FUEL
AND
OIL QUALITY
MASS-SPECTROMETRIC,
EVALUATION
ANALYSIS
METHODS
OF S Y N T H E T I C
ALKYL
BENZENES UDC 543.42 : 668.58
A. A. Polyakova, I. M . L u k a s h e n k o , R. A . K h m e l ' n i t s k i i , E. 8. B r o d s k i i , a n d K. I . Z i m i n a
The present paper gives the results of a mass-spectrometric study of alkyl benzenes which enabled us to chara c t e r i z e the group hydrocarbon composition of the alkylation products and the isomeric composition of the alkyl benzenes and establish the m o l e c u l a r - w e i g h t distribution of the hydrocarbons. The work was performed in a n M K h 1303 mass spectrometer with ionizing potential 50 V and a c c e l e r a t i n g p o t e n t i a l 2,000 V. T h e optimum temperature of the ion source and the admission system (275~ ensured a relative measurement reproducibility of 1.8%. Study of the mass spectra of a l k y l benzenes [1] showed that they have very intense m o l e c u l a r - i o n peaks, the values of which reach 6-10% of the total ion current, and peaks of "pseudomolecular" ions with a mass equal to that of the lower homologs of the alkyl benzenes. The isomeric m o n o - and dialkyl benzenes have very similar mass spectra. The s m a l l differences between them are nonsystematic and do not permit us to identify the isomers [1, 2]. The differences between the p r o b a b i l i ties of decomposition and rearrangement of the isomers increase with the length of the side chains. As a result of detachment of both alkyd groups from the m o l e c u l a r ion of d i a l k y l benzene at the 8-bonds, formation of ions with mass numbers 104, 106, and 106 is possible.
,'(
H
I~
I
9 --
9
H
H
H
H
I
H
i
,1,
I /7-%
H
I.
H--C--r + \>--C
I H
~ I #-~
H
I
H--C-- + \>-C--H
cH, m/e 104
role 105
m / e t06
Formation of an ion with mass 105 is a c c o m p a n i e d by migration of one hydrogen atom, that of an ion with mass 106 by migration of two hydrogen atoms. Of these two ions the one with mass 105 is the most intense because its formation leads to A e n e r g e t i c a l l y favorable ionic structures (a s e v e n - m e m b e r e d tropyl ring and a neutral f r a g m e n t - a n olefin). In the case of the para or ortho isomers we can assume that, ions with mass 104 have the energ e t i c a l l y favorable quinoid structure, which is absent in the case of the m e t a isomers: in the mass spectrum of p - d i - n - d e c y l b e n z e n e the IG / peak corresponding to the ion with mass 104 is approximately twice as large as in the spectrum of the m e t a isomer [1].
A
A l k y l benzenes with branched alkyl chains form basically a n a logous mass spectra. 8
8
10 12 14' 1~ 18 20
Fig. 1. Graph of the m o l e c u l a r - w e i g h t distribution of a l k y l benzenes (fraction with bp 260-360"C).
These characteristics of the mass spectra of a l k y l benzenes formed the basis for new methods of analyzing mixtures of these c o m pounds. From the peaks of the m o l e c u l a r ions we determined the m o l e c u l a r weights of the corresponding a l k y l benzenes. For these calculations we used the mono-isotopic peak values, allowing for
VNII NP. Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 9, pp. 57-61, September, 1967.
674
TABLE 1. Molecular-Weight Distribution of Alkyl Benzenes of the 260-360~ Fraction Distribution of mole cular weights, ~ J DisereNo. o f h y - Mol. droearbon ~ n ~ - - ~ - ~ [Pa ncy' atoms in the wt. f r ~ ..... overlapping ]abs % molecule alsmou-, b t~e etn " tlon curv O~ it~ [~1] "~ 13 14 15 16 17 18 19 20 21
176 190 204 218 232 246 260 274 286
2,3 2,7 7,8 17,9 20,3 29,6 8,7 8,2 8,5
0,2 0,1 0,5 0,2 0,6 0,7 0,3 0,3 0,1
2,1 2,6 8,3 18,1 20,9 28,9 9,0 7,7 2,{t
TABLE 2. Composition of the Overall Characteristics Peaks Composition of the overallpeaks arid their symbols
Groups and their symbols
alkyl benzene I alkyl benzene,II Monoalkyl benzenes (x) (with exception of 2-alkyl benzenes) 2 -Alkyl benzenes (y) Dialkyl benzenes (z)
superposition of "pseudomolecular ions." W e have put forward a graphical method of calculation, based on an appraisal of the shape of the uncorrected distribution curve. The figure shows the intensities of the mono-isotopic peaks of ions with the general formula CnH2n_6 in the mass spectra in the products of benzene alkylation by propylene polymers, with boiling ranges 260-360~ The envelope curve is distinctly bimodal and consists of two sectors, separated by a minimum: the right-hand sector reflects the true molecular-weight distribution of the alkyl benzenes, while the left-hand sector corresponds to the peaks of the ions which undergo rearrangement [3]. For the molecular ion peaks the relation between the scaling factors and the molecular weight is almost exponential [2] and for relatively high mass numbers the mean relative difference between them is 5-7% [4]. Therefore in the analysis of specimens with a narrow boiling range the molecular weight distribution of alkyl benzenes can be established from the intensities of the mono-isotopic peaks, without allowing for the scaling factors.
91, 92 (E 91)
91,92, 119, 120 (X91) I05,106,(E 105) 105, 106,(E 105)
I19, 120,133, 134 (E 119)
133, 134, 147, 148, 161, 162, 175,176, (E 133)
Note. Alkyl benzenes I were obtained by alkylating benzene with the cracking products of paraffins. Alkyl benzenes II were obtained by alkylating benzene with propylene polymers.
TABLE 3. Inverse Matrix for Determining the Isomeric Composition of Alkyl Benzenes Alk 1 benzene,, I
A lkyl benzene iii
Our method is much simpler than making an allowance for overlapping of the spectra of the higher homologs on these of the lower homologs by the use of correction factors [4, 5] and is of particular value in the absence of mass spectra of the individual compounds. Table 1 gives the molecular-weight distribution of the alkyl benzenes, calculated by the method in [4] and by normalization of the intensities of the mono-isotopic peaks, limited by the right-hand sector of the curve (the alkyl benzenes were obtained by alkylating benzene with propylene polymers).
I
Z91
x
Y Z
1,3 --0,1 --0,7
~I 105
--0, i, 0
I
I
~ 1!9
14 I _010,20" 3,0
' E 105 II
5" 91
1,18--0,35
Z 133
--1,7
--0,05
1,5 [ --0,2
--0, I
0
2,6
TABLE 4. Coefficients of Alkyl Benzene Superposition on the Analytical Peaks of Dialkyl Benzenes Mass numbers of A l k y l benzene I the analytical x y peaks 104 106
0,07 0,00
0,035 0,035
, Alkyl benzene II_ x
y
0,009 0,00
0,007 0,004
The mean discrepancy between the values is 2.7 tel. %. Our method was used for the analysis of mixtures of alkyl benzenes obtained by alkylating benzene with unsaturated compounds from the fractions obtained from thermal cracking of paraffin [6]. The different probabilities of formation of fragment ions in the mass spectra of the alkyl benzene isomers were used for determining the following groups of alkyl benzene isomers: 1) monoalkyl benzenes, with the exception of 2-alkyl benzenes (2-phenylalkanes); 2)2-alkylbenzenes; and 3) dialkyl benzenes (for which we determined the content of the recta isomer and para + ortho isomers).
For the above-listed groups of isomers in the mass spectra we found the peaks, the overall intensity of which characterized each group (Table 2).
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TABLE 5. Results of Determination of the Alkyl Benzene Composition (in wt. %) Alky.1 benzene fraction Alkyl benzenes
ibp -340 oc
340-360 ~
88,3 (89,2)
83,2 (85,2)
45,0 (45,6)
43,0
39,2
33,3
22,0
49,1
49,9
2,8 (2,7) 8,9 (8,1)
5,7
A lkyl benzene fraction
400--420oC
~
32
320--340
oC 380oC
Overall content of monoalkyl benzenes
including: monoalkylbenGenes with the exception of 2-alRyl benGenes 2-alkyl benGenes 1,4-dialkyl benzenes 1,3-dialkyl benGenes
(3,1)
11,1 (11,7)
36,3
(41,0)
58,8 )
61,1 (65,0)
41,0 (40,0)
36,2 (36,0)
(37,0)
167,
20,3
16,9
02,2
38,2
21,0
19,4
23,0
22,7
19,4
26,6
24,9
20,0
16 ,B
17,0
19,0 (20,0) 38,0
21,0 (25,0) 42,7
20,6 (18,C 20,6 (17,C
15,9 (15,0) 21,0 (20,0)
(29,0) 31,0
(16,0) 38,0 (38,4)
(39,0)
(38,0)
28,0
(31,0)
28,0
(32,0) 35,8 (32, O)
Note: The figures in parentheses are the results of the analyses of specimen from the infrared spectra. TABLE 6. Analytic Formula for the Relations between the Characteristic Intensities and Molecular Weight Characteristic sums Hydrocarbongroups
z
71
E
Paraffinic
Naphthenic t= --0,125nq-0,0518 +monoole - r finic 15Gn~<19 20~n<~24 Alkyl benGenes
(1) (2) (1) (2)
67
Z
77
=--0,0099n~+0,0054n q- l=O,O0084n~q-O,2542n +0,0695 (1) --0,2174 =--9,156nq-0,1624 (2) 15~n~24 15~n y=O,OO32nz--O,O126n ffq-1,7676 (1) t=0,0154nq-0,2141 (2) 15~
E
103
y==--0,0015n+0,0081 y=--O,0043n--0,0084 15~n~18 (1) 19~n <24 (2) y~---0,00165n-}-0,0409 9=--0,002n--0,0057 15~
(1) (2)
y~--O,O7315n y~--O,OOO2n--O,1152 15~n~18 19~nG24 y~--0,00035nq-0,3269 y = - 0,00044n+0,3364 15<~n. 18 19 -2Ln.<24
(1) (2) (1) (2) (1) (2) (1) (2)
(1) (2) (1) (2)
Note: n is the number of carbon atoms in the molecule; y is the intensity of the characteristic sums. The coefficient allowing for the mutual overlapping of the characteristic sums were determined from the mass spectra of the individual alkyl benzenes [1]. Table 3 gives the inverse matrix for calculating the isomeric composition of the alkyl benzenes. The relative content of each alkyl benzene group is determined by normalization of values obtained for the characteristic sums. To determine the isomeric composition of dialkyl benzenes (the ortho + para isomers, and the meta isomers), we used as analytical peaks the intensities of mono-isotopic peaks with masses 104 and 106; the values had to be corrected for superposition of the monoalkyl benzenes. Table 4 gives the superposition coefficients calculated from the mass spectra. The peak intensities of ions with masses 104 and 106, pertaining to the proportion of the para and meta isomers, are determined from the equations:
676
TABLE 7. Inverse Matrix for Alkyl Benzenes with an Average of 17 Carbon Atoms in the Molecule Hydrocarbon Paraffinic Naphthenes + unsam rated hydrocarbons Alkyl benzenes Alkenyl benzenes
z 71
I[
s 67
s 77
s 103
Paraffinic Naphthenic + unsatu rated Alkyl benzenes A lkenyl benzenes
U ----3.6 '11o~--'0.8" 11o6; V = --0.5.11o 4 + 2.1.11o6;
(1)
(2)
+1,2336 --0,0988 --0,0279 --0,0144 --0,15971--1,0099 --0,0099 --0,0136 --0,0011 0 0 +0,2929 --0,0732 --0,0326 +0,3328 +0,00011 olo
TABLE 8. Group Composition of the Products of Benzene Alkylation by Unsaturated Compounds (in tool. wt. %) Type of compound
U = 10.8.1104--9.2 .I106; V=- - - 1.5 .I10~ + 16.5 'II0d
Empirical formula
Boiling range of the fractions 260--360 ~
280--340~
CnH2n+•
4,0
1,2
CnH2n CnH2n-2
0,5
CnH2n-6 C~H2~-s
90,2 5,3
93,3 5,5
where U and V are the peak intensities of ions with masses 104 and 106, pertaining to the para and meta isomer fractions, respectively; I104 and I106 are these peak intensities, corrected for isotopic overlapping and monoalkyl benzene overlapping. By normalizing the U and V values obtained, we determined the relative contents of the corresponding isomers. Table 5 gives the analyses of various fractions of the products of the alkylation of benzene by olefins obtained by thermal cracking of paraffins, and by propylene polymers, from the mass spectra and the infrared specta. The discrepancy between the two methods is less than 7 rel. % for the determination of the overallcontent of mono- or dialkyl benzenes, and 15 rel. % for the determination of the individual isomers.
In the second stage of the study of synthetic alkyl benzenes we determined the groups of hydrocarbons present with the alkyl benzenes in the reaction mixture. Each hydrocarbon group has corresponding chr racteristics ions with the following mass numbers: CnHzn-6-77, 78, 79, 91, 92, 108, 106, 119, 120 . . . . (277); CnHzn-8-103, 104, 117, 118, 131, 132 . . . . (s CnHan+a-71, 88, 113, (s CnHzn-z and CnHzn-67, 68, 69, 81, 82, 88, 98, 96 (r.67). The overall intensity of the peaks of the characteristics ions in the mass spectrum is a measure of the concentration and a function of the molecular weight [2]. By mathematical processing of these relationships, we obtained equations which ensured calculation of the matrix coefficients for determining the group composition of the products of benzene alkylation by unsaturated compounds. Table 6 gives equations expressing the relationship between the characteristics intensities and n, the number of carbon atoms in the molecule; the data on the mean molecular weight are obtained from the distribution of the numbers of hydrocarbon atoms in the molecules of the alkyl benzenes. The inverse matrix, compiled for studying specimens with boiling ranges 260-360 and 280-340~ enabled us to characterize the hydrocarbon composition of the benzene alkylation products (Table 8).
(Table 7),
SUMMARY 1. It has been established that as a result of rearrangement during dissociative ionization, the distribution of the molecular ion peaks in the mass spectra of mixtures of alkyl benzenes has a bimodal character, which enables one to distinguish the peaks of the "pseudomolecular" ions from those of the true molecular ions. 2. A method is proposed for determining the molecular weights of alkyl benzenes in mixtures by normalization of the intensities of the mono-isotopic peaks of molecular ions. 3. A method has been developed for the analysis of the group hydrocarbon composition of alkyl benzenes obtained by alkylation with unsaturated hydrocarbons from the cracking products of paraffin or from propylene polymerization. 4. A method has been developed for deten-nining the isomeric composition of these alkyl benzenes, permitting determination of the content of the following groups of isomers: 1) monoalkyl benzenes (with the exception of 2-alkyl benzenes; 2) 2-alkyl benzenes; 3) dialkyl benzenes, including n-dialkyl benzenes.
677
LITERATURE CITED 1.
2. 3. 4.
5. 6.
Catalog of Mass Spectral Data, API Research Project 44, Pittsburg (1952). A. A. Polyakova, R.A. Khmel'nitskii, and A.A. Petrov, Uspekhi Khimii, No. 9 (1966). A.A. Polyakova and R. A. Khmel'nitskii, Introduction of the Mass Spectrometry of Organic Compounds [in Russian], Izd. "Khimiya" (1966). R. A. Brown, Anal. Chem., v. 31, No. 9 (1959). R. W. Boyer, Anal. Chem., v. 35, No. 9 (1968). E. K. Ivanova, L. A. Potolovskii, and A. I. Doladugin, Proc. VNII NP, No. 9, Neftekhimiya, Gostoptekhizdat
(1963), p. 241.
678