A N e w Mass Spectrometric Method for Determining Alcohols and Water in Complex Mixtures The Fischer-Tropsch Product S. H. LANGER, R. A. FRIEDEL, IRVING WENDER, and A. G. SHARKEY, Jr. Central Experiment Station, Bureau of Mines, Bruceton, Pa. Direct, mass spectrometric determination of alcohols produced in the Fischer-Tropsch synthesis is not possible because of the similarity of hydrocarbon and alcohol mass spectra. By converting the alcohols to their trimethylsilyl derivatives, characteristic mass spectra are obtained free of interference from hydrocarbon ion fragmentation. Hexamethyldisilazane is a useful reagent for the conversion. Mass spectrometric determination of alcohols as trimethylsilyl ethers is possible without prior separation from other materials and increases the range of the room temperature mass spectrometer to the Clo alcohol region. W a t e r does not interfere, because it is converted to hexamethyldisiloxane, which has its own distinctive mass spectral peak. Other compounds such as glycols, amines, and phenols might be determined as their trimethylsilyl derivatives.
from hydrocarbons. Carbon-silicon bonds in trimethylsilyl ethers are split in the mass spectrometer to give intense parent (molecule ion) minus 15 peaks, which result from the loss of a methyl group (12). CH3
x
ROSi-CH3
\
CHI I
Hexamethyldisilazane (11) is used as a converting reagent and reacts with alcohols and water to give trimethylsilyl ethers and hexamethyldisiloxane (111), respectively, as shown in Equation 1 (7). Both reactions evolve ammonia. The mixture treated with hexamethyldisilazane is analyzed directly in the mass spectrometer. H20
(CH3)3SiNHSi(CH3)a
D
mass spectrometric determination of alcohols above Cs in the presence of hydrocarbons is not possible because of a similarity in fragmentation patterns ( 5 ) . Analysis of mixtures of alcohols and hydrocarbons is of special interest, because they are the chief product of the Fischer-Tropsch synthesis (1-4, 9, 11, 14). K i t h nitrided iron catalysts, primary straight-chain alcohols may constitute the major portion of the product (2, 1 1 ) . K o r k on the Fischer-Tropsch and similar processes has been hampered by the lack of suitable procedures for determining alcohols. This paper describe? a new approach for the rapid and accurate determination of alcohols in the presence of paraffins, olefins, and other hydrocarbons without the lengthy and relatively complex distillations previously applied to Fischer-Tropsch products (6,4, 9. 14). This method is based on the quantitative conversion of alcohols t o their trimethylsilyl derivatives, trimethylsilyl ethers (I) ( 7 , 10, 1 3 ) . The niajor mass spectral peaks suitable for analysis no longer correspond t o those obtained
/
it is used in excess. Neither the reagent nor unknown needs to be measured accurately. It is convenient to use an estimated 20% excess of reagent. The reaction of alcohols with hexamethyldisilazane is catalyzed by a trace of acid, formed by adding a drop of trimethylchlorosilane ( 7 ) . The reaction of hexamethyldisilazane lyith all mixtures reported was catalyzed in this fashion. The treatment of synthetic mixture I, Table VI was typical of the procedure used. T o 11 grams of the mixture, 8.5 grams (11 ml.) of hexamethyldisilazane was added. When 1 drop of trimethylchlorosilane was added, a white precipitate formed immediately. The reaction flask was fitted with condenser and drying tube and was refluxed for 9 hours. Evolution of ammonia began a t 63" C. and continued up to the temperature of reflux. During the reflux period, the temperature in the reac-
+
( CH3)3SiOSi(CH3)3 ?TH3
I11
+ NH,
(1)
2 ROSi(CH3)3
IRECT
I1 EXPERIMENTAL
Materials. Hesamethyldisilazane n a s prepared from ammonia and trimethylchlorosilane (7). Trimethylsilanol was prepared from hesamethyldisilazane using a variation of Sauer's procedure (10); trimethylchlorosilane instead of dilute hydrochloric acid was added to the hydrolyzing mixture. Preparation of spectral standards has been reported ( 7 ) . All standards xvere carefully distilled, heart-cut material that had been stored in wax sealed vials n i t h Teflon caps. Purity and decomposition resulting from hydrolysis (Equation 2 ) could be checked by mass spectrometry, mass 31 (alcohol) and niass 147 (hexamethyldisiloxane) peaks n ere at niiiiimuni intensities.
+
2(CH3)23iOR HzO + (CH3)3SiOSi(CH3)3 2ROH (2)
+
Treatment with Hexamethyldisilazane. Hesamethyldisilazane has a distinct mass spectral peak, mass 146 (22). Khen used as a reagent in converting alcohols to trimethylsilyl ethers,
tion flask rose from 123" to 133" C. The rrhite precipitate sublimed out of the reaction flask into the condenser, leaving a clear solution. Completion of the reaction was indicated by a constant flask reflux temperature in the thermometer well. A more sensitive indication of reaction termination is the cessation of decomposition of indicating Drierite (crumbling, blue to pink color change) in the drying tube when no more ammonia is evolved. Presence of excess reagent is assured if no further ammonia evolves on addition of a fen- drops of hexamethyldisilazane a t completion of the reaction. Khere reflux time must be shortened, reaction can be speeded by initial addition of more trimethylchlorosilane. M a s s Spectra. RIass spectra were obtained on a Consolidated Model 21-103 mass spectrometer. (Consolidated Electrodynamics Corp., Pasadena, Calif.). Samples were introduced by a constant volume pipet through a mercury orifice system. Hexamethyldisiloxane was used as a reference standard (12). VOL. 30, NO. 8, AUGUST 1958
1353
DISCUSSION
groups; its valence of 4 means that tn-o addit,ional alkyl groups can be held when it is included in a hydrocarbon chain. This results in an additional shift of 2 mass units. Synthetic Alcohol Mixtures. To utilize t'rimethylsilyl ethers to determine alcohols, it is necessary to use a reagent that' quantitatively converts the alcohols t o their trimethylsilyl derivatives. The choice of hexaniethyldisilazane as a convcrting reagent was based on experience n-ith this reagent in preparing t'riinethylsilyl ether standards ( 7 ); the losses w r r from routinr handling and slight liydrol! methylsilyl ethrrs, \ h e n they n-rre not carefully protectcxl from :itniosplieric moisture (Equation 2 ) . The strong 146 mass spectral r,c:tk for lics:imetliyldisilazane allon-s detc~rminationof excess reagent and thereby a corrrction for contributions of tlir rr:igc>nt to lun-er mass peaks. h nunibcr of 1iydroc:irlic~ii-alcoliol mixtures w r e p r r p a r d :ind analyzrtl by mass spectrometry aft,('r trcatmcJnt with liesariiethyldisilazane. .A tj-pica1 result' for a mixture of hcpt:rnr, octane. pentanul, and heptanol is shown in mixture 1: Table 11. Successful determination of the hydrocarbons and alcohols as trimethylsilyl ethers indicatrs qu:inti-
Mass Spectra. Data for a large number of trimethylsilyl derivatives have been presented in detail and discussed ( I d ) . The intensities of the parent minus 15 peaks for some aliphatic trimethylsilyl ethers are shou-n in Table I with the strong mass 75 rearrangement peak. Parent minus 15 intensities begiii to drop off sharply only for tlie trimethylsilyl ethers of CIoto C l l alcohols. Thus, analyis of alcohols as trimethylsilyl ethers extends the usable range of the conventional, room temperature mass spectrometer. Ordinarily, usable mass spectra for Clo and Cli alcohols cannot be obtained in the room temperature instrument because of limited volatility and, or sorption in tlie system. The substitution of the trimethyld o x y group for the alcoholic hydroxyl group causes a shift of important mass spectral peaks into higher mass ranges n here interferences are less likely to occur. It is the inclusion of both oxygen and silicon in the ion fragments n hich leads to tlie distinctive m a s s spectral peaks 4 mass units above the principal ones obtained from alkanes. Oxygen, mass 16, is equivalent to a methylene group plus 2 mass units. Silicon. mass 28, is equivalent to tn-o methylene
tative conversion of the alcohols t o trimethylsilyl ethers. Results obtained from the t,rimet'hylsilylether analysis are reported in terms of the original alcohol composition by using standard molecular conversion factors, k n o ~ nalcohol densities, and the trimethylsilyl ether densities previously reported ( 7 ) . The use of hexamethyldi~ilazaiein the mass spectrometric analysis of alcohol mixtures having a wide boiling range is illustrated by mixtures I1 and 111, Table IT. Blending of mixturrs of t'riniethylsilyl ethers in the mass spectrometer system with oxygenate-hydrocarbon mixtures from the Fisclicr-Tropsch process dion.ed no interference from hydrocarhon ion fragmentation peaks; thus, trimethylsilyl ethers could lir determined in tlie prrsence of these addrd hydrocarbons and oxygenates. Water. As water reacts with hexamethyldisilazane t o form hesaniethyldisilosane (Equat'ion l ) , it does not interfere Ivith the conversion of the alcohols t o their triniethplsilyl ethers. Sufficient hexamethyldisilazane must lie used to rract 11-iththe v-ater, n-hich has a relatively low equivalent m i g h t . Because of the distinctive 137 mass spectral prak (12 ) hexamethyldisiloxane can be uiird for determination of water. Analysis of an alcohol-water blend,
Table I. Mass Spectra of Trimethylsilyl Ethers, Parent Minus 15 Intensities
m/e
R Group Methyl Ethyl n-Propyl n-Butyl n-Pentyl n-Hexyl 2-RIe-l-pent yl
89 1230'1b 2.7 5 3 114 192 179 54.2 326 170 97.5 54.7 33.5 4.2
-
103 1140 __
117
199 217 282 281 525 274 277 164 103 60.4
5.5 957 2.5
-
2.8 9.6 8.1 5.5 4.0
131
4.4 947 1.9 2.2 2.4 1.0 15.1 2.7 1.7 1.7 2.9
I45
I59
1354
Dev.
30.0
29.6
-0.4
25.0
24.8
-0.2
25.0 20.0
201
215
4.4 2.6 3.5 1.3 1.2 1.3 2.9
4.4 1130 550 423 4.4 0.5 0.6 3.1
~
4.4 2.0 1.2 1090 __ 3.2
2.8 686 1.0
26.3 19.3
ANALYTICAL CHEMISTRY
+1.3 -0.7
I1 Knorrn Found 34.0 30.0 20.0 12.0 4.0
229
-
438
-
265
Determination of Alcohols and W a t e r in Mixtures as Trimethylsilyl Ethers, Weight
I Known Found Ethanol 1-Propanol 1-Butanol 1-Pentanol 1-Hexanol 1-Heptanol 1-Octanol 1-Sonanol 1-Decanol 1-Undecanol Water Heptane Octane
187
5.0 1060 -
3-Me-1-pentyl n-Heptyl n-Octyl 4.0 n-Sonyl 1.7 n-Decyl 1.0 n-Undecyl 8.8 2.5 Hydrogen a n-Heptane mass 27 = 492 divisions per unit liquid volume (0,00068 cc.). (Parent mass minus 15) peaks are underscored. Sensitivity for trimethj-lsilnnol. Table II.
173
35.8 29.3 19.5 11.5 3.r
Dev. +l.,S -0.r -0.5 -0.3 -0.3
I11 Known Found
30.0 23.7 23.5 8.1 8.1
31.3 22.5 23.6 7.8
6.6
7.4
T.4
Dev.
+1.3 -1.2
43.9 __
ia
r-
19.6 750 1040 1000 1080 1050 897 536 984 569 352 212 47.2 2402c
70
IT' Known Found 30.7 27.0 18.0 10.8 3.6
30.6 26.1 18.1 10.G 3.3
Dev. -0.1 -0.9 +0.1 -0.2 -0.3
9.9
11.3
+l.4
+0.1
-0.3 -0.7 +0.8
where hexamethyldisiloxane is the basis for determining water, is shown in mixture IV, Table 11. The reaction between hexaiiiethyldisilazane and rrater is more coniplicated than indicated by Equation 1. Hoivever, it can be discussed in terms of Equations 3 and 4, where trimethylsilaiiol (IT) is an intermediate.
+
(CH,)rSiSHSi( CH3)3 2H20 2(CH3)&3iOH (IV) 2(CH3)aSiOH-, (CH3)$iOSi(CH3)3
-
+ NH3
(3)
+ H20
(4)
Table IV.
Mass Spectral Determination
Alcohols Cz C3 15 5 4 Tr.
Total illcohol,
as Trimethylsilyl Ethers, irol. %
T'ol. ';b
C1 Cj c 6 C7 C8 CQ 11.8. I.R. 3 8 2 8 10 0 5 0 4 15 4 14 6 4 66b 0-100" C.cut 24 40 100-150" C. cut 2 8 302 342 116 0 3 79 1 70 3* 150-200° c. cut 0 4 4 3 5 0 2 i 0 4 128 l5gb a .Ippro\iination from functional group determination; based o n molecular weight of Fraction Ran-ggnsoline,0-200" C.
70.
b Approximation from functional group determination; avei age molecular mated from mass spectrometer data.
syiithetir gasoline fraction known t o The n a t e r from Equation 4 can react contain predominantly primary straightwith more hexamethyldisilazane, so that chain alcohols is shown in Table IV. it is eventually converted to hcxamethInfrared functional group analysis was yldisiloxane and ammonia. When ~ a - in excellent agreement v i t h total alcoter is initially present a t concentrations hol content determined by mass specapproaching 257& a considerable trometry. amount of hexamethyldisiloxane is Data on alcohol distribution in several formed. Then, trimethylsilanol apcuts froni the distillation of a Fischerpears moderately stable, and longer peTropsch product are sh0n.n in Table IV. riods of reflux are necessary for comAlcohol distribution is as expected if the plete reaction. The condensation recomplexity of the Fischer-Tropsch prodaction is complex, but both the mechanuct and the many possibilities for azeoism and kinetics have been studied untrope forniation are considered. It is der special conditions by Grubb ( 6 ) . difficult to decide on an average molecuReaction probably could be speeded lar neight for the alcohols in a cut withby generating greater eoncentrations of out prior knon ledge of alcohol distribuhydrochloric acid by using more trition. Kithout these data, total alcohol niethylchlorosilane initially. However, by infrared analysis can only be estithe water determination need not be demated. Using these data, the infrared ferred until complete condensation of and mass spectronietric determinations trimethj-lsilanol. An alcohol-n-ater mixof Table IT' are in good agreement exture can be analyzed by taking into cept for the 100" to 150" C. cut. account the contributions of the triAnalysis of a product-water phase methylsilyl ethers and hexaniethj-ldisilohtained from a Fischer-Tropsch liquid oxane to the mass 7 5 peak and considerproduct is phown in Table V. Methyl, ing the residual portion of this peak to ethyl, and propyl alcohol give unique result from triniethylsilanol. Intenmass spectral peaks, but interfering sity of this peak is given in Table I. fragmentation from hydrocarbons and The analyses of a propanol-water oxygenated compounds allows only inistiire, after 6 and 20 hours of reflux, limits to be placed on the concentraare s h o w in Table 111. I n the first tions of higher alcohols and other oxyanalysis about 10% of the trimethylsilgenates. Using the trimethylsilyl ethers, anol formed had not reacted further, and the volume per cent unknown is resuitable correction for this was made duced from 22.2 to 9.370. Ethanol is from the intensity of the 7 5 peak. The used as an internal standard to combine second analysis was based on the presdata on the higher alcohols obtained ence of hexamethyldisiloxane and trifrom the triniethylsilyl derivatives with inethylqilyl propyl ether only. data on other components from the direct, mass spectrometric analysis. Constancy of the ethanol-propanol ratio in Table 111. Mass Spectrometric Analthe two analyses is evidence for the ysis of Propanol-Water Mixture as validity of this procedure. DeterminaTrimethylsilyl Derivative, Weight tions froni triniethylsilyl ether derivatives fall nithin the limits set by direct Found after Refluxing analysis. Data from Tables IV and V Iinown 6 hours 20 hours illustrate the detection of alcohol concentrations of less than 1%. 1-Propanol 52.2 54.1 53.4 Branching. K h e n mixtures conII-ater 47.8 45.9 46.6 taining large amounts of branched and secondary alcohols are t o be analyzed, Fischer-Tropsch Products. Three alcohol determination beconies more applications of t h e trimethylsilyl ether complex (5, 16). It would be necessary conversion technique are illustrated to fractionate the samples into relain Tables I V and V. T h e determitively narrow cuts t o use the trimethylnation of total alcohols a n d alcohol silyl ethers as a basis for mass spectrodistribution from Cz t o C8 in a raw metric analysis. For example, a C6 cut
yo
of Alcohols in Fischer-Tropsch Products
Table V.
TI
eight esti-
Analysis of Aqueous Sohtion of Alcohols
T'olume % -1s trimethylsilyl .Is alcohol ether 24 1 (24 l ) a , b
H?O ~4lcohols
C5
5 . 4 inax. ~2 mas.
c6 c 7
5.0
2.0 0.5
0.1 Acetone 3 . 3 inax. ( 3 . 3 max.) 1Iethyl ethyl 1 . 2 max. ( 1 . 2 max.) ketone Unknown 22.2d 4.8-9.3 5 H 2 0 based on (CH,),Si20 is 23.5%. * Parentheses indicate data retained froin direct analysis. c Internal standard. d Unknown based on minimum Cq value, niasimum value not included. C8
might be distilled t o contain only C5ant1 Cg or Cg and C, alcohols. Branched components in a Fischer-Tropsch product could be 2- and 3-methylpentanol. Matrices based cn the follomkg observations niight serve as a basis for determining alcohols in the presence of hydrocarbons. Parent minus 15 peaks for the nornial triniethylsilyl ethers are 145, 159, and 173. Intensities for the methyl-branched ethers are lower than the intensities for the normal ethers. Mass 103 for the 2-methylpentyl trimethylsilyl ether is more intense than for other components of the mixture under consideration. ;\lass 89 shons increased intensity for the 3-inethylpentyl trimethylsilyl ether. Ratio of 73 to 7 5 intensity is different for normal and branched-chain alcohol derivatives (12). Probably, the presence of secondary alcohols in narrow boiling fractions can be determined from cleavage a t the functional carbon atom (12). Also, identification of secondary alcohols might be possible from direct. mass spectrometry (6). Analysis of a synthetic C5,Cg alcohol cut containing an olefin and a naphthene VOL. 30,
NO. 8,
AUGUST 1958
1355
Table VI.
Mass Spectral Determination of Straight-Chain and Branched Alcohols in Presence of Hydrocarbons
I 1-Pentanol 2-Methylpentanol 3-Methylpentanol 1-Hexanol Octene Methylcyclohexane
Known 25.3 8.0
Found 26.0 8.0
Dev. +0.7
28.5 12.9 25.3
29.3 11.7 25.0
+0.8 -1.2 -0.3
is shown in Table VI. Determination of the alcohols is based on simultaneous equations utilizing intensities of 145, 103, and 159 peaks. Octene determination is based on the 112 (parent) peak and methylcyclohexane, on the mass 98 (parent) peak. I n mixture 11, Table VI, the determination of 2- and 3-methylpentyl and hexyl alcohols is based on simultaneous equations utilizing intensities of mass 89, 103, and 159. OTHER APPLICATIONS
The lower methyl ketones, especiallv acetone, undergo a variety of acid- and base-catalyzed condensation and addition reactions under the conditions of conversion of alcohols to trimethylsilyl ethers. Caution must be exercised in applying the technique described to solutions containing large concentrations of these ketones. Because trimethylsilyl derivatives give characteristic peaks in the mass spectrometer, a number of other applications are suggested. The mass spectra of the trimethylsilyl derivatives of phenol and resorcinol have been obtained (12). An analyti-
0.0
Known
I1 Found
Dev.
12.5 14.3 40.2 13.2 19.8
11.6 14.2 39.5 14.6 20.1
-0.9 -0.1 -0.7 $1.4 +O. 3
cal mass spectral procedure utilizing the trimethylsilyl derivatives of the phenols may be practical. Glycol mixtures may also be amenable to analysis as trimethylsilyl derivatives (12), As primary amines react with hexamethyldisilazane to give trimethylsilyl derivatives (7) with intense parent minus 15 peaks (8),these derivatives could be a basis for the determination of primary amines. The trimethylsilyl derivatives of mercaptans also give intense parent minus 15 peaks (12). The problem is conversion, because hexamethyldisilazane does not react readily with mercaptans ( 7 ) . Hexamethyldisilazane is an interesting possibility as a reagent for the analysis of heavy water for deuterium and oxygen-18. Ammonia formed in the course of reaction between the hexamethyldisilazane and water could be analyzed by mass spectrometry to determine deuterium. The hexamethyldisiloxane formed could be used for the oxygen-18 determination. ACKNOWLEDGMENT
The authors wish to express their ap-
preciation to Janet Shultz for helpful discussion and for the calculations associated with the determinations. LITERATURE CITED
Patent '2,629,728 (Fe6. 24, 1953). Cain, D. G Keitkamp, A. R., Bowman, 5. J., Ind. Eng. Chem. 45, 359 (1953). Friedel, R. A., Shultz, J. L., Sharkey, A. G.. Jr.. ANAL.CHEW 28. 926 (1956f. ' Grubb, W. T., J . Am. Chem. SOC. 76, 3408 (1954). Langer, S. H., Connell, S., Wender, I., J . Org. Chem. 23, 50 (1958). Langer, S. H., Friedel, R. A. Wender, Irving, Sharkey, A. &., Jr., unpublished results. Morrell, C. E., Carlson, C. S., MCAteer, J. H., Robey, R. F., Smith, P. V., Jr., Ind. Eng. Chem. 44, 2839 (1952). Sauer, R. O., J . Am. Chem. SOC.66, 1707 (1944). Schlesinger, bI. D., Benson, H. E., Murphy, E. RI., Storch, H. H., Ind. Eng. Chem. 46, 1322 (1954). Sharkey, A. G., Jr., Friedel, R. A,, Langer, S. H., AYAL. CHERI.29, 770 (1957). Sprung, 11. M., Nelson, L. S., J. Org. Chem. 20, 1750 (1955). Steitz. Alfred. Jr.. Barnes. D. K..' Ind: Eng. Chem. 45, 353 (1953). Tarborough, V. A,, A r . 4 ~ . CHEY. 25, 1914 (1953).
-
RECEIVED for review December 13, 1957. Accepted April 30, 1958. Presented in part at Symposium on Synthetic Fuels, 128th Meeting, ACS, Minneapolis, Minn., September 1955.
X-Ray Diffraction Study of n-Alkyl Malonic Acids BRAHAMA D. SHARMA' and A. 6. BISWAS National Chemical laboratory, Poona 8, India
A homologous series of n-alkyl malonic acids, from methyl to octadecyl, has been synthesized. Tables of x-ray powder diffraction data for the acids are presented in a form applicable for analytical purposes. The results are also discussed with reference to those of the n-fatty acids and their derivatives. Difficulties that may arise in the way of identification of these compounds from their long spacings are indicated.
S
( 4 , 6 )have pointed out the utility of x-ray powder
EVERAL WORKERS
1356
0
ANALYTICAL CHEMISTRY
diffraction data for the identification of compounds possessing crystalline structure. The vast majority of such published data has been limited to inorganic compounds and minerals. Organic compounds, as a rule, have more complex patterns, as they form crystals of low symmetry and often show polymorphism, thus causing difficulty in straightforward identification. I n a study of the structure and properties of soaps and soaplike molecules, the authors synthesized a homologous series of n-alkyl malonic acids. This report presents x-ray powder diffraction
data as a means of identification of these compounds, and includes a discussion of the results with reference to n-fatty acids and some of their derivatives. PREPARATION
OF
n-ALKYL MALONIC ACIDS
The acids n-ere synthesized from the normal primary alcohols according to the following scheme: 1 Present address, Chemistry Department, University of Southern California, Los Angeles 7 , Calif.
