Study of the decarboxylation mechanism of fluorobenzoic acids by strong N-bases

Article abstract of DOI:10.1002/poc.419

The kinetics of the decarboxylation reactions of pentafluorobenzoic and tetrafluorobenzoic acids by various N-bases were studied using ¹⁹F NMR spectroscopy. The rate constants of these reactions are dependent on the structure of the fluorinated acid and the pK_a values of the N-bases. Pentafluorobenzoic acid is decarboxylated about two orders of magnitude faster than tetrafluorobenzoic acid. With increasing pK_a values of the protonated N-bases these reactions became much slower. These results suggested that the rate-determining step of the studied reactions is the attack of the conjugated acid (protonated N-base) on carboxylate anion. Copyright

Full text of DOI:10.1002/poc.419

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2001; 14: 691–696
DOI:10.1002/poc.419
Study of the decarboxylation mechanism of ¯uorobenzoic
acids by strong N-bases
BtazÇej Gierczyk,1 Grzegorz Wojciechowski,1 Bogumit Brzezinski,1 Eugeniusz Grech2 and
Grzegorz Schroeder1*
1Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
2Institute of Chemistry and Environmental Protection, Technical University of Szczecin, Piasto w 42, Szczecin 71-065, Poland
Received 20 February 2001; revised 2 May 2001; accepted 21 May 2001
ABSTRACT: The kinetics of the decarboxylation reactions of pentafluorobenzoic and tetrafluorobenzoic acids by
various N-bases were studied using 19F NMR spectroscopy. The rate constants of these reactions are dependent on the
structure of the fluorinated acid and the pKa values of the N-bases. Pentafluorobenzoic acid is decarboxylated about
two orders of magnitude faster than tetrafluorobenzoic acid. With increasing pKa values of the protonated N-bases
these reactions became much slower. These results suggested that the rate-determining step of the studied reactions is
the attack of the conjugated acid (protonated N-base) on carboxylate anion. Copyright  2001 John Wiley & Sons,
Ltd.
KEYWORDS: pentafluorobenzoic acid; tetrafluorobenzoic acid; N-bases; kinetics; decarboxylation; 19F NMR;
Fourier transform infrared
INTRODUCTION
pentafluorobenzoic acid by DMAN and the difference
between this reaction and that of tetrafluorobenzoic acid
with DMAN, we performed kinetic studies of the reaction
of the two acids with various N-bases in acetonitrile using
19F NMR spectroscopy.
It is well know that the decarboxylation reactions of
carboxylic acids and the interactions between strong N-
bases and acids play very essential roles in chemistry and
biochemistry.1–10 In this respect, we have studied many
reactions of various acids and strong N-bases.11–26 In
various solvents for these reactions proton transfer from
the acid to the N-base was always observed. For some
systems a complex equilibrium including ion pairs, free
ions and hydrogen-bonded species of different stoichio-
metry was observed. A very unexpected observation was
made in the case of the deprotonation reaction between
pentafluorobenzoic acid and 1,8-bis(dimethylamino)-
naphthalene (DMAN),27 where in addition to the proton
transfer reaction decarboxylation of the carboxylic acid
occurred, as shown in Scheme 1.
EXPERIMENTAL
Pentafluorobenzoic acid (PFBCOOH) and 2,3,5,6-tetra-
fluorobenzoic acid (TFBCOOH) were purchased from
Aldrich and used without purification. N-bases such as
tetramethylguanidine (TMG), 7-methyl-1,5,7-triazabicy-
clo[4.4.0]dec-5-ene (MTBD), quinuclidine, phosphazene
(P2Et) and DMAN were obtained from Fluka. Quinucli-
dine was purified by sublimation and DMAN was
recrystallized from ethanol.
In a previous paper,27 we demonstrated that under
infrared spectroscopic conditions the decarboxylation of
tetrafluorobenzoic acid by DMAN was not observed.
This reaction was similar to other proton transfer
reactions from various acids to the DMAN molecule.
The formation of the protonated DMAN molecule and
homoconjugated anions was very well evidenced in the
FTIR spectra independent of the stoichiometry of the
substrate, which is shown in Scheme 2.
19F NMR spectra were recorded on a Varian-Gemini
300 spectrophotometer at 282.322 MHz in acetonitrile-
d3. The chemical shifts were measured with respect to
CFCl3 as an internal standard. The conditions used were
spectral width sw = 65 000 Hz, 60° pulse widths and
0.64 s acquisition time.
Kinetic measurements were performed on the basis of
the changes of the integration of the 19F signal of fluorine
in an ortho position to the carboxylic group for substrate
and products. The concentration of bases was much
higher than that of the fluorinated acids, so a pseudo-first-
To explain the decarboxylation mechanism of the
*Correspondence to: G. Schroeder, Faculty of Chemistry, Adam
Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland.
order method was used. An example of changes in the 19
F
Copyright  2001 John Wiley & Sons, Ltd.
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B. GIERCZYK ET AL.
Scheme 1
Scheme 2
NMR spectra for the reaction between PFBCOOH and
DMAN is shown in Fig. 1.
Second-order rate constants (k) for the reaction were
calculated by a linear least-squares fit of the variation of
the pseudo-first-order rate constants vs base concentra-
tion. The activation parameters (DH≠ and DS≠) were
determined using the Eyring equation [k = (kB/h)
exp(DS≠/R)exp(ÀDH≠/RT)] by a linear least-squares fit
of ln k/T vs 1/T. The DG≠ values were calculated using the
Gibbs–Helmholtz relation (DG≠ = DH≠ ÀTDS≠) for a
temperature of 298 K.
The spectra of the samples were taken with an Fourier
transform infrared (FTIR) spectrophotometer (Bruker
IFS 113 v) using a cell with Si windows (sample
thickness 0.176 mm, detector DTGS, resolution 2 cmÀ1).
The temperature of the samples was 293 K and the
concentration of the P2Et base was 0.1 mol dmÀ3
.
The measurement of the pKa values of N-bases in
acetonitrile was achieved using a published potentio-
metric procedure.28
Figure 1. 19F NMR chemical shifts vs time for solutions of
PFBCOOH 10.1 mol dmÀ3) and DMAN 11 mol dmÀ3) in
acetonitrile. 1a) PFBCOOH 1substrate); 1b) PFBCOOH and
DMAN immediately after mixing; 1c) PFBCOOH and DMAN
after 5 h; 1d) penta¯uorobenzene 1product) after 50 h
RESULTS AND DISCUSSION
The formulae of the acids and N-bases studied and their
pKa values in acetonitrile are shown in Scheme 3.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 691–696

DECARBOXYLATION OF FLUOROBENZOIC ACIDS
693
Scheme 3
In Scheme 4, the 19F NMR data for PFBCOOH and
PTBCOOH acids, their salts (as intermediate products)
and the products, i.e. 2,3,4,5,6-pentafluorobenzene and
2,3,5,6-tetrafluorobenzene, in the reaction with DMAN
are collected. These data show that the chemical shifts of
the 19F signals and the coupling constants J(H–F,F–F) are
different for fluorinated acids, their complexes with N-
bases (deprotonated species, ion pairs or hydrogen-
bonded complexes) and the end products. No other
species (i.e. carbanions of decarboxylated acids) were
Scheme 4
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B. GIERCZYK ET AL.
Table 1. Second-order rate constants 1Æ standard deviation) for reaction of TFBCOOH or PFBCOOH with N bases in acetonitrile
k (dm3molÀ1sÀ1
)
N-base
Temperature (K)
TFBCOOH
PFBCOOH
DMAN
DMAN
DMAN
DMAN
DMAN
DMAN
293
298
308
313
318
333
(2.83 Æ 0.04) Â 10À6
(9.10 Æ 0.04) Â 10À6
(2.86 Æ 0.04) Â 10À5
(7.61 Æ 0.05) Â 10À5
(1.29 Æ 0.05) Â 10À4
(1.25 Æ 0.04) Â 10À3
(1.14 Æ 0.04) Â 10À4
(2.33 Æ 0.04) Â 10À4
(9.88 Æ 0.05) Â 10À4
(1.64 Æ 0.04) Â 10À3
(3.56 Æ 0.04) Â 10À3
Quinuclidine
TMG
MTBD
P2Et
298
298
298
298
(1.06 Æ 0.05) Â 10À5
(7.72 Æ 0.05) Â 10À6
(6.61 Æ 0.02) Â 10À6
(9.86 Æ 0.02) Â 10À7
(2.41 Æ 0.04) Â 10À4
(1.73 Æ 0.04) Â 10À4
(1.57 Æ 0.03) Â 10À4
(3.60 Æ 0.03) Â 10À5
observed in the 19F NMR and FTIR spectra of the
reaction mixtures. This result suggests that such carba-
nions are not formed or they are very rapidly protonated.
The second-order rate constants and the activation
parameters for the reactions between the fluorinated acids
and DMAN are summarized in Tables 1 and 2.
The comparison of the rate constants obtained
indicates that the decarboxylation of PFBCOOH is much
faster than that of TFBCOOH. The very slow decarbox-
ylation process in the latter case explains why in previous
FTIR studies only the intermediate products were
observed (see Scheme 2)
that with increasing pKa values of the N-bases the rate
constants of the decarboxylation reaction decrease
significantly. In the case of very strong N-bases such as
P2Et the decarboxylation of both fluorinated acids is very
slow. In the 19F NMR spectra of fluorinated acids with
P2Et in acetonitrile, relatively stable free ions of the
protonated P2Et molecule and deprotonated fluorinated
acids are observed. This result is also confirmed by the
FTIR spectrum of a 1:1 mixture of PFBCOOH and P2Et
in acetonitrile [Fig. 3(a)]. In this spectrum instead of the
ꢀ(C
=O) band, observed in the spectrum of free acid at
1745 cm
,
À1 29 a band assigned to ꢀas(COOÀ) is observed
‡
The dependence of the rate constants at 25° C on the
pKa values of N-bases is shown in Fig. 2, which indicates
at 1651 cmÀ1. The ꢀ(N–H ) vibrations of free protonated
P2Et molecule are observed at 3334 cmÀ1 and of
hydrogen-bonded species to the carboxylate groups at
about 3150 cmÀ1. Furthermore, a sharp band of low
intensity is observed at 2339 cmÀ1, assigned to ꢀ(CO2)
Table 2. Activation parameters 1Æ standard deviation) for
reaction of TFBCOOH or PFBCOOH with DMAN in aceto-
nitrile
Activation parameter
TFBCOOH
PFBCOOH
DH≠ (kJ molÀ1
)
120.9 Æ 3.2
61 Æ 10
103.0Æ2.6
30 Æ 8
DS≠ (J KÀ1 *molÀ1
DG≠ (kJ molÀ1
)
)
102.8 Æ 3.2
93.8 Æ 2.6
Figure 3. FTIR spectra of a 1:1 mixture of PFBCOOH with N-
base immediately after mixing. 1a) P2Et phosphazene; 1b)
DMAN
Figure 2. Plot of rate constants of reaction for PFBCOOH or
TFBCOOH with various N-bases in acetonitrile at 25°C
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 691–696

DECARBOXYLATION OF FLUOROBENZOIC ACIDS
695
Scheme 5
vibrations, indicating the beginning of the decarboxyla-
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J. Phys. Org. Chem. 2001; 14: 691–696