DIELECTRIC PROPERTIES OF ALKYLAMIDES
509
Dielectric Properties of Alkyl Amides. 11. Liquid Dielectric Constant and Loss
by S. J. Bass, W. I. Nathan, R. M. Meighan, and R. H. Cole Chemistry Department, Brown University, Providence, Rhode Island
(Received September 13, 1963)
Static dielectric constants of dimethylformamide, formamide, N-methylformamide, acetamide, and propionamide have been measured over their liquid temperature ranges. As compared to dimethylformamide, the values for formamide are significantly larger and those for the X-methyl amides very much larger. The results are compared with predictions from models of chainwise association by hydrogen bonding. Dielectric relaxation ln the frequency range 1 to 250 Mc. for the I\’-methyl amides over a range of temperatures is described by the simple Debye function. The processes are so much faster in N,Ndimethylformamide and in formamide that only absorption conductance could be measured in the range used; the results are consistent with Debye type absorption but do not establish it. Linear Arrhenius plots of the relaxation times give activation energies from 2.7 to 7.0 kcal./mole.
Introduction Two kinds of dielectric data are reported here for five liquid amides. The first are static dielectric constants measured by a transformer bridge method over the liquid temperature range. In conjunction with recently determined vapor phase dipole moments, these data are then compared with predictions based on Kirkwood’s correlation factor calculated for various models of intermolecular association by hydrogen bonding. The second set of results are Schering bridgc measurements of somewhat lower precision in the frequency range 1 to 250 Mc. These frequencies permit fairly complete definition of the dielectric relaxation over a range of temperatures for N-methylformamide, -acetamide, and -propionamide; for formamide and K,Xdimethylformamide the relaxation times are short enough that only partial characterization is possible from the frequency dependence of conductance. Formamide from Aiatheson and Bell was distilled before use; because of its low vapor pressure a special still with large evaporation surface and short path to the receiver was employed. N-Methylformamide and K-methylacetamide from Eastman Kodak and X,Ndimethylformamide from Rohm and Haas were vacuum distilled two or more times, once over calcium
oxide. N-Methylpropionamide was prepared from propionic acid (Olin Mathieson) and a 40% solution of methylamine (Eastman) ; five distillations and drying over calcium oxide were carried out to reduce water content. Vapor fractometer measurement indicated about 0.2oj, water in the product; no other significant impurities were indicated by this or by the infrared spectrum. Only for dimethylformamide was d.c. conductance large enough to be a likely source of significant error. The static dielectric constants of N,S-dimethylformamide and of N-methylacetamide and propionamide were measured in a guarded cell with coaxial cylindrical electrodes basically similar to the design described by Hassion and Cole, but with improved shielding to ensure that the direct Capacitance measured by the transformer bridge (Cole aiid GrossZ) resulted entirely from the dielectric fluid. The geometric capacitance was 2.5 pf. and the necessary liquid volume 4 ml. Measurements in the VHF range from 1 to 250 )IC. were made using a coaxial displacement cell described by Love11 and Cole3; the parallel capacitance and conHassion and R. H. Cole, J . Chem. Phys., 2 3 , 1766 (1955). (2) R . H. Cole and P. M. Gross, Jr., Rev. Sci. Inslr., 20, 252 (1949). (1) F. X.
Volume 68, Number 3
March, 1064
S. J. BASS,W. I. NATHAN, R. M. MEIGHAN,AND R. H. COLE
5 10
ductance of this cell were measured with a commercial Schering bridge (Boonton Type 250-A RX meter). The chief difficulty of such measurements for the amides is the result of their very large dielectric constants, which are often over 100 and may exceed 300. Because of the limited capacitance range of the bridge used (20 pf.), measurements had to be made on very thin ring-shaped samples (thickness less than 0.4 mm. in several cases). This undesirable geometry and instrumental limitations in the difficult frequency range could easily cause errors in dielectric constant and loss of several per cent ; however, internal consistency and comparisons with such other data as are available suggest that the results given here may be reliable to 2 or 3%. Both cells were immersed in stirred baths; temperature control by manual adjustment of auxiliary heaters was found adequate. Temperatures were measured by a conventional copper-constantan thermocouple and potentiometer.
in the literature and are sufficiently accurate to be useful for the later discussion. Some comparisons with literature data are given In Table 11. For formamide, N,N-dimethylformamide, Table 11: Comparison of Dielectric Constants Measured for Liquid Amides with Literature Values el)
Substance
N,N-Dimethylformamide Formamide
N-Met hyIf ormamide
N-Methy lacetamide
Results Interpolated values of dielectric constants of the five amides studied are listed in Table I. For N,NN-Methylpropionamide
Table I : Static Dielectric Constanta of Liquid Amides" oc.
t,
- 60
-40 - 20
0 20 40 60 80
100 120 140 160 180
DMF
F
58.3 52.1 46.8 42.0 38.0 34.4 31.4 28.8 26.7 25.0 (23.0)
... .
I
NMF
NMAo
...
...
...
308 261 217 177 146 120
... ...
...
(119) 111 104 (96)
...
... ... ...
..,
...
...
...
...
...
.
I
.
. * .
...
...
... 162 138 118 101 85 74 64 58
NMPr
...
348 267 210 170 139 114 96 82 71
... ,.. ,..
In the column headings, D M F = N,N-dimethylformamide, formamide, N M F = N-methylformamide, NMAc = N-methylacetamide, NMPr = N-methylpropionamide. Values in parentheses are extrapolated. a
F
=
dimethylformamide, N-methylacetamide, and N-methylpropionamide, they were obtained a t 100 kc. with the transformer bridge method with precision better than 0.2y0. The data for formamide and N-methylformamide from the VHF bridge measurements are less precise as already discussed, but are reported because they cover a wider temperature range than other values The Journal of Physical Chemistry
t,
oc.
15 25 35 15 25 35 15 25 35 30 35 40 45 50 60 15 25 35
(this work)
(lit.)
Ref.
38.8 37.2 35.5 113.0 108.7 106.2 189 171 158 178.9 171.7 164.7 157.8 150.0 137.5 179.8 163.1 147.4
38.4 37.7 35.8 112.7 109.5 105.6 200 182 167 178.9 169.7 165.5 156.0 151.8 138.6 188,l 172.2 156.7
12 12 12 4 4 4 4 4 4
m
a
12 a
b a a
4 4 4
L. R. Dawson, R. H. Graves, and P. G. Sears, J . Am. Chem, SOC.,79, 298 (1957). * G. M. French and K. H. Glover, Trans. Faraday Soc., 51, 1418 (1955).
and N-methylacetamide, the values agree to within 1-20/,, but the present results for N-methylformamide and N-methylpropionamide are about 5y0 lower than those reported by The latter of these was observed to cause some corrosion of the nickel-plated cell electrodes on standing. This produced a substantial increase in conductance, but was without apparent effect on the dielectric constant. The measured specific conductancesof the several amides were of the order 10-7 to 10-5 mho/cm.; in the case of dimethylformamide, uncertainties in correction for the effect of residual circuit inductance could amount to 1 or 2% but are less in all other cases. For the three N-methyl amides, the dielectric relaxation process was slow enough that VHF measurements defined most of the dispersion to be expected a t temperatures over an appreciable range. Representative results are shown in the plots of the complex dielectric constant e* = e' - ia" in Fig. 1. These (3) S. E. Love11 and R. H. Cole, Rev. Sei. Instr., 30, 361 (1959). (4) G. R . Leader, J. A m . Chem. Soc., 7 3 , 856 (1951).
51 1
DIELECTRIC PROPERTIES OF ALKYL AMIDES
Table 111: Parameters of Dielectric Relaxation in Liquid Amides
loo-
10"ro, Substance
1 1 1
N,N-Dimethylformamide
0
"p 00
I
I
le0
50
e'
I
I50
P
I
Formamide
N-Methylformamide
N-Methylacetamide
N-Methylpropionamide
0 0
100
50
show that the dispersions can be described within experimental error by the simple Debye function =
E,
+
(Eo
- Em)/(l
+ iwro)
(1)
where and E , are limiting real dielectric constants a t low and high frequency, w is 2a frequency, and r o is the relaxation time. Values of the parameters EO, E,, and r 0 were obtained from the linear plots of E' vs. OE" and E"/W predicted by eq. 1 (see Cole$) after correction for d.c. conductance, and are listed in Table 111. For formamide and K,Ndimethylformamide, the relaxation was too fast for measurable dispersion capacitance even a t the lowest temperature, but frequericy dependent conductance G could be observed. Denoting ohmic conductance by G d c , one then has for wT0
