112
T.M. Rangarajan et al. / Journal of Fluorine Chemistry 132 (2011) 107–113
observations indicate random free radical fluorination of all the C–
H bonds.
area = 2.47 dm2) was employed. The cell and condenser tempera-
ture were maintained at 5 and ꢀ30 8C, respectively. Volatile
products were collected in a FEP (Fluorinated Ethylene Propylene)
trap kept at ꢀ80 8C using an immersion cooler at the outlet of the
cell condenser.
2.4. Carbon chain isomerization
Carbon chain isomerization is not possible in the case of PC. Even
in the case of n-BC isomerized products (XXII) and (XXIII) are formed
only with very low selectivity. Conversion of linear molecule into
isomerized product is quite difficult according to the cyclopropane
intermediate mechanism suggested earlier [12–14].
Pre-electrolysis was carried out for 48 h in order to dry AHF and
activate the electrode surfaces. Cell voltage was maintained
between 5.0 and 5.5 V during this period until the initial current
of 4 A reduced to 0.2 A. A mixture of acid chloride (40 g of PC) and
AHF was prepared separately and added to the ECF cell after the
completion of pre-electrolysis. Cell current was switched off for
30 min and about 20 ml of electrolyte mixture was drained off
from the cell to analyze the fluorinated product with out passing
current to the cell (0% charge). Electrolysis was then continued at
constant current of 4.0 A (cell voltage 5–6.5 V) until theoretical
charge was passed. About 20 ml of electrolyte was drained at every
25% of theoretical charge passed. Drained electrolytes at every
stage and the condensate from the immersion cooler trap at the
end of complete electrolysis were immediately neutralized with
aqueous sodium hydroxide solution and water was evaporated
using rotary evaporator at 60 8C until complete dryness. Dry salt
obtained contains sodium salt of poly fluorinated acids and sodium
fluoride. The fluoro organic product was extracted using anhy-
drous methanol. The methanol insoluble NaF was filtered off.
Methanol extract was evaporated to dryness under vacuum to get
sodium polyfluoro propionate. Similar procedure was followed for
the ECPF of n-butyryl chloride.
2.5. Cleavage and coupling
In the free radical pathway C–C bond cleavage leading to
compounds with less number of carbon atoms as well as radical–
radical coupling or radical–reactant coupling leading to molecules
containing more number of carbon atoms should be possible. This is
indeed observed in the case of fluorination of PC, for example,
compound (XIII) with one carbon atom less and compounds (XIV)
and(XXI) withone carbonatommoreareformed. Inthecaseofn-BC,
compound (X) with one carbon atom less and compound (XIII) with
twocarbonatoms lessarenoticed. Compoundwithone carbonmore
was also observed during ECF of n-BC with less than 1%.
The overall selectivity of perfluorinated compounds generally
decreases slightly with increasing chain length [13,14]. This type of
behaviour is noticed here as well. The overall selectivity of
perfluoro compound (X) was 92% while the selectivity of perfluoro
compound (XXI) was 81.5% obtained from immersion cooler trap
(after 100% of theoretical charge passed).
4.3. Identification of fluorinated products
3. Conclusions
Identification of partially fluorinated products was carried out
by recording 19F (both 1H coupled and decoupled spectra) and 1
H
The present study provides further evidence for the direct
involvement of high valent nickel fluorides on the electrode surface
at least during the initial stages of electrochemical perfluorination.
Free radical mechanism appears to be the major pathway as
indicated by formation of large number of partially fluorinated
compounds at every stage of electrochemical fluorination. For both
PC and nBC this mechanism also explains radical coupling, C–C bond
cleavage and carbon chain isomerization. At least in the initial
phases fluorination proceeds at the secondary carbon atom.
NMR of crude products. Assignment of molecular structure and the
identification of fluorinated products are based on,
a. chemical shifts exhibited by various fluoro groups (mono, di, tri,
etc.);
b. integral values;
c. splitting pattern obtained from both 1H coupled and decoupled
spectra; and
d. coupling constant values.
Fluorination at the
a-carbon atom may slightly be more difficult
due to steric effect and electronic effect. Carbocation pathway or
ECbECN mechanism is not absolutely necessary for explaining the
last two factors. It is thus, safe to conclude that free radical pathway
with the involvement of high valent nickel fluorides is the overall
mechanistic pathway of electrochemical perfluorination.
Acknowledgements
Authors wish to thank DRDO, New Delhi, and Ministry of
Environment and Forests, Government of India for financial
support. They also thank Mr. S. Radhakrishnan, Scientist, CIF
Division for recording NMR spectra.
4. Experimental details
References
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Synthetic grade (>98%) alkane carboxylic acid chlorides were
purchased from M/s SRL, India and used as such. Anhydrous
hydrogen fluoride (AHF) >99.9% was supplied by M/s TANFAC,
cudalore, Tamilnadu, India.
An Aplab (India) DC power supply was used for electrolysis. 19
F
NMR spectra were recorded with 376.5 MHz (400 MHz for 1H)
Bruker NMR spectrometer with D2O as solvent. CFCl3 and TMS
were used as internal reference for 19F and 1H NMR spectra
respectively.
4.2. Electrochemical perfluorination of acid chlorides
A
double walled 200 ml stainless steel electrolytic cell
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with alternate nickel anodes and cathodes (effective anode