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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for anodic coupling of perfluoroalkyl
iodides to fluorocarbons.
2. Relation to the Prior Art
Coupling of halogenated organic compounds is known. U.S. Pat. No.
3,317,618, to Haszeldine describes a process of coupling halogenated
organic compounds including perfluoroalkyl chlorides, bromides, and
iodides by a process in which the compound is subjected to energization to
raise its energy level sufficiently to cause fission of the carbon-halogen
bond with formation of a free alkyl radical. Forms of energization include
heat, molecular oxygen, ultraviolet, infrared, X, .gamma., or high energy
electron radiation, preferably in the presence of a halogen acceptor.
Bissell, J. Chem. Eng. Data, 10, 382 (1965), describes the coupling of
fluoroalkyl iodides to fluorocarbons by irradiation with ultraviolet light
in the presence of mercury.
These art processes suffer from the disadvantage that they require rigorous
conditions to bring about the homolytic cleavage of the carbon-iodine
bond, and require iodine scavengers to be present in order to go to a
practical degree of completion. In addition, they generate toxic metal
salts as by-products which present disposal problems and require
additional treatment to recover iodine values in usable form.
SUMMARY OF THE INVENTION
It has now been discovered that primary or secondary perfluoroalkyl iodides
of the structure
R.sub.f.sup.1 R.sub.f.sup.2 CFI
where R.sub.f.sup.1 and R.sub.f.sup.2 are each independently fluorine or a
perfluoroalkyl radical, may be anodically coupled in high yield to
fluorocarbons in the presence of an aliphatic carboxylic acid as
co-reactant.
Preferred as starting materials are compounds of structure I which contain
a minimum of 4 carbons and a maximum of 40 carbon atoms. Particularly
preferred are perfluoroalkyl iodides which contain 5-8 carbon atoms since
they give the highest yields of coupled products at high current
efficiencies.
Preferred as co-reactants are straight chain aliphatic carboxylic acids or
their perfluorinated analogs which contain 2-5 carbon atoms. Acetic acid
is most preferred because of its low cost and the relatively high
volatility of methyl iodide, formed as a product from this co-reactant.
The term "perfluoroalkyl" refers to a group derived from a saturated
aliphatic fluorocarbon, i.e., a hydrocarbon in which all hydrogen atoms
have been replaced by fluorine atoms, by removal of a fluorine atom.
DETAILED DESCRIPTION OF THE INVENTION
The anodic coupling process of the invention can be carried out under mild
conditions of temperature and pressure. The reaction probably proceeds by
iodine abstraction from the perfluoroalkyl iodide by alkyl radicals,
followed by combination of two perfluoroalkyl radicals to give the desired
fluorocarbon product. The alkyl radicals are in turn formed by an anodic
oxidation process in which a carboxylate is the electroactive species.
Such anodic oxidation processes, which in the absence of perfluoroalkyl
iodides lead to coupled hydrocarbon products, are generally classified as
Kolbe electrosyntheses. A recent review of Kolbe electrosynthesis
reactions by Eberson can be found in Organic Electrochemistry edited by M.
M. Baizer, Marcel Dekker, Inc., New York, 1973, p. 469.
Hence the overall process of the invention may be represented by equation
(1).
2R.sub.f.sup.1 R.sub.f.sup.2 CFI + 2R.sup.3 CO.sub.2 H .fwdarw.
R.sub.f.sup.1 R.sub.f.sup.2 CFCFR.sub.f.sup.1 R.sub.f.sup.2 + 2R.sup.3 I +
2CO.sub.2 + H.sub.2 ( 1)
it is not necessary that the process be limited to anodic coupling of a
single perfluoroalkyl iodide. Thus when the coupling reaction is applied
to a mixture of two or more iodides a mixture of all possible
fluorocarbons is obtained. Thus if the starting material is a mixture of
R.sub.f.sup.4 I and R.sub.f.sup.5 I, products R.sub.f.sup.4
-R.sub.f.sup.4, R.sub.f.sup.4 -R.sub.f.sup.5 and R.sub.f.sup.5
-R.sub.f.sup.5 would be formed. Coupling of mixtures of perfluoroalkyl
iodides is further illustrated by specific examples 8, 11 and 15 below.
The aliphatic carboxylic acid co-reactant can also be employed as a solvent
for the anodic coupling reaction and this is the preferred mode of
operation. Alternatively, a different solvent may be employed with at
least a sufficient amount of carboxylic acid present, i.e. at least an
equimolar amount based on the amount of perfluoroalkyl iodide, to satisfy
the stoichiometry of equation 1. Useful solvents include those which are
inert to the anodic oxidation process. Specific solvents include
acetonitrile, nitromethane and methylene chloride. It is preferred to
operate under essentially anhydrous conditions, but small quantities of
water, e.g., up to about 2%, do not interfere with the coupling process.
Larger quantities of water may lead to phase separation and/or
dehydrofluorination.
It is preferred to employ a concentration of perfluoroalkyl iodide of 5 to
40 weight percent of the reaction mixture.
The anodic coupling process also requires the presence of a small amount of
carboxylate ion, conveniently obtained by incorporation of an alkali metal
salt of the carboxylic acid co-reactant in an amount to give, preferably,
a 0.1M to 1.0M solution in the carboxylic acid.
The reaction temperature is not critical and temperatures of about room
temperature up to the boiling point of the solvent at atmospheric pressure
are conveniently employed, preferably 25.degree.-150.degree. C. Similarly
reaction pressure is not critical, but it is preferred to employ pressures
of atmospheric or below.
The process is conveniently carried out in an electrolysis cell. Suitable
materials of cell construction include glass, nickel and nickel alloys,
titanium, lead, polyethylene and polypropylene. Since neither starting
material nor products are reducible at less negative potentials than that
required for hydrogen evolution, it is not necessary to use a divided
cell. It is preferred to use anode/cathode area ratio >5 to prevent
reduction of R.sub.f I as a side reaction.
It is preferred to use as anode material platinum or palladium, most
preferably platinum. Preferred cathode materials include those materials
of low overvoltage for hydrogen evolution, e.g., platinum nickel, carbon
and steel.
As depicted in equation (1), the iodide values are recovered in the form of
alkyl iodides. Since these co-product iodides are also susceptible to
iodine abstraction by alkyl radicals, it is desirable to remove them from
the reaction mixture as rapidly as possible. Removal is conveniently
accomplished by fractional distillation of R.sup.3 I through a condenser
cooled to a temperature intermediate between the boiling points of
R.sub.f.sup.1 R.sub.f.sup.2 CFI and R.sup.3 I with subsequent condensation
in a cold trap. Hence the perfluoroalkyl iodide starting material
preferably has a boiling point at least 10.degree. C higher than the alkyl
iodide co-product. When reaction temperatures significantly below the
boiling point of the solvent are employed, it is desirable to employ
sub-atmospheric pressures to aid in removal of the alkyl iodide.
Since it is difficult to efficiently separate alkyl iodide co-product from
relatively low boiling perfluoroalkyl iodides, it is preferred to employ
perfluoroalkyl iodides which contain at least four carbon atoms as
starting materials.
Electrolysis is conveniently carried out at a specified constant current
density, achieved by application of an anode potential above the cricical
potential at which the Kolbe reaction starts, generally in the range of
2.1-2.8 volts (normal hydrogen electrode). Electrolysis is continued until
the pink color of the solution in the cell is discharged or until the
perfluoroalkyl iodide has been consumed as determined by a suitable
analytical technique such as gas-liquid partition chromatography. The
fluorocarbon product generally separates, upon cooling, as a separate
liquid or solid phase and the alkyl iodide co-product is recovered from
the cold trap.
The perfluoroalkyl iodides employed as starting materials may be prepared
by any of several well-known routes including oxidative decarboxylation of
perfluoroalkanoic acids, by addition of the elements of "IF" (e.g., from
IF.sub.5) to perfluoroolefins, or by telomerization of lower
perfluoroalkyl iodides with perfluoroolefins. These methods of synthesis
and specific perfluoroalkyl iodides preparable by them are discussed by
Lovelace, et al. in Aliphatic Fluorine Compounds, Reinhold Publishing
Corporation, New York, 1958.
UTILITY
The fluorocarbon products of the invention are known to possess outstanding
resistance to thermal and photochemical degradation because of the absence
of carbon-hydrogen bonds. In addition they exhibit, generally, a low
degree of mammalian toxicity. Such products are useful as thermally stable
heat transfer fluids, dielectric fluids, lubricants in corrosive and
high-temperature applications, as speciality plasticizers and as blood
substitutes.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following are illustrative examples of the invention in which all parts
and percentages are by weight and all degrees are Centigrade unless
otherwise stated. The conversions reported are calculated by the formula:
##EQU1##
EXAMPLE 1
Coupling of Perfluorohexyl Iodide
A mixture of 8.0 g of anhydrous sodium acetate, 200 g of glacial acetic
acid, 50 g of acetonitrile and 5.0 g of n-perfluorohexyl iodide was placed
in a 3-necked undivided electrolysis cell equipped with two platinum wire
electrodes. Current was passed at 270 mA while the cell temperature was
maintained at 48.degree.. After passage of 3500 coulombs of charge, the
reaction mixture was analyzed by gas-liquid partition chromatography
(glpc) on a 1/4 inch .times. 10 feet column of 15% fluorosilicone (QF-1)
on a diatomite support at 80.degree., 60 ml/min helium flow rate. The
analysis showed that .about.95% of the perfluorohexyl iodide had been
consumed, and a new peak was observed. A fluffy white crystalline solid,
formed during the electrolysis, was separated by filtration and it
amounted to 0.24 g of n-perfluorododecane, mp 73.degree.-75.degree.;
.sup.19 F nmr spectrum was identical with that of an authentic sample.
EXAMPLE 2
A mixture of 3.0 g of anhydrous sodium acetate, 50.5 g of glacial acetic
acid, and 10.28 g (0.023 mole) of perfluorohexyl iodide was placed in a
3-necked flask equipped with two platinum wire electrodes each of 2
cm.sup.2 active area. A Dewar reflux condenser, cooled by refluxing
trifluorotrichloroethane, was attached and it in turn was connected to a
trap cooled at -78.degree.. The solution was heated at reflux and current
was passed at 540 mA until passage of 3582 coulombs of charge, at which
point the solution was colorless. Glpc analysis showed 99 75.sup.(7)
11 14
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.sup.(1) R.sub.f I was added gradually from a dropping funnel during the
electrolysis.
.sup.(2) Coupled product was a mixture of C.sub.8 F.sub.18, C.sub.9
F.sub.20 and C.sub.10 F.sub.22 in a 1:5:4 ratio.
.sup.(3) Two layers were present during the electrolysis.
.sup.(4) n = 0-8; mixture obtained from telomerization of (CF.sub.3).sub.
CFI with tetrafluoroethylene.
.sup.(5) The product was a complex mixture of fluorocarbons.
.sup.(6) k = 3, 5, 7
.sup.(7) The product was a mixture of fluorocarbons.
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