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-
8.0
7.0
8.0
7.0
6.0
I I
I
6.0
Figure 1.
4.0
5.0
,
,
5.0
I
,
2.0
3.0
,
.
,
,
4.0
,
,
,
1.0
,
,
,
,
1 .o
9.0
3.0
(In chloroform, the acetic acid methyl absorbs a t 2.1 p.p.m. The upfield shift is due to solvent effects.) The two peaks due to the acetate methyl protons of the ortho and para isomers of t,he acetylated phenols absorb at about 2.1 p.p.m. The other absorptions have the same assignment as discussed above for Figure 1 . The change in the aromatic absorption is due to both removal of the hydroxyl proton and the conversion to phenol acetates. The ratio of isomers can now be established from the ratio of the two sharp methyl peaks at 2.1 p.p.m., the ortho isomer absorbing downfield. This was established by acetylating many prepared mixtures of known alkylphenols. In every case the ortho isomer acetate methyl absorbs downfield of the para isomer. Alkyl Protons Alpha to the Aromatic Ring. Column 1 of Table I gives the chemical shifts of the alpha protons in methyl, methylene, and methine groups in some ortho, para, and ineta isomers of phenols. These values are in agreement, with data reported by Crutchfield, Irani, and Toder ( 1 ) which are shown in Column 2 of Table I. There is no apparent difference, within the accuracy of measurement, in the chemical shift of methyl protons in
op.p.m.(6)
, I
0
NMR spectrum of heptyl phenol
acteristic aromatic proton absorption, but mixtures can only be qualitatively analyzed because the type of substituent (straight or branched) as well as the attachment (primary, secondary, tertiary) of the alkyl group to the ring affects the pattern of the aromatic proton absorption. The resonance at 6.1 p.p.m. is due to phenolic hydroxyl. Kote that the integral curve does not give the expected 4 : l ratio of aromatic to hydroxyl protons. This is because part of the hydroxyl absorption is under and to the left of the aromatic absorption. Usually in mixtures of isoniers the upfield hydroxyl absorption is due to the ortho isomer and the broadness is due to \Ion exchange rates. The other absorptions observed in the spectrum are due to the alkyl substituents. The multiplet centered at 3.0 p.p.m, is due to the absorption of the methine proton of the ortho isomer; the diffuse multiplet at about 2.5 p.p.m., which can also be detected from the integral trace, is due to the methine proton of the para isomer. Protons on the carbon atom beta to the ring absorb a t 1.5-1.7 P.P.m. The remaining methylene urbtons absorb at 1.25-1.4 , p.p.m. and 'the methyl protons a t 0.85 p.p.m. The data are consistent with a L straight chain secondary attachment. A The sharp peak at 1.11 p.p.m, and the shoulder a t 1.18p.p.m. are due to the methyl group beta to the ring in the %phenyl isomer
1
being split into a doublet by the methine proton. Figure 2 gives the NMR spectrum of
the same sample of heptylphenol after acetylation. The absorption at 6.1 p.p.m. due to the hydroxyl proton has disappeared, and in its place we observe several new absorptions. Excess acetyl chloride absorbs a t 2.43 p.p.m. Acetic acid, which is formed by the reaction of the acetyl chloride with water either from the atmosphere or in the sample, absorbs a t 1.93 p.p.m. in this spectrum.
Table I.
Chemical Shift" of Protons Alpha to the Aromatic Ring
Alkylphenols
Literature ( I )
2.14 2.12 2.19 2.59 2.42 2.47 3.03 2.53
2 12-2 20
o-Cresol m-Cresol p-Cresol o-Ethylphenol m-Ethylphenol p-Ethylphenol o-sec-Butylphenol p-sec-Butylphenol a
2 10-2 17 2 53-2 60 2 43-2 50 2 98-3 17 2 43-2 75
Phenol acetates
Alkyltoluenes 2 2 2 2 2 2 2 (2
13 22 20 53 56 54 83 54)b
2 2 2 2 2 2 2 2
14 24 25 50 ,55 56 83 58
Relative to internal tetramethylsilane in p.p.m.
* Chemical shift for sec-butylbenzene.
8.0 I
7.0
6.0
!
I
'
I
I
7.0
4.0
6.0
5,O
3.0 -
2.6
1 .o
I
1
0Darn. ( 6 ) I
200
300
400
Figure 2.
5.0
I
4.0
CP!
,
,HI
3.0
2.0
I
I
,
I
1.o
0
NMR spectrum of heptyl phenol and acetyl chloride reaction mixture VOL. 36, NO. 13, DECEMBER 1 9 6 4
e
2415
ortho-, meta-, and para-cresols. These chemical shifts are almost identical with those of the corresponding acetate derivative and of the xylenes which result by the substitution of a methyl group for the hydroxyl group. These data are given in Columns 3 and 4 of Table I. The results show that the hydroxyl group per se has about the same influence on the chemical shift of ring methyl protons as an acetate or another methyl group, regardless of their relative position. The chemical shifts of the methylene protons in the ethylphenols are 0.3 to 0.4 p.p.m. greater than those for the methyl proton. This difference is about the same as is usually noted (2) for the chemical shift of protons in methyl and methylene groups in the
Table 11.
Pure Phenols"
Data for the Acetate Alethyl Proton of Acetylated Pure Phenols Chemica1 Calcd. No. of shifts, *0.05 acetyl (p.p.m.1 protons Phenol 2.12 3 09 0-Cresol 2.10 3 15 m-Cresol 2.09 2 93 p-Cresol 2.12 2 95 o-E t hylphenol 2.10 2 85 m-E thy Iphenol 2 13 2 98 p-Ethylphenol 2.12 3 17 3 09b o-Phenylphenol 1.88 3 08 p-Phenylphenol 2 15 o-sec-Butyl2.09 3 01 phenol p-sec-Butyl2 92 phenol 2.09 o-tert-But,ylphenol 2.17 2 81 p-tert-Bu tyl3 24 phenol 2.16 a Eastman Chemical Co. Reaction slow.
NMR
Table 111.
same chemical environment. The difference is due to the decreased diamagnetic shielding of protons in the methylene group. The chemical shift of the ortho isomer is 0.12 p.p.m. larger than that of the para isomer; the chemical shift of the para and meta isomers is nearly the same. As shown in Columns 3 and 4 of Table I, the methylene protons of the acetate derivatives and the ethyl toluenes absorb in the same place as the ethylphenols. I t is interesting that the chemical shift of the methylene proton in the ortho isomer of the acetates is smaller than that of the para isomer, just the reverse of the phenol case. The chemical shift of the methine proton in para-sec-butylphenol is slightly larger than that of the methylene protons in para-ethylphenol. I n the ortho isomer, hoiiever, its chemical shift is 0.4 p.1i.m. greater than the methylene proton. The limited data (2) available indicate that the chemical shift of methine protons is usually about 0.25 1i.p.m. greater than methylene protons in a similar environment. However, the chemical shift of the methine proton is much more bensitive to its environment as we have observed in the case of several alkyl halides and acetates. The chemical shift of the methine protons in parasec-butylphenol acetate and sec-butylbenzene is nearly identical with that in the phenol. The corresponding chemical shifts in ortho isomers are appreciably smaller than the chemical shifts in the phenol; and their values are closer to that expected for a methine proton, based on the value found for the methylene protons in the ethylphenols. The large difference in the chemical shift of the methine proton in orthoand para-substituted phenols is useful for distinguishing the two qualitatively.
Data on Alkylphenols
Chemical shift (p.p m.)
Mole c/c and p isomers
Calcd. acetyl protons
Disubstitution
37 63
)3 17
20
36 64
)3 23
28
0-
%
Straight chain (secondary attachment) Heptylphenol p
0
Octylphenol
p 0
?Jon>lphenol p
2 2 2 2 2 2
12 16 15 20
>
08 13
34 66
)3 15
19
Decylphenol p
2 18 2 23
34 66
13 10
13
cis to C20
2 08 2 13
34 66
13 30
35
2 12
94 6
7 13 18
23
2 04 2 12
90 10
)3 40
47
0
0
p
0
Polypropylene (branched) Nonj 1 P 0
Dodecyl
2416
p
0
a
ANALYTICAL CHEMISTRY
Quantitative use of this proton for determining relative amounts of ortho and para isomer in secondary phenols is poor because the methine absorption is extremly broad (35-40 c.p.s.) due to spin-spin splitting by neighboring protons, and the low sensitivity because there is only one proton in the group. Further, the method is not, generally useful, being restricted to the secondary phenols. These shortcomings are avoided by the use of the acetylation technique. Ortho and Para Isomer Analysis. Table I1 gives the chemical shift and the number of acetate methyl protons for several nominally pure alkylphenols. With the exception of the ortho-phenyl phenols, the acetate methyl protons of all these phenols lie in a very narrow range a t about 2.1 12.p.m. T h e exception noted was expected because of the large magnetic anisotropy of the phenyl group which can interact with the methyl protons. The acetate methyl of the ortho isomer always absorbs at lower magnetic field than the acetate methyl of the para isomer, except in the case of phenylsubstituted phenols. Because spectra were not obtained under exact conditions of temperature and concentration, the absolute values given in Table I1 are not accurate enough to see this small difference, but the relative shift can always be seen in the spectra of mixtures where conditions for both isomers were, of necessit,y, identical. The absolute chemical shift difference can be measured by using the methyl group of the by-product acetic acid as an internal st,andard. K h e n this is done, the absorption of t'he para isomers is 0.19 + 0.01 p.1i.m. downfield from acetic acid; and the ortho isomer absorption is 0.23 i 0.01 p.p.in. downfield from the same reference. The second colunin in Table I1 illustrates the reliability of acetylation. I t gives the number of protons in the acetate methyl absorpt,ion as calculated from the area ratio of this group to that of the aromatic protons and assuming monosubstitution, or four aromatic protons per molecule. The agreement is somewhat outside the usual experimental error which is believed to be ~ 1 ~ 3for 7 4 this technique. Since no side reactions have been discovered in this reaction, the difference may be due to the purity of the samples. The best commercially availahle samples were used in every case without purification. Table I11 gives some data on laboratory preparation of both straight chain (secondary attachment) and branched alkylphenol.;. The resonance for the acetate methyl is given together with the amount of acetyl protons calculated from the area ratios. The calculated number of acetyl protons is invariably
higher than the expected value of 3.00. This can be explained by the presence of varj ing amounts of dialkylphenol. The presence of the disubstituted material tends to increase the acetate to aromatic proton ratio. I n some cases disubstitution was verified by mass spectrometer analysis. Hoaever, the data on disubstitution, although reasonable, must be accepted provisionally until independent checks become available. DISCUSSION
The integration accuracy of the Varian A-60 proton spectronieter used in this work is usually independent' of sweep sl'eed. However, the very sharp, closely spaced acetate methyl peaks niakes sweep speed during integration an important factor in the accuracy of the peak area measurements. > I t the 50-second sweep rate and, to a lesser extent, a t the 100-second sweep rate on the 500-c.p.s. range, the integral does not agree with planimeter area nieasuremerits, presumably because of timeconstant limitations of the integrating circuit of the spectrometer. This is based on the results which show a pronounced bias in favor of the peak integrated first regardless of the sweep direction. The bias is removed by slow sweep rates, either by lengthening sweep time or shortening the sweep range. A combination giving 2 c.p.s. per second is satisfactory. Peak height ratios agreed very a-ell with planimeter area rat,ios, which is not surprising because the two methyl peaks have equal widths. < I t low concentrations of ortho or para isomer in the presence of high con-
centrations of the other isomer, correct'ions must' be made for peak overlap. This is shown by the data in Table IT.' which gives the results on known mixtures of ortho- and para-sec-butylphenol acetates. Known mixtures were made from weighed amounts of the individual isomers and analyzed by NMR, with and without overlap corrections. Overlap was det'ermined from an expanded scale trace by manually drawing in the absorption in the overlap region. I s is usually t'he case, the overlap without correction gives high results for t,he isomer present a t the lowest coricentration. *it the ly0 range the errors become very large. I n practice, it, is easier to use a nomograph constructed from the dat,a of Table IV than to make the corrections by manual planimeter methods. I n most cases excess acetyl chloride acts as the solvent for viscous samples. Tetrachloroethylene is also very good. Benzene, toluene, and xylenes should not be used because they interfere and because they shift, t,he acetyl chloride absorption upfield far enough to somet'imes overlap the methyl grouii absorptions of the phenol acetates. During the course of this study it was noted that, t,he alkylation of phenol with straight chain olefins resulted in a preference for ortho substitution; whereas, alkylation with branched material gave predominantly para substitution. KO meta isomer is expected; if it is present, it is lumped ait,h the para isomer in this analysis on t,he basis of mefa isomers in Table I. The acetate methyl of ortho-paradisubstituted phenol acetates absorbs
Table IV. Resultsa on Known Mixtures of ortho- and para-sec-butylphenol Mole "i; ortho isomer found 90
Known
correction
10 0 13 f 1 25 0 27 zI= 1 50 0 50 f 1 75 0 73 f 1 90 0 86 i 1 Av. of three analyses
Corrected
for overlap 10 f 1 R 25 i 1 5 5 0 i 1 0 i5+15 90 i 1 *?I
in the same place as the ortho-substituted monoalkylphenol acet'ates, based on the results on dimethylphenols. Compounds \ d i large substituents in both ortho positions, as in the case of 2,6-di-tert-butylphenol, cannot be acetylated. 2,6-I)imethylpheno1 acetylates readily. In case of doubt, heating and repeated scanning in the spectrometer helps to ensure complete reaction. ACKNOWLEDGMENT
The authors are indebted to ,J. J. Shook of this labpratory for preparing many of the samples used in this study and for his valuable criticism of the manuscript. LITERATURE CITED
i l l Crutchfield. 11. M.. Irani. R 13.. Yoder, J. T.,' J . A m . hi1 Chemiats' Soc: 41, 129 (1964). ( 2 ) Jackman, I>. If., ',Suclear Magnetic Resonance Spectroscopy," p. 52, Pergamon Press, Xew York, 19.59. ~
Differential Controlled-Potential Coulometry Application to Determination of Chromium G. A. RECHNITZ and K. SRlNlVASAN Department o f Chemistry, University o f Pennsylvania, Philadelphia, Pa
'
b A new electroanalytical technique, which combines the selectivity of controlled-potential coulometry and the potential accuracy of differential methods, is proposed for the precise determination of electroactive materials in the sub-millimolar concentration range. The method is based upon the simultaneous electrolysis of the sample and a standard material in two identical cells, connected in series, under conditions which ensure selective oxidation or reduction of the substance to b e determined. The advantages of this method over direct controlled-potential coulometry are il-
I
lustrated by a study of the reduction of chrornium(V1) in sulfuric acid media. Using commercially available components, relative accuracies of better than 0.1% are easily attained.
D
techniques have been employed with advantage in such fields of analytical chemistry as spectrophotometry (1) and polarography ( 7 ) . One might expect that the combination of the differential approach with an analytical technique of high precision and accuracy such as coulometry could offer significant advantages over existing electroanalytical methods. IFFEHENTIAL
This has recently been demonstrated by the preliminary work of Monk and Goode (6), who employed c( nstant current coulometry i n a differential arrangement for the highly accurate determination of chroniium(V1) with electrogenerated iron(I1). These authors also suggested, but did not explore, the possibility of differential methods in constant' potential coulometry. Since controlled-potential coulometry has the further merit, of ensuring the occurrence of a specific electrode reaction, it was thought worthwhile to test the application of differential methods to controlledpotential coulometry. VOL. 36, NO. 13, DECEMBER 1 9 6 4
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