1426
T. S. S. R. MURTYAND KENNETHS. PITZER
Trifluoroacetic Acid. Nature of Association in Dilute Solutions in Nonpolar Solventsf. by T. S,S. R. Murty and Kenneth S. Pitzerlb Department of Chemistry, Rice University, Houston, Texas
77001
(Received October 7 , 1 9 8 8 )
Many studies on monocarboxylic acids in nonpolar solvents assume that cyclic dimeriaation is the major association process and that all monocarboxylic acids behave similarly. A detailed study of the infrared spectra in the hydroxyl and carbonyl stretching regions for trifluoroacetic acid in suitable solvents provided direct evidence for linear association.
Introduction FBm6ant2 suggested in 1952 that part of the breadth of the VOH band observed for carboxylic acids in nonpolar solutions is due to the presence of a mixture of cyclic and open-chain dimers. For a decade this idea received little support and most authors assumed only cyclic dimers. In 1963, Bellamy, Lake, and Pacea renewed the suggestion that the unusual breadth and shape of the VOH band of dimeric monocarboxylic acids may arise in part from the presence of an equilibrium mixture of open-chain and cyclic forms. Later workers concluded that cyclic dimerization is the major association process a t acid concentrations below M,4,6but linear polymerization also occurs to a significant extent especially as acid concentration increases above lo-* M.4 Underlying most of the hydrogen bonding studies of carboxylic acids in dilute solutions in nonpolar solvents is the assumption, either stated or implied, that all monocarboxylic acids behave similarly. Thus Bellamy, et ~Z.,~point out the similarity in the hydroxyl stretching region of acetic and trichloroacetic acids. There is no doubt that monocarboxylic acids are dimeric in the vapor states and in dilute solution in nonpolar solvents. The wealth of thermodynamic data seems to indicate that only cyclic dimerization occurs in the gas phase.4r6 However, in a solvent, it is reasonable to expect that solvation of the free hydroxyl group makes linear dimerization relatively more favorable, and this should be the more important the stronger the acid. The free hydroxyl stretching frequencies for trifluoroacetic acid in the vapor phase and carbon tetrachloride solutions are 3587 and 3500 cm-l, a shift of 87 cm-1, whereas for acetic acid they are 3581 and 3536 cm", respectively, a shift of only 46 cm-l. Moreover, it is well established that the stronger the acid the weaker is the hydrogen bond formed7 and the smaller the dimerization constant.8 Therefore, one can reasonably expect trifluoroacetic acid to have a better chance than other monocarboxylic acids to form open-chain dimers and polymers. In the pure liquid the dielectric constant of trifluoroThe Journal of Phyaical Chemistry
acetic acid shows an anomalous increase from 26.2 a t -11"to 43.4 a t 27.7°;8 this would be consistent with the presence of highly polar open-chain polymers at the higher temperature.
Experimental Section Trifluoroacetic acid (Aldrich Chemical Co.) was distilled (bp 72"). Glacial acetic acid was dried and distilled over anhydrous calcium sulfate. Carbon tetrachloride was dried over phosphorus pentoxide and then distilled over anhydrous calcium hydride. Benzene was dried and distilled over freshly cut sodium metal. The spectra were measured using a Beckman IR9 spectrophotometer. Solvent absorptions were removed by compensation in the reference beam. The path length of the cells varied depending upon experimental convenience. Beckman NIR silica cells of 10 and 1 om path length, and 0-6-mm Beckman variable path length sodium chloride cells were used. No quantitative measurements were attempted in this study because of the high vapor pressure of trifluoroacetic acid, 135.6 mm a t 29°.7
Results and Discussion Ideally, one should compare the carbonyl and hydroxyl stretching regions of each acid to fully under(1) (a) This work was supported by a research grant from the Robert A. Welch Foundation and was presented orally at the Symposium on Molecular Structure and Spectroscopy, Columbus, Ohio, Sept 1968. (b) Stanford University, Stanford, CaliP. 94305. (2) 8. FBmBant, Compt. Rend., 2 3 5 , 240 (1952). (3) L. J. Bellamy, R. F. Lake, and R. J. Pace, Spectrochlm. Acta. 19, 443 (1963). (4) T. C. Chaing and R. M. Hammaker, J . Phys. Chem.. 69, 2715 (1965). (5) E. S. Hanrahan and B. D. Bruce, Spectrochim. Acta, 236, 2497 (1967). (6) H. Dunken and G. Marx, Abhandl. Deut. Akad. Wiss. BerNn, Kl. Math. Physik Tech., (6), 101 (1964). (7) M. D. Taylor and M. P. Templeman, J. Amer. Chen. Soc., 7 8 , 2950 (1956). (8) F. Thyrion and D. Decroocq, Compt. Rend., 260 (10) (Group 7 ) , 2797 (1965). (9) J. H. Simons and K. E. Lorentzen, J. Amer. Chem. Sac., 71, 1426 (1950).
ASSOCIATIONIN DILUTESOLUTIONS
1427
Table I: Carbonyl Stretching Frequencies of Trifluoroacetic and Acetic Acids in the Vapor State and in Dilute Carbon Tetrachloride Solutions ,----Free
1829 1813
Vapor CCI, solution
-
Trifluoroacetic acid Bonded Avo-o
1790 1792, 1782
,
Acetic acidBonded
Free
39" (38p 3ae) 21, 31" (3%)
1783 1770
-
AVC-o
53d (513 50" (57,/ 50dMe)
1730 1720
N. Fuson, M. L. Josien, E. A. Jones, and J. R. Lamon, ibid., 20, 1827 0 This work. b R. E. Kagarise, J . Chem. Phys., 27, 519 (1957). (1952). d S. Bratoz, D. Hadzi, and N. Sheppard, Spectrochim. Acta, 8, 249 (1956). e L. J. Bellamy, R. F. Lake, and R. J. Pace, ibid., 19, 43 (1963). I J. Bellanato and J. R. Barcelo, ibid., 16, 1333 (1960).
stand the nature of association of carboxylic acids in nonpolar solvents. The absorption in the hydroxyl stretching region is very broad and complex due to overlap with -CH bands (except, of course, in the case of trihalogenated carboxylic acids), Fermi resonance effects with overtones and combinations, and Stephanov broadening.'O This makes a conclusive interpretation of the hydroxyl region difficult. On the other hand, the carbonyl stretching region usually has two major bands attributable to monomer and associated species, and is free of other interfering absorptions. Moreover, the carbonyl bands are more intense than hydroxyl and allow the use of much lower concentrations of the acid in the solvent. However, the clearest manifestation of
/
I
", 0
le28
Id3
\
1 I800
I
VI lllB
I 1780
WAVENUMBER Figure 2. Carbonyl stretching region of trifluoroacetic acid in carbon tetrachloride solut,ions: curve 1, 3.3 X lo-* M, 1-mm NaCl cell; curve 2, 1.7 X 10-2 M, 1-mm NaCl cell.
0
I
I
3qOQ
WOO
I 3300
3200
WAVE NUMBER Figure 1. Free hydroxyl stretching region of trifluoroacetic acid in benzene solutions: curve 1, 1.1 X 10-2M, 4-mm NaCl cell; curve 2, 3.7 X 10-8 M , 6-mm NaCl cell.
open-chain dimers and polymers will presumably arise from the presence in such species of terminal (nonbonded) hydroxyl groups and similarly of terminal carbonyl groups not involved in hydrogen bonding. In fact, we find that the spectral features associated with the nonbonded hydroxyl and carbonyl groups are much stronger for trifluoroacetic acid than for.acetic acid in benzene or carbon tetrachloride solutions at concentrations where association is large. While the intensities will be influenced by many factors and a quantitative treatment is not feasible, the relatively great intensity of the free hydroxyl feature near 3500 cm-I and the free carbonyl feature at 1813 cm-1 for trifluoroacetic acid is apparent. (See Figures 1-4.) (10) N. Sheppard. "Hydrogen Bonding," D. Hadzi, Ed., Pergamon Press, London, 1969,p 86.
Volume 78, Number 6 Mau 1969
1428
T. S. S. R. MURTYAND KENNETHS. PITZER
The acidity of the hydroxyl group at the end of an open-chain dimer will be expected to be greater than that of the free monomer. The solvent shift of this group in a slightly basic solvent like benzene would therefore be increased and the separation between the free hydroxyl absorptions of the two species would be greater. Free Hydroxyl Stretching Region in Benzene as Solvent. A study of the infrared spectrum of trifluoroacetic acid in benzene (without using the cancellation technique as described by Bellamy, et al.") revealed the presence of two absorption peaks in the region assigned to free
lo 0
t 1825
1
I
1715
I800
I
I750
WAVENUMBER
Figure 3. Carbonyl stretching region of trifluoroacetic acid in carbon tetrachloride solutions: curve 1,4,4X M ,6-mm NaCl cell; curve 2, 3.2 X 10-8 M , 6-mm NaCl cell; curve 3, 1.3 X M, 6-mm NaCl cell.
hydroxyl groups. Figure 1 shows these feature8 a t nearly 3400 and 3455 om-1 together with the bonded hydroxyl absorption below 3250 cm-1. At higher concentrations only a broad band is observed around 3385 cm-1 with a weak shoulder at 3455 cm-'. The effect of dilution is an increase in the intensity of the 3455-cm-' band at the expense of 3400-cm-l band. The 3455cm-1 band is assigned to the monomeric acid and the feature a t 3400-~m-~ to the terminal hydroxyl of openchain dimeric or polymeric acid species. Similar double peaks in the free hydroxyl stretching region are obtained for trichloroacetic acid (at -3400 The Journal of Physical Chemistry
and 3450 cm-') and tribromoacetic acid (at -3410 and 3455 cm-I) in benzene solutions. Very similar results are obtained for trifluoro- and trichloroacetic acids in 1,2-dichIoroethane as solvent. At higher concentrations of the acid our results agree with those reported by Reeves.12 In carbon tetrachloride solutions we find no splitting of the free hydroxyl or free carbonyl features, but we observe a dual feature for the hydrogenbonded carbonyl as shown in Figures 2 and 3. Carbonyl Stretching Region. Most of the monocarboxylic acids in carbon tetrachloride solutions show two major carbonyl bands-free and hydrogen bondedand the frequency shift (Avc-0) between them is 50 f 5 cm-l. The only effect of dilution for these acids is an increase in the intensity of the free carbonyl absorption at the expense of the bonded. The AvC=o values in the vapor phase are also 50 cm-1. However, for trifluoroacetic acid, it is clear from the Figures 2 and 3 that at the highest concentration studied the 1782cm-l band is prominent with an unambiguous shoulder near 1792 cm-', and a t lower concentrations the 1782cm-' band disappears. The band a t 1813 cm-l is the free carbonyl absorption. The bands at 1782 and 1792 cm-l can arise either from an equilbrium between cyclic and open-chain dimers or from polymers and an open-chain dimer. In the former case one would expect the relative intensities of the two bands to change little in a given solvent at a given temperature for modest changes in concentration of the acid. In the latter case, however, one would expect a dilution effect in a given solvent. A dilution effect is observed €or trifluoroacetic acid in carbon tetrachloride. Moreover, it is found, as would be expected, that the spectra are very similar in carbon disulfide and tetrachloroethylene except the "polymer" band a t 1782 cm-' starts disappearing a t higher concentrations than in carbon tetrachloride. Other workers have studied the carbonyl region of carboxylic acids including trifluoroacetic acid (see Table I ) , but this is the first time to our knowledge that a dilution effect has been noticed within the band for hydrogen-bonded carbonyl groups. A preliminary study of trichloro- and tribromoacetic acids showed only one bonded carbonyl absorption, with Avc-0 values of 35 and 37 cm-1, respectively. The presence of several maxima in the carbonyl region and the effect of different solvents on the carbonyl stretching vibration in aromatic monocarboxylic acids has been discussed in detail and explained as due to formation of various types of ~omp1exes.l~In an open-chain polymer there are three types of positions: terminal with free hydroxyl, terminal with free carbonyl, and interior with both hydroxyl and carbonyl involved in hydrogen (11) 1,. J. Bellamy and R. J. Pace, Spectrochim. Acta, 22, 525 (1966). (12) L. W.Reeves, Can. J. Chem., 39, 1711 (1961). (13) A. E. Lutskii and A. K. Kul'chitskaya, Russ. J. Phys. Chcm., 41, 519 (1967).
1429
ASSOCIATION IN DILUTE SOLUTIONS 90
80
70
u u 60
I-
t
5z
SO
4
a: I-
40
a? 30
20
IO
WAVE NU M BE R Figure 4. Hydroxyl stretching region of trifluoroacetic acid in carbon tetrachloride solutions: curve 1, 1.1 X 10-1 M, 1.5-mm NaCl cell; ourve 2,1.5 X M, 6-mm NaCl cell.
bonds. It seems probable that the 1782cm-1 peak arises from the acid units in the interior positions, the 1792-cm-l peak with terminal units with free hydroxyl and bonded carbonyl, and the 1813-cm-l peak with the terminal units with free carbonyl together with any monomeric acid. Hydroxyl Stretching Region. Figure 4 shows the spectra of trifluoroacetic acid in carbon tetrachloride at concentrations of 1.1 X 10-l and 1.5 X 10-2M. Ideally, if the equilibrium is between open-chain dimers
and higher polymers, the band at 2960 cm-' should decrease in intensity at higher dilution. It is observed that at still lower concentrations both the bands at 3100 and 2960 cm-l split further and appear as doublets. However, a conclusive interpretation of the hydroxyl region is difficult because of the possible resonance effects with overtones and combinations. A similar spectrum of trichloroacetic acid in carbon tetrachloride has been interpreted as due to a mixture of cyclic and open-chain dimers.8
Volume 78,Number 6 May ID60
