An NMR determination of constants of association in chloroform-dialkyl ketone solutions

J. inorg, nucl. Chem., 1973, Vol. 35, pp. 1239-1245.

Pergamon Press.

Printed in Great Britain

AN NMR DETERMINATION OF CONSTANTS OF ASSOCIATION IN CHLOROFORM-DIALKYL KETONE SOLUTIONS Z. B. M A K S I M O V I C and A. MIKSA-SPIRIC Laboratory of Chemical Dynamics and S. V. R I B N I K A R Faculty of Science, University of Belgrade, Yugoslavia

(Received20 June 1972) A b s t r a c t - B y means of N M R spectroscopy the strength of the hydrogen bond between chloroform and the five lowest dialkyl ketones was determined. The constants of association of the one-to-one complexes were derived, as well as the thermodynamic constants for the reaction. By use of the Hammett equation it was shown that the basicity of the ketones increases with the size of the alkyl groups. INTRODUCTION

N o SYSTEMATICdata are available on the interaction of the lower dialkyl ketones with chloroform. Even for the system acetone-chloroform, considerably differing values of the constant of association have been reported[I-4]. The values, obtained either from infrared or nuclear magnetic resonance data, in the interval 20°-30°C, are between 0.9 and 3.0 1/mol. The present investigation, using the N M R technique, was concerned with the determination of association constants of the one-to-one complexes of dialkyl ketones (DAK) with chloroform in cyclohexane solutions over a somewhat extended temperature interval. The experimental data was treated by the method developed by Mathur et al. [5], which correlates the chemical shift of the proton signal with the equilibrium constant of association. The equation is: _L=b " 1 +a, Ap CDAK

(1)

where Av is the change in the frequency of the proton signal due to the presence of the dialkyl ketone, and CDAKis its overall concentration. A plot of 1/Avvs. 1[CDAKgives a straight line with slope 1

1

b ~ m . _ _ g /)2 -- PI

I. E. R. Kearns, J. phys. Chem. 65, 314 ( 1961 ). 2. R. E. Kagarise, Spectrochirn. Acta 19, 629 (1963). 3. C. M. Huggins and G. C. Pimentel, J. chem. Phys. 23, 1244 (1955). 4. J. A. Bell and W. H. Snider,44, 200 (1967). 5. R. Mathur, E. D. Bcckcr, R. B. Bradley and N. C. Li, J. phys. Chem. 67, 2190 (I 963). 1239

1240

Z . B . M A K S I M O V I C , A. MIKSA-SPIRIC and S. V. RIBN1KAR

K is then the equilibrium constant of association, vl is the frequency of the chloroform proton signal in the pure chloroform solution, and Vz the frequency of the complex D A K . CHCIz. The ordinate intercept is a

~

-

1

V2 --//1"

The equilibrium constant is thus defined as g ~-

a

b" An alternate way is one given by Carper et al. [6]. The equilibrium constant is here obtained from the equation Av

CDAK-- K ( A v - A v o ) ,

(2)

where Av0 is the chemical shift of the pure chloroform solution. In the plot of Av/CDAK VS. Av, the slope defines the constant K. EXPERIMENTAL The instrument used was a high resolution N M R spectrometer Varian DA-60-IL. Measurements of the proton signals were made with respect to the cyclohexane solvent with an accuracy o f - 0 " 5 cps. The frequency shifts were controlled with a Varian V-3413 frequency meter. The sample temperatures were maintained using the Varian temperature controller Model V-4341/V-6040 with an accuracy of --- 1°(2. The temperatures were controlled using standards of methanol and ethylene glycol. The ketones used: diethyl (DEK), di-n-propyl (DPrK), di-n-butyl (DBK), di-n-pentyl (DPK), and di-n-hexyl (DHK), were obtained from the Fluka Co., Buchs, Switzerland. Checks on their purity were made by measuring their refractive indices, by gas chromatography, or spectrophotometry. By knowing their empirically obtained "Transition energies" in the pure state [7], from the position of the absorption bands of the solvatochromic dye pyridinium-N-phenoibetaine in each ketone, it was possible to compare their purity. Reagents containing more than 0.1 per cent of impurities were purified by fractional distillation under reduced pressure. The middle fractions were kept in bottles over dried zeolite 4A. The chloroform was purified by multiple shaking with water and dried with sodium sulfate and zeolite 4A. The handling of the reagents was done in a dry box and the solutions measured in tightly stoppered N M R tubes. The concentration of chloroform was kept at 0.05 M in all samples, while the ketone concentrations were varied between 0.55 and 1.66 M. In this concentration range the possible dimerization of the dialkyl ketones was assumed to be negligible. From the measured chemical shifts, and the appropriate concentrations, the equilibrium constants were derived using either Eqns (I) or (2). RESULTS

AND DISCUSSION

The experimental results are presented in Tables 1-5. In Fig. 1 the plots for dipenthyl ketone are shown. From the plots it is readily seen that the slopes depend on the temperature. It is also found that the slopes differ from ketone to ketone in such a way that they become smaller for the larger alkyl groups. 6. W. R. Carper, C. M. Buess and G. R. Slipp, J. phys. Chem. 74, 4229 (1970). 7. Ch. Reichardt and K. Dimroth, Fortschr. Chem. Forsch. 11(1), 1-73 (1968).

Chloroform-dialkyl ketone solutions

1241

Table 1. Experimental data of N M R measurements on the system (C2H~)2CO + CHCla in cyciohexane No

C

I/C

20°C

30°C

40°C

(tool/l)

1 2 3 4 5 6 7

0-556 0"626 0'714 0.833 1.000 1'250 1"662

1'8 1'6 1'4 1"2 1'0 0'8 0"6

vcps

Aucps

ucps

Avcps

ucps

Avcps

358-2 359-8 362-3 364'8 367.2 371-3 376'0

19'2 20'8 23'3 25'8 28'2 32'3 37'0

356.9 358"3 360.3 362"6 365"6 369.0 373.9

17'5 19'3 21'3 23'6 26"6 30"0 34'9

355.1 356.3 358'2 360'0 362.7 366.7 371"1

16.1 17'3 19"2 21-0 23.7 27.7 32-1

Table 2. Experimental data of N M R measurements on the system (CaH7)zCO + CHCl in cyclohexane No

C

I/C

20°C

30°C

40°C

(tool/l)

I 2 3 4 5 6 7

0.556 0"626 0"714 0.833 1"000 1-250 1"662

1"8 1"6 1-4 1"2 1"0 0'8 0.6

vcps

Avcps

ucps

Avcps

358.6 360"7 362'5 364.5 367'4 371.2 376"7

19'6 21.7 23'5 25"5 28.4 32'2 37"7

357.0 358'8 360'5 362.1 365"2 369.1 374.1

18'0 19'8 21"5 23-1 26"2 30"1 35"1

ucps 355'6 357.0 358'8 360"0 363-4 367"4 372.6

Avcps 16-6 18"0 19"8 21"0 24-4 28-4 33"0

Table 3. Experimental data of N M R measurements on the system (C4H9)2CO + CHC13 in cyclohexane No

C

1/C

20°C

30°C

40°C

(mol/1)

1 2 3 4 5 6 7

0"556 0'626 0"714 0'833 1'000 1'250 1"662

1'8 1"6 1-4 1"2 1-0 0"8 0"6

vcps

Aucps

359'6 361'1 363'1 365'8 368.9 372'1 377'0

20"6 22'5 24.1 26'8 29.9 33"1 38"0

vcps 358'0 359.5 361"5 364.0 366.5 370"9 374"6

Avcps 19'0 20'5 22'5 25'0 27"5 31.9 35-6

ucps 356'2 357'6 359'3 361"7 364.4 368-4 372-2

Avcps 17-2 18-6 20-3 22"7 25.4 29-4 33'2

The experimental results were treated with the least squares method. At high ketone concentrations the chemical shift of the chloroform signal tends to a constant value, irrespective of the temperature. It was therefore taken that the ordinate intercept (a) is a constant within the limits of experimental error. The value used in the calculations was the average of all intercepts found. The calculated equilibrium constants are given in Table 6.

1242

Z . B . MAKS1MOVI(~, A. MIK~A-SPIRI~ and S. V. R1BNIKAR Table 4. Experimental data of NMR measurements on the system (CJ-Ill)2CO + CHC13 in cyclohexane No

1 2 3 4 5 6 7

C (tool/l)

I/C

0.556 0.626 0.714 0.833 1.000 1.250 1.662

20°C

1.8 1.6 1.4 1.2 1.0 0.8 0.6

30°C

vcps

Avcps

360.5 362.3 364.4 366.7 370.1 373.5 378.2

21.5 23.3 25.4 27.7 31.1 34.5 39.2

vcps 358.2 360.3 362.3 364.4 367.7 371-1 375.1

40°C

Avcps

vcps

Avcps

19.2 21.3 23.3 25.4 28.7 32.1 36.1

3 5 6 .7 358.0 360.1 362.3 365.0 3 6 8 .6 3 7 3 .6

17.7 19.0 21.1 23.3 26.0 29.6 34.6

Table 5. Experimental data of NMR measurements on the system (C6H13)2CO + CHCIa in cyclohexane No

C (tool/l)

1/C

20°C vcps

1 2 3 4 5 6 7

0'556 0'626 0"714 0"833 1"000 1"250 1"668

1"8 1"6 1"4 1"2 1"0 0"8 0"6

360"5 362"5 364"4 366.9 369"5 373"3 378"3

30°C

Aucps vcps 21"5 23"5 25"4 27"9 30"5 34"3 39"3

358"0 360"1 362'3 364"3 367"3 369"6 375.6

40°C

A~cps u c p s Avcps 19'0 21'1 23'3 25'3 28"3 30"6 36"6

356"3 358"0 360.3 361'9 365'0 368"0 372"7

17"3 19'0 21'3 22"9 26"0 29'0 33'7

7

~,,~ 6

4o*c 3o*c

5

2 I

0

t

I

I

I

I

r

I

I

r

I

0'2

0"4

0'6

08

I'0

1"2

1'4

1'6

18

2'0

c Fig. 1. The di-n-pentyl ketone-chloroform system in cyclohexane. T a b l e 7 s h o w s as an e x a m p l e t h e c o m p a r i s o n o f t h e a p p l i c a t i o n o f t h e t w o E q u a t i o n s , (1) a n d (2), to t h e s y s t e m d i e t h y l k e t o n e - c h l o r f o r m . T h e d i f f e r e n c e is w e l l w i t h i n t h e limits o f e x p e r i m e n t a l e r r o r .

Chloroform-dialkyl ketone solutions

1243

Table 6. Equilibrium constants of the dialkyl ketone-chloroform complexes in cyclohexane Dialkyl ketone

K. 10 [1 mol-q 20°C 30°C 40°C

Diethyl Di-n-propyl Di-n-butyl Di-n-pentyl Di-n-hexyl

7.27 7.65 8.12 8.59 8.65

6.48 6.82 7-20 7-49 7.53

5.81 6.05 6.33 6.58 6.63

Table 7. Equilibrium constants of the diethyl ketonechloroform complex t°C K 1 mol-~ K* 1 mol-~

20

30

40

0.727 0-721

0.648 0.635

0-581 0.580

K - was calculated according to Equation (1). K* - w a s calculated according to Equation (2). F r o m a plot o f I n K a g s i n s t c a n b e f o u n d f r o m the r e l a t i o n

l/T, w h i c h is s h o w n in Fig. 2, the e n t h a l p y c h a n g e InK -

AH1 R T"

O t h e r t h e r m o d y n a m i c q u a n t i t i e s , s u c h as the free e n e r g y c h a n g e , A G , a n d the e n t r o p y c h a n g e , AS, are g i v e n i n T a b l e 8. It c a n b e s e e n t h a t b o t h the e n t h a l p y

--oz

:30 * C

-0"7

i 3"I

3'2

3'5 I 3

3"4

~ io [K-'] Fig. 2. The lnK vs.

lIT plot of the dialkyl ketones. The figures next to curves refer to the number of carbon atoms in the alkyl radical.

1244

Z. B. M A K S I M O V I ~ , A. MIKgA-SPIRIC and S. V. RIBNIKAR

Table 8. Thermodynamic data of the dialkyl ketone-chloroform complexes

AG

cal mo1-1 AH K cal mol -~ AS calmol - I ° K -~

°C

DEK

DprK

DBK

DPK

DHK

20 30 40

187 261 338

156 231 314

122 198 285

89 175 261

85 171 256

20 30 40

2.014 2.014 2.014

2-123 2.123 2"123

2.260 2.260 2"260

2.424 2.424 2"424

2.451 2"451 2.451

20 30 40

7.51 7.51 7"51

7.78 7.78 7.78

8.12 8"12 8.12

8"57 8.57 8.57

8.66 8.66 8-66

and the entropy are negative, diminishing with increase in the number of carbon atoms in the aliphatic groups. The free energy change of the system is positive, decreasing with the number of carbon atoms. With a given ketone, its value increases with the temperature. The Hammett equation and the Taft o-* values were applied to the data[8]. Numerical values of the latter are given in Table 9, and a plot of log K vs. o-* is shown in Fig. 3. It can be seen that the magnitude of the equilibrium constants follow the electron repelling power of the alkyl substituents, i.e. the absolute

DPK DHK

DBK

{~2e

-0'I0

-

0'20

-0"30

I 0-20

I 0'25

I 0-30

-20"*

Fig. 3. The relationship between log K and the Taft parameter. 8. V. A. Palm, Osnovi Kolichestvenoi Teorii Organicheskih Reakcii, p. 95. lzdatelstovo Himia Leningradskoe Odelenie (1967).

Chloroform-dialkyl ketone solutions

1245

Table 9 Dialkyl ketone

DEK DprK DBK DPK DHK

-log K 20°C

30°C

40°C

0" 143 0.121 0.095 0.071 0.067

0.193 0.169 0.147 0.130 0.127

0'240 0.223 0-203 0.186 0.183

- o-*

-2.o-*

0" 100 0.115 0.130 0.145 0-147

0-200 0-230 0.260 0.290 0.294

magnitude of the Taft parameter. Saturation is reached with dihexyl ketone, and further equilibrium constants in the series should not differ substantially from each other. Then data and conclusions are in agreement with both the theoretical and experimental values found by other authors [9, 10] on different systems and with other measuring techniques. 9. W. Gordy and S. C. Stanford, J. chem. Phys. 8, 170 (1940). 10. V. Gutman, Angew Chem. 82, 858 (1970); Angew. Chem. Internat. Ed. 9, 843 (1970); Chemistry in Britain 7, 102 (1971).