Pyrolytic decarboxylation of some derivatives of perfluorinated mono- and dicarboxylic acids

Article abstract of DOI:10.1007/s11167-005-0577-4

Pathways of pyrolysis of perfluorinated carboxylic acids are considered in relation to the structure of the acids and reaction conditions. The reaction mechanism is discussed.

Full text of DOI:10.1007/s11167-005-0577-4

Russian Journal of Applied Chemistry, Vol. 78, No. 10, 2005, pp. 1640 1645. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 10, 2005,
pp. 1668 1673.
Original Russian Text Copyright
2005 by Lebedev, Berenblit, Starobin, Gubanov.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Pyrolytic Decarboxylation of Some Derivatives
of Perfluorinated Mono- and Dicarboxylic Acids
N. V. Lebedev, V. V. Berenblit, Yu. K. Starobin, and V. A. Gubanov
Lebedev Research Institute of Synthetic Rubber, Federal State Unitary Enterprise, St. Petersburg, Russia
Received July 7, 2005
Abstract Pathways of pyrolysis of perfluorinated carboxylic acids are considered in relation to the structure
of the acids and reaction conditions. The reaction mechanism is discussed.
As shown in [1, 2], pyrolytic decarboxylation of
CF3
salts of linear perfluorinated carboxylic acids gives ter-
CnF2n + 1OCFCO2M
minal fluoroolefins in high yields, reaching 90 mol %
in the case of sodium salts:
120 160 C, diglyme
CnF2n + 1OCF=CF2 + CO2 + MF,
250 300 C
CnF2n + 1CF2CF2CO2M
CnF2n + 1CF=CF2
In this study, we examined in detail how the struc-
ture of perfluorinated carboxylic acid derivatives with
various - and -substituents affects the pyrolysis
pathway.
+ CO2 + MF; (M = Na+, K+, ..., Ca2+...).
Similarly, terminal perfluoroalkoxyalkenes are
formed in high yields by pyrolysis of sodium salts of
perfluoro- -alkoxyalkanoic acids (from butyric to
caproic) [3].
The pathway of pyrolysis of perfluoro-2-alkoxy-
propionyl fluorides in absolute diglyme at 120 C in
the presence of anhydrous sodium carbonate depended
on the ratio of perfluoroacyl fluoride and sodium
carbonate:
Perfluorinated alkyl vinyl ethers have also been ob-
tained in high yields [4] by pyrolysis of solutions of
perfluoro-2-alkoxypropionic acid salts in diglyme:
>1.0, Na2CO3
0.95RFOCF=CF2
II
CO2, NaF
CF3
RFOCFCOF
120 C, diglyme
+ RFOCFCF=CFOR
F3C
CF3
I
CF3
0.5, Na2CO3
0.45RFOCFCCFORF
IV
CO2, NaF
O
III
RF = CF3 (a), CF3CF2CF2 (b), CF3O(CF2)3 (c), C3F7OCFCF2 (d).
CF3
The figure shows how the yields of perfluorinated
The first step of the reaction involves formation of
sodium perfluoro-2-alkoxypropionate along with NaF
and CO2. The pyrolytic decarboxylation proper pre-
sumably involves formation of the reactive inter-
mediate carbanion:
alkyl vinyl ethers, their linear dimers, and the corre-
sponding bis(perfluoro-1-methyl-2-oxaalkyl) ketones
depend on the molar ratio of sodium carbonate and
acyl fluoride.
1070-4272/05/7810-1640 2005 Pleiades Publishing, Inc.

PYROLYTIC DECARBOXYLATION OF SOME DERIVATIVES OF DICARBOXYLIC ACIDS
1641
strate I to form ketone III. Also, we identified
among the pyrolysis products small amounts of the di-
mers IV of perfluorinated alkyl vinyl ethers, which
suggests the possibility of the reaction of the carban-
ion with II.
-
-
-
F3C
CF3
+
Na
RFOCF
I
RFOCF=CF2
RFOCFCCFORF
NaF
NaF
CF3
II
O
III
-
II NaF
The structures of the products were confirmed by
bromination across the double bond. The 19F NMR
data for the compounds identified are listed in the
table.
RFOCFCF=CFORF
CF3
IV
which, depending on the reactant ratio, can either
eliminate the fluoride anion to form perfluorinated
alkyl vinyl ether II or react with the initial sub-
Gubanov et al. [5] showed that dry pyrolysis
of sodium perfluoro-3-methoxypropionate follows
the scheme
270 C
CF3OCF2CF2COONa
[CF3OCF2CF2] Na+
0.26 CF3OCF=CF2
CO2
NaF
V
0.24F2C=CF2 + [CF3O] Na+
COF2
0.35CF3OCF2CF2COF.
V
NaF
NaF
Experiments on pyrolysis of V in diglyme at 120 C
showed that the perfluoro-3-oxaalkyl anion generated
by decarboxylation can eliminate either the fluoride
ion from the -position or (major pathway) the per-
fluoromethoxide anion, with its subsequent transfor-
mation into carbonyl difluoride. The yields of per-
fluorinated methyl vinyl ether and tetrafluoroethyl-
ene were 30 and 60 mol %, respectively. With Na+
replaced by K+, the yield of tetrafluoroethylene in-
creases from 60 to 95 mol %, suggesting stabilization
of the trifluoromethoxide anion with the potassium
ion, in agreement with the data of [5, 6].
The formation of tetrafluoroethylene as the major
product of pyrolysis of perfluoro-3-oxaalkanoic acid
derivatives has been observed previously [7] in pyrol-
ysis of sodium perfluoro-4-oxaheptanedioate at 250 C,
when the monodecarboxylation product, perfluoro-3-
vinyloxypropionyl fluoride, was obtained in a less
than 2% yield. This product isomerized at room tem-
An unexpected result was obtained in pyrolysis of
2-bromotetrafluoropropionyl fluoride VI in dry di-
glyme at 120 C in the presence of anhydrous sodium
carbonate. Instead of the expected trifluorobromo-
ethylene, we detected 1,1-dibromotetrafluoroethane
VII in the reaction products by 19F NMR (see table)
and mass spectroscopy. Apparently, the bromotetra-
fluoroethyl anion generated by decarboxylation of
the 2-bromotetrafluoropropionate anion is stabilized
by abstraction of the bromine cation from the second
substrate molecule, rather than by elimination of
the fluoride or bromide anion:
120 C, Na2CO3,
+
diglyme
CF3CFBrCOF
[BrCFCF3] Na
Yield
of pyrolysis products of perfluoro-2-alkoxy-
VI
propionyl fluorides vs. the molar ratio N of sodium car-
bonate to acyl fluoride: (1) perfluorinated alkyl vinyl ether
II, (2) perfluoro ketone III, and (3) dimer IV of per-
fluorinated alkyl vinyl ether.
NaF
0.45Br2CF CF + 0.45F2C=CF2
BrCF=CF2
VII
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 10 2005

1642
LEBEDEV et al.
19F NMR spectra of products formed in pyrolysis of perfluorinated carboxylic acid derivatives
*
JF
F*,
Compound
Position
CCl D, ppm
Hz
nF
3
Perfluorinated 4-oxaamyl vinyl ether IIc
1
2
3
4c
5t
6
55.5
83.9
84.2
114.2
121.5
128.3
134.8
F1 F3, 10
3
2
2
1
1
2
6
F2 F1, 10; F2 F5; 7, 6
F3 F1, 10; F3 F2, 10
F4 F5, 87; F4 F7, 67;
F5 F7, 112; F5 F4, 87; F5 F2, 6
s
5F
C1F3OC3F2C6F2C2F2OC C
7F 4tF
7
F7 F5, 111; F7 F4, 67; F7 F2,
Bis(perfluoro-1-methyl-2,6-dioxaheptyl)
1
2
3
4
5
6
7
54.6
76.5
79.7
81.1
84.6
F1 F5, 10
3
1
3
1
2
2
1
ketone IIIc
F2 F4, 150
2F
C3F3
s
F4 F2, 150
(C1F3OC5F2C6F2COC7F)2C O
4F
F5 F1, 10
127.8
135.2
s
m
Perfluoro-9-methyl-2,6,10,14-tetraoxapentadec-
1
2
3
4
5
6
7
8t
9t
55.0
76.7
79.9
81.3
84.5
127.8
130.5
133.1
165.5
m
6
1
3
1
6
4
1
1
1
7-ene IVc
F2 F4, 148
C3F3
2F
s
F4 F2, 150
C1F3OC5F2C6F2COC7F
m
m
m
C
9tF
4F
8t
C1F3OC5F2C6F2C5F2OC F
F8 F9, 120
F9 F8, 120
1,1-Dibromotetrafluoroethane VII
1
2
76.6
81.7
F1 F2, 10, q,
F2 F1, 10, d,
1
3
C2F3C1FBr2
3F 4F
Perfluoro-2-methyloxalan-3-one
1
2
3
4
5
6
79.1
81.0
92.5
120.2
120.9
129.8
F1 F3 125; F1 F5 10
s
1
3
1
1
1
1
1F C C F6
VIII
F3 F1 125; F3 F4
8
O
C O
F4 F6 280 F4 F3
s
8
5F
C
C2F3
F6 F4 280; F6 F1 10
perature under the action of CsF into a cyclic ketone,
perfluoro-2-methyloxalan-3-one VIII:
Accordingly, we detected in the experiments on
the pyrolysis of perfluoro-2-methyl-3-oxahexanedioic
acid difluoride IX (its acyl moiety is isomeric to that
of sodium perfluoro-4-oxaheptanedioate) in diglyme
at 120 C in the presence of an equimolar amount of
sodium carbonate, along with sodium trifluoroacetate
and tetrafluoroethylene, also compound VIII formed
in 15 mol % yield:
NaOOCCF2CF2OCF2CF2COONa
0.97F2C=CF2
250 C
NaF, CO2
0.015FOCCF2CF2OCF2CF2COF
0.015CF2=CFOCF2CF2COF
CF3
CF3
60 140 C,Na2CO3,
diglyme
CsF, 25 C
NaOOCCFOCF2CF2COF
FOCCFOCF2CF2COF
CO2, NaF
CF3
CF3
CF
CF
O
0.8CF2=CF2 + 0.8CF3COONa + O
CO2, NaF
O
O
VIII
F2C
F2C
CF2
CF2
VIII
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 10 2005

PYROLYTIC DECARBOXYLATION OF SOME DERIVATIVES OF DICARBOXYLIC ACIDS
1643
The product was identified by the characteristic 19F
NMR signals of magnetically nonequivalent geminal
fluorine atoms with coupling constants of 125 Hz
for the CH2O group and 280 Hz for the CF2CO group.
The 19F NMR spectrum with the signals of the OCF<
and CF3 groups is given in the table; it is quite con-
sistent with the data from [7].
was obtained. It was identified as CF3CFBrCOOCH3
by the 19F NMR [ , ppm: 77.1 d (3F, CF3, J 10 Hz),
F
134.1 q (1F, CFBrCOOCH3, J 10 Hz] and mass spec-
tra [m/z (Irel, %): 209, 207 (5.4), M+ OCH3; 181, 179
(30.5), M+ 59 (CF3CFBr+); 162, 160 (3.7), C2F3Br+;
131, 129 (13.9), CF2Br+; 128 (4.8), C2F4CO+; 109
(6.6), C2F3CO+; 100 (7.2), C2F+4; 81 (3.3), C2F+3;
69 (119), CF3; 59 (100) COOCH+3.
EXPERIMENTAL
Pyrolysis of perfluoro-2-(4-oxaamyloxy)propion-
yl fluoride Ic. (a) A 250-ml four-necked round-bot-
tomed glass flask equipped with a stirrer, thermom-
eter, and water-cooled reflux condenser was charged
with 31 g (0.3 mol) of dry Na2CO3 and 30 ml of
diglyme dehydrated over CaH2. The contents were
heated on a water bath to 70 C, after which 50 g
(0.15 mol) of Ic was added dropwise until the CO2
evolution ceased. Then the reflux condenser was re-
placed with a descending condenser, the mixture
[a suspension of Na2CO3 in a solution of sodium per-
fluoro-2-(4-oxaamyloxy)-propionate in diglyme] was
heated to 120 C, and 40 g of the product was col-
lected in the receiver. According to GLC, the product
consisted of 97.5% per-fluorinated 4-oxaamyl vinyl
ether IIc (92 mol %) and 2.5% perfluoro-4-oxaamyl
1,3,3,3-tetrafluoroethyl ether. The following products
were isolated by fractional distillation and character-
ized: CF3OCF2CF2CF2OCF=CF2, bp 64 C, d240 1.636;
mass spectrum, m/z (Irel, %): 332 (0.37), C4F12O+2 =
M+; 169 (1.83), C3F7+; 147 (1.92), C3F5O+; 135 (1.39),
C2F5O+; 131 (0.9), C3F5+; 119 (5.24), C2F+5; 100 (2.2),
C2F+4; 97 (2.37), C2F3O+; 81 (3.35), C2F+3; 78 (4.29),
C2F2O+; 69 (100), CF3+; CF3OCF2CF2CF2OCFHCF3,
bp 71 72 C, d240 1.622.
The pyrolysis products were analyzed chromato-
graphically with an LKhM-8MD chromatograph
[model 5, thermal conductivity detector, programmed
1
heating from 30 to 200 C at a rate of 6 deg min ,
3000 3-mm columns, solid support Silokhrom-2
3
(0.16 0.2 mm, packing density 0.30 0.01 g cm ),
stationary phase 5F4E (15 wt %), carrier gas helium
1
(40 ml min )].
The 19F NMR spectra were recorded on a Bruker
Spectrospin HX-90 spectrometer at a working fre-
quency of 84.67 MHz; hexafluorobenzene or hexaflu-
oro-p-xylene was used as internal reference. The 19F
chemical shifts were recalculated relative to CCl3F.
1
The H NMR spectra were recorded on the same
device at 90 MHz, with hexamethyldisiloxane as ex-
ternal reference.
The mass spectra were recorded on a Chromass
Varian CH-7M device.
Perfluoro-2-(4-oxaamyloxy)propionyl fluoride Ic
was prepared by electrochemical fluorination of meth-
yl 3-methoxypropionate [8], followed by condensation
of the resulting perfluoro-3-methoxypropionyl fluoride
with hexafluoropropene oxide on KF in diglyme.
(b) Similarly, a mixture of 8 g (0.075 mol) of dry
Na2CO3 and 30 ml of dry diglyme was heated to 60 C,
and 50 g (0.15 mol) of Ic was added, after which
the mixture was heated at 140 150 C for 2 h until
the gas evolution ceased. Then the mixture was cooled
to 80 C, the reflux condenser was replaced with
the descending condenser, and the reaction products
were distilled off in a vacuum (2 3 mm Hg). After
the separation of the lower organofluorine layer, we
obtained 43 g of the product containing, according
to GLC, 93% bis(perfluoro-1-methyl-2,6-dioxaheptyl)
ketone (90 mol %) and 7% of an impurity. We iso-
lated by fractional distillation and characterized ke-
tone IIIc, bp 74 75 C (18 mm Hg), d240 1.757, nD20
1.2836. The impurity was identified by 19F NMR as
linear dimer IVc of perfluorinated 4-oxaamyl vinyl
The synthesis of perfluoro-2-methyl-3-oxahexane-
dioic acid difluoride IX has been described previously
[9].
Perfluoro-2-methoxypropionyl fluoride Ia was pre-
pared by condensation of COF2 with hexafluoroprop-
ene oxide [10], and perfluoro-2-propoxypropionyl
fluoride Ib and perfluoro-2-(2-methyl-3-oxahexyloxy)-
propionyl fluoride Id, respectively, by dimerization
and trimerization of hexafluoropropene oxide under
similar conditions [11].
Perfluoro-2-bromopropionyl fluoride VI was pre-
pared from 67 g (0.4 mol) of hexafluoropropene ox-
ide and 43 g (0.42 mol) of dry NaBr in 75 ml of ab-
solute diglyme at 30 C with stirring in a stainless
steel autoclave. After warming up to room tempera-
ture, the lower organofluorine layer was separated,
and 80 g of a product (yield 88 mol %) with bp 20 C
ether, CF3OCF2CF2CF2OCFCF CFOCF2CF2CF2OCF3
\
CF3
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 10 2005

1644
LEBEDEV et al.
bp 70 71 C (18 mm Hg), d240 1.736, nD20 1.2805.
Its structure was also confirmed by UV-induced brom-
ination [characteristics of the dibromide: bp 73 C
(2 mm Hg), d240 1.9811, n2D0 1.3443. The mass spectrum
of the product was also consistent with the structure
of the linear dimer, m/z (Irel, %): 664 (6.4), C12F24O3 =
M+; 595 (0.63), M+ CF3; 579 (3.8), M+ CF3O;
498 (0.6), M+ C3F6O+; 479 (1.1), M+ C3F7O; 413
CF2Br; 112, 110, M+ CF3Br = CFBr+. In low-boil-
ing gases, after the uptake of CO2 with an alkali,
we identified tetrafluoroethylene and 1-hydro-1-
bromotetrafluoroethane, which was also detected as
impurity in the main product by 19F NMR spectros-
copy.
Pyrolysis of perfluoro-2-methyl-3-oxahexanedioic
acid difluoride IX. A similar flask equipped with
a water-cooled refluxed condenser was charged with
10.6 g (0.1 mol) of dry Na2CO3 and 50 ml of dehy-
drated diglyme. The mixture was heated on an oil
bath to 60 C, and 31 g (0.1 mol) of IX was added
dropwise from a dropping funnel over a period of
2 h. The mixture was stirred until the CO2 evolution
ceased. Then the mixture was gradually heated with
stirring to 140 C, with collected the pyrolysis prod-
ucts in a trap cooled to 78 C. Low-boiling gases
were passed through a trap with liquid bromine and
then through an alkaline absorber. From the trap, we
isolated 4 g (yield 15%) of a product identified as
perfluoro-2-methyloxolan-3-one VIII by comparison
of the 19F NMR and mass spectra with those reported
in [6]; bp 15 C. In the trap with bromine, 1,2-di-
bromotetrafluoroethane was obtained as a separate
phase. The nonvolatile residue contained sodium tri-
fluoroacetate.
(7.7), 498
CF3O; 325 (2.4), C7F11O+2; 275 (2.3),
C6F9O+2; 247 (2.7), C5F9O+; 219 (1.1), C4F+9; 178
(25.2), C4F6O+; 169 (98.9), C3F7+; 159 (33.0), C4F5O+;
150 (5.9), C3F6+; 147 (49.5), C3F5O+; 135 (30.3),
C2F5O+; 119 (74.8), C2F5+; 100 (37.3), C2F+4; 69 (100),
CF+3.
Similarly, we obtained by pyrolysis of perfluoro-2-
methoxypropionyl fluoride Ia, perfluoro-2-propoxy-
propionyl fluoride Ib, and perfluoro-2-(2-methyl-3-
oxahexyloxy)propionyl fluoride Id the corresponding
symmetrical ketones IIIa, IIIb, and IIId with ad-
mixture of linear dimers IV.
Pyrolysis of perfluoro-3-methoxypropionyl fluo-
ride. Similarly to the above-described procedure, a
flask with the reflux condenser cooled by a dry ice/al-
cohol mixture was charged with 22.3 g (0.21 mol) of
Na2CO3 and 40 ml of dry diglyme; 46 g (0.2 mol)
of CF3OCF2CF2COF was fed through a siphon until
the refluxing and CO2 evolution ceased. The resulting
suspension of NaF in a solution of sodium perfluoro-
3-methoxypropionate V in diglyme was heated to
120 C, with fractional distillation of the pyrolysis
products. Carbonyl fluoride was taken up by a sodium
carbonate solution, perfluorinated methyl vinyl ether
was condensed in a coil trap at 50 C, and tetrafluoro-
ethylene was taken up by liquid bromine in a trap,
with conversion into dibromotetrafluoroethane. We
obtained 10 g of CF3OCF=CF2 (30 mol %) and 31 g
of CF2BrCF2Br (60 mol %).
CONCLUSIONS
(1) Depending on the pyrolysis conditions and
structure of perfluorocarboxylic acid salts, the per-
fluoroalkyl carbanion generated by decarboxylation
is stabilized not only by elimination of the fluoride
anion to give the alkene, but also with that to give
the corresponding ketones through both inter- and
intramolecular attack at the acyl fluoride group.
(2) In the pyrolysis of potassium perfluoro-3-
alkoxypropionates, the perfluorooxaalkyl anion is
virtually quantitatively stabilized by elimination of
the perfluoroalkoxide anion with the formation of
tetrafluoroethylene.
Pyrolysis of perfluoro-2-bromopropionyl fluoride
VI. Similarly to the pyrolysis of perfluoro-3-methoxy-
propionyl fluoride, 45 g (0.2 mol) of VI was fed to
a mixture of 22.3 g (0.21 mol) of Na2CO3 and 40 ml
of dry diglyme, until the refluxing and CO2 evolution
ceased. Then the reflux condenser was replaced with
a water-cooled descending condenser, the flask was
heated to 120 C, and 23 g (45 mol %) of perfluoro-
1,1-dibromoethane VII was distilled off; bp 5 C,
d420 2.188, nD20 1.364; mass spectrum, m/z (Irel, %):
(3) An attack of the bromotetrafluoroethyl anion
at the bromine atom of another substrate molecule
yields perfluoro-1,1-dibromoethane and tetrafluoro-
ethylene.
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