1194
ANA
flushed out with carbon dioxide for a few minutes. The stationary furnace is pulled over the permanent filling and when microbubbles are obtained, the carbon dioxide is stopped. The burner furnace is slowly drawn from the side across the tube over the temporary filling, depending upon the bubble rate (not more than 3 per second), and finally placed entirely over the tube and butted against the stationary furnace. When the bubble rate diminishes to 1 every 5 seconds, the carbon dioxide is turned on and the bubble rate is adjusted t o 4 per second. The furnaces remain in position until near-microbubbles appear, then are moved back. When microbubbles are obtained, the determination is complete. The time taken for an analysis usually runs between 15 and 20 minutes.
Table I. Nitrogen Determinations % Theory
CompoundQ Sulfanilic acid
8.09
Acetanilide
10.36
Nicotinic acid
11.35
Cystine
11.66
Benaylisothidiirea €IC1
13.82
Azobenzene
15.38
2,4-Bis(benzylarnino)B,7-diphenylpteridine
1fi.99
4-(4-hlorpholinyl)-f,7-diphenplpteridine
18.96
p-Carboxyphenylazobarbituric acid
20.30
2,4-Bis(3-diethylaminopropylamino)-6,7-diphenylpteridine
20.72
2,4-Bis(2-hydroxyetliylamino)6,7-diphenylpteridine 20 88 Sulfadiazine
22 39
2,4-Bis-allylainino-G-(p-c hloroani1ino)-8-triazine
26.61
Arginine HCI
26 60
2,4-Diamino-6,i-diphenylpteridine
2fi, 74
Tetramethyldipyrimidopyrazinetetrone
27,62
4-Amino-6-hydroxy-5-nitropyrimidine
35.90
Thioguanine
41,92
a
% Found 8.06 8.13 10.35 10.61 11.35 1- -1. - xn _ 11.46 11.52 13.88 14.03 15.37 15.40 17.11 16.96 19.00 18 83 20 45 20 32 20.76 20.60 20.90 20 96 22.28 22.34 26,89 2 6 . til 26 62 26,74 26 80 2 6 . 79 27.54 27.57 36.03 35.90 42.23 41 94
Sample weights between 3 and 5 mg.
eter. The furnace temperatures are close to 675' C. for the stationary furnace, and 725" C. for the burner furnace. If numerous halogen or sulfur compounds are run, the tube should be burned out or replaced. Old used tubes should be discarded. PROCEDURE
A modified Dumas procedure is used. The combustion tube is connected to the nitrometer and the carbon dioxide source and
L Y TICA L C H E MI6 T R Y
DISCUSSION AND RESULTS
This particular modification was developed after considerable experimentation with various high temperature movable burners, etc. The advantages that appeared were several: more complete combustion, a shorter burning time, and a minimum of handling by the operator. There is no need for reburning, a8 the furnace covers the entire sample and temporary filling. There is less possibility of burning the sample too fast, thereby having incomplete combustion, because the whole tube is maintained a t a rather high temperature. Once the furnaces are in placethat is, over the tube-the apparatus requires very little attention. It is simply a matter of adjusting the carbon dioxide flow and terminating the procedure when microbubbles appear. To test the efficiency of this modification a variety of compounds were analyzed, as shown in Table I. In addition, hundreds of routine samples have been run with very good resiilts. LITERQTURE CITED
(1) Alford W. C., Ax.4~. CHEY.24,881 (1952). (2) Childs, C. E., Moore, V. A . Ibid., 25, 2 0 4 (1953) (3) Gysel, H., Helv. Chiin. Acta 35, 802 (1952). (4) Kirsten, W., A N A ICHEM. . 25,74 (1953). (5) Shelberg, E. F.. Ibid., 23, 1492 (1951). RECEIVED for review Septemher 30, 1955. Accepted March 21, 1956.
Infrared Absorption Spectra of Branched-Chain Fatty Acids DONALD L. GUERTIN, STEPHEN E. WIBERLEY, Department
o f Chemistry,
and
WALTER H. BAUER
Rensselaer Polytechnic Institute, Tioy,
N. Y.
and
JEROME GOLDENSON Chemical Corps Chemical a n d Radiological Laboratories, A r m y Chemical Center,
From a study of the infrared absorption spectra of long branched-chain fatty acids Freeman has shown that the relative intensities of the bands at 7.8 and 8.1 microns are valuable in identifying a-substitution. This correlation holds for the branched-chain hexanoic acids. In addition, the relative intensities of the bands at 6.8 and 7.1 microns are valuable in identifying a-substitution in acids containing less than 14 carbon atoms.
F
REEMAN ( 1 ) in his study of the infrared spectra of 27
branched long-chain fatty acids found that the relative intensities of the absorption bands near 7.8 and 8.1 microns could be used to distinguish fatty acids with a branched-chain in the a-position. The band a t 7.8 microns was the stronger of the two, except when a group was substituted in the a-position.
Md.
To see whether this correlation would hold for the branched short-chain fatty acids, the spectra of 15 such acids were measured on a Perkin Elmer Model 21 double-beam recording infrared spectrometer equipped a i t h rock salt optics. The liquid acids were run in a demountable liquid cell without dilution. No spacer was employed. The fatty acids vere synthesized by the Bureau of Mines and were obtained from the Chemical Corps Chemical and Radiological Laboratories. The position of branching was determined by the method of synthesis. Carbon-hydrogen analysis and neutralization equivalents were reported. Agreement between the calculated and experimental values was excellent. Freezing point data were used to determine mole per cent purity in several cases. The 2-isopropyl; 2-n-butyl-, 3-n-propyl-, 4-ethy1-, and 5-methylhexanoic acids were better than 95 mole % pure. T h e 2-ethyl- and 3-methylhexanoic acids were, respectively, 92 and 89% pure. The spectra of these acids in the region of 6.5 to 8.5 microns are plotted in Figure 1. It is apparent from this figure that the
V O L U M E 28, N O . 7, J U L Y 1 9 5 6
1195
5 METHYL
f
50
z #
‘65
&5
85
65
85
85
WAVELENGTH
65
85
65
85
M I C R O N S
3n P7OWL
5 50 m z a I-
1
85
65
65
85
Table I. Observed Wave Lengths (3licrons) of the Absorption Rands Near 8.1 Microns in Two Series of Methyl Branched-Chain Fatty Acids Wave Length, P 8.10 8.13 8.20 8.27
Acid 2-Methyloctadrcanoic 3-Methyloctadecanoic 4-hlethyloctadecanoic 5-Slethyloctadecanoic
Ware Length, p 8.09 8.14 8.23 8.25
.~
Table 11. Observed Wave Lengths of .-ibsorptionBands of Branched-Chain Fatty Acids between 12..5 and 14.0 Microns Acid 2-hiethylhexanoic 3-hlethylhexanoic 4-hIethvlliexanoic
Wave Length, p 12.66
... ...
... ... ...
4-Ethylhexanoic 2-n-Propylhexanoic 2-Isopropylhexanoic 2-n-Butylhexanoic 2-sec-Butylhexanoic 3-n-Propylhexanoic 3-n-Propylheptanoic 5-Methyliieptanoic Octanoic
... li:65 12.63 12.65 12.75
... ... ... ...
1i:is ...
13: if5 13:io .
.
I
l2:82 12.92 13.02
... ... ...
...
, . . , . .
... ... ...
... 12:95
...
... ..,
... ... ... ...
... 13 ’ i o
...
...
...
li:b6 13.55
...
...
, . .
65
85
65
85
2n P OPY
2 E~HYL
Acid 2-Xethylhexanoic 3-Methylhexanoic 4-Methylhexanoic 5-~letliylhexanoic
BE>
...
13.76
... ...
13:85
13,76 13.61
...
13:49 13.68 13.69 13:io 13:83
band near 8.1 microns is strongei than the band near 7.8 microns only when a-substitution occurs. I n addition to these two bands, the relative intensities of the bands at 6.8 and 7.1 microns are useful in identifying a-substitution. Absorption a t 7.1 microns is stronger than absorption at 6.8 microns, except when a-substitution occurs. However, because the 6.8-micron band intensity is a function of the size of the hydrocarbon portion of the molecule, a t a chain length of approximately 14 carbon atoms the 6.8-mici on hand becomes
stronger than the 7.1-micron band, regardless of the type of branching. Hence, this intensity inversion of the 6.8- and 7.1micron bands is applicable only to the shorechain fatty acids and is not as generally useful as the 7.8- to 8.1-micron region initially investigated by Freeman. It is also of interest to compare the position of the band near 8.1 microns in the methyl hexanoic acid series with those in the methyl octadecanoic acid series as reported by Freeman (see Table I). Table I shows that the band positions may be correlated with position of branching in the methyl-substituted series. I n the ethyl hexanoic acid series absorption occurs a t 8.15 microns in both the 2-ethyl- and 3-ethylhexanoic acids and at 8.25 microns in the 4ethylhexanoic acid. Changes in the position of the absorption band near 7.8 microns were not significant. Freeman also reported spectral indications for ethyl and n-propyl groups. The ethyl group is associated with a band a t 12.95 microns and the n-propyl group with one at 13.5 microns. Table I1 shows that these correlations itre confirmed by the present work. The isopropyl group may be recognized by absorption near 7.30 microns in the three fatty acids containing this group, as has been shown by Freeman and Sobotka and Stynler ( 2 ) . This investigation shows that the correlations presented by Freeman may be extended to include the lower fatty acids. The relative intensities of the bands a t 6.8 and 7.1 microns seive to identify a-substituents in acids containing less than 14 carbon atoms. LITERATURE CITED
(1) Freeman, N. K., J . Am. Chem. SOC.74, 2523 (1952). (2) Sohotka. H . , Stynler, F. E., I b i d . , 72, 5139 (1950). RECEIVED for review Pu’ovember 16, 1955. Accepted March 19, 1956 This study was conducted under contract between the Chemical Corps, c’. S. Army, and Rensselaer Polytechnic Institute.
