPA and AHF were maintained periodically.
After passing pre-determined electricity (100 or 105% of
electricity as the case may be) the electrolysis was stopped.
Crude perfluorinated products obtained were neutralised
with a dilute solution of sodium bicarbonate and washed
thoroughly with distilled water and finally anhydrous
sodium sulphate was added to remove moisture.
of the injector, column and the detector (FID) were held at
160, 100 and 200 8C, respectively. Helium was used as
carrier gas.
19F and 1H nuclear magnetic resonance spectra were
recorded for neat liquids without solvent using CD3CN
film in a Brucker WP 80SY spectrometer (80.1 MHz for
proton and 75.4 MHz for 19F). CCl3F and TMS were used
as internal references. The resonance at high field of the
reference was designated as negative.
3. Results and discussion
The AHF phase containing soluble partially fluorinated
amines was diluted with ice cold water and the partial
fluorinated amines were separated out as a heavy liquid.
They were neutralised, washed and dried.
3.1. Optimization studies
Typical results obtained during the ECF of TPA under
different experimental conditions are presented in Table 1.
In these experiments, the cell voltage was generally main-
tained around 5.0 V and even under very low reactant
concentrations, the cell voltage was not allowed to exceed
5.5 V. This was found to be a critical factor for maintaining
the activity of nickel fluoride anodic film and also for
controlling the activity of active fluorine and thus to mini-
mize decomposition products.
2.3. Separation and identification of perfluorinated
products
The mixture of perfluorinated products obtained from the
cell was separated by fractional distillation. About 64.8 g of
perfluorinated products obtained from ECF (Exp. no. 1) of
TPA was taken for fractional distillation. Distillation gave
the following three fractions.
When the theoretical quantity of electric current was
passed at 1.0 A dmÀ2, substantial quantities of partially
fluorinated products were found to remain in the HF phase
(Table 1, Exp. no. 1). This amount could be reduced and
simultaneously perfluorinated product could be increased by
increasingthetotalchargepassedto105%(Table1,Exp.no.2).
At lower current density even with 105% of theoretical
charge the partially fluorinated compound in HF phase
increased further (Table 1, Exp. no. 3). At the optimum
current density of 1.6 A dmÀ2, maximum yield of perfluori-
nated product with very little quantity of partially fluorinated
product in the HF phase could be achieved (Table 1, Exp.
no. 4). Further increase in current density, however, leads to
C–C and C–N bond cleavage and hence lower yields
although partially fluorinated products in the HF phase
are absent (Table 1, Exp. no. 5). Initial concentration of TPA
was increased from 6 to 9% to see if further improvement
2.3.1. Fraction–I (bp < 110 8C)
First drop was collected at 55 8C and there was steady rise
of temperature to 110 8C. About 3.61 g of products were
collected in this fraction.
2.3.2. Fraction-II (bp 122–127 8C)
About 48.43 g of perfluorinated products were collected
in this fraction.
2.3.3. Residue (>127 8C)
The residue contains about 12.73 g of perfluorinated
products.
Gas chromatograms were recorded on a Perkin-Elmer gas
chromatograph (10% Apiezon on chromosorb WAW, 100–
120 mesh, column length 2.5 m and 2 mm i.d.). Temperature
Table 1
Effect of experimental parameters on the yield of perfluorinated products
Exp. no.
Concentration
of TPA (%)
Total amount
of TPA added (g)
Current density
(A dmÀ2
Amount of
PFP obtained (g)
Amount of part.
fluoroamine obtained (g)
Yield of PFP
obtained (%)a
)
1
2
3
4
5
6
7
6
6
6
6
6
9
6
54.6
54.0
50.6
50.0
50.5
47.5
144.5
1.0
69.2
87.3
56.6
92.4
73.8
86.6
230.3
18.5
35
45
31
51
41
50
45
1.0
12.7
0.67
1.67
2.3
16.5
Nil
Nil
1.67
1.0
Nil
Black colour
Note: theoretical charge (100%) was passed for Exp. no. 1 and for other experiments five percent excess of charge (i.e. 105%) was passed. Cell
voltage ¼ 5:0–5.5 V. TPA: tripropylamine; PFP: perfluorinated products; part.: partially.
a Purified perfluorinated amines only.

D. Velayutham et al. / Journal of Fluorine Chemistry 115 (2002) 21–26
23
in the overall product yield could be achieved as shown in
Exp. no. 6 in Table 1. This procedure did not lead to any
improvement in overall product yield.
The optimisation study in the 200 ml capacity cell thus
suggests that under optimum experimental conditions a total
product yield of 51% can be easily achieved. These experi-
mental conditions also ensure minimum level of build up of
partially fluorinated organic compounds in the HF phase
ensuring prolonged continuous operation of the electroche-
mical reactor.
One experiment was also performed in the 1 l reactor to
obtain sufficient quantity of perfluorinated products for
further studies and also to assess the reproducibility. Under
similar conditions around 45% product yield could be
achieved (compare Exps. no. 2 and 7 in Table 1).
3.2. Product distribution
Under optimum conditions ECF of TPAwas also found to
be highly selective. An overall selectivity of 87% was
achieved (Table 2, Product 1). The product selectivity indeed
seems to depend very much on the operating conditions as
reported by different workers [3–7]. Such wide differences
are probably due to the complexity of the procedures
involved.
In the crude product obtained under the present experi-
mental conditions, proton NMR studies indicated only trace
levels of partially fluorinated compounds which could not be
quantitatively analysed. However, the presence of CHF2 and
CHF groups was confirmed by NMR studies. It, thus, appears
that under the mild experimental conditions suggested here
Table 2
19F NMR data of products obtained from ECF of tripropylamine and their selectivity
Product no.
Structure of the
products obtained
Chemical shifts (d; ppm vs.
CFCl3) observed
Angular momentum
(J; Hz)
Selectivity (%)
87.0
Reference
a
b
c
a
1
ðCF3ÀCF2ÀCF2ÀÞ3N
CF3 ¼ À81.5 br, s
3JðFaÀFbÞ ¼ 9:8 Hz
b
CF2 ¼ À121.0 sept.
c
CF2 ¼ À83.8 br, m
a
b
c
d
a
2
ðCF3ÀCF2ÀCF2ÀÞ2NÀF
CF3 ¼ À82.2
1.39
[14]
b
CF2 ¼ À126.7 d
3JðFc-dÞ ¼ 23:1 Hz
4JðFb-dÞ ¼ 19:9 Hz
4JðFa-cÞ ¼ 4:2 Hz
3JðFÀFÞ ¼ 8:6 Hz
c
CF2 ¼ À106.2 d sept.
NF ¼ À91.0 m
CF3 ¼ À84.0 t
3
4
CF3ÀCF2ÀCF3
[15]
[16]
CF2 ¼ À130.5 br, s
a
b
c
d
e
a,e
CF3 ¼ À81.8 m
CF3ÀCF2ÀCF2ÀCF2ÀCF3
1.67
b,d
CF2 ¼ À126.6 m
c
CF2 ¼ À123.3 m
5
6
CF3 ¼ À71.1 d
CF ¼ À180 m
3JðFÀFÞ ¼ 8:6 Hz
4JðFa-cÞ ¼ 9 Hz
[17]
a
b
c
d
a
ðCF3Þ2CFÀCF2ÀCF3
CF3 ¼ À72.7 d, t, q
0.56
0.84
[9,18]
CFb ¼ À186.8 m
c
CF2 ¼ À116.1 sept.
d
CF3 ¼ À82.3 d, t
a
b
c
d
a
7
ðCF3ÀCF2ÀCF2ÀÞ2NÀCF3
CF3 ¼ À81.9
3JðFa-bÞ ¼ 9:6 Hz
[19]
b
CF2 ¼ À124.7 sept.
c
CF2 ¼ À88.5 br, m
d
CF3 ¼ À50.7 m
8
9
ðCF3ÀÞ3N
CF3 ¼ À56.5 s
[8]
a
b
c
ðCF3ÀCF2ÀÞ2NÀCF3
CF3N ¼ À53.2 sept.
CF2N ¼ À91.2 m/q
3JðFa-bÞ ¼ 15:2 Hz
5JðFa-cÞ ¼ 7:6 Hz
[20]
a
CF3 ¼ À84.8 m
10
11
CF2 ¼ À133.4 s
[21]
a
CF3 ¼ À85.1 d
4JðFg-iÞ ¼ 16:6 Hz
3JðFh;iÞ ¼ 8:7 Hz
5JðFa-dÞ ¼ 7:9 Hz
7.48
b
CF2 ¼ À88.9 br
c
CF3 ¼ À75.0 d, m
CFd ¼ À156.5 d, br
e
CF2 ¼ À124 br
f
CF2 ¼ À126 d, br
CFg ¼ À162 d, br
h
CF2 ¼ À124 br
i
CF3 ¼ À80 d, t
Note: s, singlet; d, doublet; t, triplet; q, quartet; qui, quintet; sept., septet; br, broad; m, multiplet.

24
D. Velayutham et al. / Journal of Fluorine Chemistry 115 (2002) 21–26
Scheme 1. Reaction path for the formation of products from Ca–N bond cleavage.
formation of partially fluorinated compounds that are mis-
cible with perfluorinated products is indeed small. This
factor is quite important since removal of partially fluori-
nated compounds from the perfluorinated product is extre-
mely important [2] and quite sophisticated purification
procedures are necessary [11–13].
Although, the selectivity of PFTPA was quite high it was
still possible to identify at least 10 other perfluorinated
products by careful analysis of the different distilled frac-
tions. As suggested earlier during the ECPF of N,N,N0,N0-
tetramethylethylenediamine [8] and 2-fluoropyridine [9],
such a wide variety of side product formation is probably
due to bond cleavage and free radical re-arrangements of
partially fluorinated intermediates. Schemes 1–3 present the
cleavage pattern and the decomposition products identified
are marked with specific numbers. The NMR data of such
identified products are summarised in Table 2 along with
literature references wherever available.
The distribution of cleavage products clearly indicate
that all the three bond cleavages namely N–Ca, Ca–Cb
and Cb–Cg occur during ECF process. Product 2, for e.g.
is due to the cleavage of N–Ca bond (Scheme 1). Products 7
and 8 are formed due to the cleavage of Ca–Cb bond. Product
9 is obtained due to Ca–Cb and Cb–Cg bond cleavages
(Scheme 2).
During cleavage processes, perfluoromethyl, ethyl and
propyl radicals are also generated. These radicals can com-
bine among themselves to form a number of perfluorocar-
bons. The reaction pathways leading to the identified
perfluorocarbons are presented in reaction Scheme 1. Direct
combination of the radicals would lead to compounds 3 and
4. The transformation of primary perfluoropropyl radical
into a more stable secondary radical followed by further
reactions would lead to compounds 5 and 6 (Scheme 1).
Quite interestingly, cyclisation products are also formed
during the ECF of TPA. Substituted pyrrolidine compound
Scheme 2. Reaction scheme for the formation of products from b- and g-bond cleavages.

D. Velayutham et al. / Journal of Fluorine Chemistry 115 (2002) 21–26
25
Scheme 3. Schematic pathway for the formation of cyclic products from the ECF of tripropylamine.
11 is indeed found to be the most important by-product
accounting for 7.5% of the yield. This is, indeed, an isomer
of PFTPA and the possible reaction path way is indicated in
Scheme 3. Similar yield of a perfluoropyrrolidine derivative
with N-perfluoropropyl group has been indicated in an
earlier work [7]. However, our NMR data fits well with
the structure indicated in Table 2. Perfluorocyclopentane is
another interesting compound obtained in substantially
lower yield. This compound may be found through cyclisa-
tion followed by C–N bond cleavage as indicated in
Scheme 3.
boiling fractions. Most of the by-products from 2 to 10
were present in fraction-I. product 11 was identified in
fraction-II along with PFTPA. Some of the low boiling
Products like 3–6 and 8–10 were collected below 70 8C.
However, some of them remained trapped in the fluid up to
110 8C.
Since the present work only collected and analysed the
perfluorinated liquid fraction obtained through the bottom
discharge of the electrochemical reactor, the by-products
identified here should not be considered as exhaustive or
comprehensive. However, these products themselves pro-
vide substantial evidence for the free radical pathway of
overall ECF reaction [8–10]. Hence, by proper control of the
free radical generation at the electrode surface and by
optimising the reactant concentration in the electrolyte,
Table 2 contains some compounds like PFTPA which
possess fairly low boiling point. However, due to their strong
interaction with other perfluorinated products these com-
pounds still exhibit their presence in the relatively high

26
D. Velayutham et al. / Journal of Fluorine Chemistry 115 (2002) 21–26
substantial improvement in the yield and selectivity of
perfluorinated products can be achieved.
References
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4. Conclusions
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ECF of TPA in AHF gave perfluorinated amines as well as
partially fluorinated amines. Passing 5% excess of theore-
tical electricity increases the yield of perfluorinatedamine.
Higher yield of perfluorinated products (up to 51%) could be
obtained at a current density of 1.67 A dmÀ2 and by passing
105% of electricity. Under such conditions partially fluori-
nated products are almost absent while diluting the AHF
phase. Reproducible results are also obtained from a higher
capacity ECF cell.
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spectra. Selectivity of PFTPAwas about 87%. Ten other side
products were isolated, which indicated the cleavage of
C–C, C–N bonds and formation of C–C bonds, cyclisation
and isomerisation reactions. These observations lend further
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The authors wish to thank the Director, CECRI, Karaikudi
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