Energetics of the C-Cl bond in CH3CH(Cl)COOH. Enthalpy of formation of (S)-(-)-2-chloropropionic acid and of the 1-carboxyethyl radical

Article abstract of DOI:10.1021/jp020412p

The energetics of the C-Cl bond in 2-chloropropionic acid was investigated by using a combination of experimental and theoretical methods. The standard molar enthalpy of formation of liquid (S)-(-)-2-chloropropionic acid, at 298.15 K, was determined as Δ_fH°_m(C₃H₅O₂Cl, l) = - 534.6 ± 1.1) kJ?mol^-1, by rotating-bomb combustion calorimetry. The corresponding enthalpy of vaporization, Δ_vapH°_m (C₃H₅O₂Cl) = (64.9 ± 0.5) kJ?mol^-1, was also obtained from vapor pressure versus temperature measurements by the transpiration method, leading to Δ_fH°_m(C₃H₅O₂Cl, g) = -(469.7 ± 1.2) kJ?mol^-1. This value, together with the enthalpy of the isodesmic and isogyric gas-phase reaction CH₃CH(X)COOH(g) + C₂H₅(g) → CH₃CHCOOH(g) + C₂H₅X(g) (X = H, Cl) predicted by density functional theory calculations and other auxiliary data, was used to derive the enthalpy of formation of the gaseous 1-carboxyethyl radical as Δ_fH°[CH(CH₃)COOH, g] = -(293 ± 3) kJ?mol^-1, from which DH°[H-CH(CH₃COOH] = 380.7 ± 3.9 kJ?mol^-1 and DH°[Cl-CH(CH₃)COOH] = 298.0 ± 3.2 kJ?mol^-1 were obtained. These values are compared with the corresponding C-H and C-Cl bond dissociation enthalpies in XCH₂COOH, XCH₃, XC₂H₅, XCH₂-Cl, XCH(CH₃)Cl, XCH-CH₂, and XC₆H₅ (X = H, Cl). The order DH°(C-H) < DH°(C-Cl) is observed for the carboxylic acids and all other RX compounds. Comparison of DH°[X-CH(CH₃)COOH] and DH°[X-CH₂COOH] (X = H, Cl) indicates that the replacement of a hydrogen of the CH₂ group of XCH₂COOH by a methyl group leads to a decrease of both the C-H and C-Cl bond dissociation enthalpy. It is finally concluded that the major qualitative trends exhibited by the C-Cl bond dissociation enthalpies for the series of compounds addressed in this work can be predicted based on Pauling's electrostatic-covalent model.

Full text of DOI:10.1021/jp020412p

J. Phys. Chem. A 2002, 106, 9855-9861
9855
Energetics of the C-Cl Bond in CH3CH(Cl)COOH. Enthalpy of Formation of
(S)-(-)-2-Chloropropionic Acid and of the 1-Carboxyethyl Radical†,‡
Ana L. C. Lagoa, Herm´ınio P. Diogo, and Manuel E. Minas da Piedade*
Centro de Qu´ımica Estrutural, Complexo Interdisciplinar, Instituto Superior Te´cnico,
1049-001 Lisboa, Portugal
Lu´ısa M. P. F. Amaral
Centro de InVestigac¸a˜o em Qu´ımica, Departamento de Qu´ımica, Faculdade de Cieˆncias,
UniVersidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal
Rita C. Guedes and Benedito J. Costa Cabral§
Centro de F´ısica da Mate´ria Condensada da UniVersidade de Lisboa and Departamento de Qu´ımica e
Bioqu´ımica, Faculdade de Cieˆncias, UniVersidade de Lisboa, 1649-016 Lisboa, Portugal
Dmitry V. Kulikov and Sergey P. Verevkin|
Department of Physical Chemistry, UniVersity of Rostock, Hermannstrasse 14, 18051 Rostock, Germany
Michael Siedler and Matthias Epple
Solid State Chemistry, Faculty of Chemistry, UniVersity of Bochum, D-44780 Bochum, Germany
ReceiVed: February 12, 2002; In Final Form: May 13, 2002
The energetics of the C-Cl bond in 2-chloropropionic acid was investigated by using a combination of
experimental and theoretical methods. The standard molar enthalpy of formation of liquid (S)-(-)-2-
chloropropionic acid, at 298.15 K, was determined as ∆fH°(C3H5O2Cl, l) ) -(534.6 ( 1.1) kJ‚mol-1, by
m
rotating-bomb combustion calorimetry. The corresponding enthalpy of vaporization, ∆vapH°(C3H5O2Cl) )
m
(64.9 ( 0.5) kJ‚mol-1, was also obtained from vapor pressure versus temperature measurements by the
transpiration method, leading to ∆fH°(C3H5O2Cl, g) ) -(469.7 ( 1.2) kJ‚mol-1. This value, together with
m
the enthalpy of the isodesmic and isogyric gas-phase reaction CH3CH(X)COOH(g) + C2H5(g) f
CH3CHCOOH(g) + C2H5X(g) (X ) H, Cl) predicted by density functional theory calculations and other
auxiliary data, was used to derive the enthalpy of formation of the gaseous 1-carboxyethyl radical as
∆fH°[CH(CH3)COOH, g] ) -(293 ( 3) kJ‚mol-1, from which DH°[H-CH(CH3)COOH] ) 380.7 ( 3.9
m
kJ‚mol-1 and DH°[Cl-CH(CH3)COOH] ) 298.0 ( 3.2 kJ‚mol-1 were obtained. These values are compared
with the corresponding C-H and C-Cl bond dissociation enthalpies in XCH2COOH, XCH3, XC2H5, XCH2-
Cl, XCH(CH3)Cl, XCHdCH2, and XC6H5 (X ) H, Cl). The order DH°(C-H) > DH°(C-Cl) is observed for
the carboxylic acids and all other RX compounds. Comparison of DH°[X-CH(CH3)COOH] and DH°[X-
CH2COOH] (X ) H, Cl) indicates that the replacement of a hydrogen of the CH2 group of XCH2COOH by
a methyl group leads to a decrease of both the C-H and C-Cl bond dissociation enthalpy. It is finally
concluded that the major qualitative trends exhibited by the C-Cl bond dissociation enthalpies for the series
of compounds addressed in this work can be predicted based on Pauling’s electrostatic-covalent model.
Introduction
energetics of the carbon-chlorine bond in chlorocarboxylic
acids,3 as well as on the enthalpies of formation of the acids
A considerable number of applications of halocarboxylic acids
in laboratory synthesis and in industry are related to reactions
in which the carbon-halogen bond is cleaved.1,2 Among the
halocarboxylic acids, the chloro derivatives are by far the most
widely used. Surprisingly, very little information exists on the
themselves3-5 and of the radicals formed upon cleavage of the
C-Cl bond.3,6-8 These data are usually very helpful in the
rationalization of the observed reactivity trends and in devising
strategies for the synthesis of new compounds. In addition, they
are important to model the pyrolysis9,10 and the atmospheric
chemistry of chlorocarboxylic acids. These two topics have
become particularly relevant in recent years in relation to the
environmental problems associated with the incineration of
chlorinated organic compounds and with the suggestion that
industrial sources of chlorine lead to the depletion of the ozone
layer.11 We have recently applied a combination of experimental
† Part of the special issue “Jack Beauchamp Festschrift”.
‡ This article is dedicated to Professor Jack L. Beauchamp on the occasion
of his 60th birthday.
* Corresponding author: e-mail pcmemp@popsrv.ist.utl.pt.
§ E-mail ben@adonis.cii.fc.ul.pt.
| E-mail sergey.verevkin@chemie.uni-rostock.de.
E-mail matthias.epple@ruhr-uni-bochum.de.
10.1021/jp020412p CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/03/2002

9856 J. Phys. Chem. A, Vol. 106, No. 42, 2002
Lagoa et al.
301.47 K; hexatriacontane, Tfus ) 347.30 K; indium, Tfus
430.61 K; and tin, Tfus ) 506.03 K. The organic standards were
high-purity Fluka products, and the metal standards were
supplied by TA Instruments Inc. The samples were sealed, under
air, in aluminum pans and weighed to (10-7 g in a Mettler
UMT2 ultramicro balance. Helium (Air Liquide N55) at a flow
rate of 0.5 cm3‚s-1 was used as the purging gas.
(combustion calorimetry, differential scanning calorimetry, and
vapor pressure versus temperature measurements) and compu-
tational chemistry (density functional theory, DFT) methods to
study the energetics of chloro-, bromo-, and iodoacetic acids
and of the carboxymethyl radical.3 Here, these studies are
extended to chloropropionic acid and the 1-carboxyethyl radical.
)
Methods
Enthalpy of Vaporization Measurements by the Transpi-
ration Method. The enthalpy of vaporization of (S)-(-)-2-
chloropropionic acid was determined from vapor pressure versus
temperature measurements by the transpiration method.14 About
0.5 g of sample was mixed with glass beads and placed in a
thermostated ((0.1 K) U-shaped glass tube of 20 cm length
and 0.5 cm diameter. A nitrogen stream was passed through
the U-tube at a flow rate of 0.946 or 0.960 cm3‚s-1, and the
transported amount of material was condensed in a cold trap
kept at 243 K. The flow rate of nitrogen was measured with a
soap bubble flow-meter and was maintained constant with help
of a high-precision needle valve (Hoke, C1335G6 BMM-ITA).
The mass of condensed product was determined by gas-liquid
chromatography (GC) analysis with n-C11H24 as internal stan-
dard. These analyses were carried out on a Carlo Erba GC
Fraktometer (Vega Series GC 6000), equipped with a SE-30
capillary column of 25 m length, a flame ionization detector
(FID), and a Hewlett-Packard 3390A integrator. The injector
and the detector were kept at 523 K, and the column was kept
at 363 K. A nitrogen flow of 0.333 cm3‚s-1 was used as the
carrier gas in the analysis. At the end of each transpiration run,
200 µL of a standard solution of n-C11H24 in CH3CN of
concentration 4.65 g‚dm-3 was added to the trap containing the
condensed (S)-(-)-2-chloropropionic acid, and 0.1 µL of the
resulting mixture was injected in the GC apparatus. The
corresponding mass of sample (m) was then obtained from
m/mst ) a + b(A/Ast), where mst ) 0.93 mg is the mass of
n-C11H24, A and Ast are the areas of the sample and the internal
standard peaks, respectively, and a and b are constants previ-
ously found by calibration of the chromatographic apparatus
by use of solutions with known amounts of (S)-(-)-2-chloro-
propionic acid and n-C11H24.
Elemental analyses were carried out on a Heraeus XHN-O-
Rapid analyzer. The optical rotation measurements were made
on a Perkin-Elmer 341 polarimeter. The IR spectra were
recorded on a Perkin-Elmer PE 1720 apparatus with NaCl plates.
The 1H and 13C NMR spectra were obtained at ambient
temperature on a Bruker AMX (400 MHz) spectrometer. The
13C spectra were recorded at 75.5 MHz. Chemical shifts relative
to tetramethylsilane (TMS) were calculated with CHCl3 as an
internal standard. Differential scanning calorimetry (DSC)
measurements were made with a 2920 MTDSC temperature-
modulated apparatus from TA Instruments. The apparatus was
connected to a liquid nitrogen cooling accessory and was
operated as a conventional DSC.
Materials. (S)-(-)-2-Chloropropionic acid was prepared from
optically pure (S)-(+)-alanine (Merck 816000) by the method
of Fu et al.12 An amount of 50.0 g (0.56 mol) of (S)-(+)-alanine
was dissolved in 750 cm3 of HCl(aq, 6 mol‚dm-3), and the
solution was cooled to 273 K. With stirring, 62.5 g (0.91 mol)
of freshly pulverized sodium nitrite were added in small portions
and at such a rate that the temperature of the solution remained
between 273 and 278 K. About 2.5 h was required for this
purpose, and the stirring was continued at 273 K for another 4
h. The resulting solution was extracted five times with 150 cm3
diethyl ether portions. The ethereal extract was dried over CaCl2
for 10 h, and the solvent was removed by evaporation. The
slightly yellow residual liquid was fractionally distilled twice
at 100 Pa and 329 K. After distillation, 26.4 g of colorless liquid
(S)-(-)-2-chloropropionic acid was obtained, which corresponds
to a yield of 43% (lit. 43.5%12). Polarimetric analysis (l )
10 cm, pure): RD ) -17.48° (lit. ) -18.2° 12). IR (NaCl):
25
ν˜/cm-1 ) 2993, 2933 (C-H), 1729 (COO-), 1445, 1420
(COO-, C-H), 1383, 1289, 1256, 1212, 1076, 990, 921, 858.
1H NMR (400 MHz, CDCl3/TMS): δ ) 1.73 (d, J ) 7.12 Hz,
3 H, CH3), δ ) 4.46 (q, J ) 7.12, 1 H, CH). 13C NMR (75.5
MHz, CDCl3/TMS): δ ) 21.40 (s, CH3), δ ) 52.41 (s, CH),
δ ) 176.36 (s, COO). Elemental analysis for C3H5O2Cl:
expected C 33.20%, H 4.64%; found C 32.92%, H 4.83%
(average of two measurements). The temperature of fusion of
the sample obtained by DSC at the onset of the measuring curve
was Tfus ) (266.2 ( 1.2) K. The corresponding enthalpy of
A critical aspect of the transpiration method is the selection
of an adequate carrier gas flow rate across the saturation tube
at the temperature of the experiment. The flow rate, qV, must
be low enough to allow the saturation of the nitrogen stream
with the compound under study before exiting the U-tube.
However, if the flow rate is too low, the transport of material
out of the U-tube by diffusion may significantly compete with
the transport by transpiration. Tests conducted on our apparatus
with a variety of carboxylic acids and alcohols14,15 indicated
that the unsaturation and diffusion effects could be avoided
provided a flow rate in the range 0.11 cm3‚s-1 e qV e 1.5
cm3‚s-1 was selected. In the present experiments, flow rates of
0.946 or 0.960 cm3‚s-1 were used.
Combustion Calorimetry. The isoperibol rotating-bomb
combustion calorimeter used in this work has been described.16
The bomb, with a volume of 0.258 dm3, was of stainless steel
lined with platinum, and all the internal fittings were machined
from platinum. The sample container was a platinum crucible
supported by a platinum ring inside the bomb. The total mass
of the crucible and the ring was ca. 11.7 g. In a typical
experiment, the liquid sample sealed inside a Melinex bag and
the n-hexadecane (Aldrich, Gold Label) used as a combustion
aid, were weighed inside the crucible in a Mettler AE 240
balance with a precision of (10-5 g. A platinum wire of
diameter 0.05 mm (Goodfellow, mass fraction 0.9999) was
fusion was ∆fusH°(C3H5O2Cl) ) (9.8 ( 0.3) kJ‚mol-1. The
m
uncertainties quoted for both values represent twice the standard
deviation of the mean of five independent determinations.
Heat Capacity Mesurements by DSC. The heat capacity
of liquid (S)-(-)-2-chloropropionic acid was measured as
recommended by Richardson.13 This involved three sequential
runs with the same scanning program and a heating rate of 10
K‚min-1: (i) the zero line run with two empty crucibles in the
sample and reference holders; (ii) a second run, used to calibrate
the heat flow scale of the apparatus in terms of heat capacity,
with synthetic sapphire (R-Al2O3 pellets, NIST-RM 720) inside
the sample crucible; and (iii) a third run performed by replacing
sapphire by (S)-(-)-2-chloropropionic acid in the sample
crucible. The temperature scale of the apparatus was calibrated
by taking the onsets of the fusion peaks of the following
compounds: n-decane, Tfus ) 243.75 K; n-octadecane, Tfus
)

Energetics of the C-Cl Bond in CH3CH(Cl)COOH
J. Phys. Chem. A, Vol. 106, No. 42, 2002 9857
TABLE 1: Results of Vapor Pressure Measurements on
fastened between the ignition electrodes. A cotton thread fuse
of empirical formula CH1.686O0.843 was weighed with a precision
of (10-5 g and tied to the platinum wire. The crucible was
adjusted to the bomb head and the cotton thread fuse was placed
in contact with the sample, without touching the crucible walls.
A volume of 20 cm3 of a 0.09016 mol‚dm-3 As2O3(aq) was
placed inside the bomb. The presence of the arsenious oxide
solution ensured that all Cl2 formed in the combustion was
reduced to aqueous HCl.17,18 After the introduction of the bomb
solution, the bomb was closed and purged twice by charging it
with oxygen at a pressure of 1.01 MPa and then venting the
overpressure. The bomb was then charged with oxygen at a
pressure of 3.04 MPa and transferred to the inside of the
calorimeter proper. The electrical connections of the firing circuit
were attached to the bomb head, and the calorimeter proper
was filled with an amount of distilled water as close as possible
to the average mass of water used in calibration experiments
(5217.0 g). The water added to the calorimeter proper in each
experiment was weighed to (0.1 g in a Mettler PC 8000
(S)-(-)-2-Chloropropionic Acid
m/mg
VN /dm3
T/K
p/Pa
2
3.670
3.366
3.373
4.161
5.221
4.899
5.738
5.247
4.041
3.497
3.111
3.411
4.190
3.673
3.610
6.280
4.379
3.428
3.124
3.008
2.215
1.988
1.440
0.879
0.605
0.461
0.403
0.398
0.284
0.231
287.4
290.5
293.4
296.3
299.5
302.5
305.4
308.4
311.4
314.4
316.4
319.3
322.3
325.3
328.3
13.36
17.54
22.42
30.31
39.47
50.26
65.57
82.72
104.4
131.2
153.2
191.9
238.9
293.4
355.3
Results and Discussion
The heat capacity of liquid (S)-(-)-2-chloropropionic acid
balance. Calorimeter temperatures were measured to (10-4
K
obtained by DSC, at 298.15 K, was C° (C3H5O2Cl, l) )
p,m
(260 ( 6) J‚K-1‚mol-1, where the uncertainty quoted represents
with a Hewlett-Packard (HP2804A) quartz thermometer. The
duration of the fore, main, and after periods was ca. 20 min
each. Discharge of a 1400 µF capacitor through the platinum
wire referred to above ignited the cotton thread fuse and
subsequently the sample. For each experiment the ignition
temperature was chosen so that the final temperature would be
close to 298.15 K. The rotation of the bomb was started when
the temperature rise of the main period reached about 0.63 of
its total value and was continued throughout the experiment. It
has been shown that, by adopting this procedure, the frictional
work due to the rotation of the bomb is automatically accounted
for in the calculation of the adiabatic temperature rise.19,20 The
HNO3 formed from traces of atmospheric N2 remaining inside
the bomb was analyzed by the Devarda’s alloy method.21 The
extent of the reaction of Cl2 formed in the combustion with the
arsenious oxide solution was found by titrating the final bomb
solution with sodium thiosulfate (0.100 mol‚dm-3).21
Computational Details. The density functional theory (DFT)
calculations reported in this study were carried out with the
Gaussian-98 program.22 The geometry optimizations and the
calculations of the total energies were made by the Becke three-
parameter hybrid method23 with the Perdew and Wang PW9124
correlation functional (B3PW91). This hybrid functional is
known to predict accurate values for reaction enthalpies with
moderately sized basis sets.25 Total energies (E) were obtained
from26
twice the standard deviation of the mean of three independent
results. This value is in agreement with C° (C3H5O2Cl, l) )
p,m
(252 ( 25) J‚K-1‚mol-1 previously reported by Sano et al.31 in
the temperature range 318-333 K. It is also interesting to note
that two widely used versions of the Kopp method lead to
C° (C3H5O2Cl, l) ) 149 and 209 J‚K-1‚mol-1, respectively.32
p,m
These lower predictions probably reflect the inadequacy of the
method to estimate C° of strongly associated liquids. The
p,m
heat capacity of gaseous (S)-(-)-2-chloropropionic acid pre-
dicted by DFT calculations (B3PW91) with the 6-311+G(d,p)
and aug-cc-pVDZ basis sets were 101.6 and 102.1 J‚K-1‚mol-1
,
respectively. The average of these two values, C° (C3H5O2Cl,
p,m
g) ) 101.9 J‚K-1‚mol-1, was selected in the present work.
The vapor pressures p of (S)-(-)-2-chloropropionic acid as
a function of the temperature measured by the transpiration
method are given in Table 1. These results were obtained by
using the following iterative procedure. A first approximation
of p (p1) was calculated from the mass m of product collected
within a definite time period:14
mRTa
p )
(2)
MVN
2
where R ) 8.31451 J‚K-1‚mol-1 is the gas constant, M is the
molar mass of the sample, Ta ) 296 ( 1 K is the ambient
temperature, and VN is the total volume of N2 used as carrier
2
E ) VNN + HCORE + Vee + EX[F] + EC[F]
(1)
gas throughout the experiment. The obtained results were fitted
to the Clausius-Clapeyron equation
where VNN is the nuclear-nuclear interaction, HCORE is a
monoelectronic contribution to the total energy, including
electron kinetic and electron-nuclear interaction energies, and
Vee is the Coulombic interaction between the electrons. The
terms EX[F] and EC[F] represent the exchange and correlation
energies, respectively, functionals of the electronic density F.
Full geometry optimizations have been carried out with the
6-31G(d,p),27 6-311+G(d,p),28 and aug-cc-pVDZ29 basis sets.
The corresponding total energies were corrected with the zero-
point vibrational energies (ZPVEs) and thermal energy correc-
tions (without frequency scaling) calculated at the same
theoretical level. Single-point energy calculations with the aug-
cc-pVTZ30 basis set based on geometries, zero-point energy,
and thermal energy corrections obtained from the aug-cc-pVDZ
basis set were also made.
b
T
ln p ) a +
(3)
by the least-squares method, and the resulting equation was used
to derive a first approximation of the residual pressure of the
compound (pr1) at the condensation temperature (243 K). New
values of p were derived as p2 ) p1 + pr1. These were
subsequently fitted to eq 3 and a second approximation of the
residual pressure of the compound (pr2) at the condensation
temperature was computed. A third series of p values was
calculated as p3 ) p1 + pr2 and the iteration was continued
until the difference between successive values of pr was smaller
than 10-4 Pa.
Equation 2 is based on the assumption that N2 and the vapor
of (S)-(-)-2-chloropropionic acid constitute an ideal gas mixture

9858 J. Phys. Chem. A, Vol. 106, No. 42, 2002
Lagoa et al.
TABLE 2: Results of the Combustion Experiments on (S)-(-)-2-Chloropropionic Acid
m(C3H5O2Cl)/g
m(cotton)/g
0.751 36
0.002 69
0.302 26
0.044 94
1.7
8.0
5.02
2.08
94.95
93.52
297.1288
298.1398
0.017 60
1.29
25 112.41
46.03
4.78
160.68
0.51
1029.16
43.69
14 254.39
9573.17
12 741.12
0.744 81
0.002 28
0.299 59
0.042 06
0.8
5.0
4.09
2.12
94.93
93.56
297.1480
298.1464
0.017 27
1.29
24 799.00
35.50
2.98
130.93
0.52
963.36
37.03
14 128.32
9500.36
12 755.41
0.853 54
0.002 81
0.295 15
0.045 09
1.5
3.0
4.69
3.18
95.11
93.56
297.0922
298.1420
0.017 44
1.29
26 098.11
48.02
1.79
150.12
0.78
1032.58
45.63
13 918.68
10 900.51
12 770.94
0.840 12
0.002 65
0.303 57
0.044 59
1.3
4.0
4.75
1.67
95.10
93.60
297.0840
298.1420
0.017 48
1.29
26 301.23
48.27
2.39
152.36
0.41
1021.16
43.04
14 315.78
10 717.82
12 757.49
0.769 25
0.002 70
0.302 17
0.041 13
-0.5
0.792 62
0.002 69
0.294 68
0.050 03
0.1
10.0
4.36
2.24
95.01
93.59
297.1288
298.1505
0.016 88
1.29
25 393.15
46.80
5.97
139.56
0.55
1145.89
43.69
13 896.96
10 113.73
12 759.87
0.790 81
0.002 47
0.301 97
0.045 58
0.1
5.0
4.12
1.59
95.01
93.61
297.1165
298.1460
0.017 20
1.29
25 583.72
47.05
2.98
131.88
0.39
1043.78
40.11
14 240.70
10 076.83
12 742.42
m(n-C16H34)/g
m(Melinex)/g
∆m(H2O)/g
105 × n(HNO3)/mol
104 × n(As2O3)/mol
106 × n(H2PtCl6)/mol
ꢀi/J‚K-1
8.0
4.09
2.52
94.97
93.61
297.1289
298.1446
0.017 16
1.29
25 232.66
46.25
4.78
130.93
0.62
ꢀf/J‚K-1
Ti/K
Tf/K
∆Tc/K
∆Uign/J
-∆UIBP/J
∆UΣ/J
∆U(HNO3)/J
∆U(As2O3)/J
∆U(H2PtCl6)/J
∆U(Melinex)/J
-∆U(cotton)/J
-∆U(n-C16H34)/J
∆U(C3H5O2Cl)/J
-∆cuo(C3H5O2Cl)/J‚g-1
941.89
43.85
14 250.14
9814.20
12 758.14
where ∆vapC° represents the difference in heat capacity of
p,m
the gaseous and liquid compound. This leads to
∆
vapH°(C3H5O2Cl) ) 64.9 ( 0.5 kJ‚mol-1 at 298.15 K.
m
The results of the combustion calorimetric experiments are
shown in Table 2, where m(C3H5O2Cl) is the mass of (S)-
(-)-2-chloropropionic acid; m(cotton) represents the mass of
the cotton thread fuse of empirical formula CH1.686O0.843
;
m(Melinex) is the mass of the Melinex bag containing the
sample; m(n-C16H34) is the mass of n-hexadecane used as
combustion auxiliary; ∆m(H2O) represents the difference be-
tween the mass of water inside the calorimeter proper and the
average mass of water used in the calibration experiments
(5217.0 g); n(HNO3) is the amount of substance of nitric acid
formed in the bomb process; n(As2O3) is the amount of
substance of arsenious oxide consumed in the reaction with the
Cl2; n(H2PtCl6) is the amount of substance of H2PtCl6 formed
in the bomb process from the mass of platinum lost by the
crucible and its supporting ring; ꢀi and ꢀf are the energy
equivalents of the bomb contents in the initial and final states
of the bomb process, respectively; Ti and Tf represent the initial
and final temperatures of the experiment; ∆Tc is the contribution
to the observed temperature rise of the calorimeter proper due
to the heat exchanged with the surroundings, the heat of stirring,
and the heat dissipated by the temperature sensor and by the
bomb rotation; ∆Uign is the electrical energy supplied for ignition
of the sample; ∆UIBP is the internal energy change associated
with the bomb process under isothermal conditions, at 298.15
K; ∆UΣ represents the sum of all corrections necessary to reduce
∆UIBP to the standard state (Washburn corrections); ∆U(HNO3)
is the energy change associated with the formation of nitric acid;
∆U(As2O3), is the energy change due to the reaction of the
aqueous As2O3 inside the bomb with the Cl2 formed in the
combustion; ∆U(H2PtCl6) represents the energy change associ-
ated with the conversion of Pt to H2PtCl6, in the bomb process;
∆U(Melinex), ∆U(cotton), ∆U(n-C16H34), and ∆U(C3H5O2Cl)
are the contributions of the Melinex container, cotton thread
fuse, n-hexadecane, and (S)-(-)-2-chloropropionic acid, respec-
tively, to the energy of the isothermal bomb process; and finally,
∆cu°(C3H5O2Cl) is the standard specific energy of combustion
of (S)-(-)-2-chloropropionic acid.
Figure 1. Vapor pressure of (S)-(-)-2-chloropropionic acid as a
function of the temperature.
and that the total volume of the mixture is VN (i.e., the volume
2
of the acid is negligible). The values of VN were determined
2
from the selected flow rates and the duration of the experiments.
The accuracy of the VN measurements was established to be
2
(0.001 dm3 in a series of experiments where the nitrogen
volume used was obtained with a gas-clock or with a calibrated
gasometer. Because the nitrogen flow rate was measured with
a soap bubble flow-meter at ambient temperature, this temper-
ature was used in eq 2 for the calculation of p.
Least-squares fitting of eq 3 to the p vs T data in Table 1
lead to a ) (29.14 ( 0.21) and b ) -∆vapH°/R ) -(7629.8 (
m
65.7) K with a regression coefficient of 0.9999. The corre-
sponding plot of ln p versus 1/T is presented in Figure 1. From
these results, ∆vapH°(C3H5O2Cl) ) 63.4 ( 0.5 kJ‚mol-1 at the
m
mean temperature of the temperature range covered by the
experiments, Tm ) 307.9 K, is obtained. The uncertainties
quoted for a and b and ∆vapH°(C3H5O2Cl) values include
m
Student’s factor for 95% confidence level (t ) 2.145).33 The
enthalpy of vaporization of (S)-(-)-2-chloropropionic acid at
298.15 K can be calculated from the corresponding value at
307.9 K and the heat capacity data indicated above:
∆
vapH°(298.15 K) ) ∆vapH°(307.9 K) +
m m
The molar masses used in the calculation of molar quantities
are based on the 1995 standard atomic masses.34 The values of
∆
vapC° (298.15 K - 307.9 K) (4)
p,m

Energetics of the C-Cl Bond in CH3CH(Cl)COOH
J. Phys. Chem. A, Vol. 106, No. 42, 2002 9859
TABLE 3: Theoretically Calculated Enthalpies of Reaction 11 and Enthalpies of Formation of CH(CH3)COOH(g) at 298.15 Ka
X ) H
X ) Cl
method
-∆rH°m(11)
-∆fH°m[CH(CH3)COOH, g]
-∆rH°m(11)
-∆fH°m[CH(CH3)COOH, g]
B3PW91/6-31G(d,p)
40.7
37.5
36.7
38.0
293.6
290.4
289.6
290.9
56.5
54.4
54.7
55.5
295.1
293.0
293.3
294.1
B3PW91/6-311+G(d,p)
B3PW91/aug-cc-pVDZ
B3PW91/aug-cc-pVTZ//
B3PW91/aug-cc-pVDZ
a Data are given in kilojoules per mole.
Ti, Tf, and ∆Tc were calculated by using a computer program35
based on the Regnault-Pfaundler method,36-38 and ∆UIBP was
derived from39
uncertainty quoted is the standard deviation of the mean. This
result refers to
C3H5O2Cl (l) + 3O2 (g) + 598H2O (l) f
3CO2 (g) + HCl‚600H2O (l) (10)
∆
IBPU ) [ꢀo + ∆m(H2O)C° (H2O, l)](Ti - Tf + ∆Tc) +
p,m
ꢀi(Ti - 298.15) + ꢀf(298.15 - Tf + ∆Tc) + ∆Uign (5)
and leads to ∆cU° ) ∆cH° ) -(1384.22 ( 1.04) kJ‚mol-1
.
m
m
The uncertainties quoted represent twice the overall standard
deviation of the mean and include the contributions from the
calibration with benzoic acid and from the energy of combustion
The energy equivalent of the calorimeter and its standard
deviation, ꢀo ) 25177.4 ( 1.4 J‚K-1, was determined, in the
conventional way, without bomb rotation,37,40,41 from the
combustion of benzoic acid (Bureau of Analyzed Samples,
Thermochemical Standard CRM-190r), whose energy of com-
of n-hexadecane.52 From the ∆cH° value indicated above and
m
∆fH°(CO2, g) ) -(393.51 ( 0.13) kJ‚mol-1 53
,
∆fH°(H2O, l)
m
m
) -(285.830 ( 0.040) kJ‚mol-1 53
,
and ∆fH°(HCl‚600H2O, l)
bustion under certificate conditions was -(26432.3 ( 3.8) J‚g-1
.
m
) -(166.619) kJ‚mol-1 47
,
it is possible to derive ∆fH°-
m
The standard state corrections, ∆UΣ, were derived as recom-
mended in the literature for organic compounds containing
chlorine,42,43 by using the following auxiliary data: for n-
(C3H5O2Cl, l) ) -(534.6 ( 1.1) kJ‚mol-1. This value differs
by 10.9 kJ‚mol-1 from ∆fH°(C3H5O2Cl, l) ) -(523.7 ( 8.3)
m
kJ‚mol-1 4,5
,
previously measured by Smith et al.4 using static
hexadecane, c° ) 2.215 J‚K-1‚g-1 44
,
F ) 0.773 g‚cm-3 44
for (S)-(-)-2-chloropropionic
acid, c°p ) 2.396 J‚K-1‚g-1 (this work, see above), F ) 1.2585
g‚cm-3 44 -(∂u/∂p)T ) 0.20 J‚MPa1-‚g-1 46
The values of
∆U(HNO3) and ∆U(H2PtCl6) correspond to the processes
, and
p
bomb combustion calorimetry, which, in principle, does not
yield reliable results for chlorine-containing organic com-
pounds.17,19,54,55 The enthalpy of formation of liquid (S)-(-)-
2-chloropropionic acid and the corresponding enthalpy of
-(∂u/∂p)T ) 0.347 J‚MPa-1‚g-1 45
;
,
.
vaporization obtained in this work lead to ∆fH°(C3H5O2Cl, g)
m
) -(469.7 ( 1.2) kJ‚mol-1
.
1
2
1
2
5
4
HNO3(aq, 0.1 mol.dm-3) f H2O(l) + N2(g) + O2(g)
The standard enthalpy of formation of the CH(CH3)COOH
radical was derived from the enthalpy of the isodesmic and
(6)
isogyric reaction 11, ∆rH°(11), calculated by DFT for X ) H,
m
H2PtCl6(aq) + 2H2O(l) f Pt(cr) + O2(g) + 6HCl(aq)
Cl (Table 3). The ∆fH°[CH(CH3)COOH, g] values in Table 3
m
(7)
were obtained through eq 12 by using the enthalpy of forma-
tion of gaseous ClCH(CH3)COOH determined in this work
and were obtained from the following data: ∆fU°(HNO3, aq,
(-469.7 ( 1.2 kJ‚mol-1) and ∆fH°(CH3CH2COOH, g) )
m
m
0.1 mol‚dm-3) ) -59.7 kJ‚mol-1 47 and ∆rU°(7) ) 245.6
-(455.7 ( 2.5) kJ‚mol-1,5 ∆fH°(C2H6, g) ) -(83.8 ( 0.3)
m
m
kJ‚mol-1 48
.
The term ∆U(As2O3), corresponds to
kJ‚mol-1,5 ∆fH°(C2H5Cl, g) ) -(112.1 ( 1.1) kJ‚mol-1,5 and
m
∆fH°(C2H5, g) ) 119 ( 2 kJ‚mol-1 56
.
m
As2O5(aq) f As2O3(aq) + O2(aq)
(8)
XCH(CH3)COOH(g) + C2H5(g) f
It depends on the concentration of As2O5 and H+ in the final
bomb solution and was derived for each experiment by using
the equations and data given by Hu et al.42 The calculation of
∆U(cotton), and ∆U(Melinex) was based on ∆cu°(cotton) )
CH(CH3)COOH(g) + C2H5X(g) (11)
∆fH°[CH(CH3)COOH, g] )
m
∆rH°(11) - ∆fH°(C2H5X, g) + ∆fH°(C2H5, g) +
-16250 J‚g-1 39,49 and ∆cu°(Melinex) ) -(22902 ( 5) J‚g-1 50
.
m
m
m
∆fH°[XCH(CH3)COOH, g] (12)
The values of ∆U(n-C16H34) were obtained by using ∆cu°(n-
m
C16H34) ) -(47158.7 ( 3.2) J‚g-1 51
,
which was independently
As seen in Table 3, the ∆rH°(11) results are quite insensitive
measured by static-bomb combustion calorimetry. The standard
specific energy of combustion of (S)-(-)-2-chloropropionic acid
was calculated from
m
to the basis set used in the calculation. In addition, the values
of ∆fH°[CH(CH3)COOH, g] obtained from the several ∆rH°-
m
m
(11) results and the auxiliary enthalpy of formation data
indicated above are in excellent agreement. This supports the
reliability of the DFT results and indicates a very good
thermodynamic consistency between the theoretical reaction
enthalpies and the experimental standard enthalpy of formation
∆cu°(C3H5O2Cl) ) [∆UIBP + ∆UΣ + ∆U(HNO3) +
∆U(As2O3) + ∆U(H2PtCl6) - ∆U(cotton) -
∆U(n-C16H34) - ∆U(Melinex)]/m(C3H5O2Cl) (9)
data. The selected value for ∆fH°[CH(CH3)COOH, g],
m
The mean value of the standard massic energies of combus-
tion of (S)-(-)-2-chloropropionic acid indicated in Table 2 is
∆cu°(C3H5O2Cl) ) -(12755.06 ( 3.92) J‚g-1, where the
-(293 ( 3) kJ‚mol-1, is the mean of the results in Table 3.
This value, together with ∆fH°(H, g) ) 217.998 ( 0.006
m
kJ‚mol-1 53
,
∆fH°(Cl, g) ) 121.301 ( 0.008 kJ‚mol-1 53
,
and
m

9860 J. Phys. Chem. A, Vol. 106, No. 42, 2002
Lagoa et al.
TABLE 4: R-X Bond Dissociation Enthalpies in kJ‚mol-1
Cl)calc values calculated from Pauling’s arithmetic mean expres-
sion:
X
R
H (2.25)
Cl (3.10)
R
DH°(R-R) + DH°(C-Cl)
412.8 ( 3.2b 310.9 ( 2.2b 342.7 ( 4.3b
DH°(R-Cl) )
+
CH2COOH (2.74)
2
CH(CH3)COOH (2.61) 380.7 ( 3.9c 298.0 ( 3.2c 279.1 ( 6.7d
96.232(øR - øCl)2 (13)
CH3 (2.31)
C2H5 (2.32)
CH2Cl (2.55)
CH(CH3)Cl (2.46)
CHdCH2 (2.43)
C6H5 (2.49)
438.8 ( 0.6b 349.6 ( 0.6b 376.6 ( 0.6b
420.8 ( 2.0b 352.4 ( 2.3b 363.7 ( 2.9b
418.9 ( 2.5e 335.7 ( 2.6e 364.6 ( 5.3d
406.5 ( 2.2e 324.5 ( 2.4e
where øCl ) 3.10 and øR are the electronegativities of Cl and
R, respectively, by using DH°(Cl-Cl) ) 242.602 ( 0.011
kJ‚mol-1 and the DH°(R-R)3,59 and group electronegativity60
data given in Table 4, respectively. This correlation corresponds
to
465.1 ( 3.3b 400.0 ( 5.3e 489.2 ( 4.8b
465.5 ( 3.5b 399.4 ( 1.7b 478.8 ( 5.1b
a Enthalpies are given in kilocalories per mole. Pauling’s electro-
negativities60 of the R and X fragments are given in parentheses.
b Reference 3. c This work. d Reference 59. e Reference 57.
DH°(R-Cl) ) (0.833 ( 0.083)DH°(R-Cl)calc
+
(56.87 ( 29.48) (14)
with a regression coefficient of 0.98. The average error in the
estimation of DH°(R-Cl) from eq 14 is 8.8 kJ‚mol-1
.
Finally, the results obtained in this work allow us to address
the energetics of the gas-phase decompositions of chloroacetic
and 2-chloropropionic acid, which are known to occur according
to (R ) H, CH3)9
RCH(Cl)COOH(g) f RCHO(g) + CO(g) + HCl(g) (15)
From the enthalpy of formation of gaseous 2-chloropropionic
acid determined in this work (-469.7 ( 1.2 kJ‚mol-1),
Figure 2. R-H and R-Cl bond dissociation enthalpies.
∆fH°(ClCH2COOH, g) ) -(427.55 ( 1.04) kJ‚mol-1 3
,
m
∆fH°(CO, g) ) -(110.52 ( 0.17) kJ‚mol-1 53
,
∆fH°(HCl, g)
m
) -m(92.31 ( 0.10) kJ‚mol-1 53
, ∆fH°(HCHO, g) ) -(108.6 (
m
0.5) kJ‚mol-1 5
, and ∆fH°(CH3CHO, g) ) -(166.1 ( 0.5)
m
kJ‚mol-1,5 it is possible to conclude that ∆rH°(15) ) 116.1 (
m
1.3 kJ‚mol-1 (R ) H) and 100.8 ( 1.3 kJ‚mol-1 (R ) CH3).
In their theoretical study of the kinetics and mechanism of the
thermal decomposition of 2-chloropropionic acid, Safont et al.10
predicted the reaction to be endothermic with a computed
reaction enthalpy noticeably dependent on the method of
calculation: 80.42 kJ‚mol-1 (MP2/6-31G**), 82.80 (MP2/6-
31++G**), and 65.53 kJ‚mol-1 (CISD/6-31G**) with a scale
factor of 0.91, or 77.64 kJ‚mol-1 (MP2/6-31G**), 80.03 (MP2/
6-31++G**), and 62.61 kJ‚mol-1 (CISD/6-31G**) without
scale factor. Thus the calculations by Safont et al. correctly
predict the reaction to be endothermic but the computed values
Figure 3. Experimental R-Cl bond dissociation enthalpies, DH°(R-
Cl), from Table 4, versus the corresponding values calculated from eq
13, DH°(R-Cl)calc, for R ) CH2COOH, CH(CH3)COOH, CH3, C2H5,
CH2Cl, CH(CH3)Cl, CHdCH2, and C6H5.
the standard enthalpies of formation of ∆fH°[XCH(CH3)-
m
are smaller than the experimental ∆rH°(15) value by 18-38
m
COOH, g] (X ) H, Cl) indicated above, yield the bond
dissociation enthalpies DH°[H-CH(CH3)COOH] ) 380.7 (
3.9 kJ‚mol-1 and DH°[Cl-CH(CH3)COOH] ) 298.0 ( 3.2
kJ‚mol-1. These values are compared in Table 4 with the
corresponding C-H and C-Cl bond dissociation enthalpies in
XCH2COOH, XCH3, XC2H5, XCH2Cl, XCH(CH3)Cl, XCHd
CH2, and XC6H5 (X ) H, Cl) quoted or calculated from the
literature.3,57 The order DH°(C-H) > DH°(C-Cl) is observed
for the carboxylic acids and all other RX compounds in Table
4 (Figure 2). Comparison of DH°[X-CH(CH3)COOH] and
DH°[X-CH2COOH] (X ) H, Cl) indicates that the replacement
of a hydrogen of the CH2 group of XCH2COOH by a methyl
group leads to a decrease of both the C-H and C-Cl bond
dissociation enthalpy. This seems to be a general trend for DH°-
(C-H),8 and DH°(C-Cl) with the notable exception DH°(Cl-
CH3) j DH°(Cl-C2H5) (Table 4).
kJ‚mol-1
.
Acknowledgment. This work was supported by Fundac¸a˜o
para a Cieˆncia e a Tecnologia (Project POCTI/199/QUI/35406),
the Deutsche Forschungsgemeinschaft, the Fonds der Chemis-
chen Industrie, and Acc¸o˜es Integradas Luso-Alema˜s. A Ph.D.
grant from Fundac¸a˜o para a Cieˆncia e a Tecnologia (PRAXIS
XXI/BD/15920/98) is also gratefully acknowledged by R.C.G.
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Energetics of the C-Cl Bond in CH3CH(Cl)COOH
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(57) DH°(H-CH2Cl) ) 418.9 ( 2.5 kJ‚mol-1, DH°[H-CH2(CH3)Cl]
) 406.5 ( 2.2 kJ‚mol-1, DH°(Cl-CH2Cl) ) 335.7 ( 2.6 kJ‚mol-1, and
DH°[Cl-CH2(CH3)Cl] ) 325.4 ( 2.4 kJ‚mol-1 were derived as follows.
First ∆fH°(CH2Cl, g) ) 119.0 ( 2.4 kJ‚mol-1 and ∆fH°[CH2(CH3)Cl, g]
m
) 76.4 (m1.9 kJ‚mol-1 were obtained from ∆fH°(CH3Cl, g) ) -(81.9 (
m
0.5) kJ‚mol-1 (ref 5), ∆fH°(CH3CH2Cl, g) ) -(112.1 ( 1.1) kJ‚mol-1
m
(ref 5), ∆fH°(HBr, g) ) -(36.29 ( 0.16) kJ‚mol-1 (ref 53), ∆fH°(Br, g)
m
m
) 111.87 ( 0.12 kJ‚mol-1 (ref 53), and the enthalpies of the reaction
RCHCl(g) + HBr(g) f RCH2Cl(g) + Br(g) (-52.7 ( 2.3 kJ‚mol-1 for R
) H and -40.3 ( 1.5 kJ‚mol-1, for R ) CH3) reported in Seetula, J. J.
Chem. Soc., Faraday Trans. 1996, 92, 3069. Then the above-mentioned
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bond dissociation enthalpies were computed by using ∆fH°(CH2Cl2, g) )
m
-(95.4 ( 1.1) kJ‚mol-1 and ∆fH°[CH(CH3)Cl2, g] ) -(127.7 ( 1.4)
kJ‚mol-1 in ref 5. Note that in theirmrecent review Kerr and Stocker (ref 8),
give DH°(Cl-CH2Cl) ) 350.2 ( 0.8 kJ‚mol-1, a considerably different
value from that selected in the present paper. This is probably a typo since
according to these authors the value was taken from Tschuikow-Roux, E.;
Paddison, S. Int. J. Chem. Kinet., 1987, 19, 15, who in fact recommend
DH°(Cl-CH2Cl) ) 338.5 ( 4.2 kJ‚mol-1. DH°(Cl-CHCH2) ) 400.0 (
5.3 kJ‚mol-1 was derived from ∆fH°(CHdCH2, g) ) 299.6 ( 3.3
m
kJ‚mol-1 in ref 7 and ∆fH°(ClCHdCH2, g) ) 20.9 ( 4.2 kJ‚mol-1
m
recommended in Colegrove, B. T.; Thompson, T. B. J. Chem. Phys. 1997,
106, 1480.
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University Press: New York, 1960; p 92.
(59) DH°[HOOC(CH3)HC-CH(CH3)COOH] ) 279.1 ( 6.7 kJ‚mol-1
is based on the enthalpy of formation of the CH(CH3)COOH radical obtained
in this work and on ∆fH°[HOOC(CH3)HC-CH(CH3)COOH, cr] )
m
-(987.8 ( 1.4) kJ‚mol-1 in ref 5 and on ∆subH°[HOOC(CH3)HC-
CH(CH3)COOH] ) 122.7 ( 2.7 kJ‚mol-1 reported in mRibeiro da Silva, M.
A. V.; Monte, M. J. S.; Ribeiro, J. R. J. Chem. Thermodynamics 2001, 33,
23. DH°(ClCH2-CH2Cl) ) 364.6 ( 5.3 kJ‚mol-1 is based on ∆fH°-
(CH2Cl, g) ) 119.1 ( 2.4 kJ‚mol-1 in ref 57 and on ∆fH°(ClCH2m-
m
CH2Cl, g) ) -(126.4 ( 2.3) kJ‚mol-1 in ref 5.
(60) (a) Bratsch, S. G. J. Chem. Educ. 1988, 65, 34. (b) Bratsch, S. G.
J. Chem. Educ. 1988, 65, 223. The group electronegativity values in Table
4 were directly taken from these references or calculated in this work by
the method described therein.