Inf rared Examinatio n of Car bon-Hyd rogen Stretching Frequency in Pyrocatechols, GuaiacoIs, and Phenols WlLLlS BECKERING, C. M. FROST, and W. W. FOWKES Grand Forks Lignite Research Laboratory, Bureau of Mines, Grand Forks, N. D.
U. S. Department o f the Interior, P. 0. Box 82 7 3
The infrared spectra of a series of alkyl-substituted phenols, catechols, and guaiacols in carbon tetrachloride sobtion were determined in the 2800to 3000-cm.-' region. The origin and the frequency variation of several of the vibrations are discussed. The spectra are presented in a bar-type diagram and should be useful in the characterization of unknown phenols.
I
University Station,
1.1
Phenol
o -Cresol
1
I I
~
I
II] I .
m-Cresol
I
p - cresol
o - Ethyl Phenol m-Ethyl Phenol p-Ethyl Phenol
of tar acids recovered from low-temperature carbonization products of lignite, it was necessary to compile a catalog on the infrared spectra of phenolic compounds. Initially, much of the characterization work was accomplished by examining infrared bands in the 670- to 1000cm.? region. However, it was soon observed that a careful examination of the 2800- to 3000-cm.-l region can be extremely helpful in the identification of unknown phenolic compounds. A total of 78 compounds was examined. These can be classified as alkylphenols, N THE CHARACTERIZATION
o-n-Propyl
1
I
1
I
I
I
I
I j
I
I I
I
I
I
I , . , /,I,
1
I I I I I
I
I
I
II
,
I
-
-i
I
Figure 1 Alkyl-substituted phenols A. Top right B. Bottom left C. Bottom right 3,s- Xylrnol
II 2850
I I 2900
ll 2950
I 1
3000
I 1 J 3050
3100
3050
3100
WAVENUMBER. em-'
2850
2900
2950
3000
WAVENUMBER, C n i '
241 2
ANALYTICAL CHEMISTRY
3050
3100
2850
2900
2950
3000
WAVENUMBER. em-'
3 - Methyl Pyrocatechol 4 - Methyl
Ill
2850
I,II
2900
I II
2950
WAVENUMBER,
2850
2900
2950
WAVENUMBER.
Figure 2.
3000
3050
3100
Figure 3.
3050
3100
cm-l
Alkyl-substituted dihydroxybenzenes
smaller values to the remaining weaker bands. Many of the compounds were available commercially, and those that were not available were synthesized at the Grand Forks Lignite Research Laboratory. A melting (boiling) point determination or a GLC analysis was performed on those compounds whose purity was questionable. The remaining compounds were used without further purification.
EXPERIMENTAL
A Perkin-Elmer Model 112 singlebeam, double-pass infrared spectrophotometer was used for determining frequencies in the 2800- to 3000-cm.-' region. The instrument was equipped with calcium fluoride optics and had a 1.8-cm.-l spectral slit width a t 2900 cm.-' The instrument was calibrated by using the adsorption peaks of ammonia and atmospheric water vapor. The frequency values of these peaks were taken from the literature ( 2 ) . All samples were dissolved in analytical grade carbon tetrachloride. A 0.50mm. sodium chloride cell was used for those compounds sufficiently soluble in carbon tetrachloride to yield a good infrared spectrum. For the leis soluble compounds, a 10-mm. qodium chloride cell was used. I n all cases, the concentration of sample was such that no intermolecular hydrogen bonding was observed. The si ectra were recorded as per cent transmittance and are plotted as bar diagrams in Figures 1 to 3 by assigning unit absorption to the strongest band and proportionately
3000
c6'
Alkoxy-substituted phenols
alkoxy phenols, and alkyl dihydroxy benzenes. An examination of the absorption pattern in this region reveals that no two compounds exkibit the same spectrum. Hence this region can also be rei arded as a fingerprint region and therefore should become increasingly important with the high resolution infrared instruments now available.
I I Ill
DISCUSSION
Examination of Figures 1 to 3 indicates the large number of absorption bands in 2850- to 3000-cm.-' interval. Furthermore, no two compounds have the same absorption pattern throughout the region. This part of the spectrum can then be regarded as a good fingerprint region as well as one which conA tains several group frequencies. number of authors have discussed these group frequencies; hence we shall not discuss that which is already well known but shall concentrate on those bands which are less well known or are deviations from the normally reported values. Methyl Group. T h e symmetrical and asymmetrical carbon-hydrogen vibration of methyl groups in aliphatic compounds is given by Bellamy ( I ) and others (3, 5-7) as 2872 f 10 and 2962 i 10 cm.-', respectively. This is confirmed in the compounds listed in Figures 1 t o 3 whenever the methyl
group is one or more carbon atoms removed from the aromatic ring. When a methyl group is directly attached to the aromatic nucleus, as in cresols and xylenols, one generally observes four absorption bands. The origin of these bands is still under discussion (4)8). However, following the interpretation of Forel, Fuson, and we believe the following Josien (4, assignments are the probable ones for methyl groups attached directly to the aromatic ring. The band a t 2924 f 4 cm.-' (v,) arises from the symmetrical stretching vibration of the methyl group when the symmetry is Caw. The degenerate asymmetrical vibration (v,) normally observed in aliphatic methyl groups is not observed when the methyl group is attached to the ring because of the removal of the degeneracy. With the removal of the degeneracy the symmetry of the molecule changes , now an asymfrom C8,,to C, ( C I h ) and metric vibration, 2975 =t 4 cm.-I ( Y ~ ' ) , and a symmetric vibration, 2949 =t 4 cm.-l (va'), with respect to the plane of the molecule of C, symmetry is observed. The fourth band occurring a t 2862 + 8 cm.-' (26,) arises from the first overtone of the asymmetric bending vibration of C,, symmetry (4). These absorption bands are observed in the cresols, xylenols, and trimethyl phenols in Figure 1, -4 and B. When both a methyl and a methylene group are attached to the ring, the methyl vibration Y , is obscured by the asymmetrical stretching frequency of the methylene group a t 2926 =t 10 cm.-l In inany of these conipoundE VOL. 36, NO. 13, DECEMBER 1964
2413
the observed band is broader, indicating more than one absorption band present. Methoxy Groups. The methyl group, when directly attached to a n oxygen atoni as in guaiacols (omethoxyphenols), has a very strong band at 2841 i 9 cm.-l This absorption band is likely the first overtone of the symmetrical bending vibration (4). Absorption at the lower end of this range occurs in those compounds having an alkyl group in the 5-position, with 3,5-dimethyl guaiacol having the lowest frequency a t 2832 cm.-l The asymmetrical C-H vibration of the methoxy group in guaiacols occurs a t 2961 + 3 cm.-l This is in the same range as the methyl group in alkanes. However, when the methyl group is one carbon atom removed from the oxygen atom, the frequency is increased to 2988 cm.-l This is observed in the 2-ethoxy- and 3-ethoxyphenol in Figure 2. Increasing the size of the alkoxy group to four carbon atoms results in normal methyl C-H absorption, as observed in 4-n-butoxy phenol. tert-Butyl Group. Examination of 2-tert-butylphenol, 2,4-di-tert-butyl-, and 2,6-di-tert-butylphenol in Figure
1 B reveals a normal symmetrical and asymmetrical C-H vibration of the methyl group at approximately 2874 cm.-l and 2965 cm.-l, respectively. I n addition, there appears a third band a t 2914 f 3 cm.-l in all three compounds. The presence of this band is probably caused by the first overtone of the asymmetrical -CH3 deformation frequency. A similar band is observed at 2907 cm.-l in 4-tert-butylpyrocatechol (Figure 3) and in 4-tert-butyl-2methylphenol (Figure 1B). I t appears that there is a frequency shift to lower values when there is no tert-butyl group ortho to the hydroxy groups. The absorption frequencies of the methyl and methylene groups in the dihydroxy compounds in Figure 3 have essentially the same bands as the corresponding monohydroxy compounds. The authors have found the summary charts in Figures 1 to 3 very helpful for characterizing tar acids in low-temperature tar. This was especially true for those phenolics that have the same aromatic substitution pattern and hence have similar absorption bands in the long wavelength re-
gion and in the 5- to 6-micron combination-overtone region. ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance of R. IT. Youngs in preparing several of the phenolic compounds used in this work. LITERATURE CITED
(1) Bellamy, L. J., “The IEfra-red Spectra of Com lex Molecules, Methuen and
Co., Ltcf, London, 1958. (2) Downie, R., Magoon, M. C., Purcell, T., Crawford, B., Jr., J . Opt. SOC.Am. 43, 941 (1953). (3)10, Flett, 21 (1957). M. St. C., Spectrochim. Acta (4) Forel, &I. T., Fuson, K., Josien, AI. L., J . O p t . Soc. Am. 50, 1228 (1960). (5) Jones, R. N.,Sandorfy, C., “Chemical Application of S ectroscopy,” Chap. IV, Interscience, 8ew York, 1956. (6) Pozefsky, A,, Coggeshall, N. D., ANAL.CHEM. 23, 1611 (1951). ( 7 ) Shrewsbury, D. D., Spectrochim. Acta 16, 1294 (1960). (8) Wilmshurst, J. K., J . Mol. Spectr. 1, 201 (1957). RECEIVEDfor review M a y 28, 1964. Accepted September 10, 1964.
Characterization of Alkylphenols by Acetylation and Proton Magnetic Resonance L.
P. LINDEMAN and S. W. NICKSIC
California Research Corporation, Richmond, Calif. Alkylphenols are acetylated with acetyl chloride and the resulting product is analyzed by proton magnetic spectrometry. The proton absorption of the acetate methyl occurs a t a very narrow and unique region, ma king the ana!ysis for the otherwise broadened hydroxyl group much easier. The acetate resonance has three times the sensitivity of the hydroxyl proton resonance, and its precise location permits ortho-para isomer identification. The ortho-para isomer ratio can be readily determined from the ratio of the areas of the two acetate methyl peaks. The amount of disubstitution follows from the ratio of the aromatic absorption to the acetate methyl absorption(s).
are analyzed by nuclear magnetic resonance (KMR), absorptions for the aromatic, hydroxyl, and aliphatic protons are obtained in the characteristic chemical shift regions expected. The hydroxyl proton is sometimes buried under the aromatic resonance. I t can be shifted HEN ALKYLPHENOLS
24 14 *
ANALYTICAL CHEMISTRY
downfield by polar solvents such as dimethyl sulfoxide or upfield by dilution in nonpolar solvents such as carbon tetrachloride. Frequently the quantitative analysis for the hydroxyl proton is not very accurate, either because of extreme broadening of the resonance or because some of the hydroxyl resonance is not completely shifted from the aromatic resonance. For the identification of ortho and para isomers, qualitative information can be obtained by the appearance of the aromatic absorption. Additional isomer information can be obtained from the absorption of the protons on the carbon atoms alpha to the aromatic ring as discussed by Crutchfield (1j. When the alkylphenol is converted to the corresponding acetate, the analysis is much more satisfactory because the proton absorption of the acetate methyl occurs in a very narrow and unique region as a single sharp line, the sharp methyl resonance has three times the sensitivity of the hydroxyl group resonance, and the exact position of the methyl resonance permits identification of the ortho and para isomers. The
separation is good enough to permit quantitative analysis in mixtures by comparing areas of the absorption peaks. EXPERIMENTAL
The procedure for acetylation consists of dissolving the alkylphenol in excess acetyl chloride. The NMR spectrum is then obtained in a conventional high resolution spectrometer equipped with a variable temperature probe. Except for highly hindered phenols, the reaction is complete in 5 minutes as shown by no further change in the spectrum when longer reaction times are used. The excess acetyl chloride does not interfere: and free hydrochloric acid, which need not be removed, is also observed separately. RESULTS
Figure 1 gives the spectrum with integral trace of heptylphenol (n-C,, mixed secondary attachment). The aromatic proton absorption a t 6.6-7.3 p.p.m. is typical of an ortho-para mixture. Pure ortho and para isomers can often be identified by their char-
