1018
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
TABLE 11: E f f e c t i v e Dielectric Constants Acid I Acid I1 Acid I11
R
DE
7.06 7.06 1.06
40.6 53.9 51.4
acid which would be intramolecularly hydrogen bonded. If structure V, a betaine structure, is occurring in solution, the acid strength (pK2 = 2.85) would be expected to exhibit an acid strength less than a positively charged nitrogen atom, e.g., (CHJ3+NCH2C02HC1- (pK = 1.83)," and be comparable with a substituents that are strongly electron withdrawing, Le., NsCCH2CO2H, pK = 2.47; C1CH2C02H,pK = 2.85; and BrCH,C02H, pK = 2.90.19 Direct evidence for hydrogen bonding in dicarboxylic acids is established by synthesizing the half-esters of the acids. This was attempted unsuccessfully in this laboratory by several procedures; starting with the monoacid and alkylating with bromoacetic acid in a bicarbonate solution, starting with the monoester and alkylating with bromoacetic acid in a bicarbonate solution, and treating the anhydrides with a stoichiometric amount of alcohol to produce the monoester-monoacid compound. In each of these trials, after several attempts for each, the solution produced an untractable oil which failed to crystallize from many different types of solvents and mixed solvents. Therefore, the ionization constants for the half-esters of these dicarboxylic acid are not available to establish the absolute proof against hydrogen bonding. Table I1 lists the values calculated for DE,the effective dielectric constant, from the Kirkwood-Westheimer formula for dibasic acidsag The value for R, the interprotonic distance, was calculated for acid I11 by taking the
J. A. Faniran, K.
S. Patel, and L. 0. Nelson
average of the maximum extension distance (8.55 A) and the free rotation distance (5.58 A). The free rotation distance was calculated using Eyring's formulasz0 This method of estimating R has been used p r e v i ~ u s l y . ~ Bond lengths and angles used in the calculations were made considering the carboxyl group as a planar system.21 The bond lengths and angles used were as follows: bond lengths, HO (0.95); OC (1.31); CC (1.52); CN (1.48) and bond angles, HOC (107.8); OCC (119.0); CCN (112.0); and CNC (109.5).22
References and Notes (1) F. Baker, R. Parish, and L. Stock, J. Am. Chem. Soc., 89, 5677 (1967). (2) R. Golden and L. Stock, J . Am. Chem. Soc., 88, 5298 (1966). (3) J. Christensen et al., J . fhys. Chem., 69, 467 (1965). (4) D. McDaniel and H. Brown, Science, 118, 370 (1953). (5) H. Peek and T. Hill, J . Am. Chem. Soc., 73, 5304 (1951). (6) C. Wiicox and C. Leung, J . Am. Chem. Soc., 90, 336 (1968). (7) L. Eberson, Acta Chem. Scand., 13, 211 (1959). (8) H. Hall, J . Am. Chem. Soc., 78, 2570 (1956). (9)J. Kirkwood and F. Westheimer, J . Chem. fhys., 6, 506 (1938). (IO) R. C. Duty, K. Hanck, J. Roseman, and J. Jordan, Trans. Ill. State Acad. Sci., 63, 259 (1970). (11) J. Speakman, J. Chem. Soc., 855 (1940). (12) S.Ehrenson, frog. fhys. Org. Chem., 2, 195 (1964). (13)J. Christensen et al., J . Am. Chem. Soc., 89, 213 (1967). (14) M. Miles et at., J . fhys. Chem., 69, 467 (1965). (15) W. A. Bone, J. J. Sudborough, and C. H. G. Sprankling, J. Chem. Soc., 85, 534 (1904). (16) C. Tanford, J . Am. Chem. Soc., 79, 5349 (1957). (17) F. Westheimer and 0. T. Benfey, J. Am. Chem. Soc., 78, 5309 (1956). (18) C. A. &ob, E. Renk, and A. Kaiser, Chem. Ind. (London),598 (1957). (19) J. F. J. Dippy, Chem. Rev., 25, 151 (1939). (20) H. Eyring, fhys. Rev., 39, 746 (1932). (21) S. Bauer and R. Badger, J . Chem. fhys., 5, 652 (1937). (22) R. Weast, Ed., "Handbook of chemistry and Physics", 45th ed, Chemical Rubber Co., Cleveland, Ohio, 1964,pp D76-78.
Monomer-Dimer Equilibrium Study of Dichloroacetic Acid in the Vapor Phase and in Carbon Tetrachloride Solution J. A. Faniran,' K. S. Patel, and L. 0. Nelsont Department of Chemistry, University of Ibadan, Ibadan, Nigeria (Received December 1 1, 1975; Revised Manuscript Received September 9, 1977)
A study of the infrared spectra in the hydroxyl and carbonyl stretching regions is reported for monomeric and dimeric dichloroacetic acid in the gaseous state and carbon tetrachloride solution. Although cyclic dimerization is found to be predominant, evidence is provided for an open chain association in dilute solutions at acid M. Spectral constants have been calculated from the intensities of the free concentrations lower than 2 X OH absorption and the total absorbance at the carbonyl stretching band.
Introduction Monocarboxylic acids are known to exist as an equilibrium mixture of monomer and dimer in the gas phase a t moderate pressures and in dilute solutions of nonpolar solvents. The extent of association of these acids in these states has been of great research interest. During a recent study on the metal complexes of dichloroacetic acid1 it was found that dichloroacetic acid received little attention compared to acetic acid and some of its halogen derivatives.2-16 The spectral and thermodynamic constants obtained in carbon tetrachloride and benzene solutions by monitoring the OH and carbonyl stretching b a n d P are +Departmentof Chemistry, University of Calabar, Calabar, Nigeria. 0022-3654/78/2082-1018$01 .OO/O
summarized in Table I. It is seen that even in the same solvent the values reported differ very widely. Moreover, no gas phase study has been undertaken. This paper reports an investigation of the equilibrium mixture of dichloroacetic acid in the vapor phase a t various temperatures and in carbon tetrachloride solution a t different concentrations. The spectral features were obtained using the intensity of the free OH and the total absorbance at the carbonyl stretching bands by taking into consideration the overlap between the bands in the carbonyl absorption region.
Experimental Section Spectroscopic grade carbon tetrachloride (BDH) was dried and redistilled. 0 1978 American
Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978 1019
Equilibrium Study of Dichloroacetic Acid
TABLE I: Constants of Dichloroacetic Acid Constant
$, L mol-' em(OH), L mol-l cm-'
em(C=O), L mol-' cm-' eD(C=O), L mol-' cm-'
AH,kcal mol-'
Value
Solvent
Ref
1075.3 10.9 27.1 126.0 110.9 llO.Ba 108. l b 333.0a 318.0b 1227.0" 1150.0b -8.6
CC1, ( 2 5 ° C ) C6H, ( 2 0 ° C ) C6H6 ( 2 5 ° C ) CCl, C6H6 C,H, C, Hi C6H, C,H, CiHi C6H6 CC1,
2 4 5 2 4 5 5 5 5 5 5 3
Obtained using Satchel1 and Wardell's method (ref 6). Obtained using Harris and Hobb's method (ref 2).
a
Dichloroacetic acid (BDH) was purified by repeated distillation a t reduced pressure and dried over CaC1,. Titration of the purified acid with standard sodium hydroxide gave an assay of 99.8%. The refractive index observed a t 22.5 "C gave a value of 1.446 which compared very well with the reported value of 1.4658 a t 20 The infrared spectra were recorded on a Perkin-Elmer Model 577 grating double beam spectrophotometer. A Beckman-RIIC 1-mm pathlength liquid cell was used for the solution measurements. In the vapor phase a variable temperature gas cell previously described17was employed. In the range 25-120 "C, it was possible to obtain pure monomer by keeping the pressure lower than 1 mmHg. Higher vapor pressure produced a mixture of monomer and dimer. The spectra were calibrated with the usual standards. The areas under the absorption curves were measured with a planimeter.
Theory
1
I
I
I
SO0
3000
I
25bo 2000 WVEWMBER (CM-')
I
1800
I
1600
Figure 1. Infrared spectra of dichloroacetic acid vapor at (A) 60, (B)
96, and (C) 120 O C . (Pressure C0.5mm.)
Using the Beer-Lambert law we can replace C, by A,/c,l where A , is the monomer absorbance, c, the monomer molar absorption coefficient, and 1 is the cell pathlength. Equation 3 becomes
CIA,
=
1 E
(4)
-€m
Similarly in terms of the dimer absorbance AD and molar pbsorption coefficient cD we have
(5) Thus
In dilute solutions dichloroacetic acid exists as an equilibrium mixture of monomer and dimer: 2CHC1, COOH
(1)
(CHCl, COOH),
therefore
The dimerization equilibrium constant is given by
(7) where C, and CD are the concentration of the monomer and dimer species, respectively, and K , the dimerization constant in reciprocal concentration units. The total concentration (C) is
c = cm+ 2c, =
(3)
Cm + 2KcCm2
where A and t are the total absorbance and total molar absorption coefficient, respectively, in solution.
Results and Discussion Vapor Spectra. The infrared spectra of dichloroacetic acid in the OH, CH, and C=O stretching regions in the vapor phase are shown in Figures 1 and 2 while the fre-
TABLE 11: Absorption Frequencies (cm-') of Dichloroacetic Acid in the Vapor Phase and in CCl, Vapor p
< 0.5 mm 3588 s 2972 mw
1792 s
CCl, p
< 1mm 3588 s 3250 b 2972 m 2918 w 1792 vs 1763 sh
a
1M
2 x 10-3 M
Ref 2
3678 m
3678 m
3524 m 3024 s,b
3524 m 3023 m,b
3529 3046
2924 s,b 2696 m 2580 m 1788 m
2924 vs,b 2696 m 2581 w 1789 m 1764 w
2940 2720 2608
1744 vs
1744 s 1734 sh
u(CH) absorption is believed t o be strongly overlapped by the dimer OH bands in CC1,.
Assignment Free d O H ) linear dimer ' u( OH) monomer v(0H) cyclic dimer u ( CH)a v ( 0 H---0) Submaxima v ( C= 0 ) monomer
Free v(C=O) linear dimer u ( C= 0) cyclic dimer Bonded v(C=O) linear dimer
1020
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
J. A. Faniran, K.
S.Patel, and L. 0. Nelson
1
I
I
W
I
I
3 500
1
le00
3000 2500 2d00 WVENUMBER (CM-')
Figure 2. Infrared spectra of dichioroacetic acid vapor at (A) 66, (B) 80, and ((3) 100 '(3. (Pressure >1 mm.)
I
2000
I
do0 16'00 WAVENUMBER (CM?
Figure 4. Infrared spectra of dilute solutions of dichloroacetic acid in CCi.,: (A) 2 X and (6)7 X IO-' M. h
. I
v
TABLE 111: Dimerization Constants and Molar Absorption Coefficients of Dichloroacetic Acid in Carbon Tetrachloride at 29 'C
w
0
z
2 t 5, Z d
~~
Constant
l
4000
3500
I 3000
I 2500
I
20 co
WAVENJMBER (CM")
Figure 3. Infrared spectrum of a 0.1 M solution of dichioroacetic acid in CCi,.
quencies are listed in Table 11. At very low pressures, the observed bands are sharp and increase in intensity with increase in temperature without any frequency changes (Figure 1). The OH stretching band observed a t 3588 cm-l has a half-width (Avllz)of 32 cm-l while that of the C=O absorption is 64 cm-l. These rather small magnitudes of .1v1/2 would indicate that these absorptions are due to uncoupled vibrations and hence attributed to free OH and C=O stretching modes. The absence of any absorption between 3000 and 3400 cm-l associated with H bonding in carboxylic acids convinces us that the spectra are due to the monomeric dichloroacetic acid. At higher pressures there is a clear evidence of association (Figure 2). Apart from the broad feature at ca. 3250 cm-l the spectra are in general more complex. However, the position of the free OH and CH stretching modes observed in Figure 1 remain unchanged and these bands grow rapidly with increase in temperature. This behavior is similar to Bellamy's observation in the spectra of acetic acid.1° The C=O stretching band is rather broad. A t 66 "C a strong shoulder which appears to decrease in intensity as the temperature increases occurs at 1763 cm-l. We have tentatively associated this feature with the dimer component. Solution Spectra. The spectra are given in Figures 3 and 4 for three different concentrations in CC14. Two sharp interesting features are observed a t 3678 and 3524 cm-l (Figure 3). While the 3678-cm-l band appears only for concentrations higher than 0.01 M that a t 3524 cm-l persists a t all concentrations with its intensity decreasing with concentration. Harris and Hobbs' observed this latter peak a t 3529 cm-l in CCl, solution and it was reported at
Unit
Kc
L mol-'
frn(OH) e,(C=O)
L mol-' cm-' L mol-' cm-' L mol-' cm-'
et,t(C=O)
a Obtained from eq 4.
~~
Value 280 f 2a 283 f 2b 123f 2
660
2
8
1664k 5
* Obtained from eq 7 .
3410 cm-l in benzene by Nagai and S i m a r n u r ~ .These ~ authors attributed the feature to the monomer 0-H absorption. Dimerization of dichloroacetic acid can be depicted as an equilibrium mixture of the cyclic and linear forms: R-C
8-\
O--H--O
\
/F-R
e R-CHo'O-H--O\ H-0
C,
-R
The 3678-cm-l band can therefore be associated with the free OH stretching vibration of the open chain dimer. This frequency was first thought to arise from water impurity. In an independent experiment however, we found that, like acetic a ~ i d - C C 1 ,and ~ ~ formic acid-CC1, systems,l9water has no noticeable influence on the dichloroacetic acid-CC1, system. At moderate Concentrations the C=O band is resolved into two components a t 1789 and 1744 cm-l (Figure 4B). For concentrations
