The preparation and vibrational spectrum of trichloromethanesulphonic acid, CCl3SO3H

Journal of Molecular Structure, 263 (1991) 11-20 Elsevier Science Publishers B.V., Amsterdam

11

The preparation and vibrational spectrum of trichloromethanesulphonic acid, CC13S03H H.G.M. Edwards’ and D.N. Smith Chemistry and Chemical Technology, University of Bradford, Bradford, West Yorkshire, BD7 1DP (UK) (Received 24 June 1991)

Abstract The preparation of trichloromethanesulphonic acid, CCl,SO,H, is reported and vibrational assignments made on the basis of C, molecular symmetry. Raman spectra of the pure acid and of its aqueous solutions indicate that only partial dissocation into trichloromethanesulphonate ions occurs in aqueous solution; in a 2 M aqueous solution of trichloromethanesulphonic acid (Y= 0.986 and K, = 139 f 18 mol dxn3.

INTRODUCTION

Raman spectroscopic measurements of the dissociation in aqueous solution of methanesulphonic acid [ 1,2], ethanesulphonic acid [ 2,3], propanesulphonic acid [ 21 and trifluoromethanesulphonic acid [4] have been reported and the effect of the substitution of the F-, CHs- and CH3CH2- species in the methyl group of CH,SO,H have been evaluated quantitatively. It was recently concluded [ 41 from a comparison of substituted sulphonic and carboxylic acids that the carbon-sulphur bond was some 500 times more effective than the carbon-carbon bond in blocking the effects of methyl group substitution on the ionisation of the -OH group, i.e. in the pairs of sulphonic and carboxylic acids CH3S03H, CF,SO,H and CH&02H, CFBC02H, the effect of fluorine substitution on the K, values was 500 times smaller in the former case. As an extension of the quantitative spectroscopic work on fluorine-substituted methanesulphonic acid, the chloro-analogue CCl$OsH is studied here. Following its preparation, the acid species is characterised using its vibrational spectrum and preliminary results for its dissociation into trichloromethanesulphonate ions in aqueous solution, CCl$O; , are reported. ‘Author to whom correspondence should be addressed.

0022-2860/91/$03.50

0 1991 Elsevier Science Publishers B.V. All rights reserved.

12 EXPERIMENTAL

Preparation of sodium trichloromethanesulphonute Trichloromethanesulphonyl chloride (Aldrich, 25g, 0.12 mole) was stirred at room temperature with an excess of sodium hydroxide (2Og, 0.5 mole) in 250 ml water. The following reaction occurred: CC& SO, Cl + 2NaOH+ Ccl, SO, Na + NaCl+ H2 0 After 8 h the solid trichloromethanesulphonyl chloride had dissolved and the solution was filtered from particulate matter and reduced to a volume of 25 ml by rotary evaporation. On cooling, white crystals were collected, recrystallised from water and methanol solutions and air-dried at 60°C. The crystals analysed as follows and were confirmed to be sodium trichloromethanesulphonate monohydrate: calculated for CClsSO,Na*H,O; %C = 5.0, %H = 0.8, %S = 13.4, %Na=9.6; %Cl=44.4: found; %C=5.0, %H=0.8, %S=13.5, %Na=9.6, %Cl=44.3. Preparation of trichloromethanesulphonic acid Trichloromethanesulphonic acid was prepared from sodium trichloromethanesulphonate using a modification of Zuffanti’s method [5] in which dry hydrogen chloride gas is reacted with a suspension of the sodium salt of a sulphonic acid in ether. In this method, the reactant sodium salt and the sodium chloride produced by the reaction are both insoluble in ether; an incomplete reaction usually occurs and small amounts of the sulphonic acid produced have to be extracted from large volumes of ether used in the experiment. In a modification of this reaction which we report here for the first time, dry hydrogen chloride gas was bubbled into a solution of log sodium trichloromethanesulphonate in 250 ml of acetonitrile. The reaction CC1,SOsNa+HC1+CC1,SO,H+NaCl was carried out at 60 oC and was complete after 6h. The white precipitate obtained in the reaction was identified as sodium chloride and the volume of the clear solution obtained after filtration was reduced by rotary evaporation. White crystals (9g, 0.05 mole) were obtained which analysed as follows: calculated for CCl,SO,H; %C=6.0, %C1=53.3, %S=16.1, %H=0.5: found; %C=6.1, %C1=53.6, %S= 16.4, %H=0.6. The crystals do not contain acetonitrile of crystallisation.

13

Vibrational spectroscopy Raman spectroscopy Raman spectra of the pure acid and of the sodium salt in the solid state and in aqueous solution were obtained using argon laser excitation (500 mW at 488.0 nm) and a Spex Instruments 1401 double monochromator with photoncounting detection and a reciprocal linear dispersion of 21.5 cm-l mm-’ at 488.0 nm in the first order. A spectral slit width of 4 cm-’ was employed and wavenumber calibration effected using the emission lines of a neon lamp; wavenumber shifts of observed Raman lines are correct to + 1 cm-l. Spectrometer control was achieved using a Nicolet 1180 computer which facilitated multiple scanning of spectral ranges and improved signal-to-noise ratio over single-scan spectra. The use of standard programmes enabled band area measurements to be made between set wavenumber limits and this was an important requirement for the preliminary quantitative Raman spectroscopic aspects of this work [ 31. For the determination of the degree of dissociation of the acid in aqueous solution, a 2 M acid solution was made up. The Raman spectrum of this acid solution was recorded using a Raman cell which was thermostatted at 298.0 + 0.5 K. The intensity of the d0 = 1060 cm-’ band characteristic of v (SO, symmetric stretching) of the CC&SO, species was measured relative to the intensity of the AD=413 cm-l band (C-Cl symmetric stretching) taken as an internal standard to account for cell reproducibility effects and optical changes from solution to solution. The spectrum of the acid solution was consecutively recorded three times and the average band area ratios taken. Measurements of the intensity ratios for the strong bands taken in this study over the pre-selected concentration range were correct to + 1%. Using a calibration plot of the intensity of the dB = 1060 cm-’ Raman band of CC&SO; against the known concentration of the CCl,SO; species, the concentration of the trichloromethanesulphonate ions in the acid solution could be determined [ 31. The calibration graph of concentration versus band intensity for the CCL&SO; species was constructed using purified CC13S03Na material in aqueous solutions of different concentration. The actual concentration of the Na+, and hence CC&SO;, in each solution was determined from standard atomic absorption spectrophotometry. Infrared spectra The infrared spectra were recorded using a Perkin-Elmer 1760 FT-IR spectrometer over the wavenumber range 3000-400 cm-l. The samples were studied as compressed discs with potassium bromide. Calibration of the infrared spectra was effected using indene and polythene; wavenumbers are accurate to + 1 cm-‘.

14

THEORY

Trichloromethanesulphonic point group C,, for which

acid (Fig. 1) may be assigned to the molecular

all of which are active in both infrared and Raman spectra, with thirteen Raman bands polarised. The trichloromethanesulphonate ion, CCl,SO; , is of higher symmetry and belongs to the molecular point group C,,, for which

with the 5A, + 6E modes active in the Raman (5 polarised) and infrared, and the A2 mode inactive in both. Free rotation of the Ccl,- group about the C-S bond would lower the effective molecular symmetry to C3 and the A, species mode, a torsion, then becomes active in both Raman and infrared spectra. RESULTS AND DISCUSSION

The Raman spectrum of trichloromethanesulphonic acid (Fig. 2) shows features which are readily correlated with the CC&SO; species but a major difference between the spectra of CC1,S03H and CC&$03 is the absence of the Y(SO, symmetric stretching) band at 1060 cm-’ in the former. The Raman spectrum of a 2 M aqueous solution of CC13S03H (Fig. 3) is essentially that of the trichloromethanesulphonate ion. Molecular vibrational assignments supported by force-constant calculations have recently been reinvestigated [ 61 for the CCl,SO; species and a very good fit between observed and calculated wavenumbers has aided some new assignment proposals which correct previous anomalies. In the present work, the molecular assignments for CCl,SO,H followed from those of CC&SO; [6,7], CCl&SCl [B] and CCl,HgCl [9]. The wavenumbers of the Raman and infrared spectra of CC13S03H and the Raman wavenumbers of CC&$03 are given in Tables 1 and 2 respectively, along with the approximate descriptions of the vibrational modes. As with the CF3S03H species [4], the vibrational assignments of CC13S03H are necessarily somewhat tentative in the low wavenumber regions for the A’

cl\c_,,Jo / \

Cl" Cl

OAH

Fig. 1. Molecular structure of trichloromethanesulphonic acid.

15

I

100

I

500

I

I

900

Fig. 2. Raman spectrum of trichloromethanesulphonic

I

I

1300

acid in the solid state.

and A” fundamentals. Results from recent force-constant calculations on the CCl,SO; species [ 61 indicate that the approximate descriptions of the vibrational modes are not well-defined in terms of CC&, CS and SO, atomic and group motion. Because of the similarity in masses of the Cl and S atoms, for instance, the v (Ccl,) and 6( CC&) modes involve considerable contributions from v (CS ) stretching and the Ccl, wagging and rocking modes become complex vibrations involving CS and SO motion. Hence, the vibrational complexity of CCl,SO,H is comparable with that of species such as Ni(PCl,),, for which extensive force-constant and spectroscopic studies [lo-121 have been made and for which the ‘breathing’ motion of the PC& groups attached to the Ni atom was established. Despite these difficulties, the vibrational assignments proposed in Tables 1 and 2 correlate quite well with those in the published literature for related compounds such as CCl&SCl and CC13HgC1,and also with those of CC13SOc [ 71. A major inconsistency for CC13SO~ in the literature [ 71 has been the

16

L

100

900

500

1300

-1 ATi/CXl

Fig. 3. Raman spectrum of a 2 M aqueous solution of trichloromethanesulphonic acid.

negative value for the primary force constant for Y( CS) stretching, an A’ vibrational mode, which was necessitated by the constraints of the force field fitting and setting of the values of the interaction constants. Only four polarised bands were observed for the CCl,SO, species in the Raman spectrum and we conclude, in agreement with previous literature [ 71, that the vg (SO, symmetric deformation) is accidentally degenerate with the Y, (SOa asymmetric deformation) band. In Figs. 2 and 3 the Raman spectra of CCl$S03H in the solid state and in aqueous solution show that, in the 2 M acid solution, the intensity of the AD= 1060 cm-’ band characteristic of SO, symmetric stretching in the CC&$03 species increases relative to the v (C-Cl symmetric stretching) band at 418 cm-‘. This is evidence for the weak acid nature of CC13S03H, which thus resembles CH3S03H [ 11 CF,S03H [ 41, C,H,SO,H [ 31 and C,H,SO,H [ 21 in this behaviour. The proportionality between the Raman band intensity I and the species

17 TABLE 1 Observed wavenumbers (cm-‘) CCl,SOsH

and vibrational assignments of trichloromethanesulphonic acid,

Infrared solid

Baman solid

Symmetry class

Approximate description of mode

3225 s,bd 1158 m 1112 m 893 ms 828 s 621 ms 518 ms 455 mw?

3200 w,bd 1160 m 1110 m 890, sh 830 m,bd 620 m 520 ms 460 w 418 vs [315 m,bd] [260 m] 245 m 178 m

A’

vi OH stretch v2 SOPsymmetric stretch vs SOH deformation v4 SO stretch (bonded to H) vs Ccl, asymmetric stretch vs CS stretch v7 SOPdeformation vs SO wag v, Ccl, symmetric stretch v,~ SOS in-plane rock vi1 CC& asymmetric deformation vi2 CC& symmetric deformation vi3 CC& rock

1283 ms 764 ms

1275 w 766 mw [ 315 m,bd] 1260 m] 205 w [120m] [120m] 109 w?

A”

vi, SO2 asymmetric stretch vi5 CCls asymmetric stretch vi6 SO2 out-of-plane rock vi7 Ccl, asymmetric deformation vi8 CCls wag vi9 Ccl, out-of-plane rock veOCC& torsion v2i SOH torsion

concentration C is given by the following equation, assuming that the geometric and optical factors remain constant during the experiment [6]: C=JI

(1)

where J-’ is the molar intensity of the species. Then the relative intensity of the 43 = 1060 and 420 cm-’ bands is I1060

pr=

Canion J420 cyiizci =

J’a

(2)

where a! is the degree of dissociation of the acid species, CC1,S03H. In aqueous solutions of trichloromethanesulphonic acid the dissociation into trichloromethanesulphonate ions is given by CC13S03H+H20=CC13SO~ (l-ff)

CY

+H30+ (Y

for which the dissociation constant K, is

18

Kc =&/(1--(x)

(3)

in which c is the concentration of the acid. However, the true acid dissociation constant I&, is defined as

a2c

y2

(4)

Km=(l-a)B

where y is the mean molar activity coefficient of the CC&SO, anions and the H,O+ cations and p is the activity coefficient of the undissociated acid. Unlike C,H,SO,H and CH3S03H, however, activity coefficient data are not available for the CCl,SO,H species. An estimate of the extent of dissociation of the CCl,SO,H species in aqueous solution may be obtained from eqn. (2) and a knowledge of the 11060/1420ratio in the aqeous solutions of the CC&SO; species. In our previous work on the C,H,SO,H and C,H,SO; system [ 31, from eqn. (2) since a!-+ 1 as c+O then a plot of J’ (Y against C (the total concentration of sulphonic acid and sulphonate species in solution) should approach J’ at c = 0. This means that, in aqueous solutions at a concentration of about 1 M or less, the acid species is almost completely dissociated. For example, in ethanesulphonic acid at 298 K, the dissociation constant Kc has been evaluated [3] as 50 mol dmm3, and in a 1.59 M aqueous solution of the acid ~~~0.981. For (Y= 1.000, the acid would be completely dissociated into sulphonate anions and the 1’““o/1420 ratio would then be that of the sodium salt in aqueous solution. In the present work, the values of the 11060/1420ratios for 2 M aqueous soTABLE 2 Raman spectrum of a 2 M aqueous solution of sodium trichloromethanesulphonate; wavenumbers and assignments for the CCI,SOB species Wavenumbers (cm-‘) 1060 621 (548 413 247 1250 830 815 (548

vs, p mw, p m, p?) vs, p ms, p

mw,dp m, dp mw, sh, dp? m, p?) 339 ms, d 261 m, sh, dp 177 m, dp

Symmetry class

Approximate description of vibration mode SOa symmetric stretch CS stretch SO3 symmetric deformation Ccl, symmetric stretch Ccl, symmetric deformation Ccl,-SO3 torsion SO, asymmetric stretch Ccl, asymmetric stretch 2u,=826 SO, asymmetric deformation uIOSO3 rock uI1 Ccl, asymmetric deformation u12Ccl, rock

19

lutions of the trichloromethanesulphonic acid and the sodium salt are 0.373 and 0.378 respectively; the degree of dissociation of the trichloromethanesulphonic acid in a 2 M aqueous solution is, thus, estimated at 0.986. From this (Y value, a Kc value of 139 + 18 mol dram3is calculated. Because of the limited solubility of the trichloromethanesulphonic acid in water to a maximum of 2 M, it has not been possible to undertake a study similar to that which has been carried out for other acids in the series, namely CH,SO,H [ 11, CF,S03H [ 41 and C2H,S03H [ 31 where degrees of dissociation were calculated using the Raman spectroscopic method for solution concentrations in the range l-11M. However, it is interesting to compare (Table 3) the a values for each acid species in a 2 M aqueous solution evaluated by the Raman spectroscopic method in our laboratories. The Kc value for CCl,SO,H lies midway between those of the methanesulphonic and trifluoromethanesulphonic acids; Kc for CF3S03H is about twice that of CH3S03H whilst that of CH3CH,S03H is about 2/3 of the value for CH3S03H. Qualitatively, a similar trend is noted for the carboxylic acid series CH3C02H, CCl,CO,H, CF3C02H and CH,CH,CO,H but the magnitudes of the differences in Kc values is much greater for this series. For example, the Kc values of the carboxylic acids CF,CO,H to CH,CH,CO,H are 1.8~ 10V5 mol dmm3,2~ 10-l mol dmm3, 1 mol dmW3and 1.3 x 10F4 mol dmV3 respectively. There is, thus, a difference of more than lo4 between the unsubstituted ethanoic acid and its trifluoro analogue, and the trichloro-species has a Kc value which is only 0.2 that of the trifluoro acid. The methyl substituted ethanoic acid, namely propionic acid, has a Kc value which is about 2/3 that of ethanoic acid itself. Hence, we conclude that in the sulphonic acid series the effect of chlorine substitution for the methyl group hydrogen atoms in CH,SO,H produces a small increase in Kc of about 1.5 times, whereas the analogous substitution in the carboxylic acid series produces a Kc which has increased by 104. This large TABLE 3 Comparison of the degrees of dissociation, (x, and Kc" values of methanesulphonic acid and substituted methanesulphonic acids in 2 M aqueous solutions Acid

Reference

OL

K,"(mol dmm3)

CH,S03H CF,SOsH C,H,SO,H CCl,SO,H CsH,SO,H

1 4 3 This work 2

0.980 0.990 0.972 0.986 0.959

96.0 196 67.5 139 44.9

“K, values calculated for 2 M solutions; since Kc= Km pwhere p is the density of the solution, then

Kc= Km in very dilute solution.

20

difference in behaviour of the substituted carboxylic and sulphonic acids implies that the electron-withdrawing effect of the three chlorines in the CC& group transmitted through a S-O bond is almost neutralized by comparison with the C-O bond. However, the substitution of the electron-donating methyl group for a hydrogen atom in the CH; group of CHBSOBH and CH,CO,H produces a quantitatively similar change in acid dissociation for the properties of materials with electron-withdrawing substituents on carbon atoms attached to sulphur-oxygen bonds. In future work we shall consider the non-linearity of the effect of the substitution of electron-withdrawing substituents in the sulphonic acids when compared with the carboxylic acids, e.g. the progressive substitution of a Cl atom for H in CH,CO,H to CCI,CO,H results in an increase in acid dissociation constant of 80 times, 20 times and 6 times, with an overall increase from CH,CO,H to CC1,C02H of lo4 times. In contrast, the complete substitution of methyl groups for H atoms in CH,CO,H to (CH,),CCO,H results in only a twofold acid dissociation constant decrease.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12

J.H.R. Clarke and L.A. Woodward, Trans. Faraday Sot., 62 (1962) 2226. A.K. Coving-ton and R. Thompson, J. Solution Chem., 3 (1974) 603. H.G.M. Edwards and D.N. Smith, J. Mol. Struct., 238 (1990) 27. H.G.M. Edwards, Spectrochim. Acta Part A, 45 (1989) 715. S. Zuffanti, J. Am. Chem. Sot., 62 (1940) 1044. H.G.M. Edwards and V. Fawcett, to be published. M.G. Miles, G. Doyle, R.P. Cooney and R.S. Tobias, Spectrochim. Acta Part A, 25 (1969) 1515. C.O. Della Vedova and P.J. Aymonino, J. Raman Spectrosc., 17 (1986) 485. J. Mink and P.L. Goggin, J. Organomet. Chem., 246 (1983) 115. H.G.M. Edwards and L.A. Woodward, Spectrochim. Acta Part A, 26 (1970) 897. H.G.M. Edwards, J. Mol. Struct., 156 (1987) 137. R.M. Bligh-Smith and H.G.M. Edwards, J. Mol. Struct., 160 (1987) 135.