O K THE I I I E L E C T R I C C O S S T , I S T S OF SITKILES
R T EIERlI;\S S C H L r X D T
INTRODUCTION Recent in~-estigatioiison the dissociative power of solvents and the electrical conductivity o f non-aqueous solutions by I h t o i t aiid Friderich,I Kalilenberg aiid L i ~ i c o l i i ,and ~ Liiicoln,3 slion- that salts dissolved in acetonitrile, propionitrile atid beiizonitrile yield solutions that conduct well, For esaiiiple, Dutoit aiid Friclericli4 found the iiiolecular conductivity at 2 j" C of silver nitrate dissolved in acetonitrile to be 54.5 at a dilution o f I g-mol in 8 liters, and 118.3 at a dilution of I g-iiiol in I 2 8 liters. T h e inolecular condncti\-ity of aqueous solutions of silver nitrate at 18' C, according to Kolilrauscli,j is 8S.6 at a dilution of I g-mol in I O liters, and 103.3 at a dilution of I g-in01 in 166.7 liters. It appears then that dilute solutions of silver nitrate in acetonitrile 1iaT.e :i greater molecular conductivitj- tliaii the corresponding aqueous solutions. T h e inolecular conductivity of silver nitrate dissolved in benzonitrile, although lower than the conducti\.ity of aqueous solutions, is still of considerable magnitude. For esaiiiple, Lincoln6 found the inolecular condnctivity of silver nitrate dissolved in benzonitrile, at 2.j' C. to be 5.18 aiid 16.38 at the respective dilutions of I g-in01 in 9.43 and 151.96 liters. ~
~
Bull. SOC.Chixii. [Paris) ( 3 ) xg, 3 2 1 :189S). I Z j rSgg). Trans. \Vis. Acad. Sciences. Arts and Letters. Clietri. 3 , .$j7 (Xsgg). 1. c. Ivied. Ann. 2 6 , 161 (18Sg). ( j 1. c.
' Jour. Phys. Chem. 3,
12,
395
Also Jour. Phys.
.lssuming the dissociation theory, the conclusion to be drawn from these results is, that acetonitrile and benzonitrile possess dissociating power i n a high degree. On the basis of Kernst’s3 and T h o n i s o n ’ s ~ r , a u m e nthat t the greater the dielectric constant of solvents the greater is their dissociative power, the nitriles should have high values for their dielectric const ant s. T h e dielectric constant of benzonitrile was measured by Drude~anclfound to be 26.0 at 2 1 ” C. Drude measured tlie dielectric constant of another nitrile, namely, benzyl cyanide, but his extensive investigation does not incliide an!- of tlie nitriles of the aliphatic series. In view of the great dissociating power of acetonitrile, it seemed of special interest to deteriiiiiie its dielerl tric constant and that of some of its hoinologues, and at the suggestion of Prof. Kahlenberg the investigation was undertaken. Lincoln’sr tiieasiirenieiits of the electrical condnctivity of salts dissolved in pyridine show that it ninst be added to tlie list of soll-ents that possess marked dissociating power. Its dielectric constant was therefore also measured, and the value will be found appended to the results found for the nitriles investigated. T h e homologues of pyridine, the substi tuted aniinonias, together with other compounds, are now being investigated. During the progress of his researches on non-aqiieous solutions, Prof. Kahlenberg has collected a choice lot of preparations, which he kindly placed at my disposal. This greatly facilitated the experimental part of my work, and I desire to express to him my thanks for this favor. Method and Apparatus In measuring the dielectric constants the method devised and elaborated by Drude’ was used. I t is unnecessary for iiie
2S,
Zeit. phys Chem. 13, j3r (1893). Phil. Mag. 36, 320 i 1893). Zeit. phys. Chern. 23, 267 i 1897). 1. c. $1. c. Seealso n‘ied Ann. 55, 633 (1895): 58, joo i 1897).
I ;
59,
17
(1896); 60,
to give a complete description of this method and of the details of tlie apparatus eiiiplo? ed, iince Ilrude has alreadp done this. T h e reader is therefore siiiiplv referred to Drude’s original article after reading which the additional remarks 011 tlie method that are contained iii the paragraphs that now follow will he niuch better understood. T h e apparatus used for these ineasurements was a trifle larger than the oil-1 c>-aI1 i tl e C a pr o i i i t r i 1e
HCS CH,CS C,H,CS C.H.CS rCH ,CI-ICN C, H , C S t CH 1 CHCH C S
I-kiizoni trile' C, I3 C H , C S
Henz y 1 c!-aiiidt.:'
Toluonitrile ortho
Py r i d i ne j2uiiioliriei
T h e resiilts olitained \vitli tlie aliphatic nitriles a r r also pi-t:seiitetl g-1-apliicallj. i i i Curve I. iii the accoiiipaii>.iiix figire. T l i e tliclectric coiistmits are plotted as ordinates, :in(1 the iiieiiihers of tlie hoinologoiis series are iioted as abscissrtl, a tlefiiiite tlistniice being chosen for each addition of C H 2 . Crir1.e I I. represents 1)riitle's; values for tlic. tlielecti-ic coilstniits of the alcohols, wliile C1in.e 111n ) . preseiits a ~ ~ ~ ~ i - o ~ i i i i ~ ~ t e l ! ~ ~____
_.
I
,'
'
~
~~
Zeit. phj-s. Client. 35, j 8 j 1 r g o o ) . Measured- ,1 1)rude. ~ v h ofound 2 6 . 0 a t z i C . Ilrude found I j . 0 :at 1g5 C. T w n e r fouiid S . 5 with Sernst's apparatus.
5 1
c.
I 66
N e i w n i z Schhiizdf
the values obtained by Thwing,l Kernst,2 and Tereschin3 for the dielectric constants of the alcohols. Curve 111. represents the values obtained by Drude for the dielectric constants of the fatty acids. Hence in Curve I., R represents the cyanogen group (CN), in Curves 11. and II(n), R represents the hydroxyl group (OH), while in Curve 111. it represents the carboxyl group (COOH). Discussion of Results T h e high dielectric constant found for hydrocyanic acid is of special interest from a theoretical point of view, since by the Nernst-Thomson rule, that the greater the dielectric constant of a solvent, the greater is its dissociating power, bydrocyanic acid should show extraordinary dissociating power. T h e
’
Zeit. phys. Cherri. 14, 2S6 (1S94). Ibid., 14,6 2 2 (1894). \Vied. Ann. 36, 792 (18S9).
Dielectric Coizstnrzts of A'itriles
167
publication of the electrical condnctivity nieasiirenients of salts dissolved in hydrocyanic acid, which are i n progress in this laboratory, will therefore be awaited with interest. T h e marked differences in the physical properties between tlie first two members of any 1ioiiiologoiis series are well exemplified here by tlie great differences i n the dielectric constants between hydrocyanic acid and acetonitrile, water and methyl alcohol, formic acid and acetic acid. T h e ciirves sliov~that an increase in the size of the molecule by the addition of carbon and hydrogen lowers the dielectric constant. T h i s alyo holds for the aromatic nitriles and for pyridine and quinoline. T h e isomeric nitriles examined have nearly the same dielectric constant, tlie iso-compounds showing a slig-h tly higher val 11e. .I comparison of the values of the dielectric constants of the alcohols and nitriles by means of Curves I. and 11. s h o n s that for high frequency oscillations the nitriles have higher values for their dielectric constants tliroiigliout than the corresponding alcohols. For oscillations of low frequency the values for the dielectric constants of the higher members of the alcohol series are nearlj. the same as the values found for the corre4ponding nitriles with high frequency oscillations. Compare Carves II(a ) and I. Since tlie nitriles show but slight absorption, the dielectric constants when determined with Kernst's apparatus will probably agree closely with tlie vallles found with Drude's apparatus. T h e researches of Dutoit and Fridericlil on the electrical condncti\.ity of salts dissolved in acetonitrile, propionitrile, and biityronitrile show that tlie dissociating power decreases iii tlie order in wliicli the solvents are named. Their dielectric constants also decrease in the order named, thus supporting the IY ernstThomson rule. T h i s relation between the dielectric coiistant and the electrical conductivity is illustrated by the following table. T h e molecular conductivity iiieasiireiiients are taken from the work of Dutoit and Friderich. In the table p stands -
' 1. c.
-
for the molecular coiidncti\ it!- at 2 j" c', arid z the number of liters in which one grain-iiiolrcnle of silver nitrate is contained. T-IBI.1;
11 Dielectric conatant
~
I
Solvent
P-
at
210
c.
-1 cet on i t ri 1e 64 0 10.3 7 36 1 Propioilitrile 63 7 46.0 26. j But? roilitrile 7.5.6 25 4 20.3 Propioilitrile aiid beiizoiiitrile ha\ e about the same rlielectric constant ; but the niolecalar coudncti\ ities of silver nitrate dissolved i i i the\e nitriles are quite different, as a comparison of r the rcsult5 i l l the folion ilig table sho\\-s. T h e m o ~ e c u ~ aconductivitie5 for silver nitrate i n beiizoiiitrile are taken from Liiicoln's' work. T h e iiioleciilar coiiducti1 ities for silver nitrate are taken from the \\ ork of Ihitoit and Friderich.2 TABI,FI11 Henzoiiitrile
Propiorlitrile -
-
.-
Y at zjo C. _-
P at 2 5 ' C .
~~-
24.06 SS.9Y 83.92
j.66
34.9 46.0
,;2.0
63.7
11.19
73.4'
T h e inolecnlar conductivities of silver nitrate dissol\-ed in pyridine arid in bntyroiiitrile furnisli another interesting coinparison. TABLE: It' Solvent
I
Butyroriitrile" Pyridine'
i '
75.6 150.4 60.9
140.7
~
1. c., p . 4 2 2 . 330. Dutoit atid Friderich. 1. c., p. j;~. Lincoln. 1. c . , p. 423.
' 1. C.. p. :'
Iklectric con stan t
p- at 2j0
2.5.4 32. I 30.17 36.21
20.3 I
12.4
IT-liile Table I], slioivs that tlie Sernst-Tlio~iisotirille holds fcor tlie three iiieiiibers of the snine lioiiiologo~sseries, Table 111. cieiiionstrates that c1ieinic:tlly analogous substances lia\.iiig about the saiiie dielectric constants iiiay nevertheless j.ield sollitions (contairiing the same solute) with \-cry differeiit electrical coiic1uctii.itit.s. Table I\-. shows concliisively that a pyridiiie solution of sil\,er nitrate conducts better tlian a corresponding one in biit~~ronitrile, notwithstanding tlie fact tliat tlie dielectric constant of pyridine is only 6r percent of that of hutyronitrile. TVe 1iai.e here then a strikiiig case in which the Keriist-Thoiiison rille does iiot hold. It is well in this coniiection to bear in iiiiiid the fact that Good\viii and Tlioiiipsoii* have fc~iiiicl that liquid aminonia is also a strikiiig exception to tlie Sernst-Tlioiiison rule. It is evident froiii tlie above results that tlie Keriist-Thoriiscm rule holds at best only for the inernbers of the same homologous series. I t is also clear that the dielectric constaiit of a solvent is not tlie sole factor deteriiiiiiiiig whether solutiolis iii that solvent will conduct electricit). or iiot. I aiii indebted to Prof. E;ali!enberg for his suggestions a n d assistance, and I take this ineans of acknowledging the same. f ~ L 7 6 O J X t O Y Jofe ' Ph.v.iicizL c h € i J l i S i ! t ~ V ,
151ii~er-siLyof LVisromiiz, M a d i s o ~ c ,TI -is. f l l 7 2 . 3 0 , /PO/. ~
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Pliys. Review, 8 , 43 (~Sgg).
