Hetero-dimerization of acetic and trichloroacetic acids in carbon tetrachloride solution

BpectrmhimicpActa.1904,Vol.20,pp. 286 to 200.

PergsmonPreesLtd. Printedin NortheruIrehmd

Hetero-dimerization of acetic and trichloroacctic acids in carbon tetra&loride solution E. AFFSPRUNO, SHERRILD. CHRISTIAN and ARNOLDM. MELNICK* Departmentof Chemistry,The University of Oklahoma, Norman, Oklahoma

HAROLD

(Received26 June 1963) Ah&a&-The hetero-association of acetic acid and trichloroacetic acid in CCle has been investigated using near infrared spectra. Equilibrium constants and the enthalpy change have been calculatsd for the homo- and hetero-dimerization reactions over the temperature range 160-40”. Results indicate that the hetero-dimer (CH&OOHCCl,COOH) forms in preferenceto the two homo-dimers.

INTRODUCTION SEVERALgroups of workers have utiliz, 3 infrared absorption spectroscopy to determine the extent of self-association of carboxylic acids in Ccl, [l-6]. To our knowledge, however, the hetero-association of two carboxylic acids dissolved in the same solvent has not been studied previously by spectral methods. Since data were available on the self-association of acetic and trichloroacetic acids in Ccl, [J-7] it was decided to determine both homo- and hetero-dimerization constants for dilute solutions of acetic and trichloroacetic acids in Ccl,. Results are reported for this ternary system over the temperature range 15’ to 40”. EXPERIMENTAL A. Absorbance measurements Absorbance measurements were made in the 2.75 to 3.10 ,Uregion, using a Beckman custom ratio recording spectrophotometer model DK-1 with silica prism. Peak absorbancea of the O-H stretching modes of acetic and trichloroacetic acid monomer molecules at 2.822 and 2.847~ were measured to determine the respective monomer concentrations. Cells of 10 cm path length were used throughout, and concentrations of acid were varied from 1O--6to 1O-3 molal. In the experiments on hetero-dimerization, the two acids were present in solution at equal molalities, and the total molality ranged from 1O-4 to 10e3. Measured absorbancea ranged, in general, from O-1 to 0.8. * Presentaddress, Monsanto Research Corporation, Miamisburg, Ohio. Taken in part from a Ph.D. Dissertation submitted by ARNOLD M. MELNICK,University of Oklahoma, 1963. [l] A. M. BUS~ELL, W. H. RODEBUSHand M. F. ROY, J. Am. Chem. SOL, 60, 2239 (1938). [2] M. DAVIESand G. B. M. SUTHERLAND, J. Ch.em. Php., 6, 765 (1938). [3] R. M. BAJX+ER and S. H. BAUER,J. Chem. Phys., 5, 839 (1937). [4] R. A. SP~JRX and J. WENOOR~D,J. Am. Ch.em. SOL, 79, 6844 (1967). [6] J. T. HARRISand M. E. HOBBS,J. Am. Chem. Sot., 76, 1419 (1964). [S] G. M. Bll~aow and E. A. YEBOER,J. Am. Chm. Sot., 76, 5248 (1964). [‘I] R. E. KAUARISE, ZVwa.8 Rea. Lab. Rept. 4966 (August 8, 1957). 3

285

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B. Chernicula Carbon tetrachloride was purified by the method of W IUUMS and KR~~AU [8]. Residual water was removed by keeping the carbon tetrachloride in contact with anhydrous magnesium perchlorate. Ace tic acid was purified by the method of PORL et al. [S]. It was also fractionally distilled in a 30-plate Oldershaw column. ‘Trichloroacetic acid was purified by fractional freezing followed by repeated recrystallization from anhydrous benzene. Both acids were titrated with strong base to determine equivalent weight; in each case, the titration equivalent weight agreed with the theoretical molecular weight to within O-1 per cent. CALCULATIONS AND RESULTS A. Self-aseocia tion constants Self-association constants for acetic acid in Ccl, and trichloroacetic acid in CCl, were calculated using a method similar to that of HARRIS and HOBBS[6]. Assuming that only monomers and dimers of the acids exist in dilute solution and assuming that the BEER-LAMBERTlaw is valid at the monomer O-H stretching frequency, the equation : rn,/A = l/al + 2KA/(aZ)2

(1)

applicable; where m, is the formal molality of the acid, A is the absorbance at the monomer O-H frequency, Zis the path length, a is the molal absorptivity of the monomer and K is the equilibrium constant for the reaction, 2 monomer = dimer. Equation (1) requires that a plot of mf/A vs. A be linear, with slope ~K/(c~Z)~ snd intercept l/al. For each of the acid-solvent systems, data at a number of temperatures were fit in this form by a linear least squares method. Although values of the molal absorptivities obtained for the monomer showed some scatter, they did not appear to vary significantly with temperature. Consequently, an average value of l/a was calculated and this value was used in recomputing the least squares line at each temperature. In this way, K values at each of the temperatures were calculeted from the slopes of lines forced to have a common intercept. Values of AH were calculated from plots of log K vs. l/T. Table 1 summarizes results of calculations based on data for the is

Table 1. Monomer absorptivities and thermodynamic constants for sew-associationreactions X2,. (molal-l) x 10-S

System

No. of runs

a (cm-l molal-l)

C&COOHCc14

13

236 f 39 at 2.822 p

3.2 f 0.4

CCl&OOHccl,

13,

230 f 9 at 2.847~

0.43 f 0.04

[8] J. W. Wm

AH (kcal) -10.7

f I.2

-7.9

f 1.2

s and J. J. KRO~XA, J. Am. Chna. SOL, 48, 1888 (1926). [S] H. A. POEL, 116. E. HOBBS and P. M. QROSS, J. Chin. Phys., B, 408 (1941).

Hetero-dimerization of acetic and trichloroacetioacids in carbon tetrachloride solution 287

sea-&s~ciating systems. For the acetic acid dimeriz&tion constant at 29, HARRIS and HOBBS[S] report values ranging from 5-l x lo3 to 843 x lo3 molal-1, WENOGRADand SPURR[4] report 3.8 x lo3 molel-1 and BARROWand YERQER [S] list values ranging from 1.6 x 103 to 4.2 x 10s molal-1 over the range of concentrations 0.1 to IO-d molal. HARRY and HOBBSreport dimerizstion constants in the range O-76 X lo3 to 0.93 x lOa molal-l for trichloroacetic acid in Ccl, at 25’, and KAUARZSE[7] reports dimerization constants ranging from 0.62 x lo3 to 069 x 108 for the system. KAUR~SE reports AH = -10-7 kcal for the ~merization of acetic acid and -10.3 for the dimerization of triohloroacetic acid in CC&. Thus, the values in Table 1 are in reasonable agreement with those of other investigators. In order to determine hetero-dimerization constants it w&snecessary to calculate the molal absorptivity of the monomer of each acid at the monomer peak wavelength of the other acid. The method of analyzing data was the same as that used in calculating absorptivities at the peak wavelengths; except that A in equation (1) was taken to be the absorbance of the given acid at the wavelength where a maximum absorbance occurred in the spectrum of the other acid. In this way, the values a = 42 f 20 cm-l molal-l and

a = 140 f. 22 cm-l molsP

were obtained for acetic acid at 2.847 p and trichloroacetic acid at 2.822 ,z, respeotively.* From these abao~ti~ties and the measured absorbances at both wavelengths, it was possible to calculate the concentrations of both types of monomer in any given solution of acetic acid snd trichloroacetic acid in CC&. The formal concentration of trichloroacetic acid in a solution of both acids in CCL,m&y be expressed as m; = mo + 2mc, + mAo = m, + 2KomZ + EAomA%

(2)

where mo, mo3, m,o and m, are the molalities of trichloroacetic acid monomer, trichloroacetia acid dimer, hetero-dimer and acetio acid monomer, respectively, and Ko rend KAc are the self-dimerization constant for CCl,COOH and the heteroassociation constant, respectively. It is assumed that no species of higher molecular weight than the dimer is present. Equation (2) may be rearranged to give mfc/m, - 2K,mo

= 1 + KA,mA

(3)

from which it is seen that a plot of the left hand side of equation (3) vs. mA should be linear, with slope KA, and an intercept of unity. Values of the left hand side of equation (3) were calculated from measured formal concentrtitions, Kc values determined previously and mo values determined from the measured absorbancies at 2.822 and 2.847 y. These values are plotted vs. m, in Fig. 1, for~repr~en~~ve runs at 16.9,24-7 and 40-S”. In Fig. 2 are plotted values of log,, KAc vs. l/T calculated from * Although the relative uncertaintiesof the a values at the non-peak wavelengths are gfeat,

~~~~ties are approximately the same as the unee~~ti~ in the peak a values. Since the non-peak absorptivities are small compared to the peak absorptivities, these uncertainties do not lead to large uncertainties in the calculated monomer concentrations.

t,& &so&e

288

HAROLD E. AIWSPRUNU,SHERRILD.

CHRISTIAN

and

ABNOLD

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I.1

0.9

I

0

0.2

1

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0.6 m,

I

!

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0.8

i-0

1.2

x lo+4

Fig. 1. Plots used in celouIatingKAc.

I,‘Txld3

Fig. 2. Temper&we dependenceof KA,.

M.

E~LNIOK

Hetaro-dimerization of acetic and trichloroaceticacids in carbon tetrachloride solution

289

data taken at 12 temperatures ranging from 16’ to 40”. From the parameters of the least squares straight line of these data, the values AH = -7.7 f 1.5 kcal and K,,, = (7.2 f ‘9) x 10s molal-l were calculated for the reaction CH,COOH + CCl,COOH = CH,COOH~CCl,COOH. It may be noted that an equation analogous to equation (3), but involving interchange of subscripts and superscripts A and C, could have been used in the analysis of data. However, it was believed that K, had been determined somewhat more accurately than KA and that consequently more reliable values of K,, could be determined by plotting data as shown in Fig. 1. DISCUSSION Examination of the relative magnitudes of K,, Kc and K,, reveals that there is extensive hetero-dimerization in the ternary system. It is informative to consider the reacbion -&(CH,COOH), + ~(CCl,COOH),

= CH,COOHCCl,COOH,

for which the equilibrium constant is dimensionless and solvation should be relatively unimportant. The enthalpy and equilibrium constant data reported in the previous section may be combined to yield K = 6.1 f 1.2 and AH = 1.6 f 2.7 kcal for the above reaction. Obviously the value of AH is not known accurately enough to indicate with certainty whether the reaction occurs exothermically or endothermically. However, the observed equilibrium constant clearly indicates that the hetero-dimer forms preferentially to the two homo-dimers. From statistical arguments, one might predict that the equilibriulh constant for the reaction would be approximately 2, assuming that the only factor favoring products over reactants is the lower symmetry number of the hetero-dimer as compared to the homo-dimers [lo]. CHRISTIAN[ll] has shown that liquid-vapor equilibrium data for the system propionic acid-acetic acid are consistent with the assumption that K = 2 in the vapor phase. In the present system, it is clear that factors other than the symmetry number operate to promote the formation of hetero-dimers. HANSEN and CHRISTIAN [12] observed a similar tendency for hetero-dimers to form in preference to homo-dimers while studying liquid-vapor equilibrium of the systems acetic acid-trifluoroacetic acid, propionic acid-pentafluoropropiouic acid and n-butyric acid-heptafluoro-n-butyric acid. Vapor phase equilibrium constants for the reactions 44

+ @‘,

= AF

where A refers to the aliphatic acid and F to the perfluorinated aliphatic acid were found to range from 5 to 7 and AH values were of the order of -2 to -4 kcal. KOHLER et al. [ 131 have determined calorimetrically that the reaction i(CH,COOH) j$(CF,COOH), = CH,COOHCF,COOH occurs exothermically in dilute solution [lo] R. FOWLERand E. A. GUCWENEEIM, StatL&xz.lThermodynmnim p. 167, University Pm, Cambridge (1952). [ll] S. D. CHRISTIAN, J. Whys. Ctim., 61, 1441 (1957). [12] R. S. HANSENand S. D. CHRISTIAN, unpublishedwork. S. D. CHRISTIAN, Ph.D. Dissertation, Iowa State College (1956). [13] F. KOHLER,S. D. CHRWTIANand J. F. PEREIRA,unpublishedwork.

290

-OLD

E. A~~smum,

SEERRIL D. CHRISTL~LNand ARNOLD Y. MELNKE

in CCI, and that the mixing of liquid acetic acid and liquid trifluoroacetio acid is strongly exothermic. In both cases the enthalpy of mixing is of the order of -2 to -3 kcal for the reaction. It is possible that the increased tendency for formation of hetero-dimers in systems of aliphatic and perhalo-aliphatic acids results from a cooperative interaction between the eleotric moment of the perhalogenated methyl or methylene groups and the -COOH group belonging to. the aliphatic member of the heterodimer. In homo-dimers of the perhalogenated acids, the effect of this interaction would be at least partially opposed by an unfavorable interaction between the perhalogenated methyl or methylene groups of the two molecules in the dimer. The present work lends support to this hypothesis. Interactions between trichloroacetic acid and acetic acid appear to be weaker than those between perfluorinated aliphatic acids and aliphatic acids, but stronger than interactions between aliphatic acids and each other. This intermediate behavior parallels the intermediate electric moment of the perchloro-methyl group as compared to methyl or perfluoro-methyl groups. Atiwem&Thir-This

work was supportedin part by a grant from the United Statea Department of the Interior, Office of &dine Water.