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Studies in heavy metal soaps—I molecular weights of some copper n-alkanoates
J. Inorg. Nucl. Chem., 1961. Vol. 21, pp. 311 to 314. Pergamon Press Ltd. Printed in .Northern Ireland
STUDIES IN HEAVY METAL SOAPS--I MOLECULAR WEIGHTS OF SOME COPPER n-ALKANOATES A. K. RAI and R. C. Mt-:HROTRA Chemical Laboratories, University of Gorakhpur, Gorakhpur, India (Received 29 Jam~ary 1961 ; hi revisedform 19 March 1961)
Abstract --Copper alkanoates have been found to show considerably low values of magnetic susceptibility compared to other copper salts. Two alternative explanations based on formation of chelate rings either on a monomeric or dimeric structure, have been suggested for the above observations. The crystal structure as well as partition coefficients experiments tend to support the latter viev,. Actual measurements of molecular weights ebuliioscopically in benzene employing a sensitive thermistor for elevations of temperature have confirmed the dimeric nature of the following soaps butyrate laurate, palmitate and stearatc of copper in chloroform and benzene. TIIF magnetic susceptibilities of copper n-alkanoates, measured systematically by AMIEL,~11 indicated much lower values compared to those normally observed for other copper (II) salts. The following bimolecular structure in the solid state has bccn IR
\ / " \0 C
0 Cu \0
Cu
F{ C ~"
C
. / \ o / "\o/ \ , claimed by NICKARK and SKOENING.(2) MARTIN and WATERMANN~3) have tried to explain the low values of magnetic susceptibility on the basis of above dimeric structure. The dimeric structure has also been indirectly assumed to explain the partition coefficient values of copper propionate between water and chloroform, c4J NI!LSON and PINK 15) have investigated the micellar and molecular weights of some metal soaps in toluene, isobutyl alcohol and pyridine, and found that molecules exhibit a high degree of aggregration in organic solvents. In a detailed study of copper (II) alkan o a t e s , M A R T I N a n d W H I T L E Y (61 have shown that cyroscopic measurements in dioxan confirmed the dimeric structure. However, amongst the non-ionising solvents tried ~61 for ebullioscopic measurements dioxan appeared to decompose the copper alkanoates, whereas the elevations of the boiling point of benzene were remarkably small indicating molecular complexities of about six to eight. In view of a detailed scheme to investigate the behaviour of heavy metal soaps in solutions, it was considered of interest to remeasure the molecular weights of copper alkanoates ebullioscopically in some suitable solvents. The solubility of copper alkanoates was sufficiently high in dioxane and pyridine but these solvents were not tl~ AMEtL, C. R. Acad. Sci., Paris 207, 1097 (1938). ':~ NICrKARK a n d SKOENING, Acta Cryst. 6, 227 (1953). (a~ MAR nY a n d WATERMANN, J. Chem. Soc. 2547 (1957). (~ D. P. GRADDON, J. Inorg. Nucl. Chem. I I , 337 (1959). (~ Nu.SON a n d PINK, J. Chem. Soc. 1744 (1952). (6J M.*aXl~ a n d W m l ' t F V , J. Chem. Soc. 1394 (1958). 311
312
Studies in heavy metal soaps--I
employed due to the possibility of formation of addition compounds with the dissolved alkanoates. Among the other solvents attempted, the solubility in acetone was too low for accurate measurements of elevations of boiling points but benzene and chloroform were found to be suitable. Critical solution phenomenon was observed in both the solvents. Values of critical solution temperature (C.S.T.) for copper stearate were estimated to be approximately 45°C and 65°C in chloroform and benzene respectively. The molecular complexity of the soaps appeared to be highly susceptible to even traces of moisture but provided careful precautions were taken to exclude moisture, the ebullioscopic molecular weights of copper alkanoates (butyrate to strearate) corresponded to dimeric structure in both the solvents (Table 1). TABLE 1.
Name
Molecular formula
Formula weight
Molecular weight in benzene
Molecular weight in chloroform
Copper butyrate Copper laurate Copper palmitate Copper stearate
Cu(CsH¢COO)2 Cu(CuH2sCOO)z Cu(C15H~,COO)2 Cu(CITH36COO)2
237.54 461.54 573.54 629.54
514.6 857.8 1286.0 1103.0
520.0 980.0 1108.0 1162-0
I I
EXPERIMENTAL Copper laurate, stearate and palmitate were prepared by metathesis of alcoholic copper acetate (Analar, B.D.H.) with an alcoholic solution of redistilled acids (Palmitic acid, B.D.H.b.p., 222 4- 0.5°C/16 mm, stearic acid, B.D.H., b.p., 228 4- 0.5°C/10 mm). The precipitates were washed with conductivity water and then with alcohol to remove excess of free acids and other impurities, and were dried over sulphuric acid under vacuum for two to three days. The compounds were recrystailised from dry dioxane and excess of solvent was removed at 70--75°C under 2 mm pressure. Copper butyrate was prepared by treating copper carbonate with an excess of dilute butyric acid and was purified by repeated crystallization}" The measured molecular weights were found to be highly susceptible to moisture. Extra precautions had, therefore, to be taken to make the solvents (benzene and chloroform) anhydrous and to exclude moisture during the time of measurements. The molecular weights of the compounds were determined by a semi-micro ebulliometer (Gallenkamp), employing thermistor sensing. The apparatus requires 4-5 hr for each determination, therefore precautions were taken to protect the solvent and the sample from moisture. Standard joints were, therefore, used in the ebulliometer bulb which was attached to a standard joint condenser, which in turn was protected from moisture by anhydrous calcium chloride. The resistance bridge was attached with a separate sensitive Pye Scalamp Galvanometer (Cat. No. 1904/5). The metal contents of the compounds were determined and the results are shown below: TABLE 2.
Name
Copper butyrate Copper laurate Copper palmitate Copper stearate
Molecular formula Cu(CsHTCOO)2 Cu(C,,H2.COOh Cu(CIsH3tCOO)z Cu(C17Hs6COOh
cv~R.L. MARTENand H. WATERMAN.q,J. Chem. Soc. 2545 (1957).
Cu
found
Cu calc.
(%)
(%)
26.67
26"70 13"77 11 "08 10"09
13.72
11.11 10.17
A. K. RAI and R. C. MEHROTRA
313
A A copper butyrote BB copper tourote CC copper polmltote DO copper steorote
900
I 890
i\ -.,
"880
c
~
Bzo
860
850
84C O0
01
1 04
03
02
Conc. of solute,
0.5
g/16 m L
of
06
benzene
F=G. I.
1
oroform 720
700
680 o
'~ 6 6 0 o:
D
640
620
600 O0
O-I C0nc
O2 of
03 solute, FIG. 2.
O4 g/16mL
O5
07
Studies in heavy metal soaps--I
314 800
f AA
nophtholene
BR
naphthalene in chloroform
in benzene
700 ' ~ 8
\ 5OO
\
\
er 400
\
i
i
300
200
I00 0.0
i
0-1
02
03
Conc. of solute,
04
0.5
06
g/16mL.
FIG. 3. For molecular weight determination, 16 ml of dry benzene or chloroform was used each time. It was observed that stable observations can only be obtained after addition of some solute with benzene as a solvent, whereas no such addition was required with chloroform. The change in values of the resistance of the solution with concentration is shown in Figs. 1 and 2. The molecular weight can be calculated by the simple relationship, M = K W/R where M is molecular weight of the dissolved substance, W is the weight of the solute, R is the decrease in resistance due to addition of W of solute and K is a constant for the solvent. In view of the linear relationship between W and R (Figs. I and 2), the graphical method for the calculation of molecular weights has been employed, according to which M = Kin, where m is the slope of the line. All the resistances were measured within an accuracy of 4-0.20 D. The values for K were taken to be 77,200 for benzene and 127,400 for chloroform after several determinations with naphthalene as solute (Fig. 3). Acknowledgement--The authors are grateful to the Council of Scientific and Industrial Research, New Delhi, for a research fellowship to one of them (A. K. R.) during the tenure of which the present investigations were carried out.
STUDIES IN HEAVY METAL SOAPS--I MOLECULAR WEIGHTS OF SOME COPPER n-ALKANOATES A. K. RAI and R. C. Mt-:HROTRA Chemical Laboratories, University of Gorakhpur, Gorakhpur, India (Received 29 Jam~ary 1961 ; hi revisedform 19 March 1961)
Abstract --Copper alkanoates have been found to show considerably low values of magnetic susceptibility compared to other copper salts. Two alternative explanations based on formation of chelate rings either on a monomeric or dimeric structure, have been suggested for the above observations. The crystal structure as well as partition coefficients experiments tend to support the latter viev,. Actual measurements of molecular weights ebuliioscopically in benzene employing a sensitive thermistor for elevations of temperature have confirmed the dimeric nature of the following soaps butyrate laurate, palmitate and stearatc of copper in chloroform and benzene. TIIF magnetic susceptibilities of copper n-alkanoates, measured systematically by AMIEL,~11 indicated much lower values compared to those normally observed for other copper (II) salts. The following bimolecular structure in the solid state has bccn IR
\ / " \0 C
0 Cu \0
Cu
F{ C ~"
C
. / \ o / "\o/ \ , claimed by NICKARK and SKOENING.(2) MARTIN and WATERMANN~3) have tried to explain the low values of magnetic susceptibility on the basis of above dimeric structure. The dimeric structure has also been indirectly assumed to explain the partition coefficient values of copper propionate between water and chloroform, c4J NI!LSON and PINK 15) have investigated the micellar and molecular weights of some metal soaps in toluene, isobutyl alcohol and pyridine, and found that molecules exhibit a high degree of aggregration in organic solvents. In a detailed study of copper (II) alkan o a t e s , M A R T I N a n d W H I T L E Y (61 have shown that cyroscopic measurements in dioxan confirmed the dimeric structure. However, amongst the non-ionising solvents tried ~61 for ebullioscopic measurements dioxan appeared to decompose the copper alkanoates, whereas the elevations of the boiling point of benzene were remarkably small indicating molecular complexities of about six to eight. In view of a detailed scheme to investigate the behaviour of heavy metal soaps in solutions, it was considered of interest to remeasure the molecular weights of copper alkanoates ebullioscopically in some suitable solvents. The solubility of copper alkanoates was sufficiently high in dioxane and pyridine but these solvents were not tl~ AMEtL, C. R. Acad. Sci., Paris 207, 1097 (1938). ':~ NICrKARK a n d SKOENING, Acta Cryst. 6, 227 (1953). (a~ MAR nY a n d WATERMANN, J. Chem. Soc. 2547 (1957). (~ D. P. GRADDON, J. Inorg. Nucl. Chem. I I , 337 (1959). (~ Nu.SON a n d PINK, J. Chem. Soc. 1744 (1952). (6J M.*aXl~ a n d W m l ' t F V , J. Chem. Soc. 1394 (1958). 311
312
Studies in heavy metal soaps--I
employed due to the possibility of formation of addition compounds with the dissolved alkanoates. Among the other solvents attempted, the solubility in acetone was too low for accurate measurements of elevations of boiling points but benzene and chloroform were found to be suitable. Critical solution phenomenon was observed in both the solvents. Values of critical solution temperature (C.S.T.) for copper stearate were estimated to be approximately 45°C and 65°C in chloroform and benzene respectively. The molecular complexity of the soaps appeared to be highly susceptible to even traces of moisture but provided careful precautions were taken to exclude moisture, the ebullioscopic molecular weights of copper alkanoates (butyrate to strearate) corresponded to dimeric structure in both the solvents (Table 1). TABLE 1.
Name
Molecular formula
Formula weight
Molecular weight in benzene
Molecular weight in chloroform
Copper butyrate Copper laurate Copper palmitate Copper stearate
Cu(CsH¢COO)2 Cu(CuH2sCOO)z Cu(C15H~,COO)2 Cu(CITH36COO)2
237.54 461.54 573.54 629.54
514.6 857.8 1286.0 1103.0
520.0 980.0 1108.0 1162-0
I I
EXPERIMENTAL Copper laurate, stearate and palmitate were prepared by metathesis of alcoholic copper acetate (Analar, B.D.H.) with an alcoholic solution of redistilled acids (Palmitic acid, B.D.H.b.p., 222 4- 0.5°C/16 mm, stearic acid, B.D.H., b.p., 228 4- 0.5°C/10 mm). The precipitates were washed with conductivity water and then with alcohol to remove excess of free acids and other impurities, and were dried over sulphuric acid under vacuum for two to three days. The compounds were recrystailised from dry dioxane and excess of solvent was removed at 70--75°C under 2 mm pressure. Copper butyrate was prepared by treating copper carbonate with an excess of dilute butyric acid and was purified by repeated crystallization}" The measured molecular weights were found to be highly susceptible to moisture. Extra precautions had, therefore, to be taken to make the solvents (benzene and chloroform) anhydrous and to exclude moisture during the time of measurements. The molecular weights of the compounds were determined by a semi-micro ebulliometer (Gallenkamp), employing thermistor sensing. The apparatus requires 4-5 hr for each determination, therefore precautions were taken to protect the solvent and the sample from moisture. Standard joints were, therefore, used in the ebulliometer bulb which was attached to a standard joint condenser, which in turn was protected from moisture by anhydrous calcium chloride. The resistance bridge was attached with a separate sensitive Pye Scalamp Galvanometer (Cat. No. 1904/5). The metal contents of the compounds were determined and the results are shown below: TABLE 2.
Name
Copper butyrate Copper laurate Copper palmitate Copper stearate
Molecular formula Cu(CsHTCOO)2 Cu(C,,H2.COOh Cu(CIsH3tCOO)z Cu(C17Hs6COOh
cv~R.L. MARTENand H. WATERMAN.q,J. Chem. Soc. 2545 (1957).
Cu
found
Cu calc.
(%)
(%)
26.67
26"70 13"77 11 "08 10"09
13.72
11.11 10.17
A. K. RAI and R. C. MEHROTRA
313
A A copper butyrote BB copper tourote CC copper polmltote DO copper steorote
900
I 890
i\ -.,
"880
c
~
Bzo
860
850
84C O0
01
1 04
03
02
Conc. of solute,
0.5
g/16 m L
of
06
benzene
F=G. I.
1
oroform 720
700
680 o
'~ 6 6 0 o:
D
640
620
600 O0
O-I C0nc
O2 of
03 solute, FIG. 2.
O4 g/16mL
O5
07
Studies in heavy metal soaps--I
314 800
f AA
nophtholene
BR
naphthalene in chloroform
in benzene
700 ' ~ 8
\ 5OO
\
\
er 400
\
i
i
300
200
I00 0.0
i
0-1
02
03
Conc. of solute,
04
0.5
06
g/16mL.
FIG. 3. For molecular weight determination, 16 ml of dry benzene or chloroform was used each time. It was observed that stable observations can only be obtained after addition of some solute with benzene as a solvent, whereas no such addition was required with chloroform. The change in values of the resistance of the solution with concentration is shown in Figs. 1 and 2. The molecular weight can be calculated by the simple relationship, M = K W/R where M is molecular weight of the dissolved substance, W is the weight of the solute, R is the decrease in resistance due to addition of W of solute and K is a constant for the solvent. In view of the linear relationship between W and R (Figs. I and 2), the graphical method for the calculation of molecular weights has been employed, according to which M = Kin, where m is the slope of the line. All the resistances were measured within an accuracy of 4-0.20 D. The values for K were taken to be 77,200 for benzene and 127,400 for chloroform after several determinations with naphthalene as solute (Fig. 3). Acknowledgement--The authors are grateful to the Council of Scientific and Industrial Research, New Delhi, for a research fellowship to one of them (A. K. R.) during the tenure of which the present investigations were carried out.