T H E 1N.FLUENCE OF ALKYL SUBSTITUENTS ON THE ELECTRICAL CONDUCTIVITY OF MALONIC ACIDS BY WILLIAM BUELL MELDRUM
The dissociation constants of the alkyl substituted malonic acids have been determined for a few of the simpler members of the series by Ostwald,' BethmanqYand W a l d e t ~ . ~ In the present communication the investigation is extended t o some of the more complex acids. The first measurements of the electrical conductivities of the malonic acids were made by Ostwald, in 1888, who determined the constants for malonic, methyl malonic, ethyl malonic and dimethyl malonic acids. He showed that like most dibasic organic acids, the malonic acids dissociate in two stages : (1) (2)
(COOH).CH,(COOH) (COOH).CH,(COO)
i-
+H + = (C06).CH2(COO) + H.
=
(COOH).CH,(CO~)
He found that the dissociation of the second carboxyl group did not take place to an appreciable extent at low dilutions, and that therefore the dilution formula for binary electrolytes K
=
(1-WLm)v
furnished a constant value. Ostwald's research was followed by another by Bethmann in 1890. He made a special study of the saturated dibasic acids, and determined the conductivities of the first nine acids of the oxalic series, He showed that starting with oxalic acid the dissociation constant decreased, rapidly at first then more slowly, with the increase of the distance between the carboxyl groups. He repeated the measurements Zeit. phys. Chem., 32, 241,309 (1900).
* Ibid., 5 , 385 (1890). Ibid., 8, 433 (1891).
Influence of Alkyl Substituents, Etc.
475
of Ostwald on the malonic acids and added two more to his list : propyl malonic and isopropyl malonic. His determinations for the substituted derivatives of succinic acids were more extensive. For them he showed that the introduction of an alkyl group into the succinic acid molecule caused an increase in the value of the dissociation constant, the methyl and the ethyl groups having about the same effects, but two ethyl groups substituted for the two hydrogen atoms attached to one carbon atom having a greater effect than two methyl groups similarly substituted. In a later communication, Bone and Sprankling’ state that the dissociation constants of the normal alkyl-substituted succinic acids increase with the increase in molecular weight. Walden, in the year following the publication of Bethmann’s research, continued the work, studying the phenyl as well as a few of the alkyl derivatives. As his results are the most complete so far obtainable, a table is given below which contains for the purpose of comparison the results of Ostwald’s and of Bethmann’s determinations. TABLEI _____~_
1
Malonic Methyl malonic Ethyl malonic Dimethyl malonic Methyl ethyl malonic Diethyl malonic N-Propyl malonic Isopropyl malonic N-Butyl malonic Isobutyl malonic
131-132’ 128’ C
c 0.086
110-1 I 2
0.127
185 119 I 20- I 2 I
0.076 0.161 0.740
94-96 87-90
0.127
-
102
0.103
107
0.090
In 1892 James Walker2 published a paper on “The Dissociation Constants of Organic Acids, ” which included the results for some of the alkyl-substituted malonic acids Jour. Chem. SOC.,77, 1298 (1900).
* Ibid., 81,696 (1892).
476
William Buell Meldrum
together with his own results on the acid esters of the same series. The effect of substituting the ethyl radical for the hydrogen atom of one of the carboxyl groups was found to decrease the value of the constant by approximately one-half. W. A. Smith1 has recently further examined the manner in which dibasic organic acids dissociate. From his results he draws the important conclusion that as the dissocia&on constant for the dissociation,
(COOH) .CH,(COOH)
=
(COOH)
.cH,(coO.) + A
increases,
that for the second stage,
(COOH).CH,(C06.)
=
(COC.)CH,CCOO.)
+
+
H decreases.
Wegscheider' has shown that the stronger the acid as shown by its affinity constant, the lower the dilution at which dibasic dissociation beconies marked.
Experimental Carefully weighed quantities of the purified acids were dissolved in water and made up t o the volume required for a 1/16 molar, or in the case of the less soluble acids, for 1/32 molar solution. The method of measuring the conductivity was similar to that described by Ostwald,~the resistance of the solution being determined by a standard resistance box, a Wheatstone's bridge and a telephone receiver. The temperature of the thermostat was regulated automatically by the usual electrical method, and was kept at 25' C, constant t o about 0.05 of I O . The water used was specially distilled from a solution of potassium hydroxide made with the ordinary distilled water of the laboratory, and had a specific conductivity of 2.3 x IO-' reciprocal ohms. The corrections for the conductivity of the water were applied t o the molecular conductivities and amounted t o 2.35 units at a dilution of 1024 liters. Zeit. phys. Chem., 25, 193 (1898). Monatsheft, 1900,23. Zeit. phys. Chem., 2 , 561.
I n f l u e n c e of Alkyl Substituents,
Etc.
477
The bridge wire was of platinum and was carefully calibrated. The resistance capacity, or the cell constant, of the conductivity cell was found by using a 1/50 normal solution of potassium chloride which had been previously fused, the value found being 0 . 1 0 0 7 . In the following tables the letters employed have the significations noted below. ZI = dilution in cc. M, = the molecular conductivity at dilution v . M, = the molecular conductivity at infinite dilution. rn = the degree of dissociation. K = the dissociation constant.
TABLEI1 N-Propyl malonic acid. M, (observed) = 362; M, 361. M. P. 95O-96O C ~___________________~__
1
45.90 63.31
_-
0.127 0 . I73
113.90 0.316 149.72 1 0.416 191.77 1 0.533 115.64 0.627 K = 0.115 X IO-^
1
Isopropyl malonic acid. M, (observed), 360; M, 359. M. P. 88'-89' C ~ v
x 10-3
16 32 64 I 28 256 512 1024
_
_
M
48.60 67.46 91.51 121.07
158.40 201
.96
246.24
_
I
(calculated),
'0.115 0.115 0.115 0.114 0.116 0.118 0.I22
(calculated),
_
m 0 .I 3 7 0.1874 0.2542 0.3363 0.440 0.561 0.684
K X 105
William Buell Meldrum
478
N-Butyl malonic acid.
v x
10-3
1
M, (observed), 361 ; M, M. P. 98'-99' C
I
M
16 32 64 128 256
m
0.1266 0 . I748 0,239 0.324 0.412 0,532 0.657
45.56 62.93 86.70 117.08 147.57 I92 .OS 237.25
512
1024
Isobutyl malonic acid. _
_
M, (observed), 361 ; M, M. P. 104°-1050 C _
.
~
42.55 59.64 83.43 112.28 147.93 189.54 233.52 K =
Isoamyl malonic acid. vX
10-3
m
16 32 64 I 28 256 512 1024
0.I18
0.165 0.228 0.311 0.404 0.523 0.646 0.101 X
48.81 67.70 91.42 123.90 161.40 205.01
248.15
~
~
1
0
6
0.115 0.116 0.117 0.119 0.112
0 .I 1 8 0. I22
(calculated), 362. K X IO^ 0 . IO1 0.I O 1
0.105 0.109 0.107 0 .I12
0.115
IO-^
M, (observed), 362; M, M. P. 95'-96' C M
j
-
M
16 32 . 64 I 28 2 56 512 1024
(calculated), 360.
m 0 . I35 0 . I85
0,253 0.341 0.445 0,569 0.687
(calculated), 361.
K X IO^
0.131 0.131 0.134
0.138
0 . I39 0 . I47 0 .I47
Influence of Alkyl Substituents, Etc. Dimethyl malonic acid.
v x
16 32 64 128 256 512 1024
38.86 54.80 74.74 100.13 131.77 169.43 208.54 K = 0.080
I
v x
I
105.1 138. I
1
I 28
1
256 5'2
J
0.080 0.080 0.082 0.081 0.080
0.079 0.076
(calculated), 360.
I
M
10-8
16 32
o . 107 0.151 0.205 0.275 0.362 0.466 0.575 x IO-^
M, (observed), 361; M, M. P. 125'-126' C
Diethyl malonic acid.
(calculated), 362.
m
M
10-8
-
M, (observed), 363; M, M. P. 190'-191' C
479
1
222.3 264.4 298.0 I; =
Dipropyl malonic acid.
I
1)z
K X IO^
I1
0.278 0.382 0.495 ' 0.614 0.730 0.827 0.750 x IO-^
1
M, (calculated), 361 ; M, M. P. 157'-158' C
(observed), 360. K X :os
32 64 I 28 256 512 1024
146.03 205.35 247.70 288.6
0.455 0.5704 0.688 0,795
I . 187 I . 185 I . 186 I .204
1.455 2 , '34
William Buell Meldrum
480
Methyl N-propyl malonic acid. M, (observed), 357; M, ted), 361. M. P. 1 0 1 ~ - 1 0 2 ~C ~~~~
v x
10-8
I
M -I
16 32 64 128 256 512 1024
32 64 I 28 2 56 512 1024
1
60.38 82.06 109.75 '44.91 184.70
1
68.81 93.59 125.20
'
=
1
KX105
I
0.168
0.212
0.227
0.213
0.306
0.210
0.510 0.641 0.762
275.26
K
I
0.190 0.259
(1
0.449 0.568 0.684 0.141 X IO-^
1
Methyl N-butyl malonic acid. M, (observed), 358; M, ted), 362 M
16 32 64 I 28 256 512 1024
59.37 81.24 109.5I 144~02 185.88 220. I2
273.61
_~-~ _ -_-_
0.200
232.17
162.13 204.91 247 76
m
(calcula-
I
(calcula-
m
0.164 0.204 0.303 0.397 0.511 0.632 0.752
0.202
0.203 0,205
0.204 0.208 0.211 0.221
I n f l u e n c e of Alkyl Substituents,
______
~
v
x 10-3
32 64 128 256 512 1024
481
-
______~____ -~
K X ~d
m
M
101.71 130 94 176.83 220.60 261.31 307.03
0.282 0.375 0.489 0.626 0.737
'
1
Etc.
0.346 0.348 0.355 0.408 0.407 0.470
0.850
Methyl isoamyl malonic acid. M, (observed), 355; M, ted), 361. M. P. 13oO-131~C v x 10-8
82.44 110.27
'44.56 182.28 230.99 272.56 K =
I28
256 512
1024
_ _ _ ~ - -
v x 10-1
16 32 64 I28
256 1024
K x 105
m
32 64
~
'
~
0.210
(calcula-
0.228
0.209
0.305 0.401
0.210 0.210
0.508
0.205
0.640 0.754 X IO-^
0.221
0.225
~ _ _ _ _ ~ _ _ _ _ ~
l M
'
KX
m
124.39 163.00 204.72 249.67 288.64 321.29 343 76 '
0.364 0.4515 0,567 0.690 0.800
0.891 0,952
105
I . 160
1
I . 161 I . 160 I . 190 I .241 I ,420 I ,840
William Buell Meldrum
482
Ethyl isopropyl malonic acid. M, (observed), 358; M, ted), 360. M. P. 72O-73' C
I
vX1o-*
32 64 I 28 256 512 1024
m
'
168.20 210.97 253.61 2 70.02 318.26
1
(calcula-
KXIO~
0.468 0,585 0.701
I . 282 I .285 I . 284
0 ,805
1.299 ,326 1.485
0.885
I
Ethyl N-butyl malonic acid. M, (observed), 356; M, ted), 361. M. P. 1 1 5 ~ - 1 1 6C~
(calcula-
_ _ ~ - _ ~ -_ _ _ _ _ _ _ - . _ __ _ _ -_~
vx
M
10-3
-I
II
163.11 205.65 251.66 294.22 330.64 355.74
32 64 I 28 256 512 1024
m
I
~~
KXro6
I I . 161
0.452 0.570 0.686 0.812 0.916 0,987
1 . I73 I . 165
1.371 1.939
-
Ethyl isobutyl malonic acid. M, (observed), 354; M, ted), 362. M. P. 1 0 2 ~ - 1 0 3C~ vX10-8
32 64 128 256 5'2
1024
i
.___
M
190.76 234.27 274.59 291.04 329.55 324.59
I
m
0.5284 0.648 0.758 0.807 0.910 0.948
1
(calcula-
KXro6 I
.852
I ,860 I . 860 I . 720
1
'
790
-
InFuence of Alkyl Substituents, Etc.
483
Ethyl isoamyl malonic acid. M, (observed), 355; M, ted), 362. M. P. 1 1 5 ~ - - 1 1 6C~ v x 10-8
M
32 64 I28 256
156.11 196.58 237.94 279.35 310.82 334.62 K =
512
1024
m
0.432 0.545 0,659
I ,020
0.771 0.860 0.925 x IO-^
I
(calcula-
KXIO’ I
,023
I .020 1.010 I ,019 I .029 I . I21
The values of “M, (observed),” as given in the above tables, were found by subtracting the ionic conductivity of sodium 49 from the molecular conductivity of the monosodium salt of each acid at a dilution of 1024 liters and adding on the ionic conductivity of hydrogen, 325. The values obtained for the molecular conductivities of the mono-sodium salts are as follows: TABLEI11 Acid
1
Mat Acid litres
litres 1
N-Propyl malonic Isopropyl malonic N-Butyl malonic Isobutyl malonic Isoamyl malonic Dimethyl malonic Diethyl malonic Dipropyl malonic Ethyl isoamyl malonic
86. 2 I 85.60 84.78 85.64 86. I 9 86.81 84.30 79.79
1
,
Methyl N-propyl malonic Methyl isopropyl malonic Methyl N-butyl malonic Methyl isobutyl malonic Methyl isoamyl malonic Ethyl N-propyl malonic Ethyl isopropyl malonic Ethyl N-butyl malonic
,
1
8 I .40 82 . 0 7 81.69 79.94 79.86 79.62 77 . 6I 78.27
The malonic acids dissociate in two stages, the second carboxyl remaining intact until the degree of dissociation of the first carboxyl is about 50 percent. The second stage then begins and the conductivity increases more and more over what it would be if the acid were monobasic, as the dilution becomes greater. For this reason a satisfactory
484
William Buell Meldrum
value for the constant may be obtained only at low dilutions. Results indicate too that for dibasic acids of analogous constitution, as the value of the constant for the first stage of dissociation increases the value for the constant for the second stage decreases. From these considerations it is evident that in finding the molecular conductivity of the acid sodium salt at infinite dilution a considerable error may be introduced owing to the ionization of the remaining carboxyl, and that this error may be greater in the case of those acids which have a low value for K. Since then the values for M could not be exactly determined by extrapolation, the values at 1024 liters were taken and the molecular conductivities a t infinite dilution of the acids determined from them. From these the correct values were determined. Since at low dilutions,
is an identity, if MVland MV2are the molecular conductivities corresponding to the dilutions v, and v,. Since MVl, M,,, v, and v, are known, M, may be found. For calculating M, by this formula, two values of M were taken which gave approximately the same values for K when the observed value for M, was used. Only under such a condition would the formula give a correct result.
Diseussion of the Results Obtained In the following table the dissociation constants of the alkyl malonic acids are summarized. Those acids of similar structure are grouped together in the order of increasing molecular weights. Such an arrangement makes plain certain regularities in the variations depending on the nature of the substituting groups.
Influence of Alkyl
Substituents, Etc.
TABLEIV __-
Acid (1)
Malonic Methyl malonic Ethyl malonic Propyl malonic Butyl malonic (2) Isopropyl malonic Isobutyl malonic Isoamyl malonic (3) N-propyl malonic Methyl N-propyl malonic Ethyl N-propyl malonic Propyl N-propyl malonic (4) Isopropyl Methyl isopropyl malonic Ethyl isopropyl malonic (5) N-butyl malonic Methyl N-butyl malonic Ethyl N-butyl malonic (6) Isobutyl malonic Methyl isobutyl malonic Ethyl isobutyl malonic (7) Isoamyl malonic Methyl isoamyl malonic Ethyl isoamyl malonic (8) Malonic Methyl malonic Methyl methyl malonic Methyl ethyl malonic Methyl propyl malonic Methyl butyl malonic
______
Mol. wt.
K X rd
I04 I I8 132 146 I 60
0.158 0.087
146 160 I74
0.I27 0.115
0.116
'
0 . I35 0 .IO1
0.131 0.115
146 160 I74 I 88
0.212 I , 160 I . 186
146 I 60 I74
0.I35 0.141 I ,284
I 60 I74 188
0.116 0.203 I . 163
I 60 I74 I 88
0 .IO1
I74 I88
0.131
202
0.353 I ,860 0.210 1.020
104 I 18 132 146 I 60 I74
0.158 0.087 0.080 0.161
I04
0.158
132
0.127
(9)
Malonic Ethyl malonic Ethyl methyl malonic Ethyl ethyl malonic Ethyl propyl malonic Ethyl butyl malonic
48 5
146 160 I74 188
0.212
0.203
0.161 0.751 I . 160 I . 163
48 6
William Buell Meldrum
An examination of these results shows us that: I . Any alkyl group substituted alone in the malonic acid molecule causes a decrease in the value of the constant, the decrease being greatest for the methyl group and least for the ethyl group. 2 . As the number of carbon atoms in t h t substituting groups of normal structure increases the values of the constants converge. 3. The isoalkyl substituting groups do not show the same regularity in their relative effects, the isobutyl group especially having an abnormal effect in lowering the value of the constant when substituted alone, and in raising the value when substituted with other groups. 4. The acids containing isoalkyl groups are in general dissociated to a greater extent than the isomeric acids containing normal groups. 5. Dialkyl malonic acids are dissociated more than the isomeric mono-alkyl derivatives. 6. The introduction of one alkyl group into malonic acid is invariably attended by a lowering of the value of the ionization constant and the introduction of a second alkyl group is attended by an increase in the value of the constant over the value for malonic acid itself. This leads to the following conclusion : the introduction of an alkyl group into the molecule of malonic acid is attended by a fall in the value of the dissociation constant; the introduction of a second alkyl group tends to lower the value of the constant still more but this tendency is more than counterbalanced by an opposing influence due to the position of the alkyl groups in the molecule. This statement holds true no matter what alkyl groups are considered though in the case of the methyl group the tendency t o lower the value of the constant is not overcome by the influence due to spacial arrangement. That in dibasic acids the proximity of the carboxyl groups affects the ionization of one of them is proved by a comparison of the constants of maleic and fumaric acids and of
Influence of Alkyl Substituents, Etc.
487
ortho-phthalic and para-phthalic acids ; by Bethmann’s results on the dibasic saturated acids of the oxalic series, and still more conclusively by Walker’s results on the malonic acid esters, which show that the replacement of the hydrogen of the second carboxyl by ethyl causes a decrease of 50 percent or more in the value of the constant.
Fumaric Maleic
P-Phthalic M-Phthalic
1
0.093 I . 170
0 . IZI
0.029
j I
Malonic Dimethyl malonic Acid ester
Malonic Dimethyl malonic
I’ 0.163 0.086
KXIO~
1
,
0.045 0.0387
This behavior Walker explains as follows : “ The dibasic acids are always stronger than the monobasic acids from which they are derived by the substitution of carboxyl for hydrogen. This is owing to the effect of one carboxyl group on another. Carboxyl, in virtue of its acid character increases the dissociation of the hydrogen of a carboxyl group contained in the same molecule as itself, and this increase is the greater the more nearly the carboxyl groups occur together in the molecule.” If this be the only factor influencing the dissociation of the malonic acids then it must be assumed that the first alkyl group substituted occupies such a position that it interferes in the interaction of the two carboxyls; the second alkyl group causes a further change of the configuration of the molecule, which zither causes the carboxyls to actually approach more nearly to each other or else leaves the space in the molecule between them free from interfering groups. This view seems to be supported by the fact that the influence of methyl on the conductivity of malonic acid is opposite in character to its influence on the conductivity of
488
William Buell Meldrum
succinic acid, for which difference existing theory fails to account satisfactorily. In a future communication I intend extending the investigation to include substituted malonic acids of a more complex nature, after which we may be able to deduce more satisfactory conclusions. In conclusion I wish to express my thanks to Dr. Walker, a t whose suggestion this work was undertaken, and to Dr. McIntosh, for help and advice afforded during the investigation. McCill University, Montreal, February, I ~ I I
