V O L U M E 24, N O . 9, S E P T E M B E R 1 9 5 2
1391 mass spectra of the hydrocarbon types. The method is rapid, having a time requirement of about 2 hours, and is accurate t o better than &lo% of the type present. Provisions are made for an approximate determination of condensed-ring naphthenes.
Table \-. Inalyses of Synthetic Mixtures
Paruffill.; Kaphtheiies Aromatics
Olefins
29.7 36.8 33.6 0.0
io0.0
29.6 39.9 30.5 0.0
1oo.o
51.3 16.6 12.9 19.2
52.3 17.0 11.2 19.5
67.7 13.6 14.9 3.8
68.9 14.8 12.1 4.2
ioo.0
1575
10o.O
lo0.0
those shown in Table IV, combined with unit yield figures, a Ca-Ce ring naphthene split may be determined. Further data on this type of combined analysis are in the process of preparation for publication. Analyses of synthetic mixtures prepared by adding known amounts of naphthenes and olefins to stocks of,knonm composition are presented in Table 5'. Only the single determination shown was obtained on each mixture. CONCLUSION
Determination of the major hydrocarbon types in the gasoline boiling range of petroleum, applicable principally to stocks of low olefin content, involves the uee of distinctive m / e ratios in the
ACKNOWLEDGMENT
The authors express their thanks t o the management of the Humble Oil and Relining Co. for permission t o publish this material. They are indebted to Frances Lander and Maydell Williams for analyzing the samples and making many of the computations. LITERATURE CITED (1) Am. Soc. Testing Materials, D 875-461' (1946). (2) Brown, R. A., ANAL.CREM.,23,430 (1951). (3) Kurtz, S . S., Jr., Mills, I. W., Martin, C. C., Harvey, W. T , and Lipkin, hl. R., Ibid., 19, 175 (1947). (4) Lumpkin, H. E., and Thomas, B. W., Ibid., 23,1735 (1951). (5) Purdy, K. M., and Harris, R. J.,Ibid., 22,1337 (1950). (6) Rampton, H. C., Ibid., 21, 1377 (1949). (7) Rampton, H. C., J . Inst. PetToZeum, 3 5 , 4 2 (1949). (5) Washburn, H. R., Wiley, H. F., Rock, S. ill., and Berry. C. E., IND. ENQ.CHEM., AXAL.ED.,1 7 , 7 4 (1945). RECEIVEDfor review March 24, 1952.
.4ccepted June 16, 1952
ldent ificat ion of Thiophene and Benzene Homologs Mass Spectral Correlations 1. W. KINNEY, JR.', AND G. I,. COOK Petroleum and Oil-Shale Experiment Station, Bureau of Mines, Laramie. Wgo. A new method for the identification of unknown organic compounds has been developed, using correlations that relate mass spectra with molecular structure. No prior mass spectral data for the unknown compounds are necessary for identifications. The basis of the method is the use of mass spectral correlations to identify structural groups i n unknown compounds. Identifications are completed by the use of supplemental chemical and physical data. The identification procedure is illustrated by the identification of two thiophenes, isolated from Colorado shale-oil naphtha. Development of mass spectral correlations is illustrated for thiophene and benzene homologs. The use of mass spectra in qualitative identification of unknown compounds has ordinarily been limited to comparison of the spectra of the unknowns with those of reference samples. Mass spectral correlations broaden the scope of mass spectrometry, permitting establishmentof certain factors of molecular structure directly from mass spectra of unknown compounds.
I
iY THE course of an investigation of the composition of shale-
oil naphtha, a need arose for a method of identifling the thiophenes in the molecular weight range 126 t o 154. As only a few of the compounds in this range are available, classical methods (10) of identification by direct property comparison are not easily applicable. The use of mass spectra offered a promising method of attacking this problem, as the literature contains several articles (4-7, 11-15) indicating the possibility of correlations between mass spectra and molecular structure. From a consideration of the mass spectra of available thiophene and benzene homologs, correlations were developed that permit deter1
Present address, I. W.Kinney and Co., Champaign, Ill.
mination of the configuration of the alkyl substituents on the nucleus. For thiophenes, combination of this technique with the preparation of mercuric acetate derivatives (16), methylation (8),and hydrogenation ( 8 ) permits identification of any compound in the 126 to 154 molecular weight range. In the case of higher molecular weight thiophenes certain compounds may be identified, or a t least the number of possible compounds may be limited considerably. '4s the correlations for the benzenes were made primarily t o confirm the thiophene data, no extensive application of these correlations is reported in this paper. However, it should be possible to identify mono- and disubstituted benzene homologs in the 120 to 148 molecular weight range. The substituent groups could be identified by mas8 spectral correlations, and their relative positions could be found by use of infrared spectra ( 2 , 19, 2 1 ) or by oxidation to the carboxylic acid (15). EXPERIMENTAL
The mass spectra used to develop the correlations given in thia paper were obtained from published data (1)or were run in this laboratory. The spectra run in this laboratory were obtained on a Consolidated mass spectrometer, Model 21-102, equipped with automatic control of the electron current. The temperature of the ionization chamber was automatically controlled t o 300' C. The ionizing voltage used for all compounds was 70 volts, except tor the diisopropylbenzenes (50 volts), which were among the compounds not run in this laboratory. Relative intensities of mass peaks in a spectrum are expressed in terms of the largest peak, or base peak, taken as 100%. For thiophene and benzene homologs the base peak consists of the molecular fragment containing the ring as illustrated in Table I. ,is shown, these peaks result from the breaking of either an abond (C-C bond next to the ring) or a p-bond (C-C or C-H bond once removed from the ring).
ANALYTICAL CHEMISTRY
1392 CORRELATIONS OF MASS SPECTRAL PEAKS WITH MOLECULAR STRUCTURE
Correlations of mass spectra n i t h molecular structure were developed for eight peaks of both the thiophene and benzene homologs. Some correlations are similar for both groups of conipounds and are discussed together; others are discussed separately. The numerical values of the peaks in mass spectra are changed by the temperature a t which the spectra are obtained. However, the variations do not appear to affect the validity of the correlations. The peaks used for correlations are: base peak, parent peak, parent peak less mass 1, parent peak less mass 31, and the S+,841, 7 9 + , 7 8 + , 59+, 43+, and 41+ peaks. Peaks from the spectra of compounds used in establishing the correlations appear in Tables I1 and 111. Base Peak. For benzene homologs having only methyl groups as side chains, the base peak results from the breaking of an a-bond. An exception is methylbenzene, in which a 8-bond breaks. For mono- and dialkyl benzene homologs other than the methvl-substituted isomers, and for all thiophene homologs, the base peak results from the breaking of a @-bond Exceptions, in which an a-bond breaks, are polymethvl thiophenes having the 2- and 3- positions occupied. For trialkyl benzene homologs containing no nwthyl groups, the base peak results from the breaking of an a-bond. There is some evidence that the base peak results from breaking of the weakest bond in the molecule. Swarcz (17, 18) has shown that the weakest bonds in methylbenzene, ethylbenzene, and the sylenes are ,%bonds. Splitting of the molecule at the P-bond accounts for the base peak in methylbenzene and pthylbcnzene (Table 111). However, an a-bond breaks in dimcsthylbenzenes, so it is seen that factors other than the rupture of the, Treakest bond enter into base-peak formation. 'Table I. Base-Peak Formation of Selected B e n z e n e and Thiophene Homologs ComDound
Parent Peak
Base Peak
3-Methylthiophene
98
97
i,4-Dimethyibrnzene
106
91
Base-Peak Formation
C-
Ethylbenzene
1,4-Diithylbenzene
134
r-
.
119
c- c-
2,5-Diethylthmphtne
140
I25
1,3,5-Tritthylbeniene
162
133
t- c-()':c-c-
*HYDROGENS
i+
c-c-
r"+O ]+
ARE iNCLUDE0 IN THE ABOVE STRUCTURAL FORMULAS ONLY WHEN NECESSARY TO ILLUSTRATE BASE PEAK FORMATION.
If the aromatic-ring electrons are compared to double-bond electrons, a benzene and thiophene aromatic-electron rule may be postulated that is analogous to the double-bond rule of Schmidt (14). Schmidt's rule is as follows: The single bonds to a doublebonded carbon atom are strengthened by the presence of the double bond, while those bonds once removed from the double bond are weakened. The postulated aromatic-electron rule is as follows:
For monoalkyl benzene homologs and for all thiophene homologs, the a-bonds to the ring carbons are strengthened by the presence of aromatic electrons, while the &bonds are weakened. One compound shown in Table 111, tert-amylbenzene, does not have a base peak resulting from the simple rupture of a C C bond according to the previously given correlations. It is suggested that the base peak of this compound, which coincides with the parent peak less mass 43, results from an ionic rearrangement. A shift of a methyl group and a proton occurs with simultaneoue breaking of a bond, as indicated in the following diagram:
Rearrangements a t the moment of ion formation are common in the mass spectra of alkanes ( 4 1 1 )giving large peaks that cannot be formed by the simple rupture of a C-C bond. The proposed methyl and proton shifts are similar t o those postulated by Khitmore (20) in his discussion of carbonium-ion reactions. Two thiophenes through molecular weight 154, the 2- and 3- tert-amplthiophenes, are expected to exhibit this rearrangement. Parent Peak. The parent peak is above 50% of the base peak when only methyl groups are on the benzene or thiophene ring. The parent peak is below 50% of the base peak for all other benzene and thiophene homologs. The parent peak is formed by ionization of the parent molecule. Compounds nith high (over 50% of the base peak) parent peaks are aromatic ring compounds and their methyl and polymcthyl isomers. I t is suggested that high parent peaks occur in compounds having all carbon-carbon bonds of about equal strength. Parent Peak Less Mass 1. The parent peak less mass 1 is comparatively large (10 to 30% of the base peak for benzene homologs; above 50% of the base peak for thiophene homologs) w h m only methyl groups are on the ring. The parent peak less mass 1 is intermediate (1 t o 4% of the bas, peak for benzene homologs; 2 to 50% of the base peak for thiophene homologs) when methyl and ethyl groups or ethyl gioups are on the ring. An exception is 2,3,54riethyIthiophmc, Q hich has a small parent peak less mass 1. The parent peak less mass 1 is small (less than 1% of the base peak for benzene homologs; less than 2% of the base peak for thiophene homologs) when a single alkyl group other than methyl or ethyl is on the ring. The parent peak less mass 1 is formed by the loss of a hydrogen from the parent molecule. This loss of hydrogen apparently is dependent on the number of methyl groups on the aromatic ring and independent of the position of the alkyl groups on the ring (eompare dimethylbenzenes with methylethylbenzenes, Table 111). Parent Peak Less Mass 31. The parent peak less mass 31 of a benzene or thiophene homolog is larger than the parent peak less mass 29 when an isopropyl group and another group are present or when a tert-butyl group is present. The normal breakdown of alkyl-substituted thiophene and benzene homologs is the loss of a CH3and a CH2 group (mass 29) or a C2H, group (mass 29). The structures apparently re-
1393
V O L U M E 2 4 , NO. 9, S E P T E M B E R 1 9 5 2 quired for the forniation of abnormally high parent-less-31 peaks are those in which a CH3 group may be lost, leaving a carbon side chain with two methyl groups. The CHa group may be attached directly to the ring (as in methylisopropylbenzene) or may be part of a side chain (as in diisopropylbenzenes). This configuration makes possible the loss of mass 16, using one of the CII, groups plus a hydrogen. An illustration of this type of Ix-eakdown in alkanes is seen in the mass spectrum of 2,2-dimethylpropane (1). 85+ Peak (for Thiophenes). The 85+ peak is comparatively large (10 t o 15% of the base peak) for monoalkyl substitution n-hcn the carbon adjacent to the ring is tertiary. The 85+ peak is intermediate (4 t o 10% of the base peak) for monoalliyl substitution n-hen the carbon adjacent t o the ring is secondary. The 85+ peak is small (less than 4% of the base peak) for all other types of substitution. I l n 85+ peak represents a structure consisting of a thiophene ring plus a hydrogen. This peak is usually the largest of the ring-less-mass-1, ring, and ring-plus-mass-1 (83+, 84+, 85t) peaks. This is in contrast to the corresponding benzene peaks ( V f , i 8 + , 79+), of n-hich the 7 7 s peak is usually the highest. The difference is thought to result from the attraction of the side chain protons t o the sulfur electrons. 84+ Peak (for Thiophenes). The 84+ peak is larger than the 85+ peak when a single, normal alkyl group is attached to the ring. It is expected that this correlation will apply also to branched monoalkyl thiophenes having primary carbon atoms in the aposition, but a t present no thiophenes of this type are available. 79+ Peak (for Benzenes). The 79+ peak is large (7.0 to 11.0y0 of the base peak) for monoalkyl substitution when a secondary or tertiary carbon atom is attached to the ring. The 79+ peak is intermediate (1.0 to 7.5% of the base peak) for polyalkyl substitution other than dimethyl. The i9+ peak is small (less than 3.07, of the base peak) for nionoalkyl substitution when the carbon attached to the ring is primary. Ethylbenzene is an exception in that it has an intermediate value. Location o f the alkyl substituents has an effect on the size of
the 79+ peak. Thus, the i 9 + peak of 1-niethyl-2-ethylbenzene is the largest of the methylethylbenzenes and the 79+ peak of 1,2,-disopropylbenzene is the largest of the diisopropylbenzenes. 78+ Peak (for Benzenes). The 78+ peak is more than twice the size of the i 9 + peak for monoalkyl benzene homologs when a single normal alkyl group is attached to the ring. As with the thiophene 84f peak, it is expected that the above correlation will apply also t o branched monoalkyl benzenes having primary carbon atoms in the a-position. The only available compound o f this t j p , isobutylbenzene, has a 78+ peak more th:m tn-icc tlic size of the 79 peak, as expected. 59+ Peak (for Thiophenes). The 59+ peak is large (over 15% of the 1)ase peak) when methyl groups are in the 2- and 5positions. The 59+ p w k is intermediate (5 t o 15% of the base peakj when a methyl group is in the 2- position. An exception is 2methylthiophene, which has a value slightly less than 5% of the base peak. The 59+ peak is small (1 t o 5% of the base peak) when the 2- and 5- positions are unoccupied or occupied by any alkyl group other than methyl. Formation of the 59 peak, S-C--CII -3, occurs niost readily v i t h methyl groups in both the 2- and 5- positions of the thiophene ring, less readily n-ith a methyl group in the 2- position, and least readily with no methyl groups in the 2- and 5- positionc. The re~i-onfor these breakdowns is illustrated in the following diagrams:
+
+
H3c-u-c S
H3
-
5-15 %
I 5%
n
,C-%
S
ABOVE 15
59'10N
O/o
Table 11. Correlation Peaks from Mass Spectra of Thiophene Homologs Est.
.\PI
Piirity, Serial
Compound
a. b.
(1). This laboratory.
Source
%
KO.
Parent P a r e n t Parent Parent Parent Parent Parent Parent Peak, Lese 1, Less Less Less Less Less Less 8 5 + , % 70 15, % 29, % 31, % 43, % 57, % 71, % %
84+,
%
83+,
%
59+,
%
43+.
%
41+,
% '
ANALYTICAL CHEMISTRY
1394
Table 111. Correlation Peaks from Mass Spectra of Benzene Homologs .%PI Serial C o iiipo und Sollrce No. 251 hIethylbenzene 1 1 253 1 2-Ditnethylbenzene I 254 1:3-Diniethylbenzene 255 1.4-Dtniethvlbenzene 11 252 Ethylbenzene n 261 1 2 3-Trimethylbenzene it 112:4-Trimethvlbenzene it 262 263 i'3'6-Trimethylbenzene a 258 1:ihethyl-Z-ethylbenzene a 259 1-Methyl-3-ethylbenzene i: 260 1-RIethyl-4-ethylbenzenr .i 236 I ri-Propylbenzene 7 257 Isopropylbenzene 1,2,3,5-Tetramethylben1 463 zene 1,2,4,5-Tetramethylbena 486 zene 1,2-Dirnetli3.1-3-etli~~lben8 264 zene 1,2-Dlmethyl-i-ethylbena 262 zene 1,3-Diniethyl-2-ethylben1 26G zene 1,3-Dimethyl-4-ethylbenL 2(ii zene 1,3-D~methyl-5-ethylbeni 268 zene 1,4-Diiiiethyl-Z-eth3.lberrn 269 zene ~-hIethv1-3-isopro~~lb~ii46 1 zene a I-hlethyl-4-tsopropylh~~~wnP a 462 1,T-Diethylbenzene b ... 494 n-Butylbenaene a 4.59 Isobutylbenzene a 460 sec-Butylbenzene u 319 ,tert-Butylbenzene n 1-Methyl-4-tert-butylbcnzene b ... te+Amylbenzene b 1 3 5-Triethylbenzene tJ ... 1:2:4-Triethylbenzene h ' 57 1 2-Diisopropylbenzene a 5s l:3-Diisopropylbenzene a 59 1,4-Diisopropylbenzene i: a. (I). b. This laboratory.
ParPar- Parent Parent Parent Parent Parent Parent ent ent Peak, Less 1, Less Less Less Less Lese Less is+, % % % 15, % 29, % 31, % 43, % 57, % 71, % % 99.96 1 0 . 0 2 77.0 100.0 .. 0.0 99,090 =t0.007 59.8 2 4 . 1 1OO:O .. 7.4 7.7 99.94 10.04 6 5 . 3 28.6 100.0 99.94 1 0 . 0 3 6 7 . 4 .. 8.0 3 0 . 3 100.0 99.96 1 0 . 0 2 , . . .., ... .. 3.7 32.2 5 . 9 100.0 6.2 99.982 i O . 0 1 2 5 3 . 3 1 2 . 6 100.0 8.9 1.2 , . 99.67 1 0 . 2 0 58.9 5.7 9.1 1.2 1 5 . 6 100.0 99.95 1 0 . 0 2 5.8 9.2 1.2 66.5 1 6 . 3 100.0 99.73 1 0 . 0 7 10.6 1.6 . . 6.7 29.8 2 . 2 100.0 99.57 1 0 . 1 5 3 1 . 8 10.5 1 6 5.8 3 . 8 100.0 5.4 99.87 1 0 . 0 3 9.1 1.5 28.9 3 . 0 100.0 99.75 1 0 . 0 8 1.6 1.4 21.0 0.2 3.4 I 00.0 9 .3 99.93 '0.03 5.2 0 8 23:i 0 . 8 100.0 Est. Purity,
..
is+,
77+, 43+,
%
%
0.2 7.8 a.1 8.2
4.3 4.4 3.5 3.6 3.3 6.2 5.4
1.2 13.2 13.7 15.1 8.4 12.0 12.0 12.5 10.4 10.4 9.5 3 3 13.3
1.7 0.2 0.2 0.1 0.1 0.2 0.3 0.1 0.2
3 1
2.7
8.2
0.2
8.0
3.i
3 2
10.2
0.2
11.9
%
8.4 4.3
0.2 0.2 0.1 1 5
41+,
%
2.0 2.4 2.6 2.3 1.6 5.2 5.7 5.7 2.9 3.1 2.8 3.3 4.4
99.92
kO.01
22.9
10.4
100 0
3.6
8.0
123
99,8G
zzo 04
54.0
10.8
100.0
3.3
8.4
I4 5
99.6
...
30.3
1.8
100.0
11.3
3.s
II'i
..
3.2
R 0
8.4
0.2
6.8
99.6
...
.
,
2.9
2.8
i 8
o
6.7
.
2 8
2.G
i.6
0.2
5,s
2 5
i 2
0.5
6.0
29.7
2.5
100.0
8.7
3.2
11.9
99.81
27.3
1.1
100.0
.j.9
3.4
10.7
.
9Y . 9 3
26.6
1 8
100.0
5.0
3 2
104
..
2.6 3.1
3.0
8 4
0.2
6.3
..
3.2
3 0
8 6
0 2
6.3
2.2
2.4
6.9
0.6
8.4
2.1 7.4 2.8 2.8
2 2
6.6
0 5 3.0 2.8 16.9 0.1 0 5
i.6
2.7 5.3 10.2 3.7 17.0
0 4 6.7 3.5 10.7 14.5 1 2 . 5 15.3 1 1 . 1 8.2 8.2 8 . 6 30.2 R 6 17 4
21.9 17.9 18.2 17.0 18.4 17.2 12.2
99 9.3
...
35.0
3 8
100.0
9,4
3.;
12 P
99. ao
...
34.4
2.6
100.0
18 4
3.7
12.8
99.94
10.04
25.0
1.1
100.0
3.3
3 6
1a
Y Y 95
0' 03 10.02
23 7 38.4 24.5 25.4 18.0 26.3
0.7 2.0 0.1
100.0 2.7 100.0 6 9 . 6 0.9 8.5 0.9 1.2 1 . 7 100.0 0 7 100.0
3.4 7.7 2.0
99.93 99.88 99 87 99 88 99 94 99 0 5
95
95 90
99.6 99.6 99.8
iO.08 *0.09 10.06 *0.03
4~0.03 20.i ,.. 12,9 ... 38.1 , . . 40.9 ... 36.6 35.3 29 2
0.2 0.1 0.0
0 1
OS
2.4 3 0 0 1 0 6 0 6
5 8 100.0 7 3 3.6 4 7 . 1 100.0 7 2 . 8 100 0 100.0 100.0 100.0 1 0
-4 methyl group in the 3- position appears to be a factor that increases the size of the 59+ peak. As examples, this peak for 2,3-dimethylthiophene is 13.8%, whereas for 2,4dimethylthiophene it is 7%; and for 2,3,5-trimethylthiopheneit is 29.1%, whereas for 2,5-dimethyl-3-ethylthiopheneit is 19.8%. 43+ Peak (for Benzenes). The 43f peak is large (5 to 30% of the base peak) when an isobutyl group or two isopropyl groups are on the ring. The 43+ peak is small (less than 5% of the base peak) when only methyl and ethyl groups are on the ring. The large 43+ peak is thought to come mainly from the side chains instead of being ring fragments or partly ring fragments and partly side chain fragments. It is predicted that this peak will increase in benzene and thiophene homologs having long side chains. Three compounds are available sholying this increase: 2-ethyl5-wbutylthiophene (Table II), 2-n-hexylthiophene (Table 11), and 5-phenyleicosane ( 1 ) . The latter compound shoxs also that other prominent paraffin peaks (41+, 55+, 5 i + ) become larger with i n c r e w in the size of the side chain. 41f Peak. The 41+ peak is large (above 10% of the base peak for benzene and thiophene homologs) when a tert-butyl group or two isopropyl groups are on the ring. The 41f peak is small (bclo\v 10% of the base peak for benzene and thiophene homologs) for mono-, di-, aiid trisubstitution when only methyl and ethyl groups are on the ring. As with the 43+ peaks, the large 41 peaks are thought to be derived mainly from the side chains. As the 41 peak is also a prominent paraffin peak, it is predicted that this peak will become larger with long side chains.
+
+
IDENTIFICATIOS METHOD FOR THIOPHENE HOMOLOGS
The complete method of identification of thiophenes in the
126 to 154 molecular lveight range consists of identifying the side
;.;
0.9 4.6 5 0
5 9 1.9 4.0
5 4 5.2
3.7 5.4
$4
161 21.2 100.0 100.0 13.; 45
..
,. , ,
..
, .
I
:30 9 1 2 . 2 .. 100.0 3 3 . 3 5 8 . 5 39.0 3 1 . 7 4 0 . 4 35.6 3 0 . i 40.9 11.5 2 9 . 2 39.2 22.9 22.8 28 2 I4 9 17 5
r . l
9.5 1.8 7.p 7 0 7.6 7.2 5.3 3 8
3.7 6.2 2.8 3.7 4 2 2.1 4.0 5.4
5.3 2.9 3.2 2.4
13.3 5.9 3.0
10.3 9.5
i
chains by mass spectral correlations and determining their positions by formation of mercuric acetate derivatives, methylation, and hydrogenation, Final confirmation is made by synthesis of the identified thiophene and comparison of properties of the isolated and the synthesized thiophenes. Identacation of Side Chains. Mass spectral correlations are used to identify the side chains by their application to suitable peaks in the mass spectrum of the unknown thiophene.
As an example, an unknown thiophene of molecular weight 140 shows a base peak of 111+ and has a value of 3.2% of the base peak for the 85+ peak. As the base peak was formed by the loss of an ethyl group, and as the ethyl group must have broken off a t a 8-bond (see base peak correlations), the side chain must be a sec-butyl or n-propyl group. The value of 3.2% for the 85+ peak shows that the carbon attached to the ring is not secondary or tertiary, The side chain is identified as a n-propyl group. In addition, a methyl group is on the ring, because 4 carbon atoms must be attached t o the thiophene ring when the molecular weight is 140. Side Chain Location. Using a combination of mercuric acetate derivatives, methylation, and hydrogenation, the number of side chains and their relative positions on the thiophene ring may be established. Mercuric acetate derivatives are prepared following the method of Steinkopf and Killingst ad (16), taking the following precautions: The 25" C. reaction is limited to 15 minutes' reaction time and the quantity of mercuric acetate used is limited to the theoretical amount required to form the di-derivative. The quantity of mercuric acetate used in the 60' C. reaction is limited to the theoretical amount required to form the tri-derivative. If the derivative formed a t 60" C. does not precipitate, the amount of mercuric acetate is reduced to the theoretical amount required t o form the di-derivative, then if necessary to the amount required to form the mono-derivative, as monomercuric acetate deriva-
1395
V O L U M E 2 4 , NO. 9, S E P T E M B E R 1 9 5 2 tives may be soluble in an excess of mercuric acetate. Derivatives may be differentiated by determinations of either mercury or sulfur content. Preparation of mercuric acetate derivatives of thiophenes at 25' C. gives the number of 2,5-hydrogens, if any are present. If no 2,bhydrogens are present, this reaction gives the number of 3,4hydrogens. A monomercuric acetate derivative forms if only one 2,Shydrogen is available or if only one hydrogen is present on the nucleus. A dimercuric acetate derivative forms if two 2,5-hydrogens are available, or if only hydrogens in the 3- and 4 positions are present. All hydrogens present aic replaced by mercuric acetate a t 60" C. When 2,S-hydrogens are present, 3,4hydrogens are determined by the difference between , resume of the data the hydrogens replaced a t 25' and 60" C. 2 is presented in Table IT,which shows that mercuric acetate derivatives classify an unknown thiophene in one of the following groups: 2-substituted; 3-substituted; 2,5- or 3,sdisubstituted. 2,4- or 2,3-disubstituted; and 2,3,4- or 2,3,5-trisuhstituted
thiophene was thus identified as 2,3,5-trimethylt,hiophene, using mass spectral correlations only. Confirmation of trisubstitution was made with mercuric acetate derivatives. The monomercuric acetate derivative formed a t both 25' and 60" C. Only trisubstituted thiophenes react in this manner. Final confirmation of the identification was made by synthesis of 2,3,5-trimethylthiophene. A comparison of selected peaks from the mass spectra of the isolated and synthetic 2,3,5-trimethylthiophenesis as follows:
+
2-METHYL-5-ISOPROPYLTHIOPHESE. CS thiophene (molecular weight = 140), isolated from a fraction having a 168" to 175' C. boiling range mas identified as 2-methyl-5-isopropylthiophene. Mass-spectra peaks used in establishing the structure of the isolated thiophene are:
Ion 140 139 T 125 111+ 97
Table IV. Mercuric iicetate Derivatives of T h i o p h e n e s Alkyl Group(s) a t Position(s) 2
c
A t 60' C
Mono
Tri
3
I3i
Tr1
2 and 5 3 and 4
I)i D1
Ll i Di
2 and 4 2 and 3
AIono Xono
Di Di
2, 3 , and 5 2 . 3 , and 4
Nono Nono
11ono 11ono
A 2,5-disubstituted thiophene can be differentiated from a 3,4 disubstituted thiophene and a 2,3,4-trisubstituted thiophene can be differentiated from a 2,3,5-trisubstituted thiophene by methylation. The methylated thiophene is prepared by reduction of the aldehyde prepared by the method of King and Sord (8). If the 59+ peak after methylation is less than 1.5 times the original 59+ peak, then the methyl group entered the 3- or 4- position. The 2,5- positions must be occupied by side chains. If the 59+ peak is more than 1.5 times the original peak, then the methyl group entered the 2- or 5- position. The preceding methods of locating side chains are not useful for differentiating between 2,3- or 2,4disubstituted thiophenes. The distinction can be made by hydrogenating the unknown thiophrne and identifying the resulting alkane by mass spectra. The Raney nickel hydrogenation procedure of Blicke and Sheets (9)is used. Application of Method. The method has been used in the identification of a number of thiophene homologs isolated from a Colorado shale-oil naphtha. This work will be reported in detail in a subsequent publication (9). The following identification of two compounds illustrates how the method is applied: 2,3,5-TRIMETHYLTHIOPHEXE. -4 c, thiophene (molecular weight = 126), isolated from a 163-165' C. shale-oil naphtha fraction, was identified as 2,3,5-trimethylthiophene using only mass spectral data. Ion 126+ 125+ Ill+ 97 85
++
59 t
Relative Ion Intensity of Isolated Thiophene 77.9 79.1 100.0 4.2 3.1 29.9
The parent peak is above 50% of the base peak, indicating methyl substitution exclusively; the parent peak less mass 1 is above 50% of the base peak, also indicating methyl substitution exclusively; and the 59+ peak is above 15% of the base peak, indicating methyl groups in both the 2- and 5- positions. The remaining methyl group must be in the 3- position. The unknown
Relatl\e Ion Intensity 28.5 2.2 100.0 2.3 9.0 2.4 10.5
+ + p+ .I9 4-
Type of Derivative
.kt 259
Relative Ion Intensity of 2,3.~-Triniethylthiopliene Synthesized Isolated 79.5 77.9 81.5 79 1 100 0 100 0 5.,5 4 ' 2.Y 3 1 29 1 29 9
Ion 126123 Ill+ 97 4 85 T 59 ~-
As the 59+ peak is,i? the 5 to 15% range, a methxl group.is located in the 2- position, eliminating butyl- and diethylthiophenes as possible structures. Tetramethylthiophene is also eliminated, as only one methyl group is present in the 2,5- positions. The base peak is the 125 peak, showing that the isolated thiophene does not contain a n-propyl group. The only remaining Cdhiophene isomers are the methylisopropylthiophenes and dimethylethylthiophenes. The isolated thiophene must be a methylisopropylthiophene, &ith the methyl group in the 2- position or a dimethylethylthiophene, with one of the methyl groups in the 2- position. Preparation of mercuric acetate derivatives completed the identification. Dimercuric acetate derivatives formed a t 25" and 60" C. As only 2,5- and 3,4-disubstituted thiophenes react in this way and as the methyl group is in the 2position, the compound was identified as 2-methyl-5-isopropylthiophene. Confirmation of the proposed structure was obtained by synthesis of 2-methyl-5-isopropylthiophene. Comparison of the mass spectra of synthesized 2-methyl-5-isopropylthiophene and of the isolated compound gave the following results:
+
Ion
140 t 139 125+ 111497 85 59 j-
+
++
Relative Ion Intensity of 2-Methyl-5-isopropylthiophene Isolated Synthesized 28.3 28.5 1.7 2.2 100.0 100.0 0.3 2.3 8.3 9 0 2.2 2.4 10. 10.5
e.
A comparison of the 111+ peaks shows an impurity, probably a sec-butylthiophene. SUMMARY
Correlations that have been developed between the mass spectra and molecular structure of thiophene and benzene homologs offer an attractive approach to the problem of identifying compounds for which authentic reference compounds or pertinent literature data are not available. From the correlations it is possible t o identify the alkyl substituents present in a thiophene homolog in the 126 t o 154 molecular weight range or in a monoor disubstituted benzene in the 120 to 148 molecular weighb range. Application of the correlations in higher molecular wei& ranges will greatly limit the number of possible structures b h t must be considered for an unknown compound. By a combination of information from the correlations with data from the preparation of mercuric acetate derivatives, methylation, or hydrogenation, it is possible t o identify completely any thiophene in the 126 to 154 molecular weighh range..
1396
ANALYTICAL CHEMISTRY Brown, R . A , . ANAL. CHEM.,23, 430 (1951). Friedel, R. d.,and Sharkey, A. G., Jr., "Mess Spectra of Acetals and Analysis of Oxygenated Compounds," presented before 9th -4nnual Mass Spectrometer Group Meeting, Pasadena, Calif., hlay 1951. Hartough, H. D., "Thiophene and Its Derivatives," Sen. York. Interscience Publishers, 1952. King, Wm. J., and Nord, F. F., J.Org. Chem., 13, 635 (1948). Kinney, I. IV., Smith, J. R., and Ball, J. S.."Thiophenes i n Shale-Oil Naphtha," presented before Division of Petroleum SOCIETY, Chemistry, 121st meeting of AMERICASCHEMICAI. Milwaukee, IT%, 1952. lIcKittrick, D. S.,I , r ~ dEng. . Chem., 21, 585 (1929). Xlohler, F. L., TTilliamson,L.. Tyke, C. E., Wells. E. J., Dean, H. 11..and Bloom. E. G., J . Research ,Yafl. Rzcr. Standards, 44, 291 (1950). O'Xeal, 11.J.. Jr.. and Wier. T. P., J r . , . \ x ~ I , . CHEM.,23, 8:10
The application of these techniques to the identification of two unknown thiophenes is presented. ACKNOWLEDGMENT
This project was part of the Synthetic Fuels Program of the Bureau of Mines and was performed a t the Petroleum and OilShale Experiment Station under the general direction of H. P. Rue and H. M. Thorne. Suggestions and revieLy of the work were given by J. S.Ball, G. U. Dinneen, J. R. Smith, and C. M'. Bailey. The work was done under a cooperative agreement between the University of Wyoming and t,he Bureau of Mines, Department of Interior. Several of the thiophenes were made available through American Petroleum Institute Project 48 on Synthesis, Properties, and Identification of Sulfur Compounds in Petroleum. The authors wish to thank H. D. Hart,ough and the Socony-Vacuum I'aboratories for the samples of 2,3-, 2,4-, and 3,4-dimethglthiophoncs.
(1951).
Rock, S.X I . . I b i d . , 23, 261 (1951). Schmidt, O., Z . Elektrochrrii., 39, 969 (1933). Shriner, R. L.. and Fuson, R. C.. tematic Identification of Organic Compounds," p . 163, S ork. John Wiley 8- Sons, 1940.
LITER.1TURE CITED ( 1 ) .Inierican Petroleum Institute Research 1'1,oject44, Catalog of hlass Spectral Data. (2) Barnes, R. B., Gore, R. C., Stafford, R. JT-., aiid IfXliania, T, Z., AN.4L. C;(HEM., 20,402 (1948). (3) Blicke, F. F., and Sheets. 11, G.. J . d r r i . C'hem. .Sot.. 7 1 , 4010 (1949). (4) Bloom, E. (;.. Alohler, 1;. L., Lengel, J. H., arid &%e. C. E., .J. Research .\*atl. Biir. Stnndnrds. 41,129 (1948).
Steinkopf, IT,, a n d Killin Swarcs, >I.. .I. ( , ' h e m PI Ibid., 17, 431 11949;. Thompson, H. V.,,.I. Chr V I . Soc., 1948,328. Whitmore. E'.C.. C'hcm. Eng. A\-ctcs.26, ti68 (1948). Young, c'. \\-.. DuVall, R . R..and Wright. S . .-Ix.~L. THEM,,23, 709 (1951). RECEIVED for r t v i P w I'ehluary 8. 1952.
Accepted .June 23, 1952.
Ultraviolet Spectrophotometry in Detect ion of Food Product Subst it ut es New Applications R . J. RIORKIS, K. D. MACPHEEI,
iiw
E. L. K A N D Q L L
Lrnit$ersityof .Vevada, Reno, .Vet..
Rapid methods for the detection of substitution and adulteration in certain fatty food products are desirable, particularly for use by government agencies charged with protecting the public against these spurious practices. Research work was undertaken to improve existing methods for the determination of the purity of theseproducts. Ultraviolet spectrophotometric measurements showed that conjugated tetraenoic systems present in butterfats and olive oils were significantly absent in margarine fats and cottonseed oils. The previously- discovered fact
that trienoic systenis are considerablymore prevalent in horse fats than in pork or beef fats was confirmed through employing alkali conjugation and making ultraviolet absorption measurements. Ultraviolet spectrophotometrywas found to provide a successful basis for screening out substitutes among these fatty food products. Because of the rapidity with which substitutions or appreciable adulterations in these products can be deteeted by this procedure, the method will furnish a useful tool for chemists working in this field.
C
OSSIDERABLE attention has recently been dii ected toward the ultraviolet absorption characteristics of various fats and oils. Ultraviolet spectrophotometry now permits the quantitative measurement of conjugated dienoic, trienoic, and tetraenoic systems in fat and oil samples. Methods have been devised for catalytically inducing conjugation of double bonds with alkali, so that certain nonconjugated unsaturated components in fats and oils may also be estimated. .4n experimental program was decided upon at this laboratory, based upon these new techniques, and directed toward their application in distinguishing various food products from others. Ultraviolet spectrograms for a number of samples showed that by virtue of distinct differences in conjugated tetraenoic content, substitution of margarine for butter and cottonseed oil for olive
oil can quickly and routinely be detected. dlkaIi conjugation techniques, applied to pork, beef, and horse fat reaffirmed the fact that horse fat contains a much larger quantity of linolenic acid (tetraenoic system) than do pork or beef fats. Alkali conjugation coupled with spectrophotometry provides a rapid and routine method for distinguishing adulteration of ground beef or pork with horse meat. For this investigation samples of retail products were made available through the courtesy of the Weights and Measures Division of the State of Sevada. These samples were not only representative of products from this state but included foreign products as well as products from other parts of the United States.
Present address, Los Angeles County .4ir Pollution Control District, Los Angeles, Calif.
Fats were extracted from the butters and margarines quantitatively with ether a t room temperature. This procedure entailed
1
PREPARATION OF SAMPLES
