WTs = weight of sample used M W A = molecular weight of acrylate or methacrylate being analyzed for M W I = molecular weight of the respective alkyl iodide
x
Duplicate determinations should agree within i3 relative. Determination of Absorptivity Value. Polystyrene solutions were prepared between 10 and 60 g/l., and their infrared spectra determined from 650 to 750 cm-’. The true absorbance of the 700 cm-’ band was then calculated using Equation 3 (27). A = AT - Ao
where:
(3)
A = true absorbance AT = the total recorded absorbance at the absorption maxima A . = the general background absorption at the absorption maxima
The true absorbance of the 700 cm-l styrene band was then plotted against sample concentration. Using Beer’s law ( A = a h ) , an absorptivity value was readily calculated. The
absorptivity values determined in acetone and tetrahydrofuran are given in Table 111. Determination of Styrene Content. By knowing the absorbance, cell path length and absorptivity value, the concentration (g/l.) of styrene in a solution was calculated using Equation 4 : (4)
c = Ala6
where:
c =
A
=
6
=
a =
styrene concentration (g/l.) in the solution true absorbance of 700 cm-’ band cell path length (cm) absorptivity (L/g cm)
The concentration of styrene in the polymer was calculated using Equation 5 :
% Styrene where:
=
C -
W
(100)
(5)
C = styrene concentration (g/l.) in solution W = sample concentration in solution (g/l.)
RECEIVED for review November 17, 1970. Accepted March 1, 1971.
Identification of Alkanes by Pyrolysis Gas Chromatography R. A. Brown Analytical & Information Dioision, ESSOResearch & Engineering Co., Linden, N . J. 07036 In a recent study, Cramers published compositional data for thermal degradation of alkanes; individual monoolefins were measured. In a simplified representation of the Rice theory ( I ) , the monoolefins originate primarily from simple cleavage of the molecule. From this concept, correlations and working rules were found to predict alkane structures accurately. Most of the C6-Cs alkanes were identified exactly and, in all cases, the principal skeletal structure was indicated. Normal alkanes are characterized by 1-olefins. These results indicate that thermal degradation can be valuable in the determination o f ’ molecular structure. Studies of other classes of organic compounds would further clarify this picture.
There is reason to believe that pyrograms should be as useful as mass spectra in providing insight to molecular structure. Indeed, one may well complement the other, as is usually the case with apparently competitive techniques. Pyrolysis causes a molecule to dissociate into a limited number of fragments which stabilize as other compounds. These can be identified and measured. Thus, all pieces of the molecule become an integral part of the molecular data which constitute the pyrogram. In the mass spectrometer, electron bombardment occurs and the molecule behaves as below :
KEULEMANS AND PERRY (2) first suggested that the basic principles of mass spectrometry apply to pyrolysis gas chromatography. Fragmentation occurs in both techniques and then the fragments are separated by a magnetic field or gas chromatograph. It is well known that mass spectra are extremely valuable in the elucidation of structure. However, only limited progress has been made in using pyrolysis fragments, called pyrograms, for structural interpretation. This is pointed out in an excellent review of pyrolysis chromatography by Levy (3). H e states that identification is restricted mainly to the comparison of the pyrogram of the sample under test with those of known materials.
Only the ion, A+, is measured and the existence of B goes undetected. Therefore, some evidence of molecular structure is simply lost. Multiple cleavage can also occur in the mass spectrometer. This means many more fragments for the mass spectrum than for the pyrogram. The simplicity of the latter should be advantageous. A survey of literature shows that some progress has occurred in relating thermal degradation products to molecular structure, The objective of this study is to extend the understanding of thermnl degradation. In 1964 Dhont showed that each compound in a group of CrCs alcohols thermally decomposed to one or two principal olefins (4). A study by Levy and Paul (5) of n-alkanes, a-olefins, alcohols, mercaptans, and esters showed that pyrograms were a function of molecular structure. Thermal degra-
(1) F. 0. Rice, “Free Radical (Collected Papers of F. 0. Rice),”
The Catholic University Press, Washington, D. C., 1958. (2) A. I. M. Keulemans and S . G. Perry, “Gas Chromatography,” N. van Swaay, Ed., Butterworth, Inc., Washington, D. C . , 1962, p 356. (3) R. L. Levy, Chromatogr. Rec., 8 , 48 (1966). 900
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
A f B+A+ + B + e
(4) J. H. Dhont, Analyst, 89, 71 (1964). ( 5 ) E. J. Levy and D. G. Paul, J. Gas Chromatogr., 5 , 136 (1967).
Table I. Experimental Conditions Gold reactor temperature Reaction time Sample size
Cl-C7 compounds. The packed column was 2 wt Apiezon L o n Gas Chrom S, 100-200 mesh. It was 2 meters long and had a 4 mm inside diameter. A n-octadecene-1 coated capillary column was employed; it was 30 m in length, with a n inside diameter of 0.25 mm. Analysis was carried out at 25 "C.
for Pyrolysis 500 "C 9 , 5 seconds 0 . 1 p1 liquid
RESULTS
dation of hydrocarbons was studied by Fanter et al. (6), Sutton and Harris (7), Cramers and Keulemans (8). Principal objectives of these efforts were to study the reaction kinetics and pyrolyzing conditions. Detailed compositional data were obtained on products but only the latter investigators reported compositions in terms of specific compounds and their concentration. They concluded that for alkanes and olefins, the decomposition pattern is closely related to the carbon skeleton and double bond position of the parent molecule. Subsequently, Cramers (9) provided complete compositional data for the thermal degradation of all the c&S alkanes and n-alkanes over the range, C5-CI6. These data provide the basis of the study reported upon here. The study demonstrates that monoolefins from thermal degradation are characteristic of the starting alkane structure. This behavior can be used to predict accurately the structure of the original alkane. For the alkanes, then, pyrolysis gas chromatography has advanced beyond the capability of identification by comparison only. Further, it would appear that additional studies such as done by Cramers and Keulemans will result in advanced capability to identify unknowns for other classes of organic compounds.
Cramers studied the thermal degradation of normal and branched alkanes. These included eight n-paraffins covering the range C5-Clc and all of the twenty-nine isomers of the CS-Cs branched chain alkanes. Products of thermal cracking were analyzed by gas chromatography as described earlier. Thermal cracking as carried out by Cramers produced relatively large concentrations of C1-C3 alkanes and minor, if any, amounts of butanes and pentanes. The major constituents are monoolefins. In fact, fifty-five different olefins are reported among the products of the various alkanes. Any given product was usually comprised of only a moderate number of compounds, however. Among the monoolefins, there are eleven I-olefins which are almost exclusive to the thermal decomposition of n-alkanes. In addition, there are five cis-trans pairs which are grouped in our study. For the branched alkanes, then, there are data for fifty-five minus sixteen, or thirty-nine different olefins. The thoroughness of this work enabled a check to be made on almost every feasible olefin formed by the degradation of the branched alkanes. Two olefins, isobutene and 1-butene, are grouped. This detracted somewhat from the conclusions of this study as isobutene, in particular, appears to relate to specific structure(s). Data in Tables I1 to V summarize the composition of the thermally cracked alkanes. The saturates, methane-pentanes, are not reported as their occurrence does not appear to relate with structure. Also ignored are very small amounts of diolefin as found in some cases. Olefins are presented as mole of total product. Cramers shows total moles of individual product and their sum as formed by one hundred moles of starting compound. Mole % was therefore calculated by simply normalizing to 100%. Table I1 tabulates data from n-alkanes, Table 111 covers monosubstituted alkanes, whereas Tables IV and V report on the di- and tri-substituted isomers. Hydrocarbons in Table IV contain no gem substitution, whereas those of Table V have this feature in common. Compounds were grouped in this manner because of similarities in composition of product.
EXPERIMENTAL
Many details of the laboratory measurement are described (9). Specific to the data used in this study, Cramers states that pure hydrocarbons (API purity >99.6 %) were studied under the conditions listed in Table I. Samples were introduced to the reactor with a microsyringe. After pyrolysis, the product was analyzed by packed column for compounds of boiling point greater than 100 "C and by capillary for the (6) D. L. Fanter, J. Q. Walker, and C . J. Wolf, ANAL.CHEM., 40, 2168 (1968). (7) R. Sutton and W. E. Harris, Can. J . Clzem., 45, 2913 (1967). (8) C . A. M. G. Cramers and A . I. M. Keulemans, J. Gas Clzromatogr., 5, 38 (1967). (9) C. A. M. G. Cramers, Thesis, Technological University, Eind-
hoven, The Netherlands.
Table 11. Thermal Degradation of Cs-C1~Normal Paraffins
Mole n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane n-Undecane n-Hexadecane
17.6 29.0 28.6 26.5 30.2 25.4 28.0 26.0
30.0 23.8 16.7 16.4 16.4 13.8 13.9 12.3
10.4 12.3 11.6 8.9 7.6 7.6 7.3 6.9
0.4 3.3 6.4 6.9 6.5 5.7 5.8 5.1
0.1 2.4 6.3 5.5 5.5 6.2 6.0
0.0 1.6 4.3 3.9 3.1 3.5
0.0 0.2 3.2 2.4 3.2
0.0 0.8 2.0 2.8
0.5 2.7
2.2
1.9
1.7
1.4
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
0.4
901
Table 111. Thermal Degradation of C6-C8 Monosubstituted Paraffins
Other olefins 2-Methylpentane
9.6
28,5
17.7
2-Methylhexane
16.5
23.4
2-Methylheptane
14.8
19.0
2.3
15.6 15.6 21.6
3-Ethylpentane 3-Ethylhexaiie
14.0
12.8
6.2
4-Methylheptane
14.6
38.4
5.8
10.3
a
18 8
n,r.
5.0
3-Methylpentane 3-Methylhexane 3-Methylheptane
26 5 10.7
3-Methyl-1-butene = 3.1 % 4-Methyl-2-pentene = 1 , 3% 3,3-Dimethyl-l-butene = 0 . 4 % 4-Methyl-2-pentene = 0 , 1% 4-Methyl-1-pentene = 0 . 4 % 3-Methyl-1-butene = 3.7% 1-Hexene = O , l % 3-Methyl-1-butene = 3 , 7 % 4-Methyl-1-pentene = 0.5z 5-Methyl-1-hexene = 0 . 6 %
4.7
15.6 5.7 4.7
12.7
3.3
7.8
3.6
2.1
1.1 2.2
9.1
0.1 25.0
8.3 6.2
8.5
7.7
4-Methyl-1-pentene = 0 . 8 % 1-Hexene = 0.4% 3-Methyl-2-pentene = 0.1 % 4-Methyl-1-pzntene = 1.4% 3-Methyl-1-hexene = 2.5 % 4-Methyl-1-hexene = 0 . 4 z 3,3-Dimethyl-l-pentene = 0 . 1% 1-Heptene = 0.3 % 3-Methyl-2-hexene = 1 , 2 z 3-Methyl-1-hexene = 0 . 7 z
C Not predicted by simple cleavage. n.r. Not reported.
DISCUSSION
Normal Alkanes. Table I1 shows that C5-CI6 n-alkanes crack to give 1-olefins; n o branched olefins are formed. As mentioned earlier, the measurement grouped 1-butene and isobutene and it is assumed that 1-butene only is formed. Ethene and propene are present in highest concentrations. In general, olefins are observed up to one carbon less than the molecular formula of the n-paraffin. Exception to this is that 1-nonene is not reported for n-decane, whereas n-pentane shows some 1-pentene, and n-hexane shows a small amount of 1-hexene. Branched Alkanes. Most workers believe that the modified Rice theory (1) best explains the product distribution observed in thermal degradation. It has been demonstrated a number of times that products predicted by theory are in close agreement with the observed ones. According to Rice, the initial reaction is abstraction of a hydrogen atom to form a free radical. Abstraction occurs most readily for a tertiary hydrogen, then secondary, and third, primary hydrogens. A hydrogen shift then takes place caused by isomerization, and after isomerization dissociation p to the free radical site occurs. When more than one 0 bond exists, the dissociation is favored by a more stable free radical, such as a tertiary. Applied to C6-Cs iso-alkanes, the theory explains many of the observed olefins. The net effect of the theory is to predict that fragmentation is favored to be a t bonds that are CY and p to the point of branching. In gem structures, y cleavage occurs rather than p. Furthermore, if it is assumed that each cleavage produces two monoolefins, the predicted olefins are in good agreement with observed ones. For C6-Csisoalkanes, then, dissociation of a molecule can, in most cases, be simply 902
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
represented as breaking into two fragments that lose hydrogen atoms to become monoolefins. The position of the double bond is favored t o occur at a methylene in preference to a methyl carbon. Dissociation of a molecule by simple cleavage was adapted for use in this work because it does generally agree with theoretical predictions and, at the same time, provides an easily understood visual representation. As a n example, for 3-methylhexane the following reactions may be shown in an empirical manner :
p=c-c C-C-C
propene (26.5 mole %)
f C-C-C
I
C
(1)
Lc-c=c-c
2-butene (5.7 mole %)
p=c C-C-C-C
f C-C
I
Lethene (15.6 mole %)
(2)
c-c-c=c
I
C
C 2-methyl-1-butene (7.8 mole %)
c-c-c-c-c-c w
+ c-G=C-c-c-c
(3)
2-hexene (1.1 mole %)
C Reactions 1-3 illustrate the formation of olefins due t o cleavage of carbon-carbon bonds that are either CY or 6 to the branched carbon. In the first reaction, CY cleavage yields propene and 2-butene, whereas in reaction 2, p cleavage gives ethene and 2-methyl-1-butene, In example 3, a methyl group is one splinter of the molecular cleavage and so only one olefin (2-hexene) is formed. Observed mole % concentrations of
Li
s 0
i
o c ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
903
each olefin are shown in parentheses as they are given in Table 111. A gem disubstituted alkane undergoes cleavage a t a bonds as for the 3-methylhexane. However, dissociation to form olefins cannot easily occur by breaking a p bond, and so cleavage at a y-position appears to be favored. C
I
C-C
C=c-c
~
propene (27.6 mole %)
f C-C-CL
I
structure, C-C-.
(2)
C Absence or a small concentration, 7
2-ethyl-1-butene
>a
2,3-dirnethyl-1butene
>0.9
I
c-c-c-c
I I
I /
c c
c c c-c-c f c-c
2,3-Dimethyl and 2,3,4-trimethyl
2-methyl-1-butene
I
C-C-C=C
I
C
2,3-Dimethyl
C C-C-C=C
c
f
Level of concentration (mole %) 15-22
Olefin isobutene
2-methyl-2-butene
C-C=C-C
I
I 1
c c
> 10
C
C
Gem substitution
C-C
I
f R
I
>22
isobutene
C=C-C
I
C
C
C
C C-C-C
I I
I
f R
C-C==C-C
2-methyl-2-butene and
>5
C-C-C=C
2-methyl- 1-butene
>2
2-methyl-2pentene
>2
2-methyl-lpentene
>3
C
I
C C C-C-C-C
I
C
f R
C-C-C=C-C
I
and
C
C-C-C-C=C
I C C
C
I C-C-C-C-C
I C
f R
C-C-C-C=C-C
I
2,QDimethyl
I
f R
The first important clue is the 2-methyl-l-butene, which according to Table VI indicates a 3-methyl structure. The olefin, 2-methyl-l-pentene, is too low in concentration, 2.1 %, t o be significant. Presence of 2-butene, 2-pentene, and 2-hexene requires a structure such as, C-C-C-C-C-C. This mole-
I
C cule can give all three by dissociating, as shown below:
c-c-c
f
I
c-c-c
--f
2-methyl- 1hexene
I
C
c
C-C-C-C
2-methyl-2hexene
and C-C-C-C-C=C
c
I
c-c=c-c
I
4-methyl-2pentene
C-C-C=C-C
c-c-c-c-c-c L
>o. 5 >O. 5
>2
--t
c-c=c-c-c-c
C In addition, simple cleavage cannot give isobutene/l-butene (1.4zwhich ), is also a requirement. The compound is then identified as 3-methylhexane. Example 2. Olefin composition is: ethene, 13.0%; propene, 19.7 %; isobutene/l-butene, 19.5 %; 2-methyl-l-butene, 2.3; Z-methyl-2-butene, 5.5 %; 2-methyl-l-pentene, 1.1%; 2-methyl-2-pentene, 3.0 %; 3,3-dimethyl-l-pentene, 0.5 %. The presence of the methyl butenes and methylpentenes n
C
L I
c-c
+ c-c-c-c I
--f
c-c=c-c-c
C 906
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
indicates gem substitution groups of C-C-C-R
I
I
C
and
C C-C4-k-R.
Both features can be included in the mole-
I
C C I cule, c-C-C-C-C-C;
y-cleavage will explain the pres-
I
C ence of 3,3-dimethyl-l-pentene,
C
C
I c--c--c-c-c I C
I f c + c-c-c-c=c I
C plained earlier). This indicates a C-C-
I
structure. For a
c C
I I
y-cleavage would give the
C C
I I
olefin, C-C-C=C.
This olefin is absent; therefore, it can
C be presumed that a different gem olefin is formed by y-cleavage. Fragmentation of the compound, C
C
I I c-c-c-c f c c-c-c=c-c I I I C
C
RECEIVED for review December 28, 1970. Accepted March 30, 1971.
C
The alkane is 3,3-dimethylhexane. Monosubstituted alkanes and gem structures can be rather easily identified. Other polysubstituted compounds are more challenging but usually these can be handled also. Example 3 is illustrative of this situation. Example 3. Olefin composition is: propene, 7.1 %; isobutene/l-butene, 66.0%; 3-methyl-2-pentene, 0.5 %; 4,4-dimethyl-2-pentene, 3.3 %; 3-methyl-l-butene, 0.1 %; 3,4-dimethyl-1-pentene, 0.3 %. The dominant olefin is the isobutene/l-butene group but it can be assumed that the olefin is isobutene (1-butene is not found at a concentration >22 % based on an assumption ex-
structure like, C-C-C-C-C-,
The large concentration of 2-methyl-2-butene suggested 2,3or 2,3,4- structures. N o 2,3-dimethyl-l-butene is reported, which would seem t o rule out a 2,3-dimethyl configuration. The presence of 2-butene at 5.8 indicated that the molecular formula would have to explain this olefin. There were two constraints: neither a pendant ethyl nor butyl group was present. These overall requirements could not be met. A subsequent check showed the compound to be 2,3,4-trimethylpentane. Presence of the relatively large amount of 2-butene negated a correct identification.
C
(4,4-dimethyl-2-pentene) explains the only observed gem olefin. This structure meets the constraint that no pendant ethyl group exists (ethene is absent). Small concentrations of other, even unpredicted, olefins can be ignored, as this is a common occurrence. Example 4. Olefin composition is: ethene, 0.6%; propene, 26.1 %; isobutene/l-butene, 0.6%; 2-butene, 5.8%; 3-methyl-l-butene, 3.4%; 2-methyl-2-butene, 28.8 %; plus miscellaneous others of
