Determination of Hydrocarbon Functional Groups by Infrared Spectroscopy S. H. HASTINGS, A. T. WATSON, R. B. WILLIAIIIS, AND J. A. AKDERSON, J R .
Humble Oil and Rejining Co., Baytown, Tex. The use of individual component analysis as a means of characterizing hydrocarbon mixtures is not feasible in the higher boiling ranges because of the almost astronomical number of isomeric molecules. Consequently, it becomes necessary to investigate other means of characterizing these mixtures. One such method involves the determination of the functional groups, such as methyl and methylene, making up the hydrocarbon molecules. The infrared absorptivities of a number of pure paraffin, naphthene, and aromatic hydrocarbons have been meas-
T
11-0factors have contributed to the rapid advance in recent years of the compositional studies of complex hydrocarbon mixtures. First, the development of the newer physical instruments such as the infrared and mass spectrometers has resulted in an increased ability to identify and quantitatively measure the individual hydrocarbons present in fractions obtained from the distillation of petroleum products. Second, the preparation in a state of high purity of a large number of the hydrocarbons present in petroleum through the work of the American Petroleuni Institute has provided industrial and university laboratories with the compounds necessary to calibrate these new instruments accurately. However, the problem rapidly becomes too complex for individual component analysis, as is evident a hen one considers the almost exponential increase in the number of isomers with molecular weight. Not only does it become more difficult to prepare all the various isomers in the higher molecular weight ranges, but the rapid increase in the number of compounds in a given narrow-boiling fraction results in an inability to find sufficient unique spectral characteristics for accurate analysis. One method for the analysis of higher molecular weight mixtures involves the use of concepts of characteristic infrared absorption frequencies and functional group absorptivities to extrapolate available calibration data on known compounds to higher molecular weights. Characteristic infrared bands have been the subject of much investigation (I, 2, 4 , 7 ) and it is well established that vibrations involving the same functional group (such as 0-H, C=O, etc.) give rise to infrared absorption a t characteristic wave lengths. Kearest neighbors influence the position markedly in some cases, but further structural differences do not, in general, result in further shifts. For example, whereas the position of the C=O stretching vibrations appears a t different wavelengths for aldehydes, ketones, esters, and acids, this position does not vary much within each series (1). The concept of functional group absorptivities is based upon the assumption that the intensity of absorption by a given functional group is independent of the remainder of the molecule-Le., the absorbance can be related to the concentration of the absorbing group, the remainder of the molecule acting only as a diluent. The validity of this assumption has been demonstrated by Rose ( 7 ) and Hibbard (6) for the absorption of CHI, CHQ, and CH groups in paraffin, naphthene, and aromatic hydrocarbons in the 1.1- to 1.25-micron region and by Anderson (1)for the absorption by olefins in the 10- to 15-micron region, alcohols a t 3.0 microns, and oxygenated compounds in the 5.5- to 6.0-micron region. The present paper is concerned with studies conducted by the authors and directed toward a definition of the characteristic
ured under carefully controlled conditions in the 3.0- to 3.5-, 6.5- to 7.5-, and 12.5- to 14.3-micron regions. In general, these absorptivities are related to the concentrations of the various functional groups giving rise to the absorption-e.g., methyl and methylene groups in paraffins at 3.38 and 3.42 microns. This type of hydrocarbon characterization should find increasing application, especially as more complex hydrocarbons become available in a pure state, thus providing the means for more accurate calibration with a minimum of extrapolation.
infrared bands of hydrocarbon functional groups (CH,, CH,, CH) in paraffins, naphthenes, and aromatics, the determinatica of the functional group absorptivities of these groups, and finally application of the findings to analyses. Particular emphasis has been placed upon studies in the 3.0- to 3.5-, 6.5- to 7.5-, and 12.5- to 14.3-micron regions where the characteristic CH stretching, HCH bending, and CH2wagging vibrations, respectively, are located. IhSTRU\IENTS 4 V D M 4 T E R I 4 L S
Xeasurements in the 3.0- to 3.5- and 6.5- to 7.5-micron regions were made on a Model 12-B Perkin-Elmer infrared spectrometer equipped s i t h a calcium fluoride prism for high dispersion, and measurements in the 12.5- to 14.3-micron region were made on a Model 12-B Perkin-Elmer infrared spectrometer equipped with a sodium chloride prism. Both instruments employed General Electric breaker-type amplifiers. M o s t of the hydrocarbons employed in this investigation were Kational Bureau of Standards certified standards having purities of 99+ mole %. A few hydrocarbons of lower purit were utilized where high purity standards were not available and the data were deemed necessary. Baker’s C.P. carbon tetrachloride was used to prepare the necessary dilute blends. PROCEDURE
Blends of the various hydrocarbons in carbon tetrachloride ere prepared by diluting a weighed portion of the hydrocarbon to a known volume. Xeasurements in the 3.0- to 3.5- and 6.5- to 7.5-micron regions \$ere made in a 1-mm. rock salt cell and consequently concentrations of about 1 and 14 grams per liter were required for the respective regions in order to have peak absorbances in the range of from 0.2 to 0.6. PO or reference intensity measurements were made with the same cell filled with carbon tetrachloride. All measurements were made a t the point of maximum absorption in the 3.0- to 3.5-micron region rather than a t a selected characteristic wave length. Where absorption due to a minor concentration of one of the functional groups resulted in an ill-defined peak or shoulder, the weak band was located by using a value of 0.035 micron as defining th- average separation of peaks in this region. Measurements in the 12.5- to 14.3-micron region were made on the undiluted hydrocarbon in a 0.1-mm. rock salt cell. Pa measurements in this region were made by filling the cell with methylcyclohexane (purified by silica gel percolation for the removal of toluene). Methylcyclohexane has essentially no absorption in this region and this technique results in an automatic correction for general background absorption and for changes in the apparent cell density due to fogging of the vindows, etc. DISCUSSION
Absorption in the 3.0- to 3.5micron region of the infrared spectra of hydrocarbon mixtures is due to carbon-hydrogen stretching vibrations. Fox and Martin 3.0- to 3.5-Micron Region.
612
V O L U M E 24, NO. 4, A P R I L 1 9 5 2
613
( 3 )have demonstrated through extensive studies that absorption a t 3.38 and 3.42 microns in paraffin hydrocarbons is due t o unsymmetrical C-H stretching in methyl (CHI) and methylene (CH,) groups, and that absorption a t about 3.48 and 3.50 microns is due to the respective symmetrical vibrations. Use of these latter two bands is not attractive for a number of reasons: They are not so well resolved as the 3.38- and 3.42-micron bands; they are only about half as intense; and C-H stretching in paraffinic CH groups contributes t o the absorption a t 3.48 microns. As a result of these observations, no effort was made in this study to utilize these hands.
0.30
.-.-.
2.3-Dimethylbutane n-Hexane
i . 3 I I
0.20
VI ( Y r
c_
-I
g 1 I-
n. I*
0
0.10
tic hydrocarbons, when a methyl group is attached directly to the ring the position of the absorption maximum shifts from 3.38 to 3.42 microns and the observed intensity is considerably less than that which is encountered in the case of paraffin and naphthene hydrocarbons. If the methyl group is removed from the aromat,ic ring by one or more methylene groups, the absorption due to that methyl group assumes the character of. paraffinic methyl groups, in that absorption occurs a t 3.38 microns and a normal absorptivity is obtained. This is illustrated by the spectra of toluene and ethylbenzene shown in Figure 3 . Because of the anomalous behavior of methyl groups on aromatic nuclei and hecause aromatic hydrocarbons can be removed from admixture with saturat.ed hydrocarbons by silica gel percolation, the problems of characterization of aromat,ic-naphthene-paraffin mixtures by infrared spectroscopy can best be handled in tn-o partscharacterization of paraffin-naphthene mixtures and characterization of aromatic concentrates. In order to calculate the absorptivities of methyl and methylene groups in paraffins a t 3.38 and 3.42 microns, use must be made of the Beer-Lambert law : :I = abc \\-here A = log PoIP a = absorptivity b = path length c = concentration To calculate functional group absorptivities c must be expressed as the number of absorbing funct,ional groups per liter of solution, N , or its equivalent. 0.40
C
3.30
3.35
5.40
3.45
3.50
3.55
3
WAVE LENGTH, MICRONS
Figure 1. Absorption Spectra of Paraffins in 3-Rlicron Region
Figure 1 shows the absorption in this region of a normal paraffin and an isoparaffin. In the case of the normal paraffin, absorption at 3.42 and 3.50 microns is very much pronounced, whereas in the case of the isoparaffin, absorption a t 3.38 and 3.48 microns is the most pronounced. -4 study of the absorption spectra of 39 pure paraffin hydrocarbons in this region confirmed that absorption a t 3.37 to 3.38 microns can be attributed to the methyl group, whereas absorption at 3.41 to 3.42 microns is characteristic of methylene groups. Studies of the absorption characteristics of 38 pure naphthene hydrocarbons in this region have shoir-n that whereas the functional groups making up cyclohexane homologs absorb a t the same wave lengths as similar groups in paraffin hydrocarbons, such is not the case for cyclopentane homologs. Absorption due to the CHt in the C5ring is not a t 3.42 microns as would be expected, but is actually a t 3.38 microns, the characteristic wave length for CH, groups. This shift in the case of the cyclopentane homologs is undoubtedly due to the presence of strain in the C5 ring and a consequent redistribution of energy which results in changed force constants for the C H bond in cyclopentane and its derivatives. To illustrate the marked effect of this strain, Figure 2 shows the absorption spectra in this region of methylcyclohexane and methylcyclopentane. Whereas the 3.42-micron band is prominent in the case of methylcyclohexane, the major absorption in methylcyclopentane is a t 3.38 microns. This shifting of the absorption frequency in the case of cyclopentane derivatives is of considerable advantage and enables one to make an estimate of the relative proportions of cyclopentane and cyclohexane rings in naphthene-paraffin mixtures. In the case of absorption due to paraffinic side chains on aroma-
0.30
i
QI
dP
b
E
0.20
9
6 2 E
L*
0
9:
010
0.
>
3.35
3.40
3.45
3.50
3.55
3.60
WAVE LENGTH, MICRONS
Figure 2. Absorption Spectra of Naphthenes in 3Micron Region
Let
w W M p
= = = =
weight of functional group in 1 mole of compound, grams functional weight of functional group, grams molecular weight of compound, grams density of compound, grama per ml. 1000 w x---~ ,I1
w = -
Then
N
but
_W -- weight fraction of functional group = F
=
ANALYTICAL CHEMISTRY
614
therefore N
=
w
a‘ = u / W , the unit absorptivity
S o n let
Then
1000 F p IY
1000 abFp
A =
and
~
a‘
=
A lOOOpbF
but 1 0 0 0 ~= concentration of compound iii grams per liter
=
c’
Theiefore a’ = A/c’bF.
0.08
0.07
i . 2 . I I
0.06
example, in the spectrum of 2,3-dimethylbutane shown in Figure 1 it is apparent that a small absorption occurs a t 3.42 microns, although this molecule contains no CH2 groups. Whereas it is possible that this absorption is due to CH groups, it 1%as assumed in the calculations that CHI and CH, absorb at both 3.38 and 3.42 microns. Application of these calculations to the observed data for forty pure paraffins showed that the assumption of constant functional group absorptivities is not borne out. This will be seen from the data in Table I, where the coefficients Q, R, S, and T of the above equations are tabulated for normal and mono-, di-, and trisubstituted paraffins. The group absorptivities n ere determined by the “method of averages” described by Margenau and hlurphj ( 6 ) ,since 80 equations were available to determine only 4 unknowns. The data were divided into 4 groups of 20 equations each for the determination of the “average” group absorptivities shown in Table I. The group absorptivities for the normal paraffin, etc., \yere calculated in a like manner. Use of the average data shov n in this table gives reasonable results when applied to fairly complex mixtures. The errors encountered when applying the average data to the individual paraffins employed for calibration are shown in Table 11. The primary purpose in developing these correlations was for the analpis of hydrocarbon mixtures
0.05
(3
v)
u Y
Table I.
c -1 0.04
RIethyl and Methylene ihsorptivities in Paraffins Wave Length, Microns 3.38 3.42 Functional Group CHP CHo CH2 CHs Q R S T Absorptiriti , L i t e d G r a m Alm. 0.702 -0,002 0.427 -0.098 0.510 0.096 0.320 0.061 0.430 0.189 0.271 0.089 0.109 0.385 0.270 0.245 0.440 0.120 0.345 0 071
i
I-
5
I =
0.03
0
In
Paraffin Tyire n-Paraffins hIonoeubstituted paraffins Disubstituted paraffins Trisubstituted paraffins .4v.
03 2
0.08
I
0
WAVE LENGTH, MICRONS
0.3
Figure 3. Absorption Spectra of Aromatics in 3-Micron Region
0.8
Assuming that both functional groups absorb a t both wave lengths and that the functional group absorptivities are constant, one can determine these absorptivities by solving simultaneous equations of the following type:
0.7
5 (3 y
At 3 . 3 8 ~
cA- = QFcH~ RFLH,
+
2
At 3 . 4 2 ~
A _ c,b
-- SFCH,+ T F c H ~
a
where Q and T are the absorptivities of methyl groups, R and S are the absorptivities of methylene groups, and F C Hand ~ FCHZ are the weight fractions of methyl and methylene groups in the compound to which the equation applies. Because these absorption bands are not fully resolved (as illustrated in Figure l ) , it was necessary to estimate the amount of overlap by means of the band width curve shown in Figure 4. This curve was constructed from one side of the absorption mauimum of each of the compounds shown, and in using this curve for calculating the contribution of one maximum to the other it wis assumed that the bands nere symmetrical. This technique corrects only for peak overlap due to incomplete resolution of the tn-o bands and does not take into account the possibility of absorption by either CH3 or CH2 a t both vave lengths. For
k
X 0
2 , 3-Dxmethylbutane cye1opentane Cyclohexane
0.6
0.5
Z
9
0.4
6
2
0.3
0.2
~~
2
3
4
5
6
7
8
9
IO
/I
SEPARATION FROM PEAK, DRUM UNITS
Figure 4. Band Width Curve for Saturated Hydrocarbons at 3 Microns
V O L U M E 2 4 , NO. 4, A P R I L 1 9 5 2
615
wherein it is expected that a very large number of coiiipounds will be found; hence, the averaging effects should tend to increase the accuracy of the method. When one is looking at a mixture of only a very few isomers, it is likely that serious errors will be encountered unless appropriate absorptivities are used based on some knowledge of the sample.
0.3
i . I I
Q er
In order to deterniine the functional group absorptivity of methylene groups in the Cs ring, the contributions of the methgl . and paraffinic methylene groups were calculated from the absorptivities already determined for the paraffins. The residual absorptivities for the cyclopentane homologs were then plotted against weight per cent CH2 in the Cb ring. This correlation is shown in Figure 5 . The absorptivities of the methylene gioup in the Cbring a t 3.42 micions and the methylene group in the C 6 ring at 3.38 microns were calculated in a similar manner. In order to determine the absorptivity of the methylene group in C6 ring naphthenes at 3.42 microns, the absorptivities of the various cyclohexane honiologs a t this wave length were corrected for contributions froin the methyl and methylene groups in the paraffinic side chains and the residual absorptivities m-ere then plotted against the neight per cent CH, in the Cs ring as shown in Figure 6.
p 02
LL v)
-e
0.5
2
>: 5
i
c
$.
F e n.
0
0.4
Q er
0.1
B
3 0.3 W c-I / 0
20
0
40
60
BO
I
WEIGHT % CHz IN Cs RING
Figure 5. Absorption Due to RIethylene in Cyclopentane Rings at 3.38 Microns
/
I 20
Weight % CHa Present Calcd. 69 8 65.7 69.8 68.6 52 3 61.1 52 3 55.8 34.9 37.8 75 0 60.4 60.0 63.0 60 0 63.8 60 0 66.6 60 0 59,9 45 0 57.0 45 0 47.9 45 0 52.7 30 0 27.4 65 9 54.4 65.9 65.7 65 9 60.6 65.9 57.5 52.6 49.8 52.6 57.7 52.6 56.1 52 6 57.0 52.6 60.2 52.6 58.7
6.1
K e i g h t R CHz Pre.ent Calcd. 16 3 132 0 0 9.8 32.6 25.5 32.6 28.9 65.1 62.5 0.0 11.1 28.0 23 4 14.0 16.6 14.0 17.5 28 0 16.6 42.0 29.0 42.0 44.3 42.0 35.4 (0.0 72.3 12.3 20.9 12.3 8.3 12 3 14.8 0 0 16.6 36.8 37.8 24.5 24.3 24.5 21.5 24.5 30.6 36.8 20 9 24.5 20.6
A
4.1 1.2
8.8 3.5 2.9 14.6 3.0 3 8 6.6 0.1 12.0 2.9
7.1 2.6 11 5 0.2 5 3 8 4 2 8 5.1
3 5 4.4 7.6
57.3
4.7
24.5
24.3
0.2
52.6
60.6 50.0 38.7 40.7 49.1 20.5 55.3 56.3 59.7 63.8 52.7 59.4 10.3 0 0 -8.2
8.0 10.5 0.8
36.8
17.5 34.8 55,4 49.2 34..5
19.3 14.3 6.3 0.1 14.6 6.9
1.2
9.6 5.8
3 3 2.3 1.1 5.2
5.9 0.8 10.8 17.6 21.5 x5.9
49.1
49.1 49.1 49 1
i3.7 21.9 21.9 21.9 10.9 21.9 21 9
78.9 82.4 86.7
80.6
17.5 29.8 22.2
16.6 18.8 19.1 90.2
100.3 102.2
80
A
3 1 9 8 7.1 3.7 2.6 11.1 4.6 2.6 3.5 11.4 13.0 2.3 6 6 2 3 8.6 4.0 2 5 16.6 1 0 0 2 3.0 5.9 15 9 3.9
52.6 39.5 39.5 39.5 39.5 26.3 58 6 58 6 58 6 58 6 58 6 58 6 21.1 17 6 13 3
60
IN Ca RINGS Figure 6. .4bsorption Due to Methylene in Cyclohexane Rings at 3.42 Microns
Table 11. Application of Functional Group Analysis to Calibration Compounds Compound 2,P-Dimethylbutane 2.3-Dimethylbutane 2-11 ethylpentane 3-1Iethylpentane n-Hexane 2,2,3-Trimethylbutane 2,Z-Dimethylpentane 2,3-Dimethylpentane 2,4-Dirnethylpentane 3,3-Diniethylpentane 3-Ethylpentane 2-Methyihexane 3-Methylhexane n-Heptane 2,2,3-Trimethylpentanr 2,2,4-Trimethylpentane 2,3,3-Trimethylpentane 2.3,4-Trimethylpentane 2.2-Dimethylhexane 2,3-Dimethylhexane 2,4-Dimethylhexane 2,5-Dimethylhexane 3,3-Dimethylhexane 3.4-Dimethylhexane 2-Methvl-3-ethvlpentane 3-Methyl-3-ethylpentane 3-Ethylhexane 2-Nethylheptane 3-1Iethylheptane 4-Methylheptane n-Octane 2,2.4-Trirnethylhcxane 2,2,5-Trirnethylheuane 2,3,3-Trimet hyl hexane 2,3,5-Trimethylhexane 2,4,4-Trimethylhexane 3 3.4-Trimethylhexane n-Decane n-Dodecane T i - Hexadecane
40
WEIGHT % CHI
4.4
7.9 0 3 5.7 3.1 2.8 11.3 17.9 15.5 16.9
The studies on aroniatic hydrocarbons have indicated that methyl groups attached to the ring, and methylene groups attached or not attached to the ring, absorb at about 3.42 micions. Methyl groups which are not attached to the ring absorb rlose to the characteristic methyl wave length of 3.38 microns. Evaluation of the functional group absorptivities employing a limited number of calibration conipounds has given the results shown in Table 111. Although only a relatively small number of conipounds was used (14), the results are felt to be representative, because the dispeision of the data was satisfactorily IOTT.
TableJII. 3Iethyl and Methjlene Absorptivities in Aromatics Functional group coefficients CHs attached t o ring CHa not attached t o ring
CHz
Wave Length, hlicrons
Absorptivity. L/G. 1 l m .
3.42 3.38 3 42
0 151 i 0 014 0.380 z!= 0.044 0 . 2 9 3 t O 012
3.38 3 42 3.38
0.082 f O . O 1 O 0.000 0.136 1 0 . 0 3 1
6.5- to 7.5-Micron Region. Absorption of paraffins and naphthenes in the 6.5- to 7.0-micron region is due to methyl and methylene groups. Although characteristic regions of absorption in this interval by the various functional groups in paraffins, Cj ring naphthenes, and Cg ring naphthenes have been observed,
ANALYTICAL CHEMISTRY
616 Table IV.
Average Absorptivities in 7.2- to 7.5-Micron Region for Some Paraffins and Naphthenes Wt. %
CH,
Compound 5.11 n-Octacosane 5.75 n-Octadecane n-Hexadecane 5 92 5.94 n-Pentadecane 6.51 n-Dodecane n-Decane 6.28 7.05 n-Octane 9.05 n-Heptane 7.74 n-Hexane 12.91 4-Jfethylheptane 15.18 3-hlethylheptane 13.82 2-Methylheptane 11.89 3-Ethylhexane 14.06 3-Methylhexane 15.07 2-Methylhexane 11.67 3-Ethylpentane 14.55 3-Methylpentane 15.40 2-Methylpentane 16.08 2-XIet hyl-3-ethylpentane 17.91 9 3-Dimethylhexane 17.14 5'2-Dimethylhexane 16.65 2'5-Dimethylhexane 18.12 2'4-Dimethylhexane 15 57 3:Methyl-3-ethylpentane 17.14 3,4-Dimethylhexane 15.71 3,3-Dimethylhexane 2 0 . 09 3 3 4-Trimethylhexane 21.2: 2'4'4-Trimethylhexane 20 85 2:2:4-Trimethylhexane 2,2,5-Trimethylhexane 20.74 2 0 . 12 2,3,3-Trimethylhexane 24.12 2,3,5-Trimethylhexane 19,54 ?,2-Dimethylpentane 19.05 2,3-Dimethylpentane 20.48 2.4-Dimethylpentane 17.64 3,3-Dimethylpentane 24,60 2 3.4-Trimethylpentane 21.86 .>'2,3-Trimethylpentane 25.48 ~:2.4-Trimethylpentane 23.11 2,3,3-Trimethylpentane 21.09 2.2-Dimethylbutane 22.97 2,3-Dimethylhutane 27.50 ?,2,3-Trimethylbutane .4v. Av. deviation a Liter/g. mm., from 7.14 to 7.15 microns. b K' = av. K/wt. Fr. CHs. c Compounds higher in molecular weight t h a n d Hydrogenated durene.
7.6 11 8 13.3 14.2 17 6 21.1 26.3 30.0 34.9 39.6 39.5 39.5 39.5 43.0 45.0 45.0 52.3 52.3 52.6 52.6 52.6 52.6 52.6 52.6 52.6 52.6 58.6 58.6 58.6 58.6 58.6 58.6 60.0 60.0 60.0 60.0
65.8 65.8 65.8 65.8 69.8 69.8 75.0
AV.
K'
x
103b
67.1 48.7 44 5 41.8 37.0 29.8 26.8 30.2 22.2 32.7 38.4 35.0 30.1 31.2 33.5 25.9 27.8 29.5 30.6 34.1 32.6 31.7 34.5 29.6 32.6 29.9 34.3 36.3 35.6 35.4 34.3 41.9 32.6 31.8 34.1 29.4 37.4 33.2 38.7 35.1 30.2 32.9 36.7 32. 9c 19.3
AV.
Av. % deviation
Av. K' X 10'b
ioaa
0.0 11.9 13.4 15.3 17.9 23.8
46.6 36.4 37.2 38.2 41.9
Cyclopentane n-Butylcyclopentane n-Propylcyclopentane Ethylcyclopentane Methylcyclopentane Isohutylcyclopentane trans-1-Methyl-2-ethylcyclopentane cis-1-Methyl-2-ethylcyclopentane Isopropylcyclopentane 1-Methyl-I-ethylcyclopentane cis-1,3-Dimethylcyclopentsne cis-1,2-Diniethylcyclopentane trans-1.2-DimethyEcyclopentane trans-1,3-Dimethylcyclopentane I! 1-Dimgthylcyclopemtane c~s-cis-czs-1,2.3-Trimethvlcyclopentane cis-cis-trans-1,2,4-Trimethylcyclopentane cis-cis-trans-1,2,3-Trimethylcyclopentane 1,1,2-Trimethylcyclopentane cis-trans-cts-I, 2,3-Trimethylcyclopentane cis-trans-cis-1,2,4-Trimethylcyclopentane 1,1,3-Trimethylcyclopentane Cyclohexane n-Butylcyclohexane n-Propylcyclohexane Ethylcyclohexane Methylcyclohexane sec-Butylcyclohexane Isohutvlcvclohexane Isopropylbyclohexane trans-1 3-Dimethylcyclohaxane eis-1,3-bimethylcyclohexane !rans-1,2-Dimethylcyclohexane trans-I, 4-Dimethylcyclohexane cis-1,4-Dimethylcyclohexane 1,l-Dimethylcyclohexane tert-Butylcyclohexane 1,1,3-Trimethylcyclohexane 1.2,4,5-Tetramethylcyclohexaned AV.
K x
Naphthenes 1.63 5.54 4.88 5.69 6 83 9.98
....
8 40
26.8
31.3
10.40 10.43
26.8 26.8
38.8 38.9
8.35 11.24 10.76
26.8 30.6 30.6
31.2 36.7 35.2
8.24
30.6
26.9
9.72 9.70
30.6 30.6
31.8 31.7
12.76
40.2
31.7
13.83
40.2
34.4
13.67 13.98
40.2 40.2
34.0 34.8
11.32
40.2
28.1
12.51 13.70 1.33 5.36 4.63 5.49 6.19 8.87 11.13 11.20 9.75 8.77 8.76 7.68 11.25 9.59 15.08 13.52
40.2 40.2
31.1 34.1
0.0 10.7 11.9 13.4 15.3' 21.4 21.4 23.8 26.8 26.8 26.8 26.8 26.8 26.8 32.1 35.7
50.1 38.9 41.0 40.5 41.5 52.0 47.1 36.4 32.7 32.7 28.7 42.0 35.8 47.0 37.9
15.81
42.9
....
36.9 37.0 111.0
hexadecane not included in average.
shifting of peaks due to structural differences and general overlapping of all regions preclude utilization of these regions for analysis perhaps even with highest resolving power. Therefore, only a limited study was made of the absorption by saturated compounds in this region, since studies in other regions indicated more promise. .4ttenipts to utilize the region in the analysis for functional groups in aromatic hydrocarbons also met with only limited success. Absorption in the 7.1- to 7.5-micron region by paraffins, naphthenes, and aromatics is due to methyl groups only. The absorption maxima of a large number of pure paraffins and naphthenes in this region have been determined; they vary considerably in position. As this region embraces the hydrogen bending in methyl groups only, it was believed that some correlation should exist between the TT ave length of the maximum and the character of the methyl group. For instance, a methyl group attached to a methylene group absorbs consistently a t 7.25 microns and three methyl groups attached to the same carbon atom absorb a t 7.33 and 7.17 microns. However, the usefulness of this characterization is limited for those interested in petroleum mixtures by the fact that the alkyl-substituted naphthenes and aromatics also usually have a multiplicity of peaks in this same region, some of Tvhich coincide with the paraffin peaks. Because, however, absorption in the entire region appears to be due only to methyl groups, it was anticipated that the integrated hydrocarbon absorptivity nould correlate with the weight per cent methyl group. The average hydrocarbon absorptivities (which were obtained fiom absorbance measurements a t 26 equally spaced wave
lengths from 7.14 to 7.45 microns and which are linearly related to the actual integrated absorptivities in this wave-length region) of a number of paraffin and naphthene hydrocarbons are shown in Table IV and plotted in Figure 7. When these average hydrocarbon absorptivities are divided by the appropriate weight fraction of the methyl group, it is found that the methyl group absorptivity is equal to 0.0329 f 0.0031 for the paraffin hydrocarbons and 0.0370 i 0.0041 for the naphthene hydrocarbons. A definite relation between the methyl group absorptivity in normal paraffins and molecular weight is observed from the data in Table IV. This is shown graphically in Figure 8. The same trend is noted to a much less pronounced degree in the case of isoparaffins and it is believed that, in general, use of the average absorptivity (0.0350) will yield satisfactorily accurate results. Measurements on the absorptivity of aromatic hydrocarbons in this region has shown that when the methyl group is attached directly to the ring the methyl group absorptivity is only about half as great as when the methyl group is removed from the ring by one or more carbon atoms (0.0169 versus 0.0339 liters per gram mm.). The absorptivity in the latter case checks with the value found for methyl groups in paraffins. 12.5- to 14.3-Micron Region. Examination of the infrared spectra of a large number of paraffin and naphthene hydrocarbons in the 12.5- to 14.3-micron region has revealed that absorption in the entire region is due primarily to CH, groups in paraffins and paraffin side chains. The wave length of maximum absorption is a function of the number of CHA groups in a continuous chain, as has been known for some time. The following table shone the
617
V O L U M E 2 4 , NO. 4, A P R I L 1 9 5 2 wavelength regions in which continuous chains of CHJ groups of the indicated length absorb: Region of lMaximum Absorption, Microns
No. of CH? Groups in Chain 1
12.7-13.0 13 2-13.5
2
13 13 13 13
3
.! 6
APPLICATION TO ANALYSIS
Saturated Hydrocarbon Systems. With the data obtained and treated as dis'cussed above, it should now be possible to determine methyl groups, methylene groups in paraffins, and methylene groups in Cb and Cg ring naphthenes in complex mixtures contain-
.
74 80 83 85
0.07
3 m
All chains consisting of more than six groups absorb consistently at about 13.86 microns. As in the case of absorption by methyl groups in the 7.1- to 7.5micron region, it was believed that the integrated or average
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