DECOMPOSITION VOLTAGE OF ETHEREAL GRIGNARD REAGENTS
March, 1935
489
TABLE I INTERPOLATION TABLE FOR F(e) a
0.000
0.000 1.00000 ,010 .98995 ,020 .97980 ,030 ,96953 ,040 .95916 ,050 .94867 ,060 ,93805 ,070 ,92731 ,080 ,91643 .090 .go542 .loo .8942,i ,110 ,88294 .120 .8714j .130 ,85980 .140 .a797 .150 .&3594 .160 .83371 ,170 .81126 .180 .79857 . 190 .75564 .200 ,77244
0.001
0.99900 .98894 .97877 .96850 .95812 .94762 .93698 .92623 .91534 .go431 .89312 .88180 .87029 .85862 ,84677 ,83472 ,82247 .81000 .79728 ,78433 ,77110
0.002
0.003
0.004
0 * 99800 0.99700 0.99599 .98793 .98691 .98590
.97775 .96747 .95708 .94656 .93591 ,92514 .91424 ,90319 .8919g .88065 .86913 .85744 .84557 ,83350 .82123 .80874 .79599
,97672 .96644 .95603 .94550 .93483 ,92406 ,91314 .go208 .89086 .87950 .86797 .85626 ,84437 .EL3228 .81999 .go748 .79470 .78302 .78171 ,76976 ,76841
.97570 .96540 .9549g
.9.93376 .92297 .91204 .90096 .a973 .8783j .86681 .Si508 .84317 .%lo6 .81875 ,80621 .79341 .78039 .76706
0.005
0.006
0.99499 0.99398 .9848g .98387 ,97468 .97365 .9643$ .96333 .95394 .9528g .94338 .94232 .93268 .93161 .92188 .92079 .91094 .90984 .89985 .89873 .88860 .a747 .87720 .87605 .86565 .86448 .85390 .85272 .84197 .PA077 .82984 .82862 .81751 ,81626 .SO494 .80367 .79212 .79083 .77907 .77775 .76571 ,76436
0.007
0.008
0.009
0.99297 0.99197 .98285 .98184 .97263 .97160 .96229 ,96125 .95184 .95078 .9401g .93053 .92946 ,91970 .91861 .go873
.E39761 .a634 ,87490 .86331 .85154 .83957 .82740 .81501 .go240 .78954 .776Q .76301
0.99096 .98082 ,97056 ,96021 ,94973 .93912 ,92838 ,91752 ,90653 .go763 .8964g .89537 .88521 .wag .87375 .87260 .86214 ,86097 ,85035 .84916 .83836 .El715 .82617 .82494 .81376 .81251 .go113 .79985 .78824 .78694 ,77510 ,77377 .76166 .76031
Diff.
100
102 103 101 105 lo6
107 109 110 112 113 115 117 118 120 122 125 127 129 132 135
D3,a straight line i s obtained, with intercept l/Ao and slope 1/KAo2. This method, which is simply a correction for long range interionic forces of the Ostwald dilution law used by Kraus and Bray,4 (5) is very much less laborious than the previous (The term in ~ais omitted here, because it arises method' of successive approximations. (4) Kraus and Bray, THISJOURNAL, 86, 1315 (1913). from ions in contact, and these are counted associated pairs.) Finally F / A is plotted against ~ ~ ~ ~ ~ $ $ ~ ~ s OF ~ y O R Y cAP/F. For concentrations less than 3 X lo-' PROVIDENCE, R. I. RECEIVED JANUARY 10,1935 for the limiting conductance. With this tentative value, y is calculated from A (observed) by (3) and then the activity by means of the equation
[CONTRIBUTION FROM THE CHEMICAL LABORATORIES OF NORTHWESTERN UNIVERSITY AND
THEUNIVERSITY
OF NANKING]
The Decomposition Voltage of Grignard Reagents in Ether Solution BY W. V. EVANS, F. H. LEEAND C. H. LEE The Grignard reagent has been the hydrocarbons appearing as anodic products have been isolated quantitatively and identified and the faraday equivalent of the cathodic product, magnesium, determined. By assuming that the discharge potential of the magnesium is constant it should be possible to determine the discharge potentials of various organic groups in Grignard solutions of a definite composition. Experimental Preparation of Solutions.-Dry ether, purified Eastman alkyl and aryl bromides and special (1) Kondyrew and Manojew, Ber., 66, 461 (1925). (2) Evans and Lee, THIS JOURNAL, 66,654 (1934).
magnesium were used as raw materials. The magnesium for the preparation of the Grignard solutions was always in excess of the amount Mg = RMgX. necessary for the reaction RX In each preparation the reaction mixture was heated over a water-bath after reaction. A portion of the solution was analyzed by Gilman's titration method and the remainder diluted to approximately molar concentration and then transferred to an enclosed conductivity cell for decomposition measurement. The solutions during preparation and transfer were kept in contact with dry hydrogen. Measurement of the Decomposition Voltage. -The apparatus used for measuring the de-
+
W. V. EVANS,F. H. LEEAND C. H. LEE
490
composition voltage is shown in Fig. 1. The cell C was kept a t 22'. Galvanometer G, shunted in each experiment with a suitable resistance R, was used to measure the current passing through the cell. A low voltage was applied a t the beginning
VOl. 57
discharge potential of the magnesium constant in these solutions we then have a set of numbers that show us the relative discharge potential of each alkyl and aryl anion constituting the reagents. We note that the phenyl anion has a higher discharge potential than the saturated alkyl radicals while allyl has the lowest discharge potential. Among isomeric radicals the order is primary, secondary and tertiary in descending order.
Fig. 1.
and after a constant current was obtained the voltage gradually was increased usually to about five volts, and the corresponding galvanometer readings taken in a similar manner. The galvanometer readings were then plotted against the corresponding voltages. The voltage where the curve showed a distinct break was taken as the decomposition voltage. In most cases two results were obtained showing the possibility of checking such results. Check measurements were made with the current reversed. Table I shows the values obtained. TABLE I Reageut
Concentration
CeH6MgBr CH3MgBr CDHMgBr CaHsMgBr C2HsMgBr C2Hs(CHs)CHMgBr (CHa)&HMgBr (CH3)3CMgBr CH2=CHCH2MgBr
1.02 l . M 1.09 1.01 0.92 1.06 1.00
Decomposition voltage
Average
2.21 2:13 1.94
2.17 1.94 1.42 1.32 1.28 1.24 1.07 0.97
1.41 1.43 1.31 1.33 1.27 1.29 1.23 1.25 1.08 1 . M 0.91 1.03 .83 0.89
I
0
1
2 Volts. Fig. 2.
3
4
When we replace hydrogen atoms by methyl groups in a methyl radical the discharge potential of the substituted methyl radicals is gradually lowered since the discharge potential of methyl, ethyl, isopropyl and tertiary butyl radicals are, respectively, 1.94, 1.28, 1.07 and 0.97 volts.
summary 1. The decomposition potentials of molar 1.03 1.00 solutions of phenylmagnesium bromide, methyl0.95 0.98 magnesium bromide, ethylmagnesium bromide, .81 .86 .87 1.13 -86 n-, and i-propylmagnesium bromides, n, s, and Graphs for two typical results, ethylmagnesium t-butylmagnesium bromides and allylmagnesium bromide and methylmagnesium bromide, are bromide have been determined a t 22'. 2. The effect on discharge potential of subshown in Fig. 2. stituting a methyl group for hydrogen in a methyl Discussion radical has been presented. Compdson has The data indieate that the decomposition volt- been made as to the discharge potentials of isoages of the Grignard reagents tested do vary and meric alkyls. range from 2.17 to 0.86 volts. If we assume the EVANSTON, ILLINOIS RECEIVED AUGUST 1,1934 1.56 1.03
