INDUSTRIAL AND ENGINEERING CHEMISTRY
434
15 minutes. The condensers are rinsed with water and the solutions are cooled below 20" C . and titrated with approximately N sodium hydroxide. During this titration, the solution should be shaken vigorously and care should be taken not to overstep the end point. As an additional precaution against sa onification of ethyl aretate (luring titration, it is well to add the aliali slowly. DETERMINATION OF WATER
Regardless of the amount of water present in a solution, its water content may best be determined by the use of Karl Fischer reagent (1, 6). In applying this method, samples containing about 150mg. of water should be measured from a Smith weighing buret and placed directly in freshly dried 150-ml. balloon flasks containing 25 ml. of anhydrous methanol. It is preferable to make the tit,rations electrometrically, using a dead-stop end point (8). ANALYSIS OF SYNTHETIC MIXTURES
The results of a number of analyses of three synthetic mixtures, shown in Table 11, are considered sufficiently accurate for a routine ront rol method. ACKNOWLEDGMENT
The author wishes to acknowledge the wistance of the late Walter Giinther in making many of the analyses.
VoL 16, No. 7
LITERATURE CITED
(1) Alniy, E . G., Griffin, W. C., and Wilcox, C. S., IND.ENQ.CHBY., ANAL.ED., 12,392-6 (1940). (2) Babington, F. W., and Tingle, A., J. IND.ENQ.C ~ M .11, , 666-6 (1919). (3) Baernstein, H. D.,IND. ENQ. CHBM.,ANAL. ED., 15, 251-3 (1943). (4) Desmaroux, hl., A f h . poudres, 23,285-99 (1928). (5) D o h , B. H., IND.ENQ.CHEM.,ANAL.ED., 15,242-7 (1943). (6) Fischer, K.,Angew. C h a . , 48,394-6 (1935). (7) Kubins, J., Chem. Ohror, 12,543(1937). (8) McKinney, C.D.,Jr., and Hall, R . T.,IND.ENG.CHEM.,ANAL. E D . , 15,460-2 (1943). (9) Malm, C.J., Genung. L. B..and IYilliams. R. F., Jr., Zhid.. 14, 93540 (1942). (10) Masson, I.. and MrEwan. T. L., .I. SOC. Chem. Z n d . , 40,2932T (1921). (11) Moore, J. C.,and Blank, E . W., Oil and Soup, 20, 178 (1943). (12) Shaefer, W. E . , IND.ENG.CHEM., ANAL.E D . ,9,449-60(1937). (13) Simons, J. K., and Wagner, E . C., J . Chem. Education, 9, 12241 (1932). (14) Smith, D.M.,and Bryant. U'. M . D.,J . Am. Chem. Soc., 57, 61-5 (1935). (16) Verley, A,, and Bolsing, F., Be?., 34,3354-8 (1901). (16) Wilson, H.N.,and Hughes. W. C., .I. SOC.Chem. Id., 58, 747T (1939).
Determination of Carbon-Linked Methyl Groups W. U. S.
F. BARTHEL AND F. B. LAFORGE
Department of Agriculture, Bureau of Entomology and Plant Quarantine, Washington,
IK
COXXECTIOK Kith investigations of pyrethrolonc (4) the authors have drawn important conclusions from the carbon-linked methyl content'of various fractions and derivatives. For the determination of this grouping they have employed the method described by Pregl ( 5 ) , which is essentially that of Kuhn and L'Orsa (9), based on chromic acid oxidation of the sample and titration of the resulting acetic acid. Since many determinations were required, some modifications and simplifications in the original method were introduced. I n general, little use has been made of terminal-methyl determinations in analytical studies of organic compounds, perhaps because theoretical values are seldom obtained except with certain types of groupings. Although in general straight-chain compounds furnish the theoretical yield of acetic acid with great precision, other groupings, such as a single methyl group attached to an aliphatic ring, usually yield somewhat less than loo%, and the result must be considered in connection with that furnished by a reference compound. I n many cases where more than one methyl group on the same carbon atom is involved, or where methyl groups are attached to aromatic rings, this method seems to be of doubtful value. The special problem was to distinguish between the two formulas for pyrethrolone-formula I (or similar compounds with the grouping C=CH-CHa) and formula 11-and to estimate the proportions of earh in mixtures. CHI
H2-~ ~ - c H = c = c H - - C H s HO---
L
I
CHa
H
D. C,
The terminal-methyl determination of Pregl for carbon-linked methyl groups has been modified for more rapid determinations. The terminal-methyl number, or the number of mole equivalents of acetic acid produced from a mole equivalent of substance, has been determined for certain reference compounds.
In both formulas the methyl group attached to the pentenolone ring would, from analogy with similar known structures, furnish about 0.8 mole of acetic acid per mole of compound, which is about the value that should correspond t o formula 11. Formula I should furnish a value of about 1.8, because of the second terminal methyl. I n the special case of pyrethrolone and its derivatives very sharp and reproducible results were obtained, as is shown in Table I. In addition to making changes in the method, the authors have determined the amount of acetic acid furnished by a number of structures, especially of cyclic compounds. EXPERIMENTAL
The changes made in the original directions consist in employing the apparatus (Figure 1) designed. by Clark (1) for use in acetyl semimicrodeterminations, and in eliminating the redurtion of the excess chromic acid with hydrazine. From 20 to 30 mg. of sample are weighed on a iece of cigaret r p e r in the case of solids or, if a liquid, in a s m a t glass capsule. he sample is placed in the oxidation flask, A , together with 5 ml. of cold oxidizing mixture made by adding 20 ml. of concentrated sulfuric acid to 16.8 grams of chromic anhydride dissolved in 100 ml. of water. The finger condenser, R, is put in the neck of the flask, and the mixture is refluxed over a microflame for 1.5 hours. The finger condenser is then removed and washed free of acid with as little water as possible, the washings being allowed to run into the flask. Seven grams of magnesium sulfate are added, and the fiask is set up for steam distillatmion. The b e is replaced under the flask durin the distillation in order to concentrate the contentg of the flas! while 50 rnl. of distillate are being collected. The distillation is then titrated with a 0.05 N barium hydroxide solution to the neutral point of phenolphthalein.
ANALYTICAL EDITION
July, 1944
Table
mm
II.
43s
Terminal-Methyl
CaHt(CHs)C=NNHCONHi CHI
Numben of
Reference Compounds Found by Kuhn and L ’ O m by Acatio Present Method Add Titration“ 1.00 1.00 0.86 0.86
0.53
...
1.70
...
1.66
1.70
1.00
....
0.29
..
0.80
0.80
H O = O
I
I
H,-=O
Figure 1.
Diagram of Apparatus
CHI
An additional 5 ml. of distillate should not chan e the end point appreciably. A correction, determined by a bfank experiment in which the organic material is omitted, is applied. Barium hydroxide is most convenient for titration, because any trace of sulfuric acid that might have been carried over is a t once noticed. If sulfuric acid comes over, the determination must be repeated. However, the Elek and H a r k (8) iodometric procedure may be used a t this point with equally good results. An advantage of the latter would be in correcting ?or any sulfur dioxide that might be carried over. In their work the authors have always used am le oxidizing mixture and have never encountered trouble with sulfur dioxide except when the solution was concentrated to the point where charring occurred during distillation. This can be avoided by not permitting the solution to becoms thick. Should there be a tendency for the solution to thicken, the flame is removed and passage of steam continued until the solution in the flask is once more fluid. The terminal-methyl number is the number of mole equivalents of acetic acid produced from one mole equivalent of the
....
1 .oo
a Kuhn and L’Orsa results were given for general structured. Last four compound8 obtained through courteey of L. W. Buts, Bureau of Animal Industry.
compound. The calculations are made according to the following formula: Terminal-methyl No. = normality of alkali x (ml. of titer blank) X mol. wt. of sample gram of sample X lo00
-
Table
Substance
I,
Precision of Method Weight of 0.05 N Barium Terminal-Methyl Sample Hydroxide Number Gram
Ethyl methyl ketone semicarbaEone Crotonic acid hlethylcyclohexene-2 Psrethrone semicarbarone fraction Pyrethrolone methyl ether fraction Tetrahydropyrethrolone semicarbazone Pyrethrolone (fraction a) Pyrethrolone (fraction b) Sodium acetate (fused)
MI.
EXAMPLE.0.0194 gram of ethyl methyl ketone semicarbazone (mol. wt. 129.16) required 5.12 ml. of 0.05 N barium hydroxide. The blank was found to be 0.10 ml. 0.05 X (5.12 - 0.10)X 129.16 1.88 0.0194X 1000 E
1.67 1.66 0.94 0.95 0.95 0.98 0.53 0.53 0.99 0.97 1.30 1.32 1.83 1.80 1.42 1.43 1.14 1.10 0.98 1.05 0.99
Table I illustrates the precision of the method. Table I1 contains the results from various typical compounds, which may serve for reference. The results may also be expressed in terms of per cent CHI. LITERATURE CITED
(1) Clark, IND. ENQ.CHEM., ANAL.ED.,8,487 (1936). (2) Elek and Harte, I b i d . , 8,267 (1936). (3) Kuhn and L’Orsn, 2.angew. C h a . , 44,847-53 (1931). (4) LaForge and Barthel, J. Oru. C h a . , “Constituents of Pyrethrum Flowers” (submitted for publication). (5) Pregl, “Quantitative Organic Microanalysis”, 3rd ed., p. 201. Philadelphia, Blukiston Co.,1937.
