Infrared Analysis of Commercial Diethyl Ethylmalonate W. H. WASHBURN and WM. B. BROWNELL Abbott Laboratories, North Chicago, 111.
A simple infrared analysis designed for production control of diethyl ethylmalonate containing small amounts of diethyl malonate and diethyl diethylmalonate is described. The method is based on the determination of the absorbances at 11.18, 11.85, and 13.51 microns of a 70% solution of the sample in carbon tetrachloride. These bands are characteristic of diethyl ethylmalonate, diethyl malonate, and diethyl diethylmalonate, respectively. IETHYL ethylmalonate is an important intermediate in the production of several barbituric acids. The ester is usually prepared by the ethylation of diethyl malonate, and the reaction product contains variable amounts of diethyl malonate and diethyl diethylmalonate. The customary method of purification is by fractional distillation. I n large scale operations it is difficult to separate the diethylated ester cleanly from the desired monoethylated ester. If sufficient care is not exercised in the distillation operation, significant amounts of unalkylated ester may also be found in the distillate. As the purity of the barbituric acids and the economies of process yields are dependent on the quality of malonate esters used in the syntheses, it is desirable to control closely the purity of monoethylated ester. To this end, a relatively simple and speedy method of analysis was sought. It has been reported ( 4 ) that mistures of these esters may be analyzed by selective saponification with alkali. This method proved to be too erratic to be used as an effective control procedure. A study of the infrared spectra of the purified esters indicated the presence of suitable absorption bands, and a simultaneous three-component analysis of relatively pure mixtures of the three malonate esters was developed.
repeated fractional distillation, using a YO-em., glass-helix packed column a t a reflux ratio of 10 to 1. Heart-cuts of those fractions shoning no change in boiling point were chosen. The boiling points and refractive indices of the esters were: ( a ) diethyl malonate, bzo, 96"; n2,6 2 , 1.4118 ( b ) diethyl ethylmalonate, bn, 104.5'; n2,8 1.4143 and (c) diethyl diethvlmalonate, b20, 116"; nk' 2 , 1.4222. The qualitative absorption spectra (Figure 1 ) were obtained using a Perkin-Elmer RIodel 21 spectrophotometer. A PerkinElmer Model 12C spectrometer was used for all quantitative measurements. Working curves of absorbance cs. concentration n'ere established in the usual manner for a three-component analysis ( 2 ) . The ranges studied corresponded to compositions of 70 to 100% of diethyl ethylmalonate and 0 to 15% each of diethyl malonate and diethyl diethrlmalonate -4 linear relationship between absorbance and concentration was found for each component. Procedure. Adjust the amplifier gain of the Perkin-Elmer Model 12C spectrometer to obtain full scale deflection with a 4 - ~ v test . signal. Transfer e-iactlv 7.0 ml. of sample to a suitable flask, add 3 0 ml. of carbon disulfide (analytical reagent grade), stopper tightly, and mi\; well. Transfer the solution to a 0.1mm. sodium chloride cell. Using the "cell in, cell out" terhnique, determine the absorbance of the solution a t 11.18, 11.85, and 13 51 microns 2s. a 0.1-mm. sodium chloride cell containing carhon disulfide. The cells need not he matched perfectly.
Table I.
Aynt,hetic blends
Results of Analyses by Proposed Method
Sample 1 2 3
Coniinercial 1 preparations F:r
._
EXPERIRlEYTAL
3
The pure esters used in obtaining the qualitative absorption spectra and in preparing the working curves were obtained by
4
5
6
)
T DIETHYL MALONATE
Diethyl Ethylmalonate, % Theory Found 92.8 93.8 85.7 86.3 85.7 86.5
... . . ,.. , .
...
... ,. ,
. ., ...
... ...
...
Diethyl Malonate, % Theory Found 3.G 3.8 7.2 9 2 7.2 9 3
Diethyl Diethylmalonate, % Theory Found 8.6 3.0 7.2 6.0 7.2 5.7
94.0 93.8
... ...
2.0 2 8
,.. ,.,
3.8
96.7 95.6 94.0 93.3 92.7 92.7 88.8 89.9
.
1.1 2.0
,..
3.3 3.4 4.3 4.2 2.9 3.0 11.0 10.5
96.0 96.5
, . . , . .
...
...
2.8 3.5 4.0 4.5 0.0 0.0
0.0 0.0
... ... ... .. ,.. , . .
...
... ...
4.0
5.0 4.9
The difference between absorbances of solvent-filled blank and sample cells is checked frequently and the necessary corrections are applied to the absorbance determinations. Refer the corrected absorbances to the working curves and calculate the percentage of esters graphically ( 2 ) . DISCUSSION
""
\I II
d \I
DIETHYL ETHYLMALONATE
MICRONS
Figure 1. Qualitative absorption spectra of pure esters
Figure 1 shows the qualitative curves for the pure esters. The analytical bands are designated by arrows. T h e band chosen for diethyl ethylmalonate is relatively weak. This proved to be a desirable feature inasmuch as commerical preparations usually contain 90% or more of this component. Conversely, the stronger bands chosen for diethyl malonate and diethyl diethylmalonate provided for a satisfactory determination of these minor components. Table I shows the results obtained on several synthetic mixes of known ester composition, and on several samples of commercial diethyl ethylmalonate. While the relative error of the method is large, especially for the minor components, the absolute error is sufficiently small so that all the information necessary for adequate plant control may be obtained. 812
V O L U M E 27, NO. 11, N O V E M B E R 1 9 5 5 The method is satisfactory for preparations containing no significant amounts of impurities. Occasional lots have been encountered containing entrained ethylating agents or other substances which invalidate the method. Preliminary investigation has shown that in thcse cases the diethyl ethylmalonate and diethyl diethylmalonatc rontent mal- be estimated by resort to the base line method ( 1 , 3, ij), using the bands a t 11.18 and 13.51 microns, respectively. This method is less sensitive to impurities than the three-component spot method, but is not satisfactory for eqtimating dieth5-l malonate 4CK\OW LEDGMENT
T h e authors are indebted to James A. Gordon and Solomon Disman for the preparation and purification of the pure malonate
1813 esters, and t o Jerome Merkel and Frederick Scheske for technical assistance. LITERATURE CITED
(1) Barnes, R. B., Gore, R . C., Williams, E. F., Linsley, S. G., and Petersen, E. AI., ASAL. C H E Y . , 19, 620 (1947). (2) Carol, J., llolitor, J. C., and Haenni, E. O., J . A m . Pharm. Assoc., Sci. Ed., 37, 173 (1948). (3) Friedel. R. A . , and Pierce, L., . I N ~CL HE . M . 22, , 418 (1950). (4) Gamas, A . S.,Zaoodskaya Lab., 10, 155 (1941). (5) Heigl, J. J., Bell, 31. F., and White. J. U., ANAL.C H E M . ,19, 293 (1947).
RECEIVED for review March 2 , 1955. Accepted June 22, 1955.
Beta-Mercaptopropionic Acid as Colorimetric Reagent for the Determination of Cobalt EDWARD LYONS’ University o f Florida, Gainesville, Fla.
p-Mercaptopropionic acid is a useful reagent for the colorimetric determination of cobalt in dilute solution. The presence of nickel or copper in the same order of concentration i s not detrimental, as an excess of ammonium hydroxide which does not affect the cobalt test, will discharge an? color due to these metals.
T
HIOGLTCOLIC acid ( 2 ) was found t,o be a very useful reagent for bhe colo~~imetricdetermination of iron. phlercaptopropionic acid (B. F. Goodrich Co., Cleveland. Ohio), although of no partirular valuc. in the test for iron, was found to give excellent results with cobalt. Its use in the determination of nickel has been studied hy Uhlig and Freiser ( 3 ) and Lear with Mellon ( 1 j .
cobalt test and vice versa, However, in dilute solution (concentration as given for cobalt), the red color due to nickel is readily discharged by an excess of ammonium hydroxide. Consequently, in carrying out the test on nickel one must add only enough hydroxide just to neutralize the solution. Prepared unknoa ns n-ere thus readily compared with all dilutions of known concentrations, so that here too the proposed method may be used for a quantitative determination of nickel. I n testing for cobalt in the presence of nickel in equal concentration, the addition of 4 or 5 drops of ammonium hydroxide per drop of mercaptoacid will cause the color due to the nickel to be discharged, T h e green color due to the cobalt is not affected by the excess hydroxide used. On the other hand, cobalt may seriously interfere with the test for nickel. COLOR REACTION WITH PALLADIUM
COLOR REACTION WITH COBALT
FVhen a drop of mc,rcapt,opropioiiic acid is added to a concentrated solution of a cobaltous salt and the resulting mixture is made alkaline wit,h ammonium hydroxide, a deep green solut,ion is obtained. T h e dept,h of color depends on the concent’ration of cobalt and fails to appear when t,he concentration of cobalt is less t.han about, 2.5 p.p.m. Prepared “unknowns” in concenhatmionof 2.5 to 20 1i.p.m. lend themselves readily to matching with known standa.rd9. so that t,he method may be used for quantitative detmrrmin:tt8ioii. T h e green color, pirticdnrly in the more concentrated solutions, soon begin? to tun1 dark and ultimately the color becomes brown. It was found that oxidizing agents speed u p this change. I n the absence of rnetul ion? such as mercury (11)or gold (111) (which ma)- reduce t o the mPtalj, a “pinch” of sodium hyposulfite (Sa2S204.2H,O! added to the mixture prior to the addition of the ammonium h>-dro\-ide,retards the oxidat,ion. T h e resulting brilliant bluish green t’iiit makes the matching of color easier and it, is st:ihle over inany hours. COLOR REACTION WITH NICKEL
Because nickel is frequently associated with cobalt, the effect of its presence was studied. K i t h nickel the color obtained (when the test is carried out as previously described for cobalt) is deep red. Obviously, it was found to interfere with the 1
Present address, Pan-Orniond Corp., Ormond Beach, Fla.
A4cid palladium salts in concent,ration greater than 1000 p,p.m, impart a yellow color to the solution. Solutions of lesser concentration arc colorless. If to such colorless solution containing not less than about 40 p.p.ni. of palladium is added a drop of mercaptopropionic acid and the resulting mixture is made alkaline with ammonium hydroxide, the yellow color is again brought out. METAL IONS WHICH DO NOT GIVE COLORED SOLUTIONS WITH 8-4lERCAM’OPROPIONIC ACID
The following metal ions in solut,ion do not give charact’eristic color reactions with t,he reagent: silver, gold(II1 j , bismuth(II1 j ) calcium, chromium(III), copper( 11), mercury(II), magnesium, mnnganese(II), lead(II), platinum(IV), rhodiuni(III), tin(IT), thorium(IV), tit,anium(III),and zirconium(I1). Copper(I1) gives a yellow colored precipitate with thioglycolic acid and p-mercaptopropionic acid, which dissolves in ammonium hydroxide or sodium hydroxide with formation of a colorless solution. Therefore, copper should not interfere with the test for iron or cobalt when these mercapto acids are used. LITERATURE CITED
(1) Lear, J. B., with 31. G. lIellon, ANAL.C H E J ? .25, . 1411 (1953). ( 3 ) Lyons, E., J . A m . Cheni. Soc., 49, 1916 (1927). (3) Uhlig. L. J., and Freiser. H., . i h - a ~ .C H E M .23, . 1014 (1951). RELEIVED for r e n e w .January 13, 195%
Accepted J u n e 29. 1955
