Infrared Study of Methylene Group Absorptivities in Polar Straight Chain Aliphatic Compounds Wolfgang Zenker Research Division, South African Coal, Oil and Gas Corporation Limited, P.O. Box I , Sasolburg, O.F.S.,Republic of South Africa
A new experimental method, based on compensation, is introduced which corrects for the mutual overlap of methyl and methylene bands in the asymmetrical C-H stretching region, enabling accurate measurements of methyl and methylene peak intensities. The absorption bands are actually recorded individually using cyclohexane and hexamethyldisilazane as compensating agents. Values for the methylene group absorptivities in homologous series of n-alkanes, n-alcohols, n-aldehydes, and n-carboxylic acids, were obtained without lengthy procedures by simply using the ratio of methylene/methyl peak absorbances. A function relating the experimental data i s derived and discussed. It is shown that the methylene group absorptivity decreases with increasing polarity of the functional group and that the influence of a polar group is still evident twenty methylene groups away from the functional group.
I
2875
3000 WAVENUMBERS(CM-’)
SEVERAL AUTHORS have already stated that polar substituents have an influence on the absorption of neighboring methyl and methylene groups in aliphatic compounds. Pozefsky and Coggeshall ( I ) reported that the frequencies of the methyl absorption bands were shifted to higher wavenumbers by an oxygen atom in a wide variety of compounds. Gotoh and Takenaka (2) found that the C-H stretching frequencies of the methyl group in aliphatic alcohols, fatty acids, and alkyl bromides decreased with increasing length of the carbon chain. The influence of polar groups on the absorption intensities was noted by Francis (3) who reported a decrease in methyl and methylene group absorptivities in esters and ketones. More recently Drushel et a1 ( 4 ) observed a reduction in methyl and methylene absorptivities of primary alcohols in the C-H stretching region. However, no systematic study of the influence of different polar groups on the methyl and methylene group absorptivities in homologous series of aliphatic compounds is recorded in literature. This may be due to the fact that the absorption bands in the C-H stretching region overlap strongly and, hence, an accurate measurement of the methyl and methylene absorption values is practically impossible. Various attempts to correct for this overlap have been either arithmetical, graphical, or purely empirical (4-7). These methods are time consuming or are based on a number of assumptions which sometimes are rather speculative. This paper describes a new approach to the problem of overlapping bands : the overlapping bands are recorded in(1) A. Pozefsky and N. D. Goggeshall, ANAL. CHEM.,23, 1611 (1951). (2) R. Gotoh and T. Takenaka, Bull. Znst. Chem. Res., Kyoto Univ.,39,202 (1961). ( 3 ) S. A. Francis,J. Chem.Phys., 19,942 (1951). (4) H. V. Drushel, W. L. Tenn, Jr., and J. S. Ellerbe, Spectrochim. Acra, 19, 1815 (1963). ( 5 ) S. H. Hastings, A. J. Watson, R. B. Williams, and J. A. Anderson, ANAL.CHEM., 24,612 (1952). (6) D. A. Ramsay, J. Amer. Chem. Soc., 74,72 (1952). (7) R. keiicha and M. Horiik, Collect. Czech. Chem. Commun., 32, 1125 (1967).
Figure 1. Compensating agent hexamethyldisilazane, trace 2, as compared to n-decane, trace 1
dividually by using an experimental compensation method. With this method, a study of the methyl and methylene group absorptivities is based on accurate experimental values. In the present investigation, the compensation method was used to study the influence of polar groups on the methylene group absorptivities in homologous series of n-alcohols, n-aldehydes, and n-carboxylic acids as compared to n-alkanes. EXPERIMENTAL
All recordings were obtained with a Perkin-Elmer Model 221 Spectrophotometer with the following instrument settings: Amplifier gain, set to respond to 1 % deviation in transmittance; scan time, 50 cm-’/min; attenuator speed, 1100; and slit opening, 240-260 p in the recorded region. The abscissa was expanded four times and average absorption readings were obtained by using the cycling device of the instrument whereby the absorption was recorded repeatedly for at least five times. Repeatability was 0.5% transmittance. The strongest absorption bands in the infrared spectrum of aliphatic compounds, namely the asymmetrical C-H stretching vibrations of methyl and methylene groups between 3000 and 2875 cm-l, were measured. Sealed sodium chloride cells of 0.1-mm path-length were used. All substances were recorded in CC1, solution with a concentration of approximately 1 v/v %. The minimum purity of all substances was 98 %. Correction for Overlap. COMPENSATION METHOD. By applying the compensation principle to the problem of overlapping bands, the overlapping methyl and methylene absorption bands in the C-H stretching region were recorded separately. To compensate for the methyl absorption, hexamethyldisilazane was chosen, while cyclohexane was used to compensate for the methylene absorption. The spectra of these two compounds are shown in Figures 1 and 2. The positions of the methylene bands of cyclohexane and n-alkanes of n C H p 3 5 are identical. For this investigation, ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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Figure 2. Compensating agent cyclohexane, trace 3, as compared to n-decane, trace 1
3bu3
L
3000
I WAVENUMBERS~M-’)2875
Figure 4. Separated methylene absorption bands, traces 4, 4a, 4b, of n-heptane, trace 2, for different concentrations of the compensating agent hexamethyldisilazane
2t.i WAVENUMBERS(CM-’)
Figure 3. Overlapping methyl and methylene absorption bands of n-undecane, trace 2. Separated methyl and methylene absorption bands, traces 3 and 4, of n-undecane only the part of the methylene band overlapping the methyl band was important. The difference in shape on the low wavenumber side between the methylene bands of cyclohexane and aliphatic compounds does not influencethe compensation for overlap. The effect of the slightly different positions of the absorption maxima of the methyl bands of the compounds investigated and hexamethyl disilazane is discussed below. PROCEDURE. The base line was recorded with both reference and sample cells filled with CC14(trace 1, Figure 3). The overlapping methyl and methylene bands were recorded with the reference cell filled with CC14 and the sample cell filled with an approximate 1 v/v solution of the sample in CC14 (trace 2, Figure 3). To record only the methyl absorption, the reference cell is filled with a solution of cyclohexane in CCL, the concentration being adjusted to zero absorption at the frequency of the absorption maximum of the methylene band (Figure 3, trace 3), so that there is equal methylene absorption in both the reference and sample beams. To record only the methylene absorption, the reference cell is filled with the compensating agent hexamethyldisilazane of such a concentration that the methyl absorptions in reference and sample beams are identical at the frequency of the absorption maximum (Figure 3, trace 4). 1236
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Figure 5. Shoulder on the separated methyl band, traces 3, 3a, 3b, of n-octanol, trace 2, for different concentrations of the compensating agent cyclohexane APPLICABILITY. Figure 3, trace 4 shows that after compensation, a residual methyl absorption is recorded (shaded region) and that the resolved methylene band is slightly distorted on the high wavenumber side. This is due to the different positions of the methyl absorption maxima of hexamethyldisilazane and the samples. Figure 4 illustrates how the extent of this distortion and the amount of residual methyl absorption varies with the concentration of the compensating agent (traces 4,4a, 4b), but it is important that the intensity of the separated methylene band remains virtually unchanged. Figure 3, trace 3, shows that some residual methylene absorption is recorded, (shaded region). This is due to the different shapes of the methylene bands of cyclohexane and the samples on the low wavenumber side but will obviously not influencethe peak intensity of the resolved methyl band. The frequencies of the absorption maxima of the methylene bands of cyclohexane and the n-alkanes investigated, are identical. The absorption maxima of the methylene absorption bands in n-alcohols, n-aldehydes, and n-carboxylic acids are situated at a slightly higher frequency than the correspond-
Table I. Experimental Absorbance Ratios as Compared to Absorbance Ratios Calculated from ACHJACH~ = 0.52n - k
n 6i-1
(n = ~ c H , )
i=l
n-Alkanes,
n-Alcohols,
ACHZIACH~
ACdACHs
ing bands in n-alkanes. This frequency shift results in a shoulder on the low wavenumber side of the resolved methyl band of the oxygen compounds due to residual methylene absorption (Figure 5 , trace 3). This residual methylene absorption varies with varying concentrations of the compensating agent cyclohexane but the intensity of the resolved methyl band remains virtually unchanged (traces 3a, 3b). Although the frequencies of the methyl and methylene bands are not exactly the same for all compounds investigated, the method is nevertheless generally applicable as the frequency differences do not influence the intensity of the resolved bands. RESULTS AND DISCUSSION
In Beer’s law, A = abc, ( A = absorbance, a = absorptivity, b = pathlength of the cell, c = sample concentration), b and c will be the same for two absorption bands of one substance. The absorbance ratio of the two absorption bands is then equal to the ratio of their absorptivities. The methylene/methyl absorbance ratio is then given by
Using the absorbance ratios, tedious determinations of sample concentration and pathlength of the cell, which may lead to considerable experimental error, are avoided. In Equation 1, the methylene and methyl absorptivities depend only on the number of methylene and methyl groups per molecule. The absorptivity of one methylene and methyl group is the unit absorptivity of that group ( U C H ~ , UCHJ and is constant for unperturbed methylene and methyl groups, therefore &Hi ACH3
-
aCHz
aCEa
-
UCHz &Ha
x x
nCHt &Ha
(2)
In straight chain oxygen compounds where the methylene unit absorptivity varies with the distance of the methylene group from the polar group, the methylene absorptivity is the sum of the methylene unit absorptivities and the absorbance ratio is then given by
n-Carb. acids, ACHJACH~
AcdACHa
Calcd %A5 Exptl Calcd %A. Exptl 1 ... 0.40 ... ... 0.25 ... ... 2 ... 0.87 ... ... 0.52 ... ... 3 ... 1.37 ... ... 0.81 ... ... 4 ... 1.88 ... ... 1.12 ... ... 5 2.38 2.39 -0.6 ... 1.46 ... 1.28 6 2.88 2.91 -1.1 1.79 1.80 -0.8 1.61 -0.8 2.07 7 3.38 3.43 -1.4 2.15 2.17 8 3.89 3.95 -1.5 2.49 2.55 -2.3 2.37 9 4.49 4.47 0.5 3.00 2.94 2.1 2.85 10 5.06 4.99 1.4 3.51 3.34 4.9 3.11 11 5.58 5.51 1.3 3.83 3.75 2.0 3.55 12 6.12 6.03 1.5 4.11 4.18 -1.6 3.85 13 6.44 6.55 -1.6 4.72 4.61 2.3 ... 14 7.16 1.07 1.3 4.98 5.05 -1.4 ... 15 ... 1.59 .,. 5.43 5.50 -1.2 ... 16 8.00 8.11 -1.4 5.84 5.95 -1.8 ... 17 8.65 8.63 0.3 6.38 6.41 -0.4 ... 18 9.06 9.15 -0.9 6.83 6.87 -0.6 ... Av ZA: 1.1 Av %A: 1.6 a A Zis the percentage difference between experimental and calculated values. nCHz
Exptl X 2
n- Aldehydes,
Calcd ZAa 0.22 ... 0.46 ... 0.72 ... 1.02 ... 1.33 -3.8 1.66 -2.9 2.00 3.1 2.36 0.2 2.14 3.8 3.13 1.3 3.53 0.5 3.94 -2.4 4.37 ... 4.80 ... 5.23 ... 5.68 ... 6.13 ... 6.59 ... Av ZA: 2.0
Exptl
... ... ... ...
1.05 1.31 1.55 1.87 2.16 2.44 2.78 3.04 3.29 3.53 3.85 4.38 4.58 4.93
Calcd 0.19 0.38 0.59 0.82 1.05 1.30 1.56 1.83 2.11 2.39 2.69 3.00 3.31 3.63 3.97 4.30 4.65 5.00 Av %A:
%AQ
... ... ... ... 0.0
0.3 -0.5 2.4 2.6 1.8 3.3 1.5 -0.6 -2.9 -2.9 1.7 -1.5 -1.4 1.8
n
To compare the n-alkanes, which contain two methyl groups, with the straight chain oxygen compounds, the absorbance ratios of the n-alkanes were multiplied by 2. In the homologous series of straight chain aliphatic compounds investigated, the absorbance ratios then represent the methylene absorptivities relative to the methyl unit absorptivity which is assumed to be constant. The following relationship then applies
where d C H z and U ~ C Hare~ the relative methylene absorptivities and the relative methylene unit absorptivities, respectively. The need to correct the absorbance values for overlapping is clearly demonstrated in Figure 6 where the methylene/ methyl absorbance ratios of the overlapping methylene and methyl bands (curve 1) and the values obtained using the compensation method (straight line, 2) are plotted against the number of methylene groups in n-alkanes. As the overlap increases with increasing ncE2, the difference between the corrected and uncorrected values increases from 1.7 for n-heptane to 25.2 for n-eicosane. The methylene/methyl absorbance ratios of homologous series of n-alkanes, n-alcohols, n-aldehydes, and n-carboxylic acids, obtained by using the compensation method, are given in Table I and are plotted against ~ C H in , Figure 7. The nalkanes give a straight line for nCHz> 4 (trace 1). The increment of 0.52 per ~ C represents H ~ the relative methylene unit absorptivity of n-alkanes. All the oxygen compounds investigated show a lower methylene absorptivity than the corresponding n-alkanes, decreasing with increasing polarity of the substituents, Le., in the order n-alcohols (trace 2), n-aldehydes (trace 3), and n-carboxylic acids (trace 4). ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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1.0
'
5
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15
20
"CHi
Figure 6. Absorbance ratios of the overlapping methylene and methyl absorption bands us. nCH2,trace 1, as compared to the absorbance ratios of the separated methylene and methyl bands, trace 2, for n-alkanes
2825
2875 WAVENUMBERS(CM~
Figure 8. Frequency shifts of the methyl and methylene absorption bands of propane, trace 1, n-butane, trace 2, and n-heptane, trace 3 The relative methylene unit absorptivity for the n-alkanes, 0.52, is the relative unit absorptivity of the unperturbed methylene group. The relative unit absorptivity of the methylene group in aliphatic oxygen compounds can then be expressed by 0.52 - F, where F is a correction term which approaches zero with increasing ~ c H , , ie., as the relative methylene unit absorptivity approaches that of the n-alkanes. From Equation 4, it then follows that the relative methylene absorptivity of aliphatic oxygen compounds is given by n
ACH~ -= ACH~
U'CH?
= i=l
n
(0.52 - F) = 0.52~1-
i=l
F
(5)
The correction term F represents the reduction in intensity of the relative methylene unit absorptivities in oxygen compounds. This reduction must be caused by the polar substituents. The influence on the first methylene group is characteristic for each class of compound and its value is determined by the polarity of the respective functional group. Figure 7 shows that the influence of the functional group decreases exponentially with increasing ~ c H , . The correction term F can therefore be represented by F = kP-1
(n =
(6)
~ C H J
where k is a constant characteristic for each class of compound and 6 is the ratio by which the reduction in methylene unit absorptivities decreases. Substituting Equation 6 in Equation 5: Figure 7. Absorbance ratios of the separated methylene and methyl bands us. nCH, for nalkanes, trace 1, n-alcohols, trace 2, n-aldehydes, trace 3, and n-carboxylic acids, trace 4 If the polar group does not influence the methylene unit absorptivity beyond 5 or 6 carbon atoms, as assumed by other authors (2-4), the relative methylene unit absorptivity of the oxygen compounds should coincide with that of the n-alkanes fornCHI> 6. Plotting therelative methylene absorptivities against ~ C should H ~ then give parallel lines for the n-alkanes, n-alcohols, n-aldehydes, and n-carboxylic acids. The values for the oxygen compounds, however, give lines which only tend to become linear and parallel to the line for the n-alkanes (Figure 7). 1238
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
ACH, = ACH,
~
n U ~ C H =~
0.52n - k
6i-'
(7)
k=l
The following values for k and 6 were determined by fitting the function (Equation 7) to the experimental absorbance ratios for each class of compound, using a Burroughs B5500 computer:
n-alcohols n-aldehydes n-carboxylic acids
k
6
0.274 0.305 0.335
0.91 0.91 0.96
The values calculated for A C H J A C from H ~ Equation 7 for each homologous series investigated are given in Table I.
According to Ri (8),the decreasing ratio u of the induced charge on a carbon atom to that of the adjacent one along the C-C bond, has a constant value of 0.45. Gotoh and Takenaka (2), assuming that the same value is applicable to the C-H bond, correlated the inductive effect of the polar group with the frequency shift of the C-H stretching vibration of the methyl group in alcohols, fatty acids, and alkyl bromides. They showed that the methyl group absorbed at the frequency of the corresponding n-alkane for nCHt > 5 and postulated a similar effect for the methylene absorption. The values for 6 range from 0.91 to 0.96, i.e., about twice the value of u. This indicates that polar groups not only have an inductive effect on the methylene groups transmitted internally through the C-C bonds, but that external effects also exist which extend much further along the carbon chain. A small reduction in methylene absorptivity is still detectable twenty methylene groups away from the polar group. As the reduction in methylene absorptivity extends that far along the alkyl chain, the methyl absorptivity is probably also reduced and is not constant as assumed above. Correcting the methyl absorptivities would result in lower relative methylene absorptivities and the effect of the polar group would be even more pronounced, particularly in the lower members of the homologous series. N o attempt was thus made to correlate k , the reduction in absorptivity of the methylene group adjacent to the polar group, with the polarity of the polar groups, although they are obviously related. Although the polar group will have a greater effect on the absorption intensities than on the group frequencies, the methyl band of the oxygen compounds show a frequency shift of 1 to 2 wavenumbers for n C ~=2 5 to 18. These frequency shifts, which are very small and can only be detected when recording the bands separately, nevertheless indicate the extent of the influence of the polar group. A value of -0.22 for ~ C H = * 0 is obtained when extrapolating the linear function of relative methylene absorptivities us. nCH2for n-alkanes. This points to a small reduction of the relative unit absorptivity of the first 4 methylene groups. Accordingly, the values for k and 6 in Equation 7 were also determined for the n-alkanes. With k = 0.117 and 6 = 0.45, (8) T.Ri, Reti. Phys. Chem. Jap., 17, 3 (1943).
the following relative methylene unit absorptivities were obtained: nca,
U'CEt
1 2
0.40 0.47 0.50 0.51 0.52
3
4 25
This reduction in methylene unit absorptivity can only be attributed to an influence of the methyl groups which is confirmed by a downward frequency shift of 15 wavenumbers from n-propane to n-heptane (Figure 8). For ~ C H ,> 5 , the frequency of the methylene band remains virtually unchanged. Propane shows the resolved additional methyl band at 2897 cm-' which is also shown by hexamethyldisilazane (Figure 1, trace 2). CONCLUSION
The methylene/methyl absorbance ratio can be used to determine chain lengths accurately. The compensation method enables precise frequency measurements and the intensive study of methylene and methyl group absorptivities. The accurate measurement of the peak intensities of methyl and methylene absorption bands offers an improved method to determine the number of methyl and methylene groups in various substances. ACKNOWLEDGMENT
The author thanks Miss C. F. Pretorius for writing a number of computer programs and for her assistance in preparing the manuscript. Thanks are also extended to Mrs. V. Bolze for assisting with recordings and to the South African Coal, Oil and Gas Corporation Limited for permission to publish this work. RECEIVED for review October 28, 1970. Resubmitted November 3, 1971. Accepted January 5, 1972. Part of this work was presented at the sixteenth Annual Conference of the South African Institute of Physics in Cape Town, July 15, 1971.
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