by the comparative method previously described (4), in which the relative luminescence intensity is compared to that for a compound of known quantum efficiency. Quinine sulfate was selected as the reference standard for these measurements. The luminescence intensity was obtained by integration of the computer-corrected spectrum expressing the spectrum in units of relative quanta per unit frequency interval lis. frequency. h n example of the computer-corrected low-temperature fluorescence spectrum of PI3N is shown in Figure 7. The integrated area and the normalization factor (to convert the peak intensity to 1.0) from the computer program are shown in the lower righthand corner of the printed spectrum. Since the excitation wavelengths were different for each of the inhibitors and the reference standard, the ratio of excitation intensities (in relative quanta) was included in the calculations. The equation expressing these relationships is :
where
6
quantum efficiency F fluorescence intensity in quanta (integrated area) A = absorbance at the wavelength of excitation representative of the solution used for the luminescence intensity measurement E = intensity of exciting radiation in quanta. = =
Subscript 1 refers to the reference standard and subscript 2 refers to the compound being studied. Some results of quantum efficiency calculations for the inhibitors studied are listed in Table 111. The quantum efficiencies for Age Rite D and PBIS a t room temperature were determined in square 1-cm. cuvettes using quinine qulfate as the reference standard. The value for PBS a t 77" K. was obtained by use of the phosphorescence attachment with and without the liquid nitrogen in the quartz Dewar flask. Absorptivities and integrated emission intensities were determined at room temperature and a t liquid nitrogen temperatures (77" K.). Knowing 6 a t room temperature, the value a t 77' K. was calculated by direct ratio. The quantum efficiency for phosphorescence of PBK was calculated by direct ratio with the value for lowtemperature fluorescence. Likewise, the quantum efficiency for phosphorescence of Santonox was calculated using the quantum efficiency for low-temperature fluorescence of PI3S as the reference standard. Quantum efficiencies for PBX have been determined by Parker and Hatchard (9). Their value for fluorescence agrees very well with the data reported in Table 111, but the ~ a l u efor phosphorescence is much different. The low phosphorescence efficiency shown in Table 111 is not understood, but is probably related to the nature of the solvent, impurities, etc. The value for
Santonox wm determined using the rotating shutter, as this compound was observed to "saturate," yielding high values when the measurement was made without the shutter. ACKNOWLEDGMENT
The authors thank Esso Research Laboratories, Humble Oil &, Refining Co., for permission to publish this material. LITERATURE CITED
(1) Anufrieva, E. V., Saitzera, A. D.,
Izv. Akad. Nauk SSSR, Ser. Fix.24,755 (1960). (2)Brandt, H.J., ANAL.CHEM.33, 1390 (1961). (3) Drushel, H. V.,Iddings, F. A., Ibid., 35. 28 11963). (4) brushel, H. V., Sommers, A. L., Cox, R. C.,Ibid., p. 2166. (5) Forster, C. F., Rickard, E. F., Kature 197, 1199 (1963). (6)Melhuish, W. H., Hardwick, R., Trans. Faraday Sac. 58, 1908 (1962). (7) Oster, G.,Geacintov, S . ,Khan, A. U., A'ature 196, 1089 (1962). (8) Parker, C. A., Barnes, UT. J., Analyst 82,606(1957). ( 9 ) Parker,C. A.,Hatchard, C.G., Ibid., (10) 'Plitt,~K. F., Toner, S. D., J. A p p l . Polymer Sci. 5,534 (1961). (11) Provorov, V. K.,Zaftsev, V. D., Ref. Zh. K h i n . 1961, (6), Abstr. 6P338.
(12) Spell, H. L., Eddy, R. D., ANAL. CHEM. 32.1811 11961). ( 1 3 ) WahlT'P., J : Polymer Sci. 29, 375 (1958).
RECEIVED for revieff December 6, 1963. Accepted December 31,1963.
Determination of Higher Alkoxy1 Groups by Direct Chemical Reaction and Nonaqueous Titration SARAH EHRLICH-ROGOZINSKI and ABRAHAM PATCHORNIK Department of Biophysics, The Weizmann Institute of Science, Rehovoth, Israel
b A simple and rapid method i s presented for the determination of higher homologs of the alkoxyl group. The alkoxy1 compound i s reacted with hydriodic acid, and the alkyl iodide formed is extracted with a known amount of benzene and reacted with aniline. The anilinium iodide obtained is titrated in situ with sodium methoxide. The method eliminates the distillation procedure used in all modifications of Zeisel's determination and enables quantitative estimation of alkoxyl groups having four to 26 carbons.
T
HE
XUYEROUS MODIFICATIONS
Of
the classical Zeisel's method ( I d ) for the facile determination of the alkoxy1 group are restricted to methoxyl
840
ANALYTICAL CHEMISTRY
and ethosyl (1, 10). The determination of p r o p o y l and butoxyl involves further modifications of the apparatus and longer procedures (6). The main reason for the difficulties in the determination of higher homologs of the aikoxy1 group by the Zeisel method is the involatility of their corresponding alkyliodides (11). Recently Kretz (6) reported a determination of homologs of alcohols and esters u p to C16 by using fractional steam distillation of the alkyliodides in a specially designed apparatus. However, this method involves a time-consuming procedure. I n our previous studies ( 7 ) we showed a method for the determination of alkylhalogenides by reaction with aniline to yield quantitatively the corresponding phenylalkylamines and anilinium hal-
ides. The anilinium salt in the aniline solution can be titrated as an acid with sodium methoside to the blue end point of thymol blue. This method was successfully applied to the determination of benzyl bromide obtained by HBr cleavage of benzyl esters as well as that of carbobenzyloay groups (8). Our experience with both procedures indicated that the determination of homologs of higher alkoxyl groups could be performed in the same way. ,4 procedure has been worked out according to the following scheme. Converqion of the alkyloxy group into alkyliodide. R'-0-R or HI 0 RI
//
R'-C-0-R
-
Extraction into benzene of the alkyliodide formed. Neutralization of escess HI and reflux with aniline of the esiracted alkyliodide. RI
+ 2C6Hr,NH?
+
CeH,NII?.HI
+ C6HsNHR
Titration with sodium methoxide of the anilinium iodide formed.
+
C6H5SH2.HI XaOC“3 -+ C6HSxH: YaI
+
+ CH30H
Because this procedure, in contrast with all but one modification of Zeisel’s method (9), does not necessitate distillation, it enables the facile determination of alkoxy1 groups whose corresponding alkyliodides are nonvolatile.
tube of 25-ml. capacity. Liquids were weighed in capillaries. One milliliter of 70YG HI and about 20 mg. of red phosphorus were added. T h e tube was immediately closed with a greased ground-glass stopper and clamped. T h e tube was heated in boiling water for 1 hour. After cooling in an ice bath, the tube was opened and 7.00 ml. of benzene was added. The mixture was well shaken and 2 to 3 drops of a n aqueous suspension of thymolblue were added. Seutralization of excess hydriodic acid was then performed cautiously, with cooling, using 6 N aqueous sodium hydroxide (usually 2 ml. was used). Complete extraction was ensured by vigorous
Table 1.
EXPERIMENTAL
shaking. After centrifugation, a n aliquot of 5.00 ml. of the benzene layer was transferred to the boiling flask. Neutralization to the blue end point with 0.01N NaOCH3 was performed and 5 ml. of aniline, neutralized similarly, was added. The procedure was continued as described (8). Refluxing for all groups was carried out for 30 minutes in the semimicro apparatus. The titration was performed with 0.05.V sodium methoxide to the blue end point of thymolblue. MICRO. 4 sample containing 0.01 t o 0 05 mmole was weighed, 0 5 ml. of 70% hydriodic acid was used. Three milliliters of benzene was taken for extraction. An aliquot of 2.00 ml. of
Recoveries of Higher Alkyliodides Found by: __
Apparatus. Centrifuge tubes with Birect reaction Semimicro special clamps were used as described Recovery, Taken, with aniline,a procedure, previously ( 8 ) . For semimicro deferIodide 7c minations, their capitcity was 30 ml. 100.6 .56.30 56.61 1-Iodo-pentane and for microdeterminations, 10 ml. 100.0 .56.30 The apparatus for reaction of alkyl100.6 56.61 99.6 iodides with aniline was the same as 46.99 47.19 98.2 79 .95 that described for the reaction of benzyl 81.41 99. F, 40.80 41 .OO bromide with aniline in micro quantities ( 8 ) . For semimicro determinations the 98.8 92.90 91.80 1-Iodo-hexane 100.1 sidearm trar, ( E ) w:ts enlarged 92.98 - to a 99.7 92.63 capacity of 5 ml. ’ 98 . 5 110.35 112.02 Reagents. Hydriodic acid, 70Oj,, 98.1 82.83 84.42 SI). erav. 1.96. obtained from T h e 93 .3 50.32 53.92 &t;h Drug Hou.~es, Ltd., Poole, 100.5 13.88 13.92 England, reagent grade. Red phosI-Iodo-heptane 100.8 24.24 24 06 phorus, Fisher Scientific Co. Special 101 7 18.95 19.27 “RIicro.” I’ropionic 3 cid and propionic 98 .x 62 .65 63.71 anhydride, analytical grade. Aniline 98,X 79 .39 80.76 obtained from Fluka, Ltd., Uuchs, 97,8 107.28 109 .69 Switzerland, puriss. ).a. Sodium hy09.3 53.50 53.11 I-Iodo-octane droxide, Analar, G S aqueous solution. 99.8 -53.38 Sodium methoxide in benzene-methanol 100.3 63.96 63.78 3: 1, 0.05a%7and O.OlN, prepared !M.9 ,56 .39 57.04 accordinq to Fritz and Lisicki ( 3 ) . 953.4 74.64 75.85 Thymolblue, 0.1% solution in ethanol. 101 . 0 99.74 100.71 1-Iodo-nonane Thymolblue, aqueous suspension. Sili100.7 100.42 cone stopcock grease, Edwards High 100.4 100.14 Vacuum Ltd. 98.6 49 ,20 49.97 The alkyliodides 1-iodo-hentane. 197.0 49.72 51.24 99 . 0 47,18 iodo-octane and 1-iodo-hexade‘cane \;lere 47.64 obtained from The Ilritish Drug Houses, 100.3 80.61 80.38 1-Iodo-decane Poole, England, reagent grade. 09.6 80.02 1-Iodo-pentane and all alcohols with 99 . 9 80.32 97.7 the exceptions listed below were ob51 $60 52.77 97.1 55.37 57.01 tained from Fluka, Ltc ., Buchs, Switzerland, puriss. grade. 100.7 82.16 81.56 1-Iodo-undecane The alkyliodides . -iodo-hexane, 1100.7 82.16 100.7 82.16 iodo-nonane, 1-iodo-decane, l-iodo98.6 72.31 73.33 undecane, iodo-cyclohexane, and the 102.4 50.71 49 54 alcohols 1-nonanol, 1-undecanol, 199.7 88.04 88.32 pentadecanol, 1-heptadecanol, l-doco62.93 62.98 100.1 s a n d and all the benzoate esters were 1-Iodo-hexadecane 71.58 70.15 98.0 obtained from Fluka but of purum 60.90 60.38 99.2 grade. The purum grzde alcohols were 45 30 46.51 102.7 purified by distillation or recrystal32.45 32.71 100.8 lization from ethancl and benzene. 100.0 99.66 99.66 Iodo-cy clohexane Their purity as well a:>t h a t of the ben100.2 99.83 zoates was checked by C and H analysis. 99,1 98.75 The acetate esters were provided by 98.5 73.76 74.90 T h . Schuchardt, G.m.b H., Munich, Ger96.4 20.45 62.68 many, and their puritj- was determined 97.5 14.11 75.99 on the basis of saponification number Alkyliodide wt. or 2 ml. of benzene solution refluxed for 20 minutes with 2 mi. aniline and C and H analysis. Procedure for Alcohols. SEMIMICRO. and the resulting anilinium iodide titrated with sodium methoxide ( 7 ) . b As described above for alcohols. 0.05 t o 0.3 mmole of the alkoxy compound was weighed into a centrifuge (1
VOL. 36, NO. 4, APRIL 1964
841
~
Table
II.
Determination of Higher Alcohols
Wt., mg. 44.80 47.61 39.05 36.16 38.77 45.25 35.89
Alcohol 1-Pentanol ChHiiOH 1-Hexanol C6Hi3OH -Heptanol C7HIsOH
-Nonanol CgH190H 1-Decanol CiaH210H 1-Undecanol CllHz30H 1-Dodecanol CizH250H 1-Tetradecanol Ci4Hz90H 1-Pentadecanol CiaH310H 1-Hexadecanol Ci6H330H
44.10 58.78 71.22 36.78 66.33 61.93 60.06 44.28 53.78 51.38 36,78 42.39 36.35 49 .OO 7.36 10.44 1-Heptadecanol 56.18 CiTH3sOH 58.55 1-Octadecanol 63.11 CI8H370H 58.96 9.25 10.06 11.19 1-Docosanol 64.50 C22H460H 56.73 1-Hexacosanol 41 .OO C ~ H S ~ O H 37.71 37.60
Found,
Recovery,
mg.
%
43.50 46.55 38.10 36.21 38.32 44.34 35.40 54.53 61.62 19.53 15.87 43.79 57.67 70.97 36.51 65.63 61.44 59.64 44.51 53.54 50.86 36.85 43.24 36.45 49.20 7.40 10.57 57. 17 59.07 62.71 59.99 9.31 10.00 11.05 65.45 57.51 40.67 37.54 37.23
97.1 97.8 97.6 100.1 98.8 98.0 98.6 99.3 99.5 101.4 100.4 99.3 98.1 99.6 99.3 98.9 99.2 99.3 100.5 99.6 99 .o 100.2 102 .o 100.3 100.4 100.3 101.2 101.8 100.9 99.4 101.8 100.7 99.4 98.8 101.5 101.4 99.2 99.6 99 .o
benzene extract was reacted with 2 ml. of aniline in the apparatus with a sidearm trap of 2-ml. capacity. The titration was performed with 0.01N sodium methoxide.
Procedure for Esters. I n the semimicro procedure 0.05 to 0.3 mmole of the ester was dissolved in 0.5 ml. of propionic acid or anhydride and t o t h e resulting solution 0.5 ml. of 7Oy0 HI was added. The semimicro procedure as described above for alcohols was then followed. For micro analyses, 0.01 to 0.05 mmole of ester, 0.3 ml. of propionic acid or anhydride, and 0.3 ml. of 70y0 HI were used as described above. I3lanks were determined in all cases by carrying out the appropriate procedure in the absence of the alkoxyl compound. RESULTS AND DISCUSSION
Each step of the reaction scheme was tested separately and thus optimal conditions for the quantitative reaction were found. The reaction of alkyl halides with
842
0
ANALYTICAL CHEMISTRY
aniline and the titration of the anilinium halides have been evaluated in a previous paper (7). To determine the range of applicability of this method, a series of n-alkyliodides from C4 u p to C16 were refluxed with aniline and the resulting anilinium iodides were titrated in situ with NaOCH,. Table I shows that in all cases tested,. quantitative results are obtainable with a maximum relative error of 1%.
Table 111.
Determination of Esters
Wt., Ester mg. 36.92 n-Hexy1-p48.06 hydroxy57.39 benzoate 50.88 n-Hepty1-p59.69 hydroxy58.19 benzoate 11.14 9.48 7.44 59.44 n-Octy1-p63.53 hydroxy7.85 benzoate 12.44 48.15 n-Nony1-p46.30 hydroxy56.94 benzoate 15.73 12.02 55.91 n-Dodecy1-p61 .OO hydroxy72,OO benzoate 9.70 13.96 Dioctyl-phtha- 60.91 late (DOP) 38.02 58.70 n-Hexyl-acetate 43 .55 43.12 48.09 10.96 9.90 n-Heptyl46.80 acetate 49.05 49.56 n-Octylacetate 54.91 42.98 44.53 n-Nonylacetate 52.37 53.85 47.92 51.03 10.07 11.39
Found, mg. 36 .OO 46.22 55.49 51.02 58.55 57.54 11.15 9.49 7.38 58.34 62.34 8.07 12.12 47.37 44.91 56.06 16.03 12.29 54.15 58.90 68.78 9.88 14.65 58.63 38.32 57.91 43.63 43.19 48.65 11.08 10.17 46.72 49.95 49,45 55.95 42.37 43.35 51.17 53.83 48.06 50.05 10.18 11.48 64.25 49.48 39.19 44.42
Recovery,
7c
97.6 96.2 96.7 100.3 98.1 98.9 100.1 100.1 99.2 98.2 98.1 102.9 97.4 98.4 97.0 98.5 101.9 102.4 96.8 96.6 95.5 101.9 104.9 96.3 100.8 98.7 100.2 100.2 101.3 101.2 102.7 99.8 101.8 99.8 101 . 9 98.6 97.4 97.7 100.0 100.3 98.1 100.9 100.8 98.1 98.5 97.2 98.9
kyliodides above C4 can be analyzed by this procedure. Methyl-, ethyl-, propyl-, and butyliodides gave recoveries between 85 and 95%, using the method for alcohols. This may be explained by the relatively higher water solubility of these alkyliodides as well as their greater susceptibility to basic hydrolysis (4) during neutralization of exes5 hydriodic acid. As such a variety of excellent methods exist for the determination of Cl-Cd alkoxyl groups, no efforts were made to apply our procedure to the estimation of those four groups. The following reagents for conversion of alcohols and esters into alkyliodides were tested: 48y0aqueous hydrobromic acid, 33y0 hydrobromic acid in acetic acid, 57% aqueous hydriodic acid, 70% aqueous hydriodic acid and phosphorus tribromidp. The last was used as a reagent for alcohols only. Aqueous 70% hydriodic acid alone gave satisfactory results for all the compounds tested. No purification of the commercial 70% HI was needed. The quantitative results for a series of commercially obtained, analytically pure alcohols analyzed by the procedure described above are presented in Table
11.
Table IV.
ReTaken. Found. coverv. mg. mg. 70 "
Compound
I
n-Hexylacetate 28.96 28.63 37.29 42.36 47 20 49.49
29.32 28.70 36.74 42.79 47 59 49.57
101 , 2 100.2 98.5 101.0 100.8 100.2 99.6 49 48.85 -. 03 .. 41.14 i i 33 100.5 42.7,5" 42.92 100.4 42 58" 42 64 100.2 42 87= 43.54 101.6 99.0 57.63 57.04 56.02 56.85 101.5 63 12 63.74 101 0 .. -~ 53.65 54 91 102.3 12.33 12.33 100 0 12.18 12.58 103.4 8.96 9.16 102.3 Mean: 100.8
1-Hexadecanol 62.55 .54.75 58 37 46 38 52 50 53 86 65 05
61 .61 98.5 64.05 98.7 99 5 58 10 45 50 98 1 99 5 52 22 53 37 99 1 65 64 100 9 Mean: 99.2
1-Octadecanol The same series of n-alkyliodides was also treated by the semimicro procedure as described above for alcohols. Recovery, as shown in Table I, was quantitative within a maximum relative error of 2%, which verifies that n-al-
Accuracy of Method
(1
49.45 38.14 54.27 39.63 54.84 68.16
49.41 37.93 53.59 39.48 53.42 67.25 Mean: Reacted with 70% H I only.
99.9 99.5 98.7 99.6 97.4 98.7 99.0
Esters are easily converted to alkyliodides (IO) but in some cases their solubility in aqueous hydriodic acid is very low. Suitable solvents were therefore sought. Among the recommended solvents (10, 11) phenol is the best, but in our procedure it interfered with the titration as a small part of it is extracted by benzene and titrated as an acid. Propionic acid and propionic anhydride are suitable solvents which do not interfere. Both solvents give quantitative results when used with 70% hydriodic acid . n a ratio of 1:l. The results of a series of esters analyzed by this procedure appear in Table I11 and show satisfactory recoveries. Addition of red phosphorus suppresses the formation of free iodine which appears when no solvent is used, and interferes with the deterinination. Unless extremely pure phosphorus is used, there is the possibility of fclrming phosphine, which is extracted and titrated as an acid. Various solvents such as pentane, hep-
tane, toluene, and benzene were tried for extraction of the alkyliodides from the reaction mixture. Benzene was the most suitable solvent,. The accuracy of the method was tested by analysis of highly pure n-hexylacetate, 1-hesadecanol, and l-octadecanol. Table IV shows that the described procedure gives results with a relative error of about 1%. The method was applied to a plasticizer, dioctyl phthalate (DOP, Table 111), and quantitative results were obtained with and without a solvent. It is therefore reasonable to assume that in the cases where the plasticizers are not sufficiently volatile to be determined by the method developed by Esposito ($), this procedure can be used. ACKNOWLEDGMENT
The authors thank Aaron J. Kalb for help in the preparation of the manuscript and Ephraim Katchalski for his kind interest during the course of this work.
LITERATURE CITED
(1) Anderson, D. M.W., Duncan, J. L., Talanta 8, 241 (1961). (2) Esposito, G. G., ANAL. CHEM.35,
1439 (1963). (3) Fritz, J. S., Lisicki, N. M., Ibid., 23, 589 (1951). (4) Ingold, C. K., “Structure and Mechanism in Organic Chemistry,” p. 413, G. Bell and Sons, Ltd., London, 1953. (5) Kirsten, W. J., Ehrlich-Rogozinski, S., Mzcrochim. iicta 4, 786 (1955). (6) Kretx, R., 2. Anal. Chem. 176, 421 (1960). (7) Patchornik, A., Ehrlich-Rogozinski, S., ANAL.CHEM.31, 985 (1959). (8)Patchornik, A., Ehrlich-Rogozinski, S.,Ibid., 33, 803 (1961). (9)Sporek, K. F., Danyi, M. D., Ibid., 34, 1527 (1962).
(10) Steyermark, A., “Quantitative Organic Microanalysis,” p. 422, Academic Press, New York, 1961. (11) Wilson, C. L., Wilson, 1).W., “Comprehensive Analytical Chemistry,” p. 669, Elsevier, New York, 1960. (12) Zeisel, S., Monatsch. 6, 989 (1885). RECEIVED for review Xovember 5, 1963. Accepted December 30, 1963. Work supported by research grant (AM-5098) from U. S.Sational Institutes of Health, Public Health Service.
Chemical Shifts of Methyl Protons in Polymethylnaphthalenes FOCH FU-HSIE YEW, ROBERT J. KURLAND, and BEVERIDGE J. MAlR Petroleum Research hboratory, Carnegie Institute of Technology, Pittsburgh, Pa. The proton chemical shifts of the methyl groups in 24 methyl-substituted naphthalenes have been determined. Those chemical shifts are correlated with empirically derived parameters related to the effects of methyl group substitution on the aromatic nucleus and to steric effects. The prediction of chemical shifts made possible by this correlation is valuable for the identification of the polymethylnaphthalenes in petroleum.
A
a Flrogram on the identification of dinuclear aromatic hydrocarbons from petroleum, the nuclear magnetic resonance spectra of 24 polymethylnaphthalenes were obtained. -4correlation of the data has provided an insight inxo the structural factors which affect the chemical shifts of the methyl protons. s PART of
cycles per second and a field strength of 14,560 gauss. The measurements were made on solutions containing 2 to 5 mole % ’ of hydrocarbon in carbon tetrachloride, with tetramethylsilane (ThfS) as an internal reference. The T M S signal was taken at 10 7 units ( 5 ) . The chemical shifts were measured by means of the audio frequency side band technique. The effect of concentration change, in the range 1 to 10 mole %, on the chemical shifts of the methyl protons of 1- and 2-methylnaphthalenes is given in Figure 1. The chemical shifts decrease 0.02 unit as the concentration is decreased from 5 to 2 mole %. According to Pople, Schneider, and Bernstein (IO), smaller shifts due to changes in concentration may be expected with increasing substitutions-i.e., greater molecular volume-of the aromatic ring. Therefore, the uncertainty due to concentration differences for the polymethylnaphthalenes is expected to be within =tO.OI p.p.m.
EXPERIMENIAL
RESULTS
Nuclear magnetic resonance spectra were obtained for 24 methyl-substituted naphthalenes with a Varian V-4203 High Resolution N M R spectrometer, operating at a frequency of 60 mega-
The 48 chemical shift values measured for methyl groups from 24 methyl naphthalenes are given in Table I. The lettering and numbering system for the naphthalene nucleus is:
78w9,: 68
0
1
a
a
6
4
The methyl groups fall into three categories: (a) @-methyl groups, for which chemical shifts are between 7.50 and 7.70 T units; ( b ) a-methyl groups (except for dimethyl substitution in the 1,s- and 4,5-positions), for which chemical shifts are between 7.36 and 7.50 7 units; and (c) a-methyl groups, disubstituted in the 1,8- and 4,5positions, for which chemical shifts are in the range 7.14 to 7.26 T units. I n addition, two general trends may be observed: (a) The chemical shifts tend to increase as the number of substituents on the nucleus is increased; and ( b ) this increase in the chemical shifts, for a given number of substituents depends on the position of the other methyl groups. For example, the increase in the chemical shift of any methyl group with a n adjacent substitution is greater than that caused by a nonadjacent substitution in the same ring, which in turn is greater than that resulting from substitution in the VOL. 36, NO. 4, APRIL 1964
843
