Dissociation constants of polyfluorocarboxylic acids

Article abstract of DOI:10.1134/S1070427207100369

The effect of the structure and molecular weight of fluorocarboxylic acids on their dissociation constants in acetonitrile and its mixtures with water and methanol was studied.

Full text of DOI:10.1134/S1070427207100369

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2007, Vol. 80, No. 10, pp. 1770 1772.
Pleiades Publishing, Ltd., 2007.
Original Russian Text
O.N. Chechina, S.V. Sokolov, V.V. Berenblit, V.A. Soshin, 2007, published in Zhurnal Prikladnoi Khimii, 2007, Vol. 80,
No. 10, pp. 1741 1743.
BRIEF
COMMUNICATIONS
Dissociation Constants of Polyfluorocarboxylic Acids
O. N. Chechina, S. V. Sokolov, V. V. Berenblit, and V. A. Soshin
Samara State Technical University, Samara, Russia
Lebedev Research Institute of Synthetic Rubber, Federal State Unitary Enterprise, St. Petersburg, Russia
Received March 23, 2007
Abstract The effect of the structure and molecular weight of fluorocarboxylic acids on their dissociation
constants in acetonitrile and its mixtures with water and methanol was studied.
DOI: 10.1134/S1070427207100369
It is known that, owing to the strong electron-accep-
tor properties of the trifluoromethyl radical, trifluoro-
acetic acid has a dissociation constant of 5.9 10 1
in water [1] and virtually does not differ from strong
acids, such as hydrochloric acid. Its homologues, per-
fluorocarboxylic acids, are also rather strong acids.
As, however, the length of the carbon chain increases,
micelle formation leads to a decrease in the degree of
ionization [2].
Depending on a medium, the following solutions
were used as titrants: 0.1 M KOH for acetonitrile,
0.1M aqueous solution of NaOH for water and its
mixtures with acetonitrile, and 0.1 M solution of so-
dium methylate in methanol for methanol and its
mixture with acetonitrile.
The solvents were purified by the known proce-
dures [5].
Titration was applied to 0.2 M solutions of the fol-
lowing polyfluorocarboxylic acids of reagent grade
or those synthesized and purified by rectifica-
tion: CF CF COOH (I) pentafluoropropionic acid;
Electrochemical syntheses with anodic dimeriza-
tion and cross-anodic combination of various fluoro-
carboxylic acids in acetonitrile mixed with water and
methanol are of practical importance for preparation
of new organofluoric compounds possessing various
useful properties [3]. The specific role played by
acetonitrile in electrolytic media, which primarily
determines the efficiency of these processes, makes
necessary a study of the electrochemical characteristics
of fluorinated carboxylic acids in its mixtures with
protonating electrolytes in order to choose the elec-
trolysis conditions and prognosticate the results of
electrosyntheses.
3
2
CF ClCFClCOOH (II) 1,2-dichloroperfluoropropionic
2
acid prepared by the method [6]; CF CF CF COOH
3
2
2
(
III) heptafluoromaleic acid; CF CF CF OCF(CF )-
3 2 2 3
COOH (IV) perfluoro-3-oxo-2-methylhexanoic acid;
CF3OCF2CF2CF2OCF(CF3)COOH (V) perfluoro-3,7-
dioxo-2-methyloctanoic acid [prepared by condensa-
tion of CF OCF CF COF with hexafluoropropene
3
2
2
oxide and subsequent hydrolysis of acylfluoride
CF OCF CF CF OCF(CF )COF]; CF CF CF OCF-
3
2
2
2
3
3
2
2
(
2
CF )CF OCF(CF )C OOH (VI) perfluoro-3,6-dioxo-
,5-dimethylnonanoic acid; F(CF CF O)CF COOH
2 2 2
3 2 3
EXPERIMENTAL
(
VII) perfluoro-3,6,9,12-tetraoxotetradecenoic acid
The dissociation constants of fluorocarboxylic acids
were determined by potentiometric titration [4] with
a pH-673M millivoltmeter, using a glass working elec-
trode and the standard calomel electrode. The solu-
tions were agitated with a magnetic stirrer.
prepared by hydrolysis of the tetrafluoroethene oxide
pentamer.
The equivalence point pHeq was determined by
differentiating the titration curve. The determination
accuracy was 2%.
The dissociation constants of the acids were mea-
sured in individual solvents (acetonitrile, water, and
methanol) and in their mixtures with relative solvent
concentrations commonly used in electrochemical
syntheses [3].
The negative logarithm of the dissociation con-
stant, pKHA , was calculated by the equation [4]:
pKHA = 2pHeq pK
+ log cHA = 2pHeq 13.3010.
H O
2
1770

DISSOCIATION CONSTANTS OF POLYFLUOROCARBOXYLIC ACIDS
Calculated dissociation constants of polyfluorocarboxylic acids in various solvents
KHA 102
1771
Acid
CH CN H O, vol % CH CN CH CN CH OH,
3
2
3
3
3
CH CN
H O
CH OH
10 vol %
3
2
3
9
0
50
40
30
CH CN
3
CF CF COOH (I)
3.66
3.12
1.80
1.30
2.50
3.98
1.58
1.58
1.26
3.55
3.16
1.25
1.26
1.12
1.07
3
2
CF ClCFClCOOH (II)
2
CF CF CF COOH (III)
3
2
2
CF CF CF OCFCOOH (IV)
1.40
0.85
0.68
1.12
0.65
0.93
0.41
3
2
2
\
CF3
CF OCF CF CF OCFCOOH (V)
1.17
2.75
1.20
2.45
0.79
1.25
0.01
3
2
2
2
\
CF3
CF (CF ) OCFCF OCFCOOH (VI)
0.70
0.45
3
2 2
2
\
\
CF3
CF3
F(CF CF O)CF COOH (VII)
0.88
0.80
2
2
2
*
CH CH CH COOH
1.58
1.25
3
2
2
*
5
K
10 .
HA
For comparison, the dissociation constant of nor-
mal maleic acid was determined and found to coincide
with the published value [7].
compared with acetonitrile. However, the dipole
moment of the acetonitrile molecule is twice that
of water and methanol [9]. Apparently, it is because
of this circumstance that its capacity for solvation
of fluorocarboxylic acid exceeds that of water and
methanol. Presumably, the solvation-induced polar-
ization of the O H bond in carboxylic acid facilitates
the dissociation by making the difference in their
dielectric constants less pronounced.
Table lists the dissociation constants of the acids
studied, calculated from the experimental data.
It follows from the experimental data that, in
the systems studied, the dissociation constants of
fluorocarboxylic acids are lower by at least an order
of magnitude than the dissociation constant of tri-
fluoroacetic acid. Therefore, they cannot be classed
with strong acids. This conclusion is supported by
Just this circumstance provides solvation of the car-
boxylate anion of fluorocarboxylic acid with aceto-
nitrile in mixtures with water and methanol, despite
the hydrophobic nature of its fluoroalkyl radical.
Probably, this is a manifestation of the specific role
played by acetonitrile in the reactions of anodic con-
densation. It is the presence of acetonitrile in an elec-
trolyte that provides synthesis of fluoroalkyl dimers
the data of [8], according to which the K values on
HA
2
the order of 10 are reported for perfluoromaleic and
perfluorocaprylic acids in water. However, unsubsti-
tuted N-maleic acid has, similarly to most of hydro-
carbon carboxylic acids [7], K
orders of magnitude (1.58 10 ).
higher by three
HA
5
[3] in high yield because of the displacement of water
It is noteworthy that the dissociation constants of
perfluorocarboxylic acids with different molecular
weights and chain structures differing in substituents,
oxygen heteroatoms, and pendant trifluoromethyl
moieties are close in order of magnitude in water and
acetonitrile. In the latter case, pentafluoropropionic
acid is even stronger, as regards KHA , in acetonitrile
as compared with water.
and methanol from the double layer via adsorption on
the anode of carboxylate anions solvated by aceto-
nitrile.
Presumably, the dependence of the dissociation
constant on the molecular weight and structure of
fluorocarboxylic acids could not be observed because
of the possibility of micelle formation, which creates
a heterophase colloid system even at a concentration
of 0.25 M. The influence of oxygen heteroatoms,
which make lower the rotation barrier of the fluoro-
alkyl chain and create a certain steric hindrance to
The dielectric constant of a medium, to which
the dissociation of a polarized O H bond in carbox-
ylic acid is related, is considerably higher for water,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 10 2007

1772
CHECHINA et al.
pendant trifluoromethyl groups, also gives no way of
observing this dependence.
3. Chechina, O.N. and Tomilov, A.P., Elektrokhimiya,
1999, vol. 35, no. 3, p. 149.
4
5
. Khe’fets, V.L., Avdeev, D.K., and Reishakhrit, L.S.,
Praktikum po teoreticheskoi elektrokhimii (Practical
Course of Theoretical Electrochemistry), Leningrad:
Len. Gos. Univ., 1954.
CONCLUSIONS
(
1) It was shown that the dissociation constants of
fluorocarboxylic acids are lower by at least an order
of magnitude than the dissociation constant of tri-
fluoroacetic acid. Therefore, the above compounds
cannot be classed with strong acids.
. Ptitsina, O.A., Kupletskaya, N.V., Timofeeva, V.N.,
et al., Laboratornye raboty po organicheskomu sintezu
(
Laboratory Works on Organic Synthesis), Moscow:
Prosveshchenie, 1979.
(
2) In water and acetonitrile, the dissociation con-
6
7
. Paleta, O., Havin, V., and Dedek, V., Org. Chem.
Technol., 1978, vol. 25, p. 105.
stants of fluorocarboxylic acids of various structures
and molecular weights differ only slightly, although
their dielectric constants are strongly different.
. Kratkii spravochnik fiziko-khimicheskikh velichin (Con-
cise Reference Book of Physicochemical Constants),
Ravdel’, A.A., Ed., Leningrad: Khimiya, 1983.
REFERENCES
8
. Radell, J., Brodman, B.W., Hirshfeld, A., and Berg-
mann, E.D., J. Phys. Chem., 1965, vol. 69, p. 928.
1
2
. Sheppard, W.A. and Sharts, C.M., Organic Fluorine
Chemistry, New York: W.A. Benjamin, Inc., 1969.
. Kauck, E.A. and Diesslin, A.R., Ind. Eng. Chem.,
9. Osipov, O.A. and Minkin, V.I., Spravochnik po di-
pol’nym momentam (Reference Book of Dipole Mo-
ments), Moscow: Vysshaya Shkola, 1965.
1
951, vol. 43, no. 10, p. 2.
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