Methyl Substitution Effects on the Proton Chemical Shifts in Benzene * G. S. REDDY E. I. du Pont de Nemours & Company, Inc. Explosives Department, Eastern Laboratory, Gibbstown, New Jersey, U.S.A. (Z. Naturforschg. 21 a, 609—615 [1966] ; received 16 December 1965) Methyl substitution effects on aromatic and methyl proton chemical shifts in several mono-, di-, and trimethyl benzenes are studied. A new method for obtaining the changes in the ring proton chemical shifts from those of methyl proton shifts at the corresponding positions is used. The extra jr-electron densities in toluene are calculated using the already known relation between the jr-elec- tron densities and the proton shifts in aromatic systems. An inverse relationship is obtained between the ionization potentials and the total methyl effects on the chemical shifts in this series of com- pounds as one would expect. Dipole moment of toluene is calculated, and a reasonably good agree- ment is found between the experimentally observed and the calculated dipole moment. Several efforts have been made from time to Considerable work has also been done in estimat- time to study the substitution effects on chemical ing ir-electron densities from chemical shift meas- shifts and coupling constants. One of the earliest urements in unsaturated systems. This study in- attempts in this line are those of CAVANAUGH and volves extension of the substitution effects and also DAILEY 1 who tried to study the effect of multiple estimating jr-electron densities in methyl benzenes. methyl substitution in methane. They encountered Eight mono-, di-, and trimethyl substituted benzenes negative shifts contrary to expectations based on have been studied, and a new technique has been inductive and hyperconjugative effects of the methyl deployed to obtain the methyl substitution effects group which eventually was attributed to the an- on the chemical shifts of ring protons from proton isotropy effect of the added C — C bonds 2-7. SPIES- chemical shifts and methyl group shifts. ECKE and SCHNEIDER have studied the substituent ef- fect on both proton and carbon-13 chemical shifts Experimental in alkyl 8 and aromatic 9 compounds and concluded that diamagnetic anisotropy contributions are very All the spectra were obtained in very dilute solu- tions (ca. 2 — 3%) in CC14 with about \% tetramethyl- serious and have to be corrected for before attempt- silane added to serve as internal reference. Preliminary ing to use these parameters for correlations. dilution studies indicated no serious concentration ef- REDDY and coworkers 10-13 have studied the me- fects on the chemical shifts. Calibrations were carried out by side band technique and the allowed uncer- thyl substitution effects on proton chemical shifts tainties in the chemical shifts are + 0.5 cps. All the in ethylenic and heterocyclic systems and showed measurements were made on V a r i a n Associates that methyl group releases electrons into the n- 4311-B High Resolution Spectrometer operating at system which then rearranges according to the sub- 60 mc/sec and equipped with a Flux Stabilizer. All stituents in the system. Their results also showed the the chemical shifts are expressed in cycles per second with respect to tetramethylsilane as internal reference. extent of electron release by the methyl as measured The centers of the peaks were taken as the chemical by the proton chemical shifts is approximately the shifts wherever the pattern is broad or symmetrically same in all the systems studied. split. * Contribution No. 207 from the Research and Development 7 G. S. REDDY and J. H. GOLDSTEIN, J. Chem. Phys. 39, 3509 Division of the Explosives Department of E. I. du Pont de [1963], Nemours & Company. 8 H. SPIESECKE and W. G. SCHNEIDER, J. Chem. Phys. 35, 722 1 J. R. CAVANAUGH and B. P. DAILEY, J. Chem. Phys. 34, 1094, [1961], 1099 [1961]. 9 H. SPIESECKE and W. G. SCHNEIDER, J. Chem. Phys. 35, 731 2 J. TILLIEU, Ann. Phys. Paris 2, 471, 631 [1957]. [1961]. 3 P. T. NARASIMHAN and M. T. ROGERS, J. Chem. Phys. 31, 10 G. S. REDDY and J. II. GOLDSTEIN, J. Am. Chem. Soc. 83, 1302 [1959]. 2045 [1961]. 4 G. S. REDDY and J. H. GOLDSTEIN, J. Chem. Phys. 38, 2736 11 G. S. REDDY and J. H. GOLDSTEIN, J. Am. Chem. Soc. 83, [1963]. 5020 [1961]. 5 J. I. MUSHER, J. Chem. Phys. 35, 1159 [1961]. 12 G. S. REDDY, R. T. HOBGOOD, and J. H. GOLDSTEIN, J. Am. 6 A. A. BOTHNER-BY and C. NAAR-COLIN, Ann. N.Y. Acad. Sei. Chem. Soc. 84, 336 [1962]. 70, 833 [1958]. 13 G. S. REDDY, L. MANDELL, and J. H. GOLDSTEIN, J. Chem. Soc. 1963, 1414. Results and Discussion thyl effect on the ring protons, the average effect due to one methyl group calculated as described in Table 1 shows all the chemical shifts in the eight the following pages, and the ionization potentials of methyl benzenes, including that of benzene. coar and some of these compounds. Figure 1 represents a plot come are the chemical shifts in cycles per second of of the ionization potentials and the total methyl sub- the aromatic protons and methyl protons respective- stitution effects. Figure 2 shows the structure of ly from tetramethylsilane. AcoaT is the shift of the toluene and the additional ^-electron distribution aromatic protons for each compound from benzene used in calculating the dipole moment. and denotes the methyl substitution effect. Where It has been shown that when a methyl group re- there is more than one peak due to different pro- places a proton in ethylene, substituted ethylenes, tons shifting to different extents, the number of pro- and some unsaturated ring systems, the other pro- tons representing that peak is shown in the paren- ton chemical shifts move to higher fields 10-13? in- theses after the chemical shift. Acome is the differ- dicating that the carbon atoms to which these pro- ence in the chemical shift of the methyl protons in tons are attached gain electronic charge released into the compound under consideration and that in the system by the methyl group by inductive and/or toluene. This denotes the effect of methyl substitu- hyperconjugative mechanism. The redistribution of tion on another methyl group already present in the this additional charge in the molecule is different in ring. The last three columns represent the total me- different molecules, depending on the properties of Total Me- Total Effect Ionization thyl Effect Compound War A A>a,T Aoime No. of Potential* Wme (327/lcüme + Methyl Groups (eV) CO ar) - 433.0 9.56 0 A-CH3 -424.2 —136.2 + 8.8 44.0 44.0 9.23 U A-CH3 -418.3 -130.5 + 14.7 + 5.7 93.0 46.5 9.04 \) ~CH3 A-CH3 -415.5 (1) -133.8 + 17.5(1) + 2.4 92.5 46.3 9.05 -412.8 (3) + 20.2 (3) CH3 A-CH3 -416.2 -132.8 + 16.8 + 3.4 87.6 43.8 8.99 HAC-IJ H3C-A-CH3 -400.3 -131.7 + 32.7 + 4.5 138.6 46.2 8.74 V CH3 A-CH3 - 409.8 -131.3 (1) + 23.2 + 4.9(1) 131.7 43.9 HGC-'^JJ-CHA -128.3 (2) + 7.9(2) A-CH3 -410.8 -130.7 (2) + 22.2 + 5.5 (2) 135.6 45.2 - -124.2 (1) + 12.0(1) CH3 * Taken from F. H. FIELD and J. L. FRANKLIN, J. Chem. Phys. 22, 1895 [1954]. Table 1. Methyl Substitution Effects on Proton Chemical Shifts in Benzene. Numbers in parentheses denote the number of protons giving that peak. the atoms present in the ring. Moreover, it has been 7r-orbitals of the carbon and 10 ppm for the directly shown in five- and six-membered heterocyclic sys- bonded hydrogen. These results can be incorporated tems the methyl effects are long-range, showing that into the methyl substitution effects on proton chemi- the mechanism of electron release is predominantly cal shifts studied by REDDY and coworkers 10-12 and hyperconjugative. All these observations are in ac- the expected effects of methyl substitution on C13 cordance with the expectations that there should be chemical shifts can be calculated. These calculations a linear relationship between the cr-electron density in ethylenic systems give a shift of + 4.8 to + 6.8 on the carbon and the shifts of the carbon atom ppm for the effect of methyl substitution on ^-car- and the protons attached to this carbon 14-16. bon in ethylenic systems and about — 7.2 ppm for FRIEDEL and RETCOFSKY 17 have studied the methyl the a-carbon. These calculated values agree well with 17 substitution effects on the C13 chemical shifts in the observed values of + 4.4 to + 6.8 ppm for ethylenic systems. They have observed in ethylene /?-carbon and —3.3 to — 7.3 ppm for a-carbon de- that when a proton is replaced by a methyl group, pending on the other substituents. This is considered as an evidence for the validity of the methyl substi- the ß-C13 resonance shifts to higher field while the tution effects on the proton chemical shifts. a-C13 resonance shifts to lower field. This parallels the methyl substitution effect studies carried out by In toluene the pattern of the aromatic proton REDDY and coworkers 10-12 on proton resonances in region is a broad peak with clear indications of ethylenic and some heterocyclic systems. It has been complicated structure, suggesting that the chemical shown 10-12 that in ethylene, when a proton is re- shifts of these protons are different and the center placed by a methyl group, the /?-proton resonances of this broad peak is at higher field than in benzene.