Electrochemical fluorination: State of the art and future tendences

Article abstract of DOI:10.1016/j.jfluchem.2003.07.002

A brief survey of the ECF process for the preparation of perfluorinated organo-compounds is given. The yield of perfluorinated products depends on the experimental conditions and on the nature of the starting material. Some useful rules are reported in order to optimise the ECF process. An improved operational technique allowed to obtain high yield of perfluorodimorpholinepropane (PFDMP) by simultaneous fluorination of dimorpholinepropane and N -methylmorpholine.

Full text of DOI:10.1016/j.jfluchem.2003.07.002

Journal of Fluorine Chemistry 125 (2004) 139–144
Electrochemical fluorination: state of the art
and future tendences
Lino Conte*, GianPaolo Gambaretto
Dipartimento Processi Chimici dell’Ingegneria Via Marzolo 9, 35100 Padua, Italy
Received 5 February 2003; received in revised form 17 July 2003; accepted 28 July 2003
Abstract
A brief survey of the ECF process for the preparation of perfluorinated organo-compounds is given. The yield of perfluorinated products
depends on the experimental conditions and on the nature of the starting material. Some useful rules are reported in order to optimise the ECF
process. An improved operational technique allowed to obtain high yield of perfluorodimorpholinepropane (PFDMP) by simultaneous
fluorination of dimorpholinepropane and N-methylmorpholine.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Electrochemical fluorination; Simons process; Perfluorodimorpholinepropane
1. Introduction
2. Results and discussion
Fluorine containing organic compounds and perfluori-
nated organic compounds are useful starting materials for
a wide range of practical and industrial applications. Sur-
factants, refrigerants, lubricants, fire extinguishes, oxygen
carriers, and polymers are only some of the most important
application.
As to perfluorinated compounds, the Simons process
[1,2] plays a key role in the synthesis of these materials.
By this method a wide range of organic substrates can be
perfluorinated in a single step. Since nearly 60 years,
electrochemical fluorination is used as a versatile method
in the production of perfluorinated compounds bearing
different functional groups. Results in this field are
reported in a great number of papers, patents and reviews
[3–8].
2.1. Electrochemical fluorination technique
According to Simons method, an organic substance
undergoes electrolysis in anhydrous hydrogen fluoride at
cell voltages of 5–6 V. Slightly soluble materials can be
fluorinated using a suspension or emulsion of the substrate in
the presence of conductivity additives.
In our experience, when slightly soluble material are used
forced circulation of the electrolyte increases the yield. With
cell voltage in the range 5–6 V the organic substrate can be
oxidised at the anode and hydrogen can be discharged at the
cathode, under these conditions the evolution of gaseous
fluorine does not normally occur.
To perform the ECF process it is important that Fꢀ is the
only anion in solution. If the electrolytic solution contains
others ions, e.g. Clꢀ and OHꢀ, their discharge is favoured
despite the organic compound oxidation and reduced yield
of the desired perfluorinated product was obtained.
Compounds containing nitrogen, sulphur, phosphorous
and other heteroatom, can be protonated in hydrogen fluor-
ide giving rise to conductive solutions.
At the ‘‘Fluorine Chemistry Laboratory’’ of Padua Uni-
versity the electrochemical fluorination was widely studied.
Several perfluorinated carboxylic and sulfonic acid fluor-
ides, amines and some heterocyclics have been prepared.
Considerable efforts are also devoted to understand the
reaction mechanism.
In some cases, compounds providing high electric con-
ductivity solutions can not be fluorinated efficiently. Car-
boxylic acid, aldehydes, ketones, and alcohols are very
soluble in hydrogen fluoride but, during the electrofluorina-
tion, they may forms water that may have deleterious effects
on the process yield. The ECF process presents a number of
* Corresponding author. Tel.: þ39-049-8272555;
fax: þ39-049-8272557.
E-mail address: fluoro@unipd.it (L. Conte).
0022-1139/$ – see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2003.07.002

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L. Conte, G. Gambaretto / Journal of Fluorine Chemistry 125 (2004) 139–144
advantages over other fluorination methods: among these,
the possibility to obtain perfluorinated compounds main-
taining the functional group of the starting compound
represents one of the most relevant one. On the contrary,
the main disavantage is that low yields are often obtained
and this asks that reaction conditions are optimized in
relation to the specific substrate under study.
ꢁ before charging the organic substrate, the anode surfaces
must be activated by a pre-electrolysis carried out at 5–
6 V, until the current drops to zero;
ꢁ a non-stop experiment is better than discontinuous ones.
In any case, if the experiment is interrupted, a voltage of
3–4 V must be applied to prevent the depolarisation of the
electrodes;
Another serious problem is products purification, in par-
ticular in the case of perfluorocarbons to be used for artificial
blood formulations or other pharmaceutical applications,
which must not contain toxic impurities.
ꢁ the electrodes must be completely dept into the electro-
lyte, otherwise in the emerging part a film of materials
forms. It cannot be removed also after a new immersion,
but only through a mechanical polishing;
ꢁ anhydrous hydrogen fluoride must be charged during the
experiment. Water forms F2O and degradation processes
of the substrate occur;
2.2. Experimental conditions
The electrolytic cells used are fairly simple and instru-
mentation and operating conditions have been described.
We focused some features and details of the electrolytic
cell construction on the basis of our experiences in ECF
processes. Table 1 reports some prototype substrates the
electrofluorination of which has been performed in Padua.
Iron and nickel are commonly used as material for cathode
and anode. Platinum anodes have been also used in our
experiment in the morpholine fluorination, but this material
rapidly dissolved forming hydrogen hexafluoroplatinate.
To optimise the ECF process, simple rules should be taken
into account. The electrodes packs were arranged to give a
uniform anode current density and to act as baffles for the
mixed flow. The length/width ratio of the electrodes can be
in the range from 2/1 to 3/1 in laboratory cells and increased
to 5/1 in industrial cells.
In monopolar electrodes assembly, Teflon spacer were
used to keep the electrodes apart at 3.5–4.5 mm distance.
In large industrial cells, a series of smaller electrodes
packs has to be preferred over single large pack. This simple
trick in the electrodes assembling, reduces concentration
polarization phenomena: in fact hydrogen evolved at the
cathode favours the electrolyte mixing.
ꢁ for a better control over the fluorination process the
substrate must be added to the solution at the same rate
of the fluorination reaction.
The ECF process efficiency is strictly correlate to the
experimental conditions, i.e. temperature, substrate concen-
tration, stirring and cell voltage (or current density).
Substrate concentration ranging from 1 to 50% are
reported; however, when higher concentrations are used,
lower yields occur. The best concentration is characteristic
of the substrate: concentration greater than 15% should be
avoided in any case.
According to our experience, the best value is determined
from the need to obtain electrolyte with high conductivity,
but at the lowest possible concentration.
In the ECF of some sulphonylfluorides the yields can be
improved starting with substrate concentration of about 10%
and adding every three hours the quantity of substrate
consumed, calculated on the current flowed.
As for regard the cell voltage, values ranging from 5 to
6 V are preferred. It is not reasonable to run the process at
very low voltage, even if yields might be higher. Since as the
conductivity of the solution is fixed the voltage determines
the current; the current determines the reaction time and for
practical reasons it is not desirable that the reaction takes a
longer time.
However the lower voltage must be sufficient to produce a
quantity of hydrogen sufficient to stir the solution and to
reduce the concentration polarisation. For any cell with
Different electrodes assembling, but with the same anode
surfaces, exhibit a minimum current density value, under
whichextensiveconcentrationpolarizationphenomenaoccur.
An electrode pack can be used over a period that depends
from the substrate to be fluorinated but that can be increased
according to the following rules:
Table 1
Typical substrate submitted to ECF at fluorine chemistry laboratory, Padua University
Carboxylic acid
Sulfonic acid
N-containing compounds
Others
Acetic
Methanesulfonic
Ethanesulfonic
Sulfolane
Tripylamine
Butyl ether
Phenyl ether
Butyl sulphide
Uracil
Propionic
Butyric
Tributylamine
N-ethylpiperidine
Octanoic
2-Ethylhexanoic
Benzoic
Hexanesulfonic
Octanesulfonic
Benzenesulfonic
Toluenesulfonic
Ethylbenzenesulfonic
N-methylmorpholine
1,3-Dimorpholinepropane
N,N-diethylcyclohexylamine
N-ethyldicyclohexylamine

L. Conte, G. Gambaretto / Journal of Fluorine Chemistry 125 (2004) 139–144
141
Table 2
Effects of current density in the ECF of octanoyl fluoride
Experiment
number
Organic
Voltage
(V)
Current
density (mA/cm2)
Current
Products
Yield on
Yield on organic
added (g)
passed (%)
obtained (g)
current (%)
added (%)
1
2
3
756
756
5.1–5.25
5.2–5.35
5.4–5.7
22
30
40
61
70
70
1050
1264
1851
80.7
85.2
90.3
49
58.7
62.8
1035
electrodes fixed area, applied voltage, substrate concentra-
tion and temperature are the operating conditions which
determine current density.
In some cases high current density increase the yield in
perfluorinated products. Table 2 collects our results obtained
in the ECF of octanoyl fluoride carried on at different current
density; the increase in current density was obtained increas-
ing the applied voltage.
There are two opposing factors effecting the choice of
temperature. Higher operating temperature allowed to
obtain higher conductivities of the HF solutions, but, at
the same time increased the by-products formation. Experi-
mental data [9] showed that the temperature is the parameter
with a remarkable influence over perfluorotripropylamine
yield and by-products formation; low temperature operation
resulted in a less coloured system, due to less tar formation.
radical-cation that after a proton elimination in a chemical
step (Cb) forms a radical, which is then further electroche-
mically oxidized (E) to a carbocation which reacts with a
fluoride ion (Cn) forming a C–F bond.
Our experiments [20–28] carried out in the usual Simons
cell, in particular over carbonyl and sulfonyl fluoride, tri-
propylamine and N-methylmorpholine are in agreement
with the hypothesis that fluorination reaction proceeds via
a series of alternate electrochemical-chemical steps.
During the ECF experiments over N-methylmorpholine
and tripropylamine, samples of electrolyte were collected
from the cell after passing a quantity of electricity corre-
sponding to the discharge of about 1.5 and 2.5 mol of
fluorine per mole of the substrate.
In the case of N-methylmorpholine in Table 3 the products
identified in the reaction mixture are reported.
The substitution of hydrogen atoms by fluorine, takes
place in N-methylmorpholine preferably at the carbon atoms
close to the oxygen, where the electron density is greater.
Thus the substitution of the first hydrogen atom could
occur as reported in Scheme 1, according to the ECbECn
mechanism.
The second fluorine atom introduction occurs through a
similar mechanism and the repetition of the process, until the
organic remains on the anode surface, leads to perfluorina-
tion of the molecule Then the molecule is desorbed and
moves into the solution.
That mechanism proposed explains two important obser-
vations, i.e. N-methylmorpholine was always found in the
reaction mixture together with the perfluoro derivative,
therefore the fluorination proceeds until perfluorinated pro-
duct is formed.
2.3. ECF mechanism
The precise mechanism of the Simons process remains a
matter for discussion despite the considerable efforts.
Simons [10] proposed a radical mechanism in which
fluoride ion is oxidized at the anode to produce fluorine
atom that reacts with hydrogen in the organic substrate by
homolytic substitution.
This pathway does not explain all the experimental
observations. For this reason a great number of workers
studied the ECF and proposed different explanations of the
reaction evidences.
A group of authors thought that fluorination is carried out
by inorganic fluorinating agents [11–14], i.e. NiF2 and NiF3,
electrochemically formed on the surface of the nickel anode
or as consequence of anodic absorption of complex formed
between nickel fluorides and the substrate [15]. The
observed induction period has been attributed to the forma-
tion of such fluorinating agents.
Other authors proposed that fluorination proceeds via a
series of steps; started by the direct anodic oxidation of the
substrate [16–18].
Table 3
Percentage of identified products in the reaction collected samples after the
discharge of 1.5 mole (column I) and 2.4 mole (column II) of fluorine per
mole of N-methylmorpholine
Compound
Column I
(%)
Column II
(%)
Rozhkov, on the basis of hypothesis of Burdon et al. [19],
proposed the ECbECn mechanism based on experimental
results. However must be observed that Rozhkov drown his
conclusions on the ECF mechanism from experiments car-
ried out using aprotic solvents and platinum anodes, i.e.
experimental conditions different from the Simons process.
In the ECbECn mechanism, the organic substrate adsorbed
on the anode is electrochemically oxidized (E) to form a
N-methylmorpholine
61.3
19.6
0.9
38.3
28.2
1.9
2-Fluoro-N-methylmorpholine
2,2-Difluoro-N-methylmorpholine
2,5-Difluoro-N-methylmorpholine
2,6-Difluoro-N-methylmorpholine
2,2,6-Trifluoro-N-methylmorpholine
2,2,6,6-Tetrafluoro-N-methylmorpholine
Perfluoro-N-methylmorpholine
0.2
0.6
4.4
9.1
1.6
0.5
4.4
1.5
6.5
11.0

142
L. Conte, G. Gambaretto / Journal of Fluorine Chemistry 125 (2004) 139–144
compounds the ECF can be performed charging a partially
fluorinated substrate. The presence of a fluorine atom
increases the acidity of the hydrogen atom and the solubility
of the compound. The use of partially fluorinated precursor
also prevents isomerisation and cyclisation reactions [30].
For the same aim, the insoluble substrate can be added to
the cell as a mixture with an hydrogen fluoride soluble
solvent, a simultaneous fluorination occurs.
In a similar manner can be carried out the fluorination of
volatile compounds not very soluble in hydrogen fluoride. A
mixture of volatile compound with a higher boiling com-
ponent can be charged in the cell [31]. Mixed component
technique is still interesting in the fluorination of substrates
soluble in HF but forming solid perfluorinated materials.
An example of this technique is the ECF of dimorpho-
linepropane, which when perfluorinated, is a solid that
cannot be removed from the cell and for this reason must
be produced in a discontinuous manner.
Scheme 1.
In the case of tripropylamine see Table 4 the fluorination
takes place at the mostly hydrogenated position of the
molecule that is the farthest position from the nitrogen atom
and proceeds step by step until perfluorotripropylamine is
formed. Unreacted tripropylamine and perfluorotripropyla-
mine are still present in the mixture. Also in the ECF of N,N-
diethylcyclohexylamine residual hydrogen is always located
in the cyclic part of the molecule and bonded to the carbon
atom close to nitrogen atom.
Increasing the number of fluorine atoms in the molecule,
the basicity of the substrate decreases. As a consequence
desorbtion of the molecule is in competition with the
complete fluorination of the molecule. This fact can explain
that partially fluorinated molecules are always detected in
the reaction mixture.
3. Experimental details
3.1. ECF cell
Furthermore, during the ECF process, the observed iso-
merisation and cyclisation of the substrate, and the sequence
of yields observed in the series benzenesulphonyl, p-tolue-
nesulphonyl chlorides or tripropylamine–tripentylamine can
be moreover explained with the ECbECn mechanism.
However, the complete understanding of the Simons ECF
process mechanism is far from completeness. In fact, the
radical mechanism cannot be completely excluded, as well
as ECbECn mechanism completely confirmed.
A 900 ml stainless steel cylindrical cell with a pack of
alternate nickel cathodes and anodes was employed in the
present study. The effective anodes area was 630 cm2.
The cell was provided with a liquid level indicator and
with a condenser maintained at ꢀ40 8C to condense the
hydrogen fluoride in the gas stream exiting from the cell.
3.2. Analytical procedures
2.4. New technologies
Gas-chromatrograms were recorded on a Carlo Erba
instruments GC 6000 Vega Series 2 chromatograph, using
a stainless steel column (2:5 m ꢂ 2 mm) fitted with Rtz100
on silcoport P100-20 helium was used as carrier gas.
1H NMR and 19F NMR were recorded on Varian XL200
instrument, using TMS and CFCl3 as internal standard.
GC–MS were recorded on a Carlo Erba Instruments
MFC500/QMD 1000 using a silica fused capillary PS264
column (30 m ꢂ 0:25 mm).
The research in the ECF is mainly devoted to assembly
cells characterized by an efficient stirring of the electrolyte.
Advantages of innovations into the circulatory system are
reported [29] and recently we have built a cell with a natural-
circulation system (thermosiphon circulation) that enabled
us to obtain an increasing in the yield of perfluorosulfonyl
compounds. Experiments are still in progress.
Moreover interest in the fluorination of compounds which
are not very soluble in hydrogen fluoride goes on. For these
3.3. ECF of dimorpholinepropane
Table 4
Percentage of fluorinated groups determined by 19F NMR in the samples
obtained after discharge of 1.5 mole (column I) and 2.5 mole (column II)
of fluorine per mole of tripropylamine
PFDMP is a suitable material in forming and stabilizing
emulsions for applications in artificial blood formulations.
ECF experiments carried out by usual Simons technique,
charging dimorpholinepropane, allowed us to obtain yields
from 20 to 30% in the perfluorinated product. A wide range
of partially fluorinated compounds were also present.
Because of a solid fluorinated material forms in the reaction,
it cannot be collected from the cell bottom. The experiments
must be stopped and the organic compound recovered after
hydrogen fluoride remotion. In order to complete the fluor-
ination of the recovered partially fluorinated products a
Identified fluorinated groups
Column I (%)
Column II (%)
CH2F–CH2–CH2–
CHF2–CH2–CH2–
CH3–CHF–CH2–
CH2F–CHF–CH2–
CHF2–CHF–CH2–
CH2F–CHF–CH2–
CHF2–CHF–CH2–
44
17
39
–
26
15
21
7
–
4
–
Total 7

L. Conte, G. Gambaretto / Journal of Fluorine Chemistry 125 (2004) 139–144
143
Scheme 2.
mixture of dimorpholinepropane (80%) and partially fluori-
nated material (20%) was feed into the cell and the electro-
fluorination carried out. The perfluorodimorpholinepropane
was obtained with a yield of about 20%. This low yield is
probably due to removal of the formed partially fluorinated
products favoured by the charged partially fluorinated mate-
rials.
If dimorpholinepropane is charged as a mixture with N-
methylmorpholine or sulpholane surprising results were
obtained. A mixture of liquid fluorinated material can be
continuously collected from the cell bottom. For this reasons
the experiments can be performed for a longer time.
Improved yields of perfluorodimorpholinepropane were
obtained and the partially fluorinated dimorpholinepropane
can be recycled.
After distillation of perfluoro N-methylmorpholine
(PFNMM) 392 g of crude fluorinated DMP were obtained.
Treating the crude product with acetone and then cristalliz-
ing from Freon 113, 200 g of white crystalline 96% pure
PFDMP were obtained with a yield of 37.4%. GC–MS, m/z
(relative intensity): 591 [M–F]þ (1), 380 [C7F14NO]þ (3),
280 [C5F10NO]þ (82), 169 [C3F7]þ (55), 164 [C3F6N]þ (27),
119 [C2F5]þ (100), 114 [C2F4]þ (75), 100 [C2F4]þ (68).
The acetone soluble material (160 g) is a mixture of
partially fluorinated DMP and it was solubilized with
160 g of NMM and then submitted to ECF; 370 g of crude
mixture containing 20% of PFDMP were obtained (PFDMP
total yield 52%), Scheme 2.
In the reported cell, 340 g (53% mixture of dimorpholi-
nepropane DMP with N-methylmorpholine NMM) were
charged and electrolysed in 7 days with a voltage of
5.1–5.4 V and a current density of about 19 mA/cm2.
When 102% of the theoretical quantity of electric current
was passed, a total of 640 g of fluorinated mixture was
collected, containing 34% of perfluorodimorpholinepropane
(PFDMP).
4. Conclusions
Electrochemical fluorination will securely continue to be
arguments of investigation because of the industrial interest
toward completely fluorinated materials, often difficult to
obtain by other methods. This intrinsic feature of the Simons
process together with relatively simple equipment, the use of
relatively cheap raw material to produce high added value

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L. Conte, G. Gambaretto / Journal of Fluorine Chemistry 125 (2004) 139–144
[9] L. Conte, M. Napoli, G.P. Gambaretto, J. Fluorine Chem. 30 (1995)
89.
products bring advantages to industrial chemistry also if the
perfluorinated products are obtained with low yield.
The method is still an art, yields and ratios of products
obtained vary not only from compound to compound but
also from experiment to experiment for reasons not well
understood.
Understanding of the mechanism is far from complete-
ness. Despite these difficult the ECbECn concept can help to
explain the experimental evidence in ECF of various classes
of compounds.
[10] J.H. Simons, Fluorine Chemistry, Accademic Press, New York,
1650.
[11] T. Gramstad, R.N. Haszeldine, J. Chem. Soc. 173 (1956).
[12] R.N. Haszeldine, F. Nyman, J. Chem. Soc. (1956) 2684.
[13] P. Sartori, N. Ignat’ev, S. Datsenko, J. Fluorine Chem. 75 (1974) 157.
[14] P. Sartori, N. Ignat’ev, J. Fluorine Chem. 87 (1998) 157.
[15] J. Burdon, J.C. Tatlow, Advances in Fluorine Chemistry, vol. 129,
Butterworths, London, 1960.
[16] H. Meinert, R. Fackler, J. Mader, R. Beuter, W. Rohlke, J. Fluorine
Chem. 51 (1991) 53.
[17] H. Schmidt, H. Meinert, Angew. Chem. 72 (1960) 109.
[18] I.N. Rozhkov, Russian Chem. Rew. 45 (1976) 615.
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The experimental conditions to improve the yield must be
checked for each organic substrate.
Advantages can be obtained mixing the electrolyte and
performing ECF with the mixed component technique as
reported for the production of PFDMP.
[21] G.P. Gambaretto et al., Atti e Memorie Accademia Patavina di
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[23] C. Fraccaro, M. Napoli, L. Conte, Atti. Ist. Ven. SS.LL.AA.
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